# SHEETMETAL

• posted
Can someone give me a formula to calculate a sheetmetal flat pattern.
Thanks,
• posted
Add the outside dimensions of adjoining flanges and subtract the bend deduction. The bend deduction is the difference between the outside dimensions (to a point coincident to both outside surfaces of the sheet metal) and the actual length of the material required to make the part. The radius in the formula below is the inside radius.
BEND ALLOWANCE = (.01745 X RADIUS) + (.0072 X THICKNESS) X DEGREES IN ANGLE OSSB = RADIUS + THICKNESS ISSB = RADIUS X TAN 1/2 DEGREES IN ANGLE BEND DEDUCTION = (2 X OSSB) - BEND ALLOWANCE
The radlius will be somewhat different for press brake work than stamping, and bottoming bending or air bending parts. So there is variation depending on your process, but the formula is pretty close for general fabricating.
Hope this helps. Diego
• posted
I think in my drafting class we used a line at 1/3 the thickness from the outside length of the bend to compensate for the stretching of the metal. But, it will vary with the material, thickness and bending method. dimensions for sheet metal are usually given to sharp corners and the die maker usually deals with it.
Bob
• posted
Yeah, in most cases, flat patterns should be avoided on product drawings. You are specifying the final condition of your product to you vendor, and it is up to them to match it by whatever means, per current ASME Y14.5M.
Matt Lorono