Sheet metal design.pptx

Sheet metal presentation

1. Sheet metal design

2. cold rolled sheet hot rolled sheet Galvanised sheet cold rolling can be rolled with 0.007mm metal aluminum foil At present, the thinnest hot-rolled strip can be rolled out with a steel strip of about 0.78mm Hot-dip galvanizing is one of the most effective means to delay the environmental corrosion of steel materials. while the highest accuracy of cold rolling can reach 0.6%. The thickness accuracy of hot rolling can generally reach 5% It is to immerse the steel products after the surface has been cleaned and activated in molten zinc solution. The surface is coated with a zinc alloy coating with good adhesion. Generally, Cold Rolled Steel Sheet requires annealing, pickling and surface smoothing before being delivered to customers. For a long time, the hot-dip galvanizing process has been favored by people for its low plating cost, excellent protection characteristics and beautiful appearance. It is widely used in automobiles, construction, home appliances, chemicals, machinery, petroleum, metallurgy, light industry , Transportation, power, aviation and marine engineering. The maximum thickness of cold-rolled steel is below 0.1-8.0MM. For example, the thickness of cold-rolled steel plates in most factories is below 4.5MM. 1.0mm-2.0mm cold rolled steel sheet.

3. Sheet metal calculations Bend allowance Distance from bend line to mold line Bend allowance of corner flanges At angles other than 90 deg

4. Bend line calculation

5. Terms

6. Bend allowance

7. Design guide lines

8. Bends Bends should be toleranced plus or minus one-half degree at a location adjacent to the bends. For the ease of manufacturing, multiple bends on the same plane should occur in the same direction. Avoid large sheet metal parts with small bent flanges. In low carbon steel sheet metal, the minimum radius of a bend should be one-half the material thickness or 0.80 mm (0.03 inch), whichever is larger. Counterbores The minimum distance between two counterbores is eight times the material thickness. The minimum distance from a counterbore to an edge is four times the material thickness. The minimum distance from a counterbore to a bend is four times the material thickness plus the bend radius. Countersinks The maximum depth is 3.5 times the material thickness at an angle of the hardware. A minimum of 50% contact between the hardware and the countersink is required. The minimum distance between two countersinks is eight times the material thickness. The minimum distance from one countersink and an edge is four times the material thickness. The minimum distance from a countersink and a bend is four times the material thickness plus the bend radius.

9. Curls The minimum radius is two times the material thickness with an opening to a minimum of one material thickness. The minimum distance between a curl and the edge of a hole is the radius of the curl plus the material thickness. The minimum distance a curl should be from an internal bend is six times the material thickness plus the radius of the curl. The minimum distance a curl should be from an external bend is nine times the material thickness plus the radius of the curl. Dimples The maximum diameter should be six times the material thickness, and a maximum depth of one-half the inside diameter. The minimum distance that a dimple should be from a hole is three times the material thickness plus the radius of the dimple. The minimum distance that a dimple should be from the edge is four times the material thickness plus the inside radius of the dimple. The minimum distance that a dimple should be from a bend is two times the material thickness plus the inside radius of the dimple plus the radius of the bend. The minimum distance between one dimple and another is four times the material thickness plus the inside radius of each dimple. Embossments The maximum depth is proportional to the internal radius or material thickness. The maximum depth for a flat embossment is equal to the internal radius plus the external radius. The maximum depth for a V embossment is equal to three times the material thickness

10. Extruded Holes The minimum distance between two extruded holes is six times the material thickness. The minimum distance from an extruded hole to an edge is three times the material thickness. The minimum distance from an extruded hole to a bend is three times the material thickness plus the bend radius. Flanges The minimum height of a bent flange is directly related to the material thickness, bend radius, and length of bend. The minimum width of a bend relief is one material thickness or 1.50 mm (0.06 inch), whichever is greater. Gussets The width and depth, recommended at an angle of 45 degrees, is directly proportional to the radius and material thickness. The minimum distance that a gusset should be from the edge of a hole in a parallel plane is eight times the material thickness plus the radius of the gusset

11. Hems The minimum diameter of a teardrop hem is equal to the material thickness, with a return flange height equal to or greater than four times the material thickness, and a minimum opening of 1/4 of the material thickness. The minimum diameter of an open hem is equal to the material thickness with a return flange height equal to or greater than four times the material thickness. The minimum return flange height of a closed hem is equal to or greater than four times the material thickness (the diameter is zero). NOTE: Closed hems tend to fracture at the bend and cause entrapment of solutions during the finishing process. The minimum distance from a hole to a hem is two times the material thickness plus the radius of the hem. The minimum distance a hem should be from an internal bend is five times the material thickness. The minimum distance a hem should be from an external bend is eight times the material thickness.

