Sheetmetal design Consideration

Precision sheet metal tolerances Typical Tolerances Fifty years ago, "build-to-order" sheet metal tolerances were +/-.06" (1.5mm), primarily due to the accuracy of the equipment. Today, normal production tolerances are around 10% of that. that . In the following article, we are discussing tolerance ranges that are "reasonable" or "practical". If tighter tolerances are needed, they can be achieved with dedicated tooling and special processing. The most significant improvement has been in the machinery. Some modern high speed equipment can position within .004" (.10mm) and repeat within .002" (.05mm). Materials and methods continue to play a role in the limits of precision, however. They greatly influence practical considerations for tolerances between holes (hole-to-hole), between folds (fold-to-fold), and so forth.

Hole Size Holes are produced by mechanically pressing a punch tool through the sheet metal in order to rip out a slug. When the punch retracts the slug remains stuck in the die tool and a hole is left in the sheet metal. The size and shape of the punch and die tooling govern the size and shape of the hole produced in the sheet metal. For strength, the punch tip must usually be at least as large as the sheet metal thickness. The die tool must be slightly larger than the punch to minimize tooling wear and to reduce the pressure required to punch the hole (see Punching Force Calculation.) Calculation.) The die clearance is generally about 10% of the material thickness. For example, the material is .100 aluminum and the punch diameter is 1.000", the die diameter would be 1.010". The size of the hole on the punch side will be the same size as the punch tool. The size of the hole on the die side will be the same size as the die tool. Except for tooling wear, there is very little variation from one hole to the next. DSM has an extensive library of tooling, but we do not stock all possible tool sizes. We look to the engineers and draftsmen to give us a tolerance range that allows us to use our existing tooling. When that is not possible, a capital investment in new tooling is required. Generally speaking, +/-.003" (.08mm) is a reasonable hole size tolerance. Keep in mind, however, that we are measuring what will pass through the hole, not the "rim sizes" of the hole. Hole to Hole The accuracy of the distance from one hole to another hole is largely dependent upon the machinery. DSM's equipment will hold better than +/-.005" (.13mm) with little difficulty. However, each hole that is punched introduces stress into the sheet metal. If the part has many holes closely spaced, the result can resemble a baker rolling cookie dough -- the sheet metal can be expanded causing unwanted variation between holes. Whenever possible, engineers should allow +/-.010" (.25mm) hole-to-hole. Resort to +/-.005 (.12mm) only when absolutely necessary. 

Distance between hole should be +/- .010” tolerance but +/-.005 can be produced.

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Hole to Edge Because the profile (or edges) of the part are generally punched just like any hole, the same considerations for hole-to-hole apply. When punching very near to an edge (less than 2X material thickness) the edge will be pushed out by the stress of punching the metal. This edge migration introduces variables in the accuracy of the hole location. Whenever possible, engineers should allow +/-.010" (.25mm) hole-to-edge. Resort to +/-.005 (.12mm) only when absolutely necessary. 

Distance between hole to edge should have +/- .010” tolerance but +/-.005 can be produced.

Hole to Fold There are several variables introduced leading up to this stage in the fabrication process. The part has been punched on a CNC Turret Press, sanded or tumbled to remove burrs, and is now being bent on a Press Brake. The deburring process may remove .003" (.08mm) when cosmetic appearance is a priority. Precision Press Brakes will position and repeat in less than .001" (.025mm). Well trained and skilled operators will be able to load the parts for forming consistently from bend to bend. Nevertheless, engineers must consider the natural variation in material thickness (5% of nominal thickness), the +/-.005" (.12mm) from the turret press, the effects of cosmetic graining, and the variation introduced by the press brake. Whenever possible, engineers should allow +/-.015" (.38mm) hole-to-fold. Resort to +/-.010 (.25mm) only when absolutely necessary. 

Distance between hole to fold should have +/- .015” tolerance but +/-.010 can be produced.

Fold to Fold All of the considerations of "hole to fold" apply, compounded by the fact that multiple material surfaces and thicknesses are involved. Whenever possible, engineers should allow +/-.020" (.50mm) fold-to-fold. Resort to +/-.010 (.25mm) only when absolutely necessary. 

Distance between fold to fold should have +/- .020” tolerance but +/-.010 can be produced.

Best Practice in Tolerance Specification Although the machinery and tooling will repeat within .004" (.10mm), it is a mistake to simply engineer all mating parts expecting +/-.005" (.13mm) accuracy. Such over kill forces additional labor in sorting and inspection. The result of tolerances that are too tight is simply higher cost and lower productivity. Correctly toleranced parts still have excellent fit and function, with the added benefit of efficiency.







