definitive guide to dfm success

8/17/2019 Definitive Guide to DFM Success

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2A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS

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Contents

Machining Design Guidelines ..................................................................................... 6

Drilling Guidelines 6

Standard Hole Sizes ................................................................................................................................................. 7

Holes with Flat Bottoms .......................................................................................................................................... 8

Holes Intersecting Cavities ...................................................................................................................................... 9

Partial Holes .......................................................................................................................................................... 10

Deep Holes ............................................................................................................................................................ 11

Entry/Exit Surface for Holes .................................................................................................................................. 12

Milling Guidelines 13

Deep Radiused Corners ......................................................................................................................................... 14

Sharp Internal Corners .......................................................................................................................................... 15

Tool Accessibility ................................................................................................................................................... 16

Narrow Regions in Pockets.................................................................................................................................... 17

Side Radius and Bottom Radius ............................................................................................................................ 18

Tool Clearance Check ............................................................................................................................................ 19

Angular Milling Faces ............................................................................................................................................ 20

Turning Guidelines 21

Blind Hole Relief .................................................................................................................................................... 22

Keyways ................................................................................................................................................................. 23

Long - Slender Turned Parts .................................................................................................................................. 25

Minimum Internal Corner Radius .......................................................................................................................... 26

Outer Diameter Profile Relief ................................................................................................................................ 27

Symmetrical Axial Slots ......................................................................................................................................... 28

Casting Design Guidelines ........................................................................................ 29

Fillet Radius ........................................................................................................................................................... 30

Uniform Wall Thickness......................................................................................................................................... 31

Mold Wall Thickness ............................................................................................................................................. 32

Wall Thickness Variation ....................................................................................................................................... 33

Undercuts .............................................................................................................................................................. 34

Draft Angles ........................................................................................................................................................... 35

Injection Molding Design Guidelines ........................................................................ 36

Wall Thickness Design Guidelines 36Mold Wall Thickness ............................................................................................................................................. 37


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Uniform Wall Thickness......................................................................................................................................... 38

Wall Thickness Variation ....................................................................................................................................... 39

Ribs Design Guidelines 40

Recommended Rib Parameters ............................................................................................................................ 41

Minimum Radius at Base of Ribs ........................................................................................................................... 42

Draft Angle for Ribs ............................................................................................................................................... 43

Spacing Between two Parallel Ribs ....................................................................................................................... 44

Bosses Design Guidelines 45

Minimum Radius at Base of Boss .......................................................................................................................... 46

Spacing between Bosses ....................................................................................................................................... 47

Radius at Base of Hole in Boss .............................................................................................................................. 48

Minimum Draft for Boss OD .................................................................................................................................. 49

Minimum Draft for Boss ID ................................................................................................................................... 50

Boss Height to OD Ratio ........................................................................................................................................ 51

Minimum Radius at Tip of Boss ............................................................................................................................. 52

Chamfer at Top of Boss ......................................................................................................................................... 53

Wall Thickness of Boss .......................................................................................................................................... 54

Standalone Boss .................................................................................................................................................... 55

General Design Guidelines 56

Minimum Draft Angle ............................................................................................................................................ 57

Undercut Detection ............................................................................................................................................... 58

Sharp Corners ........................................................................................................................................................ 59

Hole Depth to Diameter Ratio ............................................................................................................................... 60

Sheet Metal Design Guidelines ................................................................................. 61

Form Feature Design Guidelines 61

Minimum Distance from Dimple to Bend ............................................................................................................. 62

Minimum Distance from Dimple to Cutout .......................................................................................................... 63Minimum Distance between Dimples ................................................................................................................... 64

Minimum Distance from Dimple to Hole .............................................................................................................. 65

Minimum Distance from Dimple to Part Edge ...................................................................................................... 66

Maximum Embossment Depth ............................................................................................................................. 67

Gussets .................................................................................................................................................................. 68

Curl and Lance Design Guidelines 73

Curl Radius............................................................................................................................................................. 74

Minimum Distance between Curl and Hole .......................................................................................................... 75

Minimum Depth of Lance ...................................................................................................................................... 76

Minimum Distance from Bend to Lance ............................................................................................................... 77


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Machining Design Guidelines

Drilling Guidelines


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Holes with Flat Bottoms

It is recommended to make blind holes with conical and not flat bottoms. Flat bottomed holes cause

problems in subsequent operations (for example: reaming). Also, flat bottomed holes require special

tooling operations leading to increase in manufacturing cost and time.

A standard twist drill is used to create a hole with conical bottom.

Example

In this example, hole size is less than 45 mm; preferred operation will be drilling. It is recommended

to use conical bottom holes.