12. Holes The minimum diameter of a hole should be equal to the materials thickness or 1.00 mm (0.04 inch), whichever is greater. The minimum distance between holes is directly proportional to the size and shape for the hole feature and the material thickness The minimum distance the edge of a hole should be from a form is three times the material thickness plus the form radius. The minimum distance between a hole and the edge of the material is directly proportional to the size and shape of the hole and the material thickness. The minimum distance between the leading edge of a hole through a bend should be equal to the thickness of material plus the bend radius or two times the material thickness, whichever is greater Lances The minimum width of an open lance is two times the material thickness or 3.00 mm (0.125 inch), whichever is greater, with a maximum length of five times the width. The minimum width of a closed lance is two times the material thickness or 1.60 mm (0.06 inch), whichever is greater, and a maximum height of five times the material thickness at a 45-degree angle. The minimum distance from a lance to a bend in a parallel plane is eight times the material thickness plus the radius of the bend. The minimum distance from a lance to a bend in a perpendicular plane is ten times the material thickness plus the radius of the bend. The minimum distance from a lance to a hole is three times the material thickness.

13. Notches The minimum width is equal to the material thickness or 1.00 mm (0.04 inch), whichever is greater. The maximum length for a straight/radius end notch is equal to five times the width. The maximum length for a V notch is equal to two times the width. The minimum distance between a hole and the edge of a notch is directly proportional to the size/shape of the hole and the material thickness. The minimum distance from a notch to a bend in a parallel plane is eight times the material thickness plus the radius of the bend. The minimum distance from a notch to a bend in a perpendicular plane is three times the material thickness plus the radius of the bend. The minimum distance beyond the bend on the side edge is equal to the thickness of the material plus the bend radius, or two times the material thickness, whichever is greater. The minimum distance between two notches is two times the material thickness or 3.200 mm (0.125 inch), whichever is greater.

14. Ribs The maximum inside radius is equal to three times the material thickness, with a maximum depth of the inside radius. The minimum distance from a center line of a rib to the edge of a hole is three times the material thickness plus the radius of the rib. The minimum distance a rib should be from an edge in a perpendicular plane is four times the material thickness plus the radius of the rib. The minimum distance a rib should be from an edge in a parallel plane is eight times the material thickness plus the radius of the rib. The minimum distance a rib should be from a bend perpendicular to the rib is two times the material thickness, plus the radius of the rib, plus the radius of the bend. The minimum distance between two parallel ribs is ten times the material thickness plus the radii of the ribs Semi-Pierced Hole The minimum distance from a semi-pierced hole and a form is three times the material thickness plus the form radius. The minimum distance from a semi-pierced hole and a bend is two times the material thickness plus the bend radius. The minimum distance between semi-pierced holes is eight times the material thickness

15. Slots The minimum width of a slot is equal to the material thickness or 1.00 mm (0.04 inch), whichever is greater. The minimum distance from the inside surface of a bend to the edge of a slot is directly proportional to the length of the slot, material thickness, and radius of the bend. When using slots and tabs the maximum width of the slot must be greater than the thickness of the tab and the tab length should equal the material thickness. Tabs The minimum width is equal to two times the material thickness or 3.200 mm, whichever is greater, while the maximum length is five times the width. The minimum distance between tabs is equal to the material thickness or 1.00 mm (0.04 inch), whichever is greater. Welding Spot welding should be restricted to joining coplanar surfaces. The minimum distance between welds is 10 times the material thickness. Using 20 times the material thickness is ideal. The minimum distance between a weld and the edge is two times the diameter of the spot weld. The minimum distance from a weld to a form is the spot diameter plus the bend radius. Use PEMs instead of threaded inserts

16. Plating Outside sharp corners receive twice as much plating as flat surfaces. Allow for pitch diameters for screw threads, which can increase four times the plating thickness. Tapped holes may need to be re-tapped after plating to ensure accuracy. Projections accumulate more plating than other areas. Recessed areas may be difficult to plate, resulting in little or no coverage. Lap-welded joints trap plating solutions. One solution is to raise welds on embossed areas by 0.015 in. (0.3 mm) to allow for flushing and blow drying between the surfaces. Masking of stampings and fabrications to anodize certain areas is not recommended. Design drain holes/vent holes for plating solutions and rinsing. Design tabs/holes for attachment to part racks.

