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Speeds and Feeds for Milling and Drilling 03/19/12

Brent Dunn/BCIT of 14

Module - Speeds and Feeds for Mil ling and Dril ling

Objectives

• compute the speed, feed, and depth of cut for milling

• compute the speed and feed for drilling

• describe methods used to make threads

• compute the speed and feed for tapping





Overview

Prior to writing a program, the following must be calculated

• the speed (rpm) of the cutter

• the feed rate (how fast the cutter moves through the material)

•

the depth of cut

Speed

• cutting speed is the velocity of the cutting edge

• depends on the material you are cutting and the material that the tool is made from

• cutting speed greatly affects tool life

Calculating Speed

• you set the machine's speed in revolutions per minute (RPM)

• in manufactures’ data or other handbooks, the cutting speed is specified in surface

feet/minute (SFM) or meters/min

Calculation relating RPM to CUTTING SPEED (V)

• procedure

• find desired cutting speed in SFM from manufacturer’s or machinist's data book

• compute RPM

RPM = (12 * SFM) / (π * tool_diameter)OR

RPM = 4 V / D (easy to remember in the shop)

where D is in inches, V is in feet/minute





Feed Rate

• feed rate is how fast the cutter is fed through the material

• you need to specify the feed rate in inches per minute (ipm) or mm/min

• the feed rate from a handbook is usually specified as the chip load per tooth in inches per

tooth (ipt) or mm/tooth• the chip load is the amount of material that each tooth of the cutter removes in each

revolution

• too high a chip load can lead to broken cutting edges because the force on thecutting edge will be large

• too low a chip load can lead to dull edges since the cutter tends to rub on thematerial rather than shearing it off

Calculation relating FEED to CHIP LOAD

• procedure

• find desired chip load per tooth from manufacturer’s or machinist's data book

• compute feed rate

feed rate = chip load per tooth * number of teeth * RPM

where chip load is in inches/tooth or mm/tooth

Depth of Cut

• axial depth of cut has the least affect on tool life - maximize it in order to minimize machiningtime but don’t go so deep that you break the tool or stall the machine!

• depends on radial depth of cut (width of cut), tool diameter, rigidity of setup and machine,

and available horsepower • to determine depth of cut, you must look at the volume of material removed since it affects

how much pressure (and force) is on the tool and how much horsepower it takes to turn thetool

• in general, the smaller the radial depth of cut, the larger the axial depth of cut

• when the radial depth of cut is large such as when you are slotting or roughing apocket with large stepover, and you are using the suggested chip load, the depth of cut is smaller than the tool diameter. The rule of thumb for our shop is





depth of cut = 1/3 x too l diameter

Note: This is conservative for regular milling. Some shops may go higher.

• when the radial depth of cut is small, you can go much deeper. For example, for a

finishing cut where the tool is taking 0.020” (0.5mm) from a wall, the depth of cutcould be several times the tool diameter.

Other Considerations for cutting

• you should normally remove most of the material by taking roughing cuts which have a highmaterial removal rate but do not machine accurately due to higher tools loads and may notgive the required surface finish.

• you will then take light finishing passes to remove the remainder of the material (this givesa more accurate surface due to lower loads and deflections and a better surface finish)

• finish cuts generally remove from 0.010” to 0.030”

•

some materials (e.g. stainless steel, titanium) have a very thin but hard surface layer dueto work hardening. On these materials, you must take heavier finish cuts so that the cutpenetrates below the hard surface layer into the softer material.

• on a CNC machine where there is very little backlash, you should climb mill when finishingto get the best surface finish. This requires you to select the correct feed direction. In somematerials, especially where the surface is hard or tough on the cutting edges, it is better touse conventional milling.

• you should create lead-in and lead-out moves

• for a lead-in, the cutter should plunge to cutting depth while it is away from the finishedsurface (in air if possible) then feed into the material tangential to the first cutting path

• for lead-out, the cutter should move away from the part tangential to the last path beforemoving up





Milling cutters

A milling machine can perform a variety of tasks. Each task requires a different type of cutter. Asample of popular cutters is shown below.

flat end mill • most popular cutter

• used for profiling, pocketing,slotting

• available in high speed steel(HSS) or carbide

• most popular have 2 to 4cutting flutes

• many configurationsavailable (single ended,short, long, double ended)

• regular and high helix

availableBall nose endmill

• used mainly for cuttingsurfaces

Carbide insertend mill

• replaceable inserts are costeffective

• available in large sizescompared to solid end mills

Carbide insertface mills

• used for facing (flattening) asurface

Carbide insertball nose endmills

• used for cutting surfaces





Speeds and Feeds for End Mills

The following tables are examples of recommended speeds and feeds for HSS and carbide endmills. Check with your tool supplier for recommendations or for other tool types.