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Holes Intersecting Cavities

In drilling operation, it is recommended to avoid holes intersecting with cavities.

If not, there are chances that the drill tool will wander. This also increases the chances of drill tool

breakage.

Example

In this example, if an intersection is unavoidable, at a minimum, the centerline of the hole should be outside the cavity as

shown in image. Order of machining can also impact the drilling condition.


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Partial Holes

Avoid partial holes as there are high chances that drill will wander if a large portion of the hole is

outside the material. The problem can become even more severe if the axis of hole is on or near the

edge of the material. If partial hole is unavoidable, then ensure that at least 75% of hole area should

be within the material.

Example


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Deep Holes

Deep, small diameter holes should be avoided as they are difficult to machine. Small diameter drills

tend to wander and are prone to breaking. Chip removal also becomes difficult while drilling deep

holes. Therefore it is recommended that the hole diameter to depth ratio should be less than 3.

Example

In this example, recommended hole diameter to depth ratio is less than 3.


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Entry/Exit Surface for Holes

Drills should enter and exit surfaces that are perpendicular to the centerline of the hole. If the drill tip

contacts the non-planer surface, then tip will wander as its axis is not perpendicular.

Also exit burrs will be uneven around the circumference of the exit hole, which can make burr

removal difficult.

Example


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Milling Guidelines


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Deep Radiused Corners

Flute engagement in the milling operation is important because it directly influence the forces. When

the axial depth of cut is increased, the length of engaged flutes increases, and the milling forces also

increase.

Longer end mills are prone to breakages and chatter, requires longer machining time and results in

increased tool vibrations.

Vibration creates uneven wear on cutting tools and thereby shortens tool life.

Designers should design milling areas such that longer end mills are not required to machine it.

Example


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Tool Accessibility

Features should be accessible to the cutting tool in the preferred machining orientation.

Example


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Narrow Regions in Pockets

It is recommended to avoid features that are too close to each other such that the gap between them

is too narrow to allow milling cutter to pass through them. If narrow regions are unavoidable, then

they should not be very deep.

The size of the milling cutter is constrained by the smallest distance between the faces of the feature.

Small diameter cutters are prone to breakage and chatter. Hence larger diameter, shorter cutters are

generally preferred.

Example


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Side Radius and Bottom Radius

Use of standard side radius and bottom radius for milling features will ease manufacturing of milling

features with standard available milling tools. For reducing, machining cycle time and tool setup cost,

it is recommended to avoid non-standard side radius and bottom radius.

As a general guideline use single standard side radius and single standard bottom radius.

Example


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Tool Clearance Check

Machining Features or slots should be accessible to the cutting tool in the preferred machining

direction and at the same time there should not be any clash between tool holder and component

while machining the feature.

Tool clash with components leads to adverse effect like tool damage, component damage, and it will

be unsafe for machine operator as well.

Example


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Angular Milling Faces

Side and bottom faces of milling features separated by bottom fillet should be at 90º to each other to

allow production with an end mill having bottom corner radius. Machining of angular faces require

multi-axis machining, which leads to higher machining cost.

It is recommended that side and bottom faces of milling features separated by bottom fillet should

be at an angle 90º to each other.

Example


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Turning Guidelines


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Keyways

The most commonly used type of keys are woodruff, round-end machine and square-end machine.

Out of the three, square-end machine key is most difficult and costly to generate.

For generating radiused keyways, end milling cutter or a slotting cutter has to be used and can be

done much faster compared to other processes. To suit a cutting tool, blind axial keyways should be

radiused at the end.

It is recommended to use radiused keyways at the end of a key-slot. Radiused keyways need to be

milled with end milling cutters, so that the rounded end or ends of the key may fit the ends of the

keyway. The cutter’s diameter should always be equal to the width of the key.

A blind keyway that is square at the end is very difficult to achieve by milling process.

Keyways should be radiused at the end to suit the cutting tool.


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If the end of keyway is radiused in such a way that it could be cut by a slotting cutter it will improve

the speed of machining keyways.


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Long - Slender Turned Parts

The axial-length of the part should be proportional to part’s minimum diameter to avoid deflection

towards center direction.

For long-slender part, it is recommended to center drill the free end and use a dead or live center in

the tailstock to support it. Without support, the work piece would bend and displace away from the

tool. Also, it may loosen the grip of chuck jaws on work piece and cause injuries or accidents.

However a very long slender part, even if supported by tail stock, may deflect toward the center.

Where possible, turned parts should be designed such that a tail stock support is not required. This is

done by designing the part to be stubby rather than long with a high aspect ratio. It is recommended

that, ratio of total length of part to its minimum diameter should be less than or equal to 8.