Aluminum Pure aluminum is soft and ductile and most commercial uses require greater strength than pure aluminum affords. So, strength is achieved by the addition of other elements to produce alloys. Further strengthening is possible by means which classify the alloys into roughly two categories, non-heat-treatable (alloyed with manganese, silicon, iron, and magnesium) and heat-treatable (alloyed with copper, magnesium, zinc, and silicon). Aluminum is available in a wide variety of alloys to meet specific applications. This article focuses on the alloys commonly used by DSM in precision sheet metal. 1100-H14 (QQ-A-250/1d) Commercially pure aluminum, highly resistant to chemical attack and weathering. Excellent for chemical processing equipment and other uses where product purity is important. Easily worked and welded, ductile enough for deep draws, but the lowest strength aluminum alloy. Uses include light reflectors, decorative and jewelry parts, name plates. Seldom used in precision sheet metal -- see 5052-H32. 3003-H14 (QQ-A-250/2c) General purpose manganese alloy. Stronger than 1100 with same good formability and low cost. Fine corrosion resistance and weldability. Used in stampings, spun and drawn parts, mail boxes, cabinets, tanks, fan blades. For higher strength, consider 5052-H32. 5052-H32 (QQ-A-250/8d) Main alloy is magnesium. Far stronger than any of the alloys described above, yet forms well with reasonable inside bend radii. Corrosion resistance and weldability is very good. Better salt water corrosion resistance than 1100. Used for electronic chassis, tanks, pressure vessels and any number of parts requiring considerable strength and formability at reasonable cost. Anodizing may be slightly yellowish. 6061-T6 (QQ-A-250/11d) Alloyed with magnesium & silicon. Heat treatable to improve strength. A widely used structural alloy for light to medium strength applications. Requires much larger inside bend radii than 5052-H32, but can be formed. Combines good weldability, corrosion resistance, and strength after heat treatment. Since it looses appreciable strength when welded, the 5000 series alloys replace it in dump body and some marine applications. Quick comparison chart Alloy

Yield

Formable

Weldable

Corrosion

Color code

1100-H14

17000

A

A

A+

WHITE

3003-H14

21000

B

A

A

GREEN

5052-H32

28000

B

A

A

PURPLE PURP LE







Minimum bend radius When aluminum is bent around too small of an inside radius, cracking will occur. Cracking is most pronounced when the bend runs parallel to the natural grain of the material (formed as the sheet is rolled from molten ingot). It is generally a good idea to keep the inside radius at least equal to the material thickness. The more the merrier!. Min bend radii for thickness (in 1/32nds of inch): ALLOY

.032

.040

.050

.063

.090

.125

.190

.250

1100-H14

0

0

0

0

0

0

3

6

3003-H14

0

0

0

0

1

2

4

8

5052-H32

0

1

1

2

3

4

8

12

6061-T6

2

2

3

4

6

8

18

32

Aluminum sheet vs steel sheet The following table gives a quick point of reference when you need the approximate thickness of aluminum sheet to use in replacing steel sheet. The designated aluminum thickness will give you about the same stiffness. Or, putting it another way, the deflection will be about equal. As a rule of thumb, plan on using an aluminum sheet about 40% thicker than steel. Since aluminum weighs only 1/3 as much as steel, this means that the equivalent aluminum sheet will weigh only half as much as the steel sheet it replaces. Approximate stiffness equivalence: Steel LB/SF

Steel Thick

Alu Thick

Alu LB/SF

.975

.024

.032

.452

1.22

.029

.040

.564

1.47

.035

.050

.705

1.80

.044

.063

.890

2.44

.059

.080

1.13

2.56

.062

.090

1.27

2.86

.070

.100

1.41

3.66

.089

.125

1.76







Aluminum sheet thickness tolerances During the rolling process that produces the sheet stock, a certain amount of "bowing" occurs in the rollers. This results in the sheet being slightly thinner at the edges than at the center of the sheet. Aluminum thickness tolerances: Thickness

36" Sheet

48" Sheet

.018-.028

.002

.0025

.029-.036

.002

.0025

.037-.045

.0025

.003

.046-.068

.003

.004

.069-.076

.003

.004

.077-.096

.0035

.004

.097-.108

.004

.005

.109-.125

.0045

.005

.126-.140

.0045

.005

.141-.172

.006

.008

.173-.203

.007

.010

.204-.249

.009

.011

Aluminum temper designation Aluminum is specified with a 4 digit alloy followed by a temper designation. For example, 5052H32 indicates an aluminum/magnesium alloy that has been strain hardened and stabilized by low temperature heating and is 1/4 hard.