Cutting Speed for End Mills

Material

HSS

Speed

(SFM)

Carbide

Speed

(SFM)

Aluminum / Aluminum Alloys 300 600

Soft Cast Irons 200 250

Medium Cast Irons 125 150

Hard Cast Irons 65 80

Brass/Bronze 200 200

Coppers / Copper Alloys 150 300

Magnesium 300 800Nickel Alloys 75 150

Free Machining Stainless Steels 100 150

Work Hardening StainlessSteels 50

75

Low Carbon Steels 150 200

Medium Carbon Steels 100 100

High Tensile Steels (35-40 Rc) 50 75

Tool Steels (40-50 Rc) 40 60

Titanium 80 100

Plastics 300 600

Notes on speeds and feeds:

• the speed shown is a good starting point, you can increase the speed by up to 100%when taking light finish cuts

• for slotting applications where a full cutter width of material is being removed, reduce thespeed to 75-80% of the value shown

• when milling steels, chip color indicates correct speed (tan – good, blue or dark – toofast, white – too slow)

Tool

diameter

Chip Load

(inch/tooth)

1/16" 0.0002 - 0.0005

1/8" 0.0005 – 0.001

1/4" 0.001 – 0.002

1/2" 0.002 - 0.004

3/4" 0.004 - 0.006

Note: Recommended chip load varies with

material type. The chip loads shown in thetable are suitable for softer materials. For

hard materials, reduce the chip load up to

50%. The chip load may need to be

reduced further if end mills are long or the

setup lacks rigidity.





Class Exercise

Compute a speed (rpm) and feed (ipm) for the following situations (all tools ½” diameter):

Tool Material Speed (rpm) Feed (ipm)

HSS end mill, 2 flutes Aluminum 6061-T6

Carbide end mill, 2flutes

Aluminum 6061-T6

HSS end mill, 2 flutes Steel 125 BHN

Carbide end mill, 2flutes

Steel 125 BHN

For the following situations, would you mill clockwise or counterclockwise in order to climb mill ?





Drilling

Drills are available in a variety of styles and materials. The most common drill is a high speedsteel twist drill with a 118 degree drill point angle. Solid carbide twist drills are available whichallow for faster drilling speeds and longer tool life at increased tool cost. Insert drills (areplaceable carbide insert) can also be advantageous in certain situations.

Drill Point

Most drills have a 118 degree angle which is fine for general purpose use in softer materials.Hard materials require a blunter drill point (e.g. 150 degrees) while soft materials like plasticsmight use a sharper point (e.g. 90 degrees)

Drill Depths

The depth of a hole on a drawing is the depth for the full diameter hole. For a blind hole, the

bottom of the hole will not be flat because the drill tip is not flat. When you write a CNCprogram, you program to the tip of the drill. Therefore, you have to calculate the height of the tipof the drill and add it to the hole depth. For through holes, the drill should pass completelythrough the part. To calculate the depth, add the drill tip amount plus a breakthrough amount of approximately 0.1”. Depths of drilled holes are not precise so you rarely need to calculate tomore than a couple of decimal points.

Drill tip calculation





Center drilling

Center drilling forms an entry point for subsequent drill operations. It is important when:

• the subsequent drill has a drill point (twist drill, spade drill)

• the hole size is small (generally < 1/2") since larger drills don't deflect as much

•

the hole must be precisely located

The hole must be center drilled deep enough so the drill tip does not make contact before theedge of the drill.

Spot Drilling

If a chamfer is desired on a hole, the hole can be spot drilled before drilling. Spot drilling willalso help to locate the hole so there is no need to center drill. A spot drill has a 90 degreeincluded tip angle which produces a 45 degree chamfer.

To determine the spot drill depth, first determine the diameter of the top of the cone that resultsfrom the spot drilling operation. For example, if you are drilling a 1/2" hole and want a 0.050"chamfer on the hole, the spot drilled hole diameter is 0.5 + 2x0.050=0.6 inches. The tip depth ishalf this amount or 0.3 inches.





Speeds and feeds for dr ill ing

The following table is an example of recommended speeds and feeds for drilling. Check withyour tool supplier for recommendations.

In drilling, the feed rate is given in IPR (inches per revolution). The feed rate in IPM isIPM = IPR * RPM

Material Speed Feed Rate (I.P.R)

SFM 1/16" 1/8" 1/4" 1/2" 3/4"

Aluminum / Aluminum Alloys 300-600 .0008 .003 .007 .012 .015

Aluminum Alloyed Si > 10% 150-400 .0008 .002 .006 .01 .012

Soft Cast Irons 200-300 .001 .003 .005 .01 .012

Medium Cast Irons 75 -150 .001 .003 .005 .008 .01

Malleable Cast Irons 65-200 .0005 .002 .004 .007 .01

Brass 100 .0007 .002 .003 .004 .006

Bronze 100 .0007 .002 .003 .004 .006

Coppers / Copper Alloys 150-300 .001 .003 .006 .01 .012

Magnesium 300-600 .001 .003 .007 .012 .015

Nickel Alloys 75-200 .001 .003 .005 .009 .012

Free Machining Stainless Steels 100-150 .001 .003 .005 .008 .012

Work Hardening Stainless Steels 50-100 .0005 .002 .004 .006 .01

Low Carbon Steels 100-200 .001 .002 .004 .007 .012

Medium Carbon Steels 100-200 .001 .002 .003 .006 .01

High Tensile (35-40 Rc) Steels 50-75 .001 .002 .003 .004 .005 Tool Steels 40-100 .001 .0015 .003 .005 .008