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Minimum Internal Corner Radius

It is recommended to avoid sharp internal corners and provide large inside radius so that a tool with

large nose radius can enter easily. A tool with large radius is less prone to breakage. A turn-down

surface at right angles to an un-machined (cast) surface might lead to burrs. The minimum radius on

internal corners of a turned part determines the cutting inserts that can be used.

It is always recommended to use cutting inserts with larger radii.


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Outer Diameter Profile Relief

Profile relief on the outer diameter of a turned part helps in smooth turn-machining of part.

The turned contour should allow easy tracing with a minimum number of changes of stylus and

cutting tool. Grooves with parallel or steep sidewalls are not feasible in one operation.

Contours inclined upto an angle of 58° from the axis of the part are feasible for a cutting tool (and

stylus). Following image shows feasible contours when they are perpendicular to the axis.


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Casting Design Guidelines


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Fillet Radius

Sharp corners, edges and rapid changes in cross section should be avoided in cast parts. Fillets should

be added to sharp corners and edges.

Inside corners should be designed with fillets and outside corners should have radii as large as

possible. Depending on the casting process, minimum fillet radii should be provided on inside and

outside corners of the components. For example, in die casting, a minimum radius of 1.5 times the

wall thickness should be provided.

Example

.


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Uniform Wall Thickness

Wall thickness should be kept uniform as it helps to create high quality cast parts. Sudden variations

and geometry changes and in wall thickness affects metal flow, resulting in air enclosures and poor

surface finish of parts.

The recommended range of wall thickness is two times the thinnest wall section. The transition from

thick to thin walls should also be as gradual as possible.

Example


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Mold Wall Thickness

Mold wall thickness is an important aspect to be considered in casting. If the mold wall is too thin and

elongated, stresses are developed in the mold, reducing mold life. Also, special materials are required

to create the molds and they may need regular replacement and service. Ribs and bosses which are

too close to each other can result in thin mold walls.

Hence the minimum allowable mold wall thickness should be decided based on process and material

considerations. The minimum clearance between features of a cast component will be based on the

casting process, component material and tool material.

Example


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Wall Thickness Variation

Wall thickness variations in the casting result in differing rates of cooling, shrinkage, warping and

distortion. Ideally, the wall thickness should be uniform throughout the part (equal to the nominal

wall thickness).

In reality, this variation is unavoidable due to functional and aesthetic requirements. However, the

amount of variation should be minimized and within a certain tolerance limit.

Example


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Undercuts

It is recommended that undercuts should be avoided for ease of manufacturing. Undercuts require

additional mechanisms, adding to mold cost and complexity. Clever part design or minor design

concessions can often eliminate complex mechanisms for undercuts.

Example


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Draft Angles

Draft is the taper given to core and cavity for easy removal of casting (or pattern). Adding proper

drafts on the cast parts improves cycle time and quality of surfaces. The sidewalls of the castings and

other features perpendicular to the parting line must be drafted as much as possible.

The draft angle will depend upon the type of material and varies inversely with height of the wall.

Example


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Injection Molding Design Guidelines

Wall Thickness Design Guidelines


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Uniform Wall Thickness

Non-uniform wall sections can contribute to warpage and stresses in molded parts. Sections which

are too thin have a higher chance of breakage in handling, may restrict the flow of material and may

trap air causing a defective part. Too heavy a wall thickness, on the other hand, will slow the curing

cycle and add to material cost and increase cycle time.

Generally, thinner walls are more feasible with small parts rather than with large ones. The limiting

factor in wall thinness is the tendency for the plastic material in thin walls to cool and solidify before

the mold is filled. The shorter the material flow, the thinner the wall can be. Walls also should be as

uniform in thickness as possible to avoid warpage from uneven shrinkage.

When changes in wall thickness are unavoidable, the transition should be gradual and not abrupt.


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Wall Thickness Variation

Wall thickness variation should be within tolerance so as to allow for smooth filling of the mold.

Ideally, the wall thickness should be uniform throughout the part (equal to the nominal wall

thickness). In reality, the variation is unavoidable due to requirements of functions and aesthetics.

However, the amount of variation has to be minimized.

Non-uniform wall thicknesses may cause uneven plastic flow and cause different parts of the part to

cool at different rates. This can cause warpage toward the heavier portion of the model. If an uneven

wall thickness is unavoidable, it may be necessary to provide additional cooling for the heavier

sections. This increases tooling complexity and adds to production costs.

In general, gradual change of 25% and 15% is acceptable in amorphous (PC, ABS, etc.) and semi

crystalline (Nylons, PE, etc.) materials respectively.