-H

Strain hardened (cold worked) with or without thermal treatment.

-H1

Strain hardened without thermal treatment.

-H2

Strain hardened and partially annealed.

-H3

Strain hardened and stabilized by low temperature heating.







2nd Digit

A second digit denotes the d egree of hardness. -Hx2 = 1/4 hard. -Hx4 = 1/2 hard. -Hx6 = 3/4 hard. -Hx8 = full hard.

-O

Full Soft (annealed).

-T

Heat treated to produce stable tempers.

-T1

Partially solution heat treated and naturally aged.

-T3

Solution heat treated and cold worked.

-T4

Solution heat treated and naturally aged.

-T5

Partially solution heat treated and artificially aged. -T51 Stress relieved by stretching. -T510 No further straightening after stretching. -T511 Minor straightening after stretching. -T52 Stress relieved by thermal treatment.

-T6

Solution heat treated and artificially aged.

-T7

Solution heat treated and stabilized.

-T8

Solution heat treated, cold worked, and artificially aged.

-T9

Solution heat treated, artificially aged, and cold worked.

Effects of alloying elements Series

Main Alloy

Effect of Alloying Element

1000

none (99% alu)

Unalloyed aluminum is highly corrosion resistant, low strength, workable, conductive. Non-heat-treatable.

2000

Copper

Gives strength, hardness, machinability. Heat-treatable.

3000

Manganese

Adds moderate strength, good workability. Non-heat-treatable.

5000

Magnesium

Moderate to high strength. Corrosion resistant. Non-heat-treatable.

6000

Magnesium & Silicon

Increases strength, formability, corrosion resistance. Heat-treatable.

7000

Zinc

For greatest strength. Heat treatable.









Steel Steel sheet is commonly categorized as either "hot rolled" or "cold rolled" and by varying the amount of carbon, the manufacturer can produce a wide range of material characteristics. Tool steels have a much higher carbon content than the mild steels used in sheet metal work. The hot rolling process is generally less expensive, but results in a surface slag that is not always acceptable. Pickled and Oiled Hot Roll Steel has had most of the mill oxide removed and has a better surface appearance. Cold roll steel is commonly used in precision sheet metal applications due to its excellent surface condition, material consistency, and accuracy in thickness. ASTM-A366 specifies a cold roll steel with a maximum carbon content of .10 for improved welding and forming. It is soft enough to bend back on itself in any direction without cracking. Typical applications include refrigerators, ranges, washing machines, auto and truck bodies, signs, panels, shelving, furniture, and stamped parts. One main advantage of steel over aluminum is the ease of resistance spot welding. Steel also has a lower cost per pound than aluminum, although adding corrosion protection (plating and painting) may consume a great deal of the cost savings over aluminum. Steel sheet is available in a wide range of pre-finished products, including galvanized, paint primered, and fully painted. DSM generally works with bare steel sheet and plates the finished part to assure that all perforations and bends are adequately covered. Steel sheet tolerances The following table shows the commercial quality gauge thicknesses. DSM generally purchases "HCQ" (Half Commercial Quality) which tightens the tolerance range on each gauge by approximately 50%. Note that some gauge thicknesses actually overlap in decimal range.Steel sheet thickness tolerances: gauge

Nominal

Max

Min

LB/SF

10

.1345

.1405

.1285

5.625







28

.0149

.0169

.0129

0.625

Stainless Steel Stainless steel does rust, but in a minuscule amount compared to steel sheet. This is accomplished by alloying elements like nickel to reduce the amount of iron exposed on the surface. There is a variety of stainless steel alloys. This article focuses on those alloys commonly used in sheet metal applications.

Stainless steel alloys Alloy

Characteristics

303

NOT FOR SHEET METAL. For use in automatic machining applications (screws). Corrosion resistant to atmospheric exposures, sterilizing solutions, most organic and many inorganic chemicals; most dyes, nitric acid and foods.

304

The most widely used of the stainless steel and heat resisting steels. Offers good corrosion resistance to many chemical corrodents as well as industrial atmospheres. Has very good formability and can be readily welded by all common methods. ASTM A240 Cold rolled, annealed and pickled. Finishes: 2B, #3, and #4.