17. Bending

18. Bending is a process by which metal can be deformed by plastically deforming the material and changing its shape. The material is stressed beyond the yield strength but below the ultimate tensile strength. The surface area of the material does not change much. Bending usually refers to deformation about one axis. Bending is a flexible process by which many different shapes can be produced. Standard die sets are used to produce a wide variety of shapes. The material is placed on the die, and positioned in place with stops and/or gages. It is held in place with hold-downs. The upper part of the press, the ram with the appropriately shaped punch descends and forms the v-shaped bend. Bending is done using Press Brakes. Press Brakes normally have a capacity of 20 to 200 tons to accommodate stock from 1m to 4.5m (3 feet to 15 feet). Larger and smaller presses are used for specialized applications. Programmable back gages, and multiple die sets available currently can make for a very economical process

19. Air Bending is done with the punch touching the workpiece and the workpiece , not bottoming in the lower cavity. This is called air bending. As the punch is released, the workpiece ends up with less bend than that on the punch (greater included angle). This is called spring-back. The amount of spring back depends on the material, thickness, grain and temper. The spring back usually ranges from 5 to 10 degrees. Usually the same angle is used in both the punch and the die to minimize setup time. The inner radius of the bend is the same as the radius on the punch. Bottoming or Coining is the bending process where the punch and the workpiece bottom on the die. This makes for a controlled angle with very little spring back. The tonnage required on this type of press is more than in air bending. The inner radius of the workpiece should be a minimum of 1 material thickness in the case of bottoming; and upto 0.75 material thickness, in the case of coining.

20. Design Considerations for bending The bend radius should be kept the same for all radiuses in the part to minimize set up changes. Bend radius guidelines are as follows: For most materials, the minimum inner radius should be at least 1 material thickness. As a general rule, bending perpendicular to rolling direction is easier than rolling parallel to the rolling direction. Bending parallel to the rolling direction can often lead to fracture in hard materials. Thus bending parallel to rolling direction is not recommended for cold rolled steel > Rb 70. And no bending is acceptable for cold rolled steel > Rb 85. Hot rolled steel can be bent parallel to the rolling direction. The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or operator due to slippage.

21. Shearing

22. Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock. Shears are used as the preliminary step in preparing stock for stamping processes, or smaller blanks for CNC presses. Material thickness ranges from 0.125 mm to 6.35 mm (0.005 to 0.250 in). The dimensional tolerance ranges from ±0.125 mm to ±1.5 mm (±0.005 to ±0.060 in). The shearing process produces a shear edge burr, which can be minimized to less than 10% of the material thickness. The burr is a function of clearance between the punch and the die (which is nominally designed to be the material thickness), and the sharpness of the punch and the die. Design Considerations Material selected for shearing should be standard stock sizes to minimize the extra costs associated with special slitting. Burrs and hold down marks which are inevitable, should be considered in the design of the end product. Burrs should be kept away from handling areas, preferably folded away, or in some obscure area. The same can be done with hold down marks too.

23. Staking

24. Staking is a method of fastening (usually sheet metal) by squeezing protrusion formed in one part inside a hole in the second part, and then deforming the protrusion. The act of deformation causes radial expansion of the inner part and locks it in the hole.

25. Stamping

26. Introduction The operations associated with stamping are blanking , piercing , forming , and drawing . These operations are done with dedicated tooling also known as hard tooling . This type of tooling is used to make high volume parts of one configuration of part design. (By contrast, soft tooling is used in processes such as CNC turret presses, laser profilers and press brakes). All these operations can be done either at a single die station or multiple die stations — performing a progression of operations, known as a progressive die . Equipment Types The equipments of stamping can be categorized to two types: mechanical presses and hydraulic preses Mechanical Presses : Mechanical presses has a mechanical flywheel to store the energy, transfer it to the punch and to the workpiece . They range in size from 20 tons up to 6000 tons. Strokes range from 5 to 500 mm (0.2 to 20 in) and speeds from 20 to 1500 strokes per minute. Mechanical presses are well suited for high-speed blanking , shallow drawing and for making precision parts.

27. Hydraulic Presses : Hydraulic Presses use hydraulics to deliver a controlled force. Tonnage can vary from 20 tons to a 10,000 tons. Strokes can vary from 10 mm to 800 mm (0.4 to 32 in). Hydraulic presses can deliver the full power at any point in the stroke; variable tonnage with overload protection; and adjustable stroke and speed. Hydraulic presses are suitable for deep-drawing, compound die action as in blanking with forming or coining , low speed high tonnage blanking, and force type of forming rather than displacement type of forming.