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Ribs Design Guidelines


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Recommended Rib Parameters

Ribs are used to improve melt flow into sections, like corners or a large boss and to increase stiffness

of the part. The most economical way to increase stiffness of the part is by adding reinforcing ribs.

Ribs should be placed on the mounds opening direction. For parts under bending, ribs should be

placed perpendicular to the bending moment to achieve good stiffness.

However, projections like ribs can create cavity filling, venting, and ejection problems. These

problems become more troublesome for taller ribs. Ribs need to be designed in correct proportion to

avoid defects such as short shots and provide the required strength. Thick and deep ribs can cause

sink marks and filling problems respectively. Deep ribs can also lead to ejection problems. If ribs are

too long or too wide, supporting ribs may be required. It is better to use a number of smaller ribs

instead of one large rib (which can lead to voids or sink marks).

Generally, the rib height is recommended to be not more than 2.5 to 3 times the nominal wall

thickness. Similarly, rib thickness at its base should be around 0.4 to 0.6 times the nominal wall

thickness.

Example

t = Nominal Wall Thickness

W = Width of the Rib

H = Height of the Rib

T = Thickness at base of the Rib


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Minimum Radius at Base of Ribs

A fillet of a certain minimum radius value should be provided at the base of a rib to reduce stress.

However, the radius should not be so large that it results in thick sections. The radius eliminates a

sharp corner and stress concentration. Flow and cooling are also improved.

Fillet radius at the base of ribs should be between 0.25 to 0.4 times the nominal wall thicknesses of

the part.

Example


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Draft Angle for Ribs

Draft angle design is an important factor when designing plastic parts. Such parts may have a greater

tendency to shrink onto a core. This creates higher contact pressure on the core surface and

increases friction between the core and the part, thus making ejection of the part from the mold

difficult. Hence, draft angles should be designed properly to assist in part ejection. This also reduces

cycle time and improves productivity. Draft angles should be used on interior or exterior walls of the

part along the pulling direction.

A general guideline suggests that draft angle for rib should be around 1 to 1.5 deg. Minimum draft

should be 0.5 per side.

Example


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Spacing Between two Parallel Ribs

Mold wall thickness gets affected due to spacing between various features in the plastic model. If

features like ribs are placed close to each other or the walls of the parts, thin areas are created which

can be hard to cool and can affect quality. If the mold wall is too thin, it is also difficult to

manufacture and can also result in a lower life for the mold due to problems like hot blade creation

and differential cooling.

A general guideline suggests that spacing between ribs should be at least 2 times the nominal wall.

Example


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Bosses Design Guidelines


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Minimum Radius at Base of Boss

Bosses find use in many part designs as points for attachment and assembly. The most common

variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or

other types of fastening hardware. Under service conditions, bosses are often subjected to loadings

not encountered in other sections of a component. Provide a generous radius at the base of the boss

for strength and ample draft for easy part removal from the mold.

A fillet of a certain minimum radius value should be provided at the base of boss to reduce stress. The

intersection of the base of the boss with the nominal wall is typically stressed and stress

concentration increases if no radii are provided. Also, the radius at the base of the boss should not

exceed a maximum value to avoid thick sections. The radius at base of boss provides strength and

ample draft for easy removal from the mold.

It is recommended that the radius at the base of boss should be 0.25 to 0.5 times the nominal wall

thickness.

.


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Spacing between Bosses

When bosses are placed very close to each other, it results in creating thin areas which are hard to

cool and can affect the quality and productivity. Also, if the mold wall is too thin, it is very difficult to

manufacture and often results in a lower life for the mold, due to problems like hot blade creation

and differential cooling.

The general guide line suggests that spacing between bosses should be at least 2 times the nominal

wall thickness.


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Radius at Base of Hole in Boss

Providing a radius on the core pin helps in avoiding a sharp corner. This not only helps molding but

also reduces stress concentration.

It is recommended that the radius at base of hole in boss should be 0.25 to 0.5 times the nominal wall

thickness.


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Minimum Draft for Boss OD

An appropriate draft on the outer diameter of a boss helps easy ejection from the mold. Draft is

required on the walls of boss to permit easy withdrawal from the mold.

It is recommended that minimum draft on outer surface of the boss should be greater than or equal

to 0.5 degree.


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Minimum Draft for Boss ID

Designs may require a minimum taper on the ID of a boss for proper engagement with a fastener.

Draft is required on the walls of boss to permit easy withdrawal from the mold.

It is recommended that minimum draft on the hole in boss should be greater than or equal to 0.25

degree.


