1. Bend Allowance Forumlas

Drawings of sheet metal parts usually show the part in its finished (bent) state. However, the designer should supply information about the layout of the flat sheet stock before it is bent, which requires the calculation of the amount of material used at each bend. This amount of material at each bend is referred to as the bend "allowance." The individual bend allowances are shown in a developed (flat) view of the part which can thus include overall dimensions and the locations of key features prior to bending.

A. The Formulas

The bend allowance for any bend is the length of the neutral arc, as shown below. This neutral arc can be calculated from the material thickness (T), inside radius (R), and the angle through which the material is bent (A).

The neutral arc, in a side view of the bend, represents the theoretical cylindrical plane of material that is neither stretched nor compressed. The material on the outside of this plane is stretched, while the material on the inside of this plane is compressed. But the length of this arc is the same before and after bending.

This plane occurs a certain fraction of the material thickness into the material measured from the inside surface. This fraction ranges between 1/4 and 1/2, but is never greater than 1/2. For rough calculations, some designers use a fixed fraction for all bends (such as .3, or .333, or .4). However, the fraction actually varies from one bend to the next because the fraction depends on the ratio between the size of the inside radius and the thickness of the material. The fraction is closer to 1/4 when the the bend is "profound" or "tight," that is, the inside radius is relatively small and the material is relatively thick. The fraction moves from 1/4 toward 1/2 as the radius gets larger or the material gets thinner.

The first formula below provides a fraction (K) that is based on experimental results with soft steel as reported in various reference manuals.

FIRST FORUMLA (applies to soft steel)

K = ((R/T)/16) + .25 (K never greater than .5)

The above formula applies to "medium hard" materials such as soft steel and aluminum. For softer materials such as soft copper and soft brass replace the .25 with .21. For harder materials such as hard copper, bronze, CRS, and spring steel, replace the .25 with .28.

After K is determined, a second formula is used to calculate the bend allowance.

SECOND FORUMLA

Bend allowance for any angle = A • π • (R + K•T) / 180

Thus, the bend allowance for a 90° angle = π • (R + K•T) / 2

These formulas can be used whether you are working in inches or millimeters. The derivation of both formulas is explained at the end of this article

B. Print Reading Pitfalls

Four cautions are necessary when working with sheet metal drawings and calculating bend allowances.

Four cautions are necessary when working with sheet metal drawings and calculating bend allowances.

• Thickness:

  Because of the variety of sheet metal gages used in the past, use the actual decimal thickness, which should appear on the drawing along with the gage.

• Straight sections:

  Straight sections are measured only up to the center of the bend radius (not the middle or "center" of the actual bend). Any dimensions on straight sections which go beyond the center of the bend will have to be adjusted by subtracting the inside radius and perhaps the material thickness.

• Radius:

  Since the designer is free to dimension sheet metal bends either on the inside or the outside, remember that both formulas above require the inside radius.

• Angle:

  Be careful how you interpret angular dimensions on a sheet metal drawing. The angle needed for the second formula above is the angle the material actually bends through, which may not be the same as the angle dimensioned on the drawing. For example, in the following drawing, the dimensions are correct in the sense that they define the bends. However, none of the numbers should be used for A in the second formula.

C. Inaccurate Tables in Machinery's Handbook

• The three bend allowance tables in the Machinery's Handbook (Industrial Press) are based on three different formulas which use a constant fraction K for each different material.

• For soft materials such as soft copper and soft brass, K is always .350.

• For medium-hard materials such as soft steel, K is always .408.

• For hard materials such as bronze, CRS, and spring steel, K is always .452.

• The fact that these tables do not recognize variations in K (as the radius and the thickness change) makes them unacceptably inaccurate. The use of a different K for different radius bends (determined by the first formula above) yields much more accurate results.