28. Tooling Considerations Optimum clearance (total = per side × 2) should be from 20 to 25% of the stock thickness. This can be increased to 30% to increase die life. Punch life can be extended by sharpening whenever the punch edge becomes 0.125 mm (0.005 in) radius. Frequent sharpening extends the life of the tool, cuts down on the punch force required. Sharpening is done by removing only 0.025 to 0.05 mm (0.001 to 0.002 in) of the material in one pass with a surface grinder. This is repeated until the tool is sharp. If it is done frequently enough, only 0.125 to 0.25 mm (0.005 to 0.010 in) of the punch material is removed. Grinding is to be done with the proper wheel for the tool steel in question. Consult with the abrasive manufacturer for proper choice of abrasive material, feeds and speeds, and coolant. After sharpening the edge is to be lightly stoned to remove grinding burrs and end up with a 0.025 to 0.05 mm (0.001 to 0.002 in) radius. This will reduce the chance of chipping.

29. Punching Force: Punching can be done without shear or with shear Punching without shear. This is the case where the entire punch surface strikes the material square, and the complete shear is done along the entire cutting edge of the punch at the same time. Punching Force = Punch Perimeter × Stock thickness × Material Shear Strength. e.g., Punch Diameter = 25 mm (1 in), Circumference = 78.54 mm (3.092 in) Thickness = 1.5 mm, (0.060 in) Material Shear Strength (Steel) = 0.345 kN /mm 2 (25 tons/in 2 ) Punching Force = 78.54 × 1.5 × 0.345 (3.09 × 0.060 × 25)       = 40.65 kN (4.64 tons)       = 4.14 Metric Tons (4.64 US Tons) Punching with shear. This is the case where the punch surface penetrates the material in the middle, or at the corners, first, and as the punch descends the rest of the cutting edges contact the material and shear the material. The distance between the first contact of the punch with the material, to when the whole punch starts cutting, is the Shear Depth. Since the material is cut gradually (not all at the same time initially), the tonnage requirement is reduced considerably.

30. The Punching Force calculated above is multiplied by a shear factor, which ranges in value form 0.5 to 0.9 depending on the material, thickness, and shear depth. For shear depths of 1.5 mm (0.060 in) the shear factor ranges from 0.5 (for 1.2 mm / 0.047 in stock) to 0.9 (for 6.25 mm / 0.25 in stock). For shear depth of 3 mm (0.120 in) the shear factor is 0.5. Punching Force = Punch Perimeter × Stock thickness × Material Shear Strength × Shear Factor. Since shear factor is about 0.5, the Punching Force is reduced by about 50%. For the same example above, Punching Force = 78.54 × 1.5 × 0.345 (3.09 × 0.060 × 25) × 0.5 (Shear Factor)       = 40.65 kN (4.64 tons) × 0.5       = 2.07 Metric Tons (2.32 US Tons)

31. Blanking

32. Blanking is cutting up a large sheet of stock into smaller pieces suitable for the next operation in stamping, such as drawing and forming. Often this is combined with piercing. Blanking can be as simple as a cookie cutter type die to produce prototype parts, or high speed dies that run at 1000+ strokes per minute, running coil stock which has been slit to a specified width. For production parts, the final configuration of the drawn or formed shape needs to be established before the blank die can be built-since the blank size and the slit width size needs to be established precisely.

33. Design considerations Corners should have a minimum radius of 0.5 x material thickness or 0.4 mm (0.016in) whichever is greater. Sharper corners can be produced but at a greater die maintenance costs and more burrs. Slots or tabs widths should be greater than 1.5 X stock thickness. The length can be a maximum of 5 times slot/tab width. These rules can be violated at an increased tooling cost-- width as low as 1 X thickness and length as high as 7 X thickness can be achieved.

34. On cutoffs, avoid full radiuses across the width of stock. A square cut-off is best. If a radius is necessary, then an angle-blended radius is best.

35. Drawing

36. In drawing , a blank of sheet metal is restrained at the edges, and the middle section is forced by a punch into a die to stretch the metal into a cup shaped drawn part. This drawn part can be circular, rectangular or just about any cross-section. Drawing can be either shallow or deep depending on the amount of deformation. Shallow drawing is used to describe the process where the depth of draw is less than the smallest dimension of the opening; otherwise, it is considered deep drawing. Drawing leads to wrinkling and puckering at the edge where the sheet metal is clamped. This is usually removed by a separate trimming operation.