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Boss Height to OD Ratio

A tall boss with the included draft will generate a material mass and thick section at the base. In

addition, the core pin will be difficult to cool, can extend the cycle time and affect the cored hole

dimensionally.

General guidelines suggest that the height of boss should be less than 3 times of outer diameter.


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Minimum Radius at Tip of Boss

Bosses are features added to the nominal wall thickness of the component and are usually used to

facilitate mechanical assembly. Under service conditions, bosses are often subjected to loadings not

encountered in other sections of a component.

A fillet of certain a minimum radius value should be provided at the tip of boss to reduce stress.


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Chamfer at Top of Boss

Boss should have chamfer on top. A chamfer at top of boss is good lead in for the fasteners.


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Wall Thickness of Boss

Wall thicknesses for bosses should be less than 60 percent of the nominal wall to minimize sinking.

However, if the boss is not in a visible area, then the wall thickness can be increased to allow for

increased stresses imposed by self-tapping screws.

It is recommended that wall thickness of boss should be around 0.6 times of nominal wall thickness

depending on the material.

.


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Standalone Boss

Bosses and other thick sections should be cored. It is good practice to attach the boss to the sidewall.

In this case the material flow is uniform and provides additional load distribution for the part.

For better rigidity and material flow, the general guideline suggests that boss should be connected to

nearest side wall.


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General Design Guidelines


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Minimum Draft Angle

Draft angle design is an important factor when designing plastic parts. Because of shrinkage of plastic

material, injection molded parts have a tendency to shrink onto a core. This creates higher contact

pressure on the core surface and increases friction between the core and the part, thus making

ejection of the part from the mold difficult. Hence, draft angles should be designed properly to assist

in part ejection. This also reduces cycle time and improves productivity. Draft angles should be used

on interior and exterior walls of the part along the pulling direction.

It is typically recommended that the draft angle for sidewall should be at least between 0.5 to 2

degrees for inside and outside walls, although a larger angle will make it easier for part release.


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Undercut Detection

Undercuts should be avoided for ease of manufacturing. Undercuts typically require additional

mechanisms for manufacture adding to mold cost and complexity. In addition, the part must have

room to flex and deform.

Clever part design or minor design concessions often can eliminate complex mechanisms for

undercuts. Undercuts may require additional time for unloading molds. It is recommended that

undercuts on a part should be avoided to the extent possible.


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Sharp Corners

Generously rounded corners provide a number of advantages. There is less stress concentration on

the part and on the tool. Because of sharp corners, material flow is not smooth and tends to be

difficult to fill, reduces tooling strength and causes stress concentration. Parts with radii and fillets

are more economical and easier to produce, reduce chipping, simplify mold construction and add

strength to molded part with good appearance.

General design guideline suggests that corner radii should be at least one-half the wall thickness. It is

recommended to avoid sharp corners and use generous fillets and radii whenever required.

In addition inside and outside radii should have same center so as to avoid stresses during cooling as

shown in following image.


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Hole Depth to Diameter Ratio

Core pins are used to produce holes in plastic parts. Through holes are easier to produce than blind

holes which don’t go through the entire part. Blind holes are created by pins that are supported at

only one end; hence such pins should not be long. Longer pins will deflect more and be pushed by

the pressure of the molten plastic material during molding.

It is recommended that hole depth-to-diameter ratio should not be more than 2.


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Sheet Metal Design Guidelines

Form Feature Design Guidelines


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Minimum Distance from Dimple to Bend

A certain minimum distance must be maintained between dimple and the bend feature to avoid

deformation and fracture of the metal.

It is recommended that the minimum distance between dimple to bend should be two times sheet

thickness plus the inside radius of the dimple plus radius of the bend.

t = Sheet metal thickness


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Minimum Distance from Dimple to Cutout

A minimum distance must be maintained between dimple and cutout edge to avoid deformation and

fracture of the metal.

It is recommended that a minimum distance of four times the sheet thickness plus the inside radius

of each dimple must be maintained between a dimple and cutout edge.



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Minimum Distance between Dimples

Care need to be taken when placing formed features close to each other. If a station does not clear a

form already placed in a part, the form will get flattened out.

It is recommended that the minimum distance between dimples should be four times sheet thickness

plus radius of the dimple.



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Minimum Distance from Dimple to Hole

To avoid deformation and fracture of the metal a certain minimum distance should be maintained

between dimple and adjacent holes

It is commonly recommended that the minimum distance between dimple and hole should be three

times sheet thickness.



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Minimum Distance from Dimple to Part Edge

It is recommended that the minimum distance between dimples to part edge should be four times

material thickness plus radius of the dimple to avoid deformation and fracture of the metal.



