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Thread: Bend Allowance questions
11-15-2007, 03:28 PM #1
I am working with .060" SS and I calculated the bend allowance to be around .286" for a 90* bend.
Ok that was easy! Now actually putting that formula to work is what is driving me crazy.
I am working with a .010" tolerance. I have a really good Brake. Just two 90* bends, which I can and did figure out the correct deminsions.
I would like someone to educate me on how to apply the bend allowance formula.
Thanks in advance!
11-15-2007, 03:44 PM #2
How did you come up with that? That BA for 90° would be for a .152 inside radius with .060 stock.
Worst part about leaf brakes is that the stock, especially tough to bend stock, just "ooches" along under the clamp as the leaf tries to bend it. Ruins dimensions.
I had to make a back stop blade for my brake, adjustable of course. Stock stayed put then.
11-15-2007, 04:54 PM #3
our brake does have a stop adjustable with a push of a button on the display. My material isn't moving, this is not a whimpy leaf brake!
Yes that is the correct inside radius, I just bent a piece of scrap to 90* and measured with radius gages.
I am just asking how to apply this allowance, so that you can cut to exact length, and then run up against the stop on the brake and get a accurate part.
11-15-2007, 05:21 PM #4
If it is a two bend .06 part with .152 inside radii, the first leg will have (assuming part is dimensioned outside) .182 taken away from it and the other leg the same. The middle or lenght will be .364 shorter than outside.
These dimensions are to bend zones, each of which occupies .286.
So, if legs were 1" and overall bent lenght was 6", the dimensioning scheme would be:
.818 from each end to commencement of bend zone, and 5.636 in the middle between the bend zones.
The flat part before bending would be:
.818+.286+5.636+.286+.818 for an overall of 7.844
Note - this is all theoretical and assumes bend is equal on either side of neutral axis of the .060 thickness.
Your machine/dies may not perform exactly this way, so make enough test pieces for you to see what needs to be done to satisfy both your requirements and its operating characteristics.
If my math is off some this is your opportunity to say so
11-15-2007, 05:39 PM #5
11-15-2007, 05:49 PM #6
Thanks alot! Still trying to figure out a couple of those numbers. Sorry for my ignorance, but where did .182 