316

Better corrosion and pitting resistance as well as higher strength at elevated temperatures than T304. Used for pumps, valves, textile and chemical equipment, pulp & paper and marine applications. ASTM A240 Cold rolled, annealed and pickled. Finishes: 2B, #3, and #4.

410

Heat-treatable stainless used widely where corrosion resistance is not severe (air, fresh water, some chemicals). Frequently used in cutlery. This series is martensitic (magnetic). ASTM A240 Hot rolled, annealed and pickled. Finishes: Dull

Stainless sheet thickness tolerances During the rolling process that produces the sheet stock, a certain amount of "bowing" occurs in the rollers. This results in the sheet being slightly thinner at the edges than at the center of the sheet.







.116-.131

.005

.0075

.132-.146

.006

.009

.147-.187

.007

.0105

Stainless sheet surface finishes Sheet stock from the rolling mill is available in several finishes. To protect the finish, a static adhering PVC film may be applied. DSM normally handles "2B bare" and grains or polishes the part after all perforation is completed. Stainless sheet finishes: Finish

#1

Description

Hot rolled, annealed and pickled.

#2D

Dull cold rolled, annealed, and pickled.

#2B

Bright cold rolled, annealed, and pickled.

#3

Grained 100-120 grit.

#4

Grained 150-180 grit

BA

Bright Annealed -- highly reflective.

Theoretical gauge thickness The emphasis on theoretical is due to the fact that most rolling mills (manufacturers of sheet metal) will maximize the square footage and minimize the thickness. Note that nonferrous gages (aluminum) are not the same as ferrous gages (steel & stainless)! not the Material thickness:







2.588

.1019

10

.1345

3.416

2.304

.0907

11

.1196

3.030

2.052

.0808

12

.1046

2.657

1.829

.0720

13

.0897

2.278

1.628

.0641

14

.0747

1.897

1.450

.0571

15

.0673

1.709

1.290

.0508

16

.0598

1.519

1.151

.0453

17

.0538

1.367

1.024

.0403

18

.0478

1.214

0.912

.0359

19

.0418

1.062

0.813

.0320

20

.0359

0.912

0.724

.0285

21

.0329

0.836

0.643

.0253

22

.0299

0.759

0.574

.0226

23

.0269

0.683

0.511

.0201

24

.0239

0.607

0.455

.0179

25

.0209

0.531

0.404

.0159

26

.0179

0.455

0.361

.0142

27

.0164

0.417

0.320

.0126

28

.0149

0.378

0.287

.0113

29

.0135

0.343

0.254

.0100

30

.0120

0.305







Hole size What is the minimum hole size? The size? The material thickness and hardness limits the size of the hole we can punch. In mild steel, we can go down to the material thickness as the smallest hole -- the smallest CNC punch tooling we have is .031" (.79mm). (see Theoretical Gage Thicknesses and Sheet Metal Materials) Materials) What is the maximum hole size? Tricky question. If multiple hits are allowed, we could CNC punch out a hole 50" x 72" (1270mm x 1828mm). The largest single CNC tool we can hold is 3.5" dia (88.9mm). The CNC equipment can press up to 30 tons (see Punching Force Calculation), Calculation), so the material thickness and hardness create another limit.

Punching force calculation How to calculate punching force in tons Punching force = 25 x Land distance x Material thickness x Material factor x Shear factor Note: 1 Ton = 8.90 kN. 1kN = .112 Tons

Station Size & Land Distance

The larger the perimeter of the cut hole, the larger the tonnage required. required. Shape

"A" Dimension

"Ld" Land Distance

A=Diameter

Ld= 3.14 x A

A=B x 1.414

Ld= 4 x B









Material factor The higher the shear strength of the material, the higher the tonnage required. required.

Material thickness

Material

Factor

Aluminum 3003-H14

.30

Aluminum 5052-H32

.38

Aluminum 6061-T6

.50

Copper

.57

Brass 260-HH

.70

Mild Steel

1.00

Cold Drawn Steel

1.20

Stainless Steel 304

1.40

Tempered Spring Steel

4.00







The face of the punch tip may be ground flat, roof-topped, or concave to reduce the amount of tonnage required. required. Size & Shape

Factor

0.0 Flat

1.00

.062" (1.6mm) Concave

.30


.55


.60


.75

Hole-to-fold How close can a hole be to a bend? The hole can be right in the middle of a bend, but it will be distorted as the material stretches during forming. In general, the edge of the hole should be at least 2 material thicknesses away from the start of the inside bend radius 

Distance between hole to fold should have atleast 2 times the material thickness.