37. Design Considerations Round shapes (cylinders) are easiest to draw. Square shapes can also be drawn if the inside and outside radiuses are at least 6 X stock thickness. Other shapes can be produced at the cost of complexity of tooling and part costs.

38. The corner radiuses can be reduced further by successive drawing operations, provided there is sufficient height for the draw Perpendicularity can be held to ±1º, flatness can be held to 0.3%. This can be improved by performing extra operations.

39. forming

40. Forming is similar to bending . Complex parts such as U-sections, channel sections of different profiles can be produced by doing multiple bends. There is no change in thickness. Good dimensional repeatability as well as close tolerances is possible with this process. Design Considerations On bends, the short leg (inside length) should be a minimum of 2.5 X stock thickness + radius.

41. Minimum hole (and short slot) to bend distance should be 2.5 X the stock thickness + bend radius. For long slots, the distance should be 4 X the stock thickness + bend radius

42. Bending using tight radiuses or in hard materials often results in burrs and fractures on the outside of the bends. These can be eliminated by using larger bend radiuses and by providing relief notches at the edges on the bend line.

43. Bend relief notches should be provided = 2 X stock thicknesses in width (minimum 1.5mm / 0.060 in) and radius + stock thickness in length.

44. Generally, bending perpendicular to rolling direction is easier than rolling parallel to the rolling direction. Bending parallel to the rolling direction can often lead to fracture in hard materials. Thus bending parallel to rolling direction is not recommended for cold rolled steel > Rb 70. And no bending is acceptable for cold rolled steel > Rb 85. Hot rolled steel can be bent parallel to the rolling direction

45. Gage for common sheets

46. Metal Gages are often used in the sheet metal industry. The following table illustrates the sizes of the most used sizes Gage Number Brass mm (in) Mild Steel mm (in) Stainless Steel mm (in) 8 3.3 (0.129) 4.2 (0.164) 4.4 (0.172) 9 2.9 (0.114) 3.8 (0.150) 4.0 (0.156) 10 2.6 (0.102) 3.4 (0.135) 3.6 (0.141) 11 2.3 (0.091) 3.0 (0.120) 3.2 (0.125) 12 2.1 (0.081) 2.7 (0.105) 2.8 (0.109) 13 1.8 (0.072) 2.3 (0.090) 2.4 (0.094) 14 1.6 (0.064) 1.9 (0.075) 2.0 (0.078) 15 1.4 (0.057) 1.7 (0.067) 1.8 (0.070) 16 1.3 (0.051) 1.5 (0.060) 1.6 (0.063) 17 1.1 (0.045) 1.4 (0.054) 1.4 (0.056) 18 1.0 (0.040) 1.2 (0.048) 1.3 (0.050)

47. 19 0.9 (0.036) 1.1 (0.042) 1.1 (0.044) 20 0.8 (0.032) 0.9 (0.036) 1.0 (0.038) 21 0.7 (0.029) 0.8 (0.033) 0.9 (0.034) 22 0.6 (0.025) 0.8 (0.030) 0.8 (0.031) 23 0.6 (0.023) 0.7 (0.027) 0.7 (0.028) 24 0.5 (0.020) 0.6 (0.024) 0.6 (0.025) 25 0.5 (0.018) 0.5 (0.021) 0.6 (0.022) 26 0.4 (0.016) 0.5 (0.018) 0.5 (0.019) 27 0.4 (0.014) 0.4 (0.016) 0.4 (0.017) 28 0.3 (0.013) 0.4 (0.015) 0.4 (0.016) Gage Number Brass mm (in) Mild Steel mm (in) Stainless Steel mm (in)

48. Galvanized sheet gage (inches) 9 0.1532 10 0.1382 11 0.1233 12 0.1084 13 0.0934 14 0.0785 15 0.0710 16 0.0635 17 0.0575 18 0.0516 19 0.0456 20 0.0396 21 0.0366 22 0.0336 23 0.0306 24 0.0276 25 0.0247 26 0.0217 27 0.0202 28 0.0187 29 0.0172 30 0.0157 31 0.0142 32 0.0134

49. Piercing

50. Piercing is the operation of cutting internal features (holes or slots) in stock. Piercing can also be combined with other operations such as lance and form (to make a small feature such as tab), pierce and extrude (to make an extruded hole). All these operations can be combined with blanking. Piercing of all the holes is best done together to ensure good hole-to-hole tolerance and part repeatability. However if the material distorts, the method described below can be done. When there are large numbers of holes, in a tight pitch, there could be distortions, due to the high amount of tension on the upper surface due to stretching and compression on the bottom surface. This causes the material not to lay flat. This can be avoided/lessened by staggering the piercing of the holes. Holes are punched in a staggered pattern; then the other holes are punched in the alternate staggered pattern.