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Maximum Embossment Depth

Embosses are small, shallow formed projections on the surface of stamped parts. During this

operation, stretching is the main deformation mode resulting in high tension. Thereby the metal is

subject to excessive thinning or fracturing. Consequently, the depth of the embossed feature is

restricted by the material's thickness and ability to stretch in addition to the emboss geometry.

It is recommended that the maximum depth of embossment be less than or equal to three times

material thickness.



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Gussets

Gussets are used to strengthen a flange without the need for secondary processes such as welding.

A general guideline suggests that the width and depth of gusset at an angle of 45 degrees is directly

proportional to the radius and material thickness.


h = depth of gussets

W = width of gussets


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Minimum Distance from Hole to Gusset

A certain distance must be maintained between a hole and gusset to avoid metal deformation and

fracturing.

It is recommended that minimum distance between hole edge to gusset should be at least eight

times the material thickness.



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Minimum Distance from Extruded Hole to Part Edge

Extruding metal is one of the most extreme pressure applications in press working and generates lot

of friction and heat. If an extruded hole is too close to the part edge, it can lead to deformation or

tearing of the metal.

It is recommended that the minimum distance between the extruded holes to part edge should be at

least three times the thickness of sheet.



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Minimum Distance between Extruded Holes

Certain distance should be maintained between two extruded holes in sheet metal designs. If

extruded holes are too close it can lead to metal deformation.

It is recommended that the minimum distance between two extruded holes should be six times the

thickness of sheet metal.



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Minimum Distance from Extruded Hole to Bend

A certain distance must be maintained between extruded hole to bend to avoid metal deformation

and fracturing.

It is recommended that the minimum distance between extruded hole and bend should be three

times the thickness of sheet metal plus bend radius.



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Curl and Lance Design Guidelines


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Curl Radius

Curling is the process of forming the sheet metal flange into a rolled shape. Curling strengthens the

edges and provides smoothness to the surface. It is commonly used as a means of joining two

components.

Curls are often added to avoid sharp edges and make parts safer for handling and use.

It is recommended that the outside radius of curl should be minimum 2X of the material thickness.


R = Outside radius


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Minimum Distance between Curl and Hole

It is recommended that the minimum distance between a curl and the edge of a hole should be sum

of curl radius and material thickness.


D = Distance


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Minimum Depth of Lance

Lancing is a piercing operation in which the work piece is sheared and bent with strike of a die. In this

process there is no material removal however it only modifies the geometry. Lancing can be used to

make partial contours and free up material for other operations.

Lancing is used to make tabs, vents and louvers.

It is recommended that the minimum depth of lance should be 2X the material thickness.


H = Depth of lance


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Minimum Distance from Bend to Lance

During lancing operation a sufficient degree of clearance should be given around the lance feature

and bend.

It is recommended that minimum distance between lance and bend should be 3X the material

thickness plus bend radius.


r = Inside bend radius

r

d


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Minimum Distance from Hole to Lance

During lancing operation we need to maintain sufficient degree of clearance around the lance feature.

It is recommended that the distance between lance and hole should be 3X the material thickness.



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Minimum Spacing between Lances

During lancing operation sufficient degree of clearance should be maintained around the lance

feature as the punch and die will need some degree of clearance around the feature in order to hold

down the work piece during operation. If another lance is placed inside this working envelope it will

be crushed by the punch and die, potentially damaging the work piece and tools.

It is recommended to maintain sufficient clearance between two lance features by considering die

and punch clearance allowance.



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Hem Design Guidelines


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Rolled Hem

It is recommended that the ratio of the external radius of a rolled hem to material thickness should

be greater than or equal to 2.

Also, the ratio of the rolled hem opening to material thickness should be greater than or equal to 1.

R = Rolled hem's external radius

D = Rolled hem's opening


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Tear Drop Hem

In tear drop hem the bend angle is greater than 180 degrees. It is recommended to consider the

following guidelines while designing a tear drop hem -

The ratio of the radius of a tear drop hem to the sheet metal thickness should be greater than or

equal to 0.5

The ratio of the return flange height to the sheet metal thickness should be greater than or equal to

4.0

The ratio of the hem opening (spacing between the hem edge and the part) to the sheet metal

thickness should be greater than or equal to 0.25

Where,

d = Diameter of a tear drop hem.

H = Return flange height of the tear drop hem.

D= Hem Opening

T = Sheet Metal Thickness


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Rolled Hem to Hole Edge Distance

The ratio of the distance between the rolled hem and the edge of a hole to the sheet thickness

should be greater than or equal to 1.