D. Other Sources of Inaccuracy

Even when using the more accurate fraction K from the first formula above, we must keep in mind that any formula based on experience is still only a theoretical approximation. In the case of sheet metal bends, the above formulas do not take into account several facts.

• The thickness of the material changes slightly at the bend.

• The center of the neutral arc is not exactly coincident with the center of the inside radius.

• Material deformation (stretching and compressing) is not entirely limited to the theoretical boundaries of the bend.

• The neutral plane does not form a perfect cylinder, but "bells out" at its edges (this effects bends in very narrow strips more than wide strips).

• The fact that these tables do not recognize variations in K (as the radius and the thickness change) makes them unacceptably inaccurate. The use of a different K for different radius bends (determined by the first formula above) yields much more accurate results.

Even so, the error introduced by these factors is far less than the error introduced by the assumption of a constant K.

2. Laser Cutting Sheet Metal Benefits

Standards The British Standard for weld symbols is BS EN 22553. When identification of the weld process is required as part of the weld symbol the relevant weld process code is listed in BS EN ISO 4063.

Basic Weld Symbol The weld symbol always includes

• An arrow line

• A reference line

• A symbol

Laser cutting is a manufacturing technique describing the use of lasers to cut metal and other products. Used frequently in sheet metal fabrication , lasers allow us to achieve a higher degree of accuracy, tighter tolerances, increased speed of manufacturing and reduced waste by combining several orders into one cut. Laser cutting works by using a computer to target a high power laser at the material to be cut. The material then burns, melts, vaporizes away or can be blown away by a jet of gas. The edge is left with a high quality surface finish.

3. Types Of Laser Cutters

There are three major types of laser cutters:

• CO2 lasers

• Neodymium (ND) lasers

• ND YAG lasers

CO2 lasers are used for industrial cutting of many materials including mild steel, stainless steel, paper, titanium, wax, aluminum, plastics, fabrics and wood. YAG lasers are used primarily for cutting and scribing metals and ceramics. At Ceative Laser Fab Tech Pvt Ltd , we use three Bystronic lasers with up to 4,000 watts of laser power each to achieve unmatched tolerances in our custom manufacturing!

4. Advantages Of Laser Cutting

The main advantages of laser cutting over mechanical cutting include:

• Easier workload

• Reduced contamination

• Better precision = tighter tolerances

• Reduced chance of warping

• Can cut materials resistant to tradition cutting means

• Consistency

• Improved yields

• Superior edge condition

• Elimination of tooling

• Nearly unlimited creative freedom

Laser cutting is more precise than plasma cutting and uses less energy cutting sheet metal. Plasma cutters can cut through thicker materials than lasers, but newer lasers are approaching the capabilities of plasma cutters.

5. Tolerance

Hole Sizes

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. 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. SMI 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. SMI'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, like a perforated pattern area, the result can resemble a baker rolling cookie dough -- the sheet metal can be expanded causing unwanted variation between holes. If this condition exists, a greater tolerance should be applied to certain areas surrounding this characteristic

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 (.13mm) only when absolutely necessary.

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" (.13mm) 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.

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.

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.

6. Welding Symbol

Standards The British Standard for weld symbols is BS EN 22553. When identification of the weld process is required as part of the weld symbol the relevant weld process code is listed in BS EN ISO 4063.

Basic Weld Symbol The weld symbol always includes

• An arrow line

• A reference line

• A symbol

Note: Weld symbols on the full reference line relates to welds on the near side of the plate being welded. Weld symbols on the dashed line relates to weld on the far side of the plate. If the welds are symmetrical on both sides of the plate the dashed line is omitted. If the dashed line is above the full line then the symbol for the nearside weld is drawn below the reference line and the symbol for the farside weld is above the dashed line

More Detailed Symbolic Representation of Weld

Table of Weld Symbols

Dimensioning Welds

7. Useful Links

• https://www.engineersedge.com/sheet_metal.htm
• http://en.wikipedia.org/wiki/Sheet_metal