51. Design Considerations Minimum hole diameter should be at least 20 % greater than stock thickness. In the case of stainless steels, it should be 2 times the material thickness. Minimum wall thickness (distance from hole to edge or hole to hole) should be at least 2 times stock thickness. For non-round slots, the minimum wall thickness should be 2 times thickness for short slots < 10 thicknesses long; and 4 times thickness for long slots > 10 thicknesses long. Minimum hole (and short slot) to bend distance should be 2.5 × the stock thickness + bend radius. For long slots, the distance should be 4 × the stock thickness + bend radius.

52. Machining

53. Drilling , tapping , counterboring , and countersinking are the usual operations done in sheet metals. Drilling : Drilling is done in sheet metal only when piercing cannot deliver the accuracy required. For example, on a formed part, when holes on different features need to be coaxial, the accuracy obtained by machining may be required. Tapping : Tapping can be done using cut threads or formed threads. Formed threads (thread rolling) is preferable for the following reasons: Thread rolling is faster than cutting. Fewer burrs are generated, so no clean up is required or risk of future hazards such as shorting with electronic components. Larger sized holes are required for thread rolling vs. tapping, resulting in improved tap life. Rolled threads are stronger due to cold working. Typically, rolled threads are 20% stronger than cut threads. For very thin stock, either threaded fasteners such as clinch nuts, or forming thread in extruded holes is recommended. OR The material is upset in the sheet metal hole to form one thread pitch Counterboring : Counterboring is often done to provide clearance and a bearing surface for the fastener's head. Countersinking : Countersinking allows for flush mounting of flat head fasteners. Countersinking cannot always be done for very thin stock or for very large fasteners.

54. Welding

55. Welding is the process of permanently joining two or more metal parts, by melting both materials. The molten materials quickly cool, and the two metals are permanently bonded. Spot welding and seam welding are two very popular methods used for sheet metal parts. Spot welding is primarily used for joining parts that normally upto 3 mm (0.125 in) thickness. Spot-weld diameters range from 3 mm to 12.5 mm (0.125 to 0.5 in) in diameter Materials Low carbon steel is most suitable for spot welding. Higher carbon content or alloy steels tend to form hard welds that are brittle and could crack. This tendency can be reduced by tempering. Austenitic Stainless steels in the 300 series can be spot welded as also the Ferritic stainless steels. Martensitic stainless steels are not suitable since they are very hard. Aluminums can be welded using high power and very clean oxide free surfaces. Cleaning the surface to be oxide-free, adds extra costs (that can be avoided with low carbon steel). Dissimilar materials cannot be spot welded due to different melt properties and thermal conductivities. Plated steel welding takes on the characteristics of the coating. Nickel and chrome plated steels are relatively easy to spot weld, whereas aluminum, tin and zinc need special preparation inherent to the coating metals

56. Design considerations Thickness of the parts to be welded should be equal or the ratio of thicknesses should be less than 3:1. Spacing of welds Min. Weld to weld spacing = 10 x Stock thickness. Thickness of the parts to be welded should be equal or the ratio of thicknesses should be less than 3:1. Spacing of welds Min. Weld to weld spacing = 10 x Stock thickness. Center of weld to edge distance = 2 x weld diameter, minimum. Weld to form distance = Bend Radius + 1 weld diameter, minimum.

57. Adequate access for spot welding should be considered. Small flanges in U channels for example may restrict the electrode from entering the part. Flat surfaces are easier to spot weld due to easy access. Multiple bends impose access restrictions, and special fixtures may have to be designed to handle the parts, if access is not a problem. Prior to finishing, the spot welds have to be sanded or ground to blend the welds with the rest of the surface. It is best to choose the same spot weld size, to minimize setups and increase throughput. Plating of spot welded assemblies can cause problems when the sheet metal is overlapped. This can cause plating salts to be trapped-requiring special cleaning, or potential long-term corrosion problems. By carefully designing the assembly to allow easy draining of plating solutions this can be avoided. The mating parts can be self-jigged for easy location prior to welding. This can be done by lancing one part and locating in a corresponding slot in the other part; or by boss type extrusion, weld buttons, in part locating to a slot in the other. This type of design can often eliminate the need for external fixtures.