Repeatability of box break?

If you make each bend referencing the edge used to reference the holes, you should have no trouble holding your tolerance, assuming the holes themselves are well within said tolerance. If not, there's something wrong with your setup or technique.

Your ultimate accuracy will depend on the accuracy and consistency with which the holes are punched. If this varies, might see if there's some way you can set up to register off one of the holes.

Jon

Bend first, punch holes later?

Christopher A. Young Learn more about Jesus

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Picture a piece of 18- or 16-gauge aluminum, bent in a square "U" shape. The arms of the "U" have holes in them that need to be at fairly equal height up from the base of the "U" (20 mil is good, 50 mil is acceptable,

100 mil is definitely not). Dimensions are about 8 inches deep (i.e., we start from a sheet 8" wide) by six inches tall by four or five inches wide.

Assuming that we can reference the holes with perfect accuracy to the edge of the sheet that will be at the top of the "U", how closely can we get the hole heights, if we bend the part in a pan brake?

I'm thinking that if the break has a stop that registers that "top" edge of the U the same for both sides, that we should be able to do better than 20 mil most of the time, and never exceed 50 mils unless there's just something wrong with the equipment.

Does this sound reasonable to you, or am I all wet? Is this something that a normal sheet metal shop should be able to do, if they were provided with suitably cut sheets?

Where accuracy of hole placement on the final, bent piece is tight, then this is the only way to do it. If .05" is acceptable, then you CAN do it by milling/punching it flat and bending after. You may have to make a test article and bend, then adjust the work stop for additional parts. You may need to do this "bend calibration" step for each batch of aluminum. You will also need to make sure EACH piece of aluminum is oriented with the rolling "grain" in the same direction. Once set up and calibrated like that, you can probably hold .020" tolerance pretty consistently, if the brake is in good shape.

I do this sort of stuff in a home shop with well-used equipment, and it works out pretty well. But, if the hole location relative to the bend is critical, I bend first, then machine.

Jon

Actually, I can easily design the piece so that the absolute height of the holes can vary by 1/4 inch up or down -- it's just the relative height that's important, so that a shaft stuck through them is more or less level to the bottom.

When I became a draftsman the sheet metal foreman told me to assume a tolerance of 1/32" (0.031") on bend locations, though they usually could and did do better. I think the presence or absence of shearing burrs on the edge made a substantial contribution. The Strippit that punched the holes located on a small area near the corner rather than the full width like the press brake's back gauge.

Ask the ones you plan to send the quotes to. When I was designing circuit boards I invited the sales reps to stop by to discuss their normal and extra-cost tolerances in considerable detail. I suspect they saw MITRE as a big defense contractor and didn't realize my

12-board prototype orders were all they would ever get. jsw

OK, then you only need the two sides to bend the same way. If your brake is working properly, that should happen with a pretty small variation.

Jon

Is the quantity big enough to justify making a drill jig plate with edge locating pins that protrude from both sides? jsw