T = Sheet metal thickness

D = Distance


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Closed Hem

Hems without any inside radius are called as closed hem. Closed hems are not recommended if the

hem geometry is to be painted or if the material used is SST or Aluminum.

In closed hems, return flange length from outside the bend should be equal to or greater than four

times the material thickness.

H = Return flange height of the tear drop hem

T = Sheet Metal Thickness


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Knife Edge

Knife edge features on sheet metal parts should be avoided for ease of manufacturing. To achieve

knife edges on sheet metal parts, additional operations may be required which increases the cost of

the product. Knife edge conditions can also lead to fatigue failures in certain conditions.

It is recommended to avoid knife edge conditions as it can either increase the manufacturing cost and

complexity or lead to fatigue failures.


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Notch and Tab Parameters


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Notch Parameters

Notching is a shearing operation in which a section is removed the outside edge of metal strip or part.

Notching is typically manual and low-cost process performed with a small range of standard punches.

Appropriate notch parameters results in use of standard punch sizes

Notch Width ( W) should not be narrower than 1.5 x t

Notch Length (L) should be maximum up to 5 x t

Recommended corner radius for notches should be 0.5 x t

t = Sheet Metal Thickness

L = Length/ Depth of Notch

W = Width of Notch


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Notch to Bend Distance

In case distance between notches to bend is very small then distortion of sheet metal may take place.

To avoid such condition notch should be placed at appropriate distance from bend with respect to

sheet thickness.

It is recommended that minimum distance from notch to bend should be 3 times the material

thickness plus inside bend radius.



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Minimum Distance between Notches

It is recommended that minimum distance between two Notches should be two times the material

thickness.


D = Distance between Notches


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Minimum Distance between Notch to Hole

It is recommended that minimum distance between Notch and Hole should be 1.2 times the material

thickness.


D = Distance between Notch and Hole


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Tab Parameters

A protrusion from an edge of the sheet metal part is called a tab.

It is recommended that Tab Width [C] should be greater than two times the sheet thickness.

It is recommended that Tab Depth [B] should be less than or equal to five times the tab width.


B = Tab Depth

C = Tab Width


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Minimum Distance between Tabs

It is recommended that minimum distance between tabs should be 1.5 times the sheet metal

thickness.


D = Tab Distance


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Burring Hole Height

Sufficient burring holes height is needed to accommodate the stress during loading. It is

recommended that the minimum burring height should be at least 2 times the sheet metal thickness.

Where,

h = Burring hole height



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Burring Hole Inner Diameter

Burring hole inner diameter should be sufficient enough to accommodate the pins, bolts etc.

Where,

d1 = Burring hole inner diameter


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Burring Hole Distance to Bends

Burring holes that are too close to the bend will distort during the bending process. Distortion will be

minimal if the distance between the edge of the hole to the beginning of the inside bend radius is at

least four times the material thickness.

It is recommended that the minimum distance between the burring hole edge and the bend should

be at least 4 times the sheet thickness.

Where,

A = Minimum distance from a burring hole edge and bend



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Burring Hole Distance to Part Edge

If the distance from a burring hole edge and the part edge falls below a minimum value, then it can

lead to distortion or breaking of the sheet. If a burring hole is too close to an edge, the edge can

distort, forming a bulge.

It is recommended that the minimum distance from a burring hole edge and the part edge should be

at least 4 times the sheet thickness.

Where,

B = Minimum distance from a burring hole edge and the part edge



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Burring Hole Distance to Cutout

If the distance from a burring hole edge and the cutout edge falls below a minimum value, there is a

chance of distortion or cracking of the sheet in the area under consideration. If a burring hole is too

close to an edge, the edge can distort, forming a bulge.

It is recommended that the minimum distance from a burring hole edge and the cutout edge should

be at least 4 times the sheet thickness.

Where,

D = Minimum distance from a burring hole edge and cutout edge



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Burring Hole Spacing

Sufficient, space should be mentioned between the two adjacent burring holes to avoid distortion or

else harder materials may crack during fabrication.

It is recommended that the minimum distance between the burring hole edge and other burring hole

edge should be greater than or equal to 4 times the sheet metal thickness.

Where,

C = Burring hole spacing



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General Design Guidelines


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Preferred Sheet Sizes

Length and width of a flattened sheet metal part should be as per the standard sheet sizes that are

available in the inventory.

Each organization has its standard or preferred sheet sizes which are in accordance with the

machine’s capability and setup. At the initial stage of the design, designer should be aware of

unfolded sheet size and availability of corresponding appropriate sheet sizes in the inventory. This

will help designer to take decision related to inventory management. Indirectly it helps to reduce

inventory of non-standard sheet sizes and cost of manufacturing.