Soft Titanium 80-125 .001 .002 .004 .006 .01

Hard Titanium 40-100 .0007 .001 .002 .005 .008

For reaming, start at 1/2 to 2/3 of the drilling speed and two times the drilling feed rate.





Class Exercise

Compute a speed (rpm) and feed (ipm) for the following situations. Use the lower end of thecutting speed.

Tool Material Speed (rpm) Feed (ipm)

1/8” HSS drill Aluminum 6061-T6

1/2” HSS drill Aluminum 6061-T6

1/8” HSS drill Low Carbon Steel

1/2” HSS drill Low Carbon Steel

Calculate the drill tip depths for the following operations. Assume that the Z 0 position is at thetop surface of the workpiece.

Tool Operation Tip depth

1/8” HSS drill ½” deep hole in 1” plate

1/2” HSS drill ½” deep hole in 1” plate

1/2” HSS drill Through hole in ½” plate,0.1” breakthrough

82o

countersink 0.26” diameter

90o

spot drill 0.025” chamfer on 3/8”-16UNC tapped hole





Threading

Common methods of forming threads include

• taps

• thread mills

Taps

standardhand tap

• Most common especially for hand tapping

• Used for machine tapping as well

spiral pointtap

• pushes chips ahead of tap

• good for through holes

spiral tap • draw chips out of the hole

• good for deep, blind holes

The end of a standard tap is tapered (chamfered). This is desirable for manual tapping as ithelps to align the tap to the hole and start the thread cutting process. The 3 types of standardtaps are:

• Taper Tap – first 8 to 10 threads are tapered. Used for threading through material and

for the first threading pass in a blind hole.

• Plug Tap – first 3 to 5 threads are tapered. Used for threading through or for threadingcloser to the bottom of a blind hole.

• Bottom Tap – first 1 to 2 threads are tapered. Used for threading to the bottom of ablind hole.

Depth for tapping

When you compute the tap depth in CNC, you must account for the tapered portion of the tap.For example, if you are tapping a through hole with a plug tap, you must go at least 5 threads

beyond the hole depth in order to get fully formed threads. For a 3/8-16 tap, this would be 5/16of an inch.

For a blind hole

• you cannot form threads right to the bottom of the hole since each tap has taper

• you can make the drilled hole deeper than required so the threaded portion is thedesired amount





For example, Tapmatic (a producer of tapping attachments) recommends that the hole bedrilled at least Chamfer Teeth + One Pitch + 1mm beyond the required thread for clearance.

Example:1/4-20 bottom tap, 0.5" deep

Chamfer Teeth = 2 x pitch (.050) = 0.10Chamfer Teeth + one pitch + 1 mm = drill depth clearance0.10 + .050 + .039 = .19 inchesso, drill 0.5 + 0.19 = 0.69 inches deep if you require a 0.5” deep threaded portion

Thread milling

Another common method used to make threads is the thread mill. It looks similar to a tap but isoften used for larger (about >0.5”) or for external threads. The tool cuts the threads by movingin a circular path around the area to be threaded while also moving down.





Tapping speeds and feeds

The following table is an example of recommended speeds and feeds for tapping. Check withyour tool supplier for recommendations.

Speeds for machine tapping using HSS taps

MATERIAL

SPEED

(SFM) LUBRICANT

Aluminium 70-90 Soluble oil

Aluminium alloy 50-70 Soluble, light base or lard oil

Brass 60-100 Light base oil

Bronze 30-40 Light base oil

Copper 60-80 Light base oil

Gun metal 50-60 Soluble, light base or lard oil

Grey cast iron 30-60 Dry or soluble oil

Alloy cast iron 15-30 Sulphur based oilMalleable iron 20-40 Soluble or sulphur based oil

Magnesium alloy 50-70 Soluble oil or paraffin with lard oil

Nickel-based alloy 10-12 Very high pressure cutting oil

Plastics 50-70 Dry, freeze spray, liquid soap

Mild steel 30-50 Sulphur based oil

Carbon steel to 4% 20-40 Sulphur based oil

Carbon steel to 7% 20-30 Sulphur based oil

Carbon steel to 7%+ 15-25 Sulphur based oil

Steel alloys to 60T 15-25 Sulphur based oil

Steel alloys to 60T+ 10-15 Sulphur based oilStainless steels 10-20 Sulphur based oil

Tool steels 15-25 Sulphur based oil

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