Flange size What is the smallest flange width? The quick answer is to keep the inside of the flange at least 3 times the material thickness + the inside bend radius. We don't like to do it, but we can "coin" flanges as small as 1.5 times the material thickness in thin ductile material (brass, mild steel,







Quick comparison of plating types: Applied On

Anodize

Black Oxide

Purpose

Specification

Aluminum

Durable decorative coating. Nonconductive. Available in many colors. Thickness build up: .00005" .0010"

MIL-A-8625 TYPE II CLASS 1 (NON-DYED 600mg/sf) OR CLASS 2 (DYED 2500mg/sf)

Steel (ferrous metals)

Thin decorative coating. Very little corrosion protection. Good on moving parts. Should be oiled.

MIL-C-13924 CLASS 1 (ALKALINE OXIDIZING) for plain steel. OTHER CLASSES ARE AVAILABLE.

AKA Chem Film, Iridite, Alodine, etc. MIL-C-5541 CLASS 1A (MAX Aluminum Corrosion control or paint primer. Chromate PROTECTION) OR CLASS 3 (MAX (non ferrous) Conductive. Easily scratched. Normally CONDUCTIVITY) yellow. Specify CLEAR if needed.

Chrome

Hard, durable, decorative finish. Low Copper over coefficient of friction. Pretty, but Steel pricey.

Copper

Ferrous or nonferrous

Gold

nonferrous

QQ-C-320 TYPE I (BRIGHT) OR TYPE II (SATIN) CLASS 1 (DECORATIVE). OTHER CLASSES AVAILABLE.

Used for undercoating, to eliminate buffing, to lubricate, to prevent case hardening, to enable tinning, etc.

MIL-C-14550 CLASS 0,1,2,3, or 4 (APPLICATION DICTATES WHICH CLASS TO USE)

Conductive decorative corrosion resistance. Really pricey.

MIL-G-45204 TYPE I, II, OR III CLASS 00 THRU 6 (APPLICATION DICTATES WHICH TYPE AND CLASS TO USE)







Painting Properly applied, paint is an excellent finish when considering cost, corrosion control, and appearance. To minimize air borne pollutants, "powder coat" paints are becoming widely used. Solvent based paints use a liquid to suspend the paint solids during application. The carrier solvent evaporates, leaving the paint film behind. Powder coat paints, on the other hand, take advantage of static electricity to cause the fine, dry powder of paint solids to cling to the part being coated. Heat is used to liquify the power to form a film. As the part cools, the paint returns to a solid. Several brands of powder coat are available (Morton, Tiger Drylac, Spraylat, HB Fuller, Cardinal, O'brien to name a few). The following comparison of materials is taken from an O'brien catalog. The ranking system: 1=NOT RECOMMENDED, 2=FAIR, 3=GOOD, 4=VERY GOOD, 5=EXCELLENT. Quick comparison of powder coat materials:

Epoxy Hybrid

Aromatic Urethane

Aliphatic Urethane

TGIC Polyester

Hardness

5

4

3

3

4

Flexibility

5

5

5

5

5







To prepare the screen mesh (so the ink only goes where it should) it is completely sealed with a photo-resist. A high contrast master art film layout is used to expose the photo-resist, which is then removed by rinsing. Most of the screen mesh remains sealed after exposure. Only the desired lettering, lines, and graphics are open to pass the ink to the work piece. The screen mesh, typically a high quality stainless steel cloth, can be reused many times for various projects by rinsing to remove all of the photo-resist. The art film master is kept on-file for preparing the screen frame for the next batch of parts. The Pantone Color Chart is an excellent reference for specifying shades & hues. Each color of ink must be applied separately and requires its own art film master and screen mesh setup. The preparation of the art film (produced with a Line-o-type film printer) can take advantage of a variety of CAD and Graphics software. A camera can be used to "shoot" a cut & paste layout. We can also design an art layout from just a concept sketch or outline. The inks used are generally epoxy based and will adhere to virtually any clean, dry surface (anodize, chromate, zinc, paint, plastic, etc.). Rough surfaces (highly textured paint) can make it very difficult to apply fine detail.

Drafting Revision control: Always show a revision level on the print. Change the revision every time the drawing is changed! Even if all you did was fix a spelling error, change the revision of the print! The effort will reduce brain damage in the long term.

Sheetmetal design Consideration

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