Length and width of the flattened sheet metal part should be within the preferred sheet sizes used

for manufacturing, depending on the material and sheet thickness.

L = Sheet metal length

W = Sheet metal width

CL = Trimming clearance on length side

CW = Trimming clearance on width side

Note: For nested components, additional considerations will be required.


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Minimum Hole Diameter

If the punch diameter becomes too small to bear the shear force required to punch the hole over a

small area, it may lead to failure.

Where,

d1 = Hole diameter



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Interference in Flat Pattern

It is not feasible to manufacture a sheet metal part when its design involves interference during

unfolded state of sheet metal.

Interference in flat pattern should be avoided for ease of manufacturing as there should not be any

interference in sheet metal component design when sheet metal part is unfolded.


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Half-Shear Parameters

A Semi-shear or half-shear is a blind hole that is punched on the sheet metal surface. The ratio of

half-shear depth to sheet metal thickness should not be too large to avoid deformation and fracture

of the metal.

It is recommended that a maximum ratio of half-shear depth to sheet metal thickness should be less

than or equal to 0.6 times the sheet metal thickness.


h = Half-shear depth


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Hole To Part Edge Distance

To prevent distortion or tearing, hole should be sufficiently away from the part edge. It is

recommended that minimum distance from a hole to edge of a part should be at least 2 times sheet

thickness

T= Sheet metal thickness


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Minimum Bend Radius

Minimum bend radius is a function of the ductility and thickness of the material being worked. The

minimum bend radii requirements can vary depending on applications and material. For aerospace

and space applications, the values may be higher. When the radius is less than recommended, this

can cause material flow problems in soft material and fracturing in hard material. Localized necking

or fracture may also occur in such cases.

It is recommended that minimum inner bend radius should be at least 1 * material thickness.

r = Inside Bend radius



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Bend Relief

Bend relief is the notch that needs to be created for sheet metal bending. Bend relief helps control

the sheet metal material behavior and prevents unwanted deformation.

The flange which does not have relief will result in a greater amount of distortion or tearing of the

adjacent material.

It is recommended that the depth of bend relief should be greater than or equal to the inside bend

radius of the bend and also width of the bend relief should be equal to or greater than the sheet

metal thickness.


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Minimum Hole Diameter

The diameter of the hole in sheet metal part should not be very small, small holes are created by

piercing operation and for manufacture small holes, small sizes punches are required.

Small hole size in sheet metal requires smaller size punching tool which may leads to break during the

operation.

It is recommended that the diameter of the hole should be equal or more than the thickness of the

sheet metal.

A = Minimum Hole Diametert = Sheet metal thickness


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Curls Feature

Curling sheet metal is the process of adding a hollow, circular roll to the edge of the sheet. The curled

edge provides strength to the edge and makes it safe for handling. Curls are most often used to

remove a sharp untreated edge and make it safe for handling.

It is recommended that:

The outside radius of a curl should not be smaller than 2 times the material thickness.

A size of the hole should be at least the radius of the curl plus material thickness from the curl

feature.

A bend should be at least the radius of the curl plus 6 times the material thickness from the curl

feature.


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Hem Feature

Hemming is nothing but to fold the metal back on itself. In Sheet Metal Hems are used to create folds

in sheet metal in order to stiffen edges and create an edge safe to touch.

Hems are most often used to remove a sharp untreated edge and make it safe for handling. Hems are

commonly used to hide imperfections and provide a generally safer edge to handle. a combination of

two hems can create strong, tight joints with little or minimal fastening. Hems can even be used to

strategically double the thickness of metal in areas of a part which may require extra support.

It is recommended that:

For Tear drop hems, the inside diameter should be equal to the material thickness.

For Open hem the bend will lose its roundness when the inside diameter is greater than the

sheet metal thickness.

For bends, the minimum distance between the inside edge of the bend and the outside of the

hem should be 5 times material thickness plus bend radius plus hem radius.

http://sheetmetal.me/sheet-metal-hems/http://sheetmetal.me/sheet-metal-hems/


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Notch Feature

Notching is a shearing operation that removes a section from the outer edge of the metal strip or

part. In case, distance between the notches to bend is very small then distortion of sheet metal may

take place.

To avoid such condition notch should be placed at appropriate distance from bend with respect to

sheet thickness.

Notching is a low-cost process, particularly for its low tooling costs with a small range of standard

punches.

Recommendations for Notch Feature:

Notch Width should not be narrower than 1.5 * t

Length of notches can be up to 5 * t

Recommended corner radius for notches should be 0.5 * t

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