Aluminum Extrusion Manual S E C T I O N S I X4

There are any number of ways inwhich extruded aluminum can beapplied to meet design challengesmore effectively, more efficiently, ormore economically than alternativemethods of manufacture. Thefollowing illustrations offer just a fewcommon examples.

1. As shown, several rolled shapes,riveted together, can be replacedby a single extruded profile,resulting in higher strength whileeliminating joining costs.

2. Machining costs often can bereduced by extruding the desiredcomponent to exact (or near net)size and shape requirements.

3. Weight can be greatly decreasedby putting the metal only whereneeded. The extrusion processcan put the metal exactly whereneeded.

Design Decision – Advantages withAluminum Extrusions

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Advantages

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4. Welded assembliesfrequently can be eliminatedby designing an appropriateextrusion. In this way, costscan be reduced while bothstrength and accuracy areincreased.

5. Sturdy multi-void hollowprofiles are available toreplace roll-formedalternatives, often atreduced set-up costs andshortened lead times.

6. Improved stiffness andstrength can be achievedthrough extrusion. Here, adetailed hollow profilereplaces a crimped tubularsection, at a reducedmanufacturing cost.

Advantages

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Circumscribing Circle Size

One common measurement ofthe size of an extrusion is thediameter of the smallest circlethat will entirely enclose its cross-section—its circumscribing circle.

This dimension is one factor inthe economics of an extrusion.In general, extrusions are mosteconomical when they fit within amedium-sized circumscribingcircle: that is, one with a diameterbetween one and ten inches.

Most common profiles are less than18 inches in diameter, but a fewextruders are capable of producingextrusions with a much largercircumscribing circle diameter (CCD),some as large as 32 inches.

The example shown here would beclassified as a 3-to-4-inch circle-sizeshape.

Designing with Aluminum Extrusions 11

To develop a good extrusion design, thefollowing key characteristics should beaddressed:

u Specify the appropriate metal thicknessu Keep metal thickness as uniform as

possibleu Use metal dimensions for tolerancesu Design with surface finish in mindu Smooth transitionsu Use webs where possibleu Use ribs to straightenu Round corners wherever possible,

avoiding sharp edgesu Incorporate indexing marks.

Specify the Most Appropriate MetalThicknesses

Specify metal thicknesses that are just heavyenough to meet your structural requirements.Even in low stress areas, however, keepsufficient thickness to avoid risking distortionor damage. Some shapes tend to invitedistortion during the extrusion process (such

as an assymetrical profile or thin details atthe end of a long flange); such tendenciesexert more influence on thin-walled shapesthan on those with typical metal thickness.

Keep Metal Thickness As Uniform As Possible

Extrusion allows you to put extra metal whereit is needed—in high-stress areas, forexample—and still save material by usingnormal dimensions elsewhere in the samepiece. Adjacent-wall thickness ratios of lessthan two-to-one are extruded withoutdifficulty, but large differences between thickand thin areas may create dimensionalcontrol problems during extrusion. It is bestto maintain near uniform metal thicknessthroughout a shape if possible. When adesign combines thick and thindimensions, streamline thetransitions with a radius (acurve, rather than a sharpangle) at junctions wherethe thickness changessharply.

Design Decision - Practices

Rounded corners ease the flowof metal.

Metal thicknesses should beappropriate.

Practices

Aluminum Extrusion Manual S E C T I O N S I X12

Use Metal Dimensions forBest Tolerance

Dimensions measured across solidmetal are easier to produce to closetolerances than those measuredacross a gap or angle. So rely onso-called metal dimensions as muchas possible when designing close-fitted mating parts or other shapesrequiring closer tolerances.Standard industry dimensionaltolerances are entirely adequate formany applications, but specialtolerances can be specified ifnecessary.

Design with Surface Finish in Mind

Always indicate “exposed surfaces”on your design drawing so theextruder can give them specialattention and protect their finishduring both extrusion and post-extrusion handling.

As a general rule, the narrower theexposed surface, the more uniformits finish.

Webs, flanges, and abrupt changesin metal thickness may show up asmarks on the opposite surface of anextrusion, particularly on thinsections. The marking of exposedsurfaces can be minimized bythoughtful design.

A “Metal Dimension”can be extruded toclose tolerances.

An “Open SpaceDimension” is moredifficult to hold toclose tolerances.

This shape, with sharpangular transitions,risks show-throughstreaks on theopposite surface.

Modifying the shapeby rounding thetransitions reducesthe chance ofopposite-sidestreaking.

Designing with Aluminum Extrusions 13

Smooth All Transitions inThickness

Transitions should be streamlined by agenerous radius at any thick-thinjunction.

Web Gives Better DimensionalControl

Metal dimensions are more easily heldthan gap or angle dimensions. The webalso allows thinner wall sections in thisexample.

Ribs Help StraighteningOperation

Wide, thin sections can be hard tostraighten after extrusion. Ribs help toreduce twisting, and to improve flatness.

Rounded Corner StrengthensTongue

The die tongue is less likely to snap offwhen the corners of the profile arerounded at the narrowest area of thevoid.

Built-In Indexing Mark

Shallow extruded grooves make drilling,punching, and assembly easier byeliminating the need for center-punching. An index groove can also beused to help identify pieces that aresimilar in appearance, or to distinguishan inside (rather than an outside)surface.

Smooth transitions canbe achieved throughrounding corners.

The hollow conditionof the part can beavoided by makingthe component intwo pieces as shownby the dotted line.

Rounded corners arestronger corners.

Ribs reduce twisting.

An extruded groovecould eliminate theneed for center-punching.

Aluminum Extrusion Manual S E C T I O N S I X14

Extruded shapes can incorporateessential design features such asscrew bosses, card slots, or drillguides. Thus, aluminum profilesenhance the usefulness of thepart produced.

The joining of aluminumextrusions can be accomplishedby way of nine distinct methodsthat can be designed into theprofiles themselves:

1. Nesting2. Interlocking3. Snap-fit4. Three-piece interlock5. Combination6. Slip-fit7. Hinge joint8. Key-lock joint9. Screw slot

Nesting Joints

Nesting joints which include lap joints and tongue-and-groove joints, have mating elements that are shaped tobe assembled with little or no self-locking action.

Interlocking Joints

The interlocking joint is, in effect, a modified tongue-and-groove. But instead of being straight, the twomating elements are curved, therefore, they cannot beassembled or (more to the point) disassembled bysimple straight-line motion. They are assembled by arotating motion and will not separate without acorresponding counter-rotation. As long as the partsare held in their assembled position, they stronglyresist separation and misalignment in both thehorizontal and the vertical directions.

Design Decision - Assembly

A nesting joint.

An Interlocking joint.

Assembly

Designing with Aluminum Extrusions 15

Snap-Fit Joints

A “snap-fit” or “snap-lock” joint is one which isself-locking and requires no additionalfasteners to hold the joint together.

The mating parts of a snap-fit joint exert a camaction on each other, flexing until one part slipspast a raised lip on the other part. Once pastthis lip, the flexed parts snap back to theirnormal shape and the lip prevents them fromseparating. After it is snapped together, thisjoint cannot be disassembled unintentionally.

A Three-Piece Interlocking Joint

A three-piece joint can be designed with ablind (hidden) fastener interlocking the twoprincipal extrusions. Such a design presentsone side with a smooth appearance and novisible mounting hardware.

Combination Joints

Nesting, interlocking and snap-fit joints can becombined in the same extruded assembly.

For example, snap-fit elements can easily becombined with rotating elements.

A snap-fit joint.

A three-pieceinterlocking joint.

Assembly In example at above, a single extruded shape is designed formating with identical parts that are rotated into assembly andthen snap-locked rigidly into position without auxiliaryfastening. The tight surface-to-surface contact in this designalso provides resistance to sliding between the parts.

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Slip-Fit Joints

Slip-fit joints are assembled by slidingtwo extruded mating parts together inthe direction of their length. They aregenerally classified either as close-fitting, rigid dovetail joints or as loose,freely-rotating hinge joints.

Dovetail joints are useful in manyproducts where a simple, strong,permanent connection is required.

Hinge Joints

The cross-section of the componentsof a hinge joint have ball-and-socketshapes that allow them to rotatewithout separating. Hinge actionthrough 60 to 90 degrees is easy toobtain; incorporating adequatereinforcement, hinge joints may bedesigned to rotate beyond 90degrees. Since the hinge joint isrelatively “loose,” provisions shouldbe made to prevent lateral (side-toside) slippage.

This dovetail joint is atype of slip-fit joint.

A pair of extruded componentsmake up this hinge joint.

A number of profiles could be assembled inseries for this type of hinge joint.

Designing with Aluminum Extrusions 17

Key-Locked Joints

These unusual joints have two or moreprimary elements which are lockedtogether only when an additionalspecialized part, the key, is slid intoposition.

The joint shown here is used to connecttwo or more panels. In the illustration, twopanels and their extruded joining elementsare seen edge-on from the top or bottom.The three hook-profiled extrusions nesttogether, but are not in fact joined until anextruded pin with a special profile isinserted into the space at the center,locking them in place.

Keyed interlocks of this type permit rapid,easy assembly and disassembly, makingthem particularly adaptable to temporaryand portable installations, as well asrelatively permanent structures.

This unique key-locked jointwon an award in aninternational extrusiondesign competitioncosponsored by theAluminum Association andthe Aluminum ExtrudersCouncil.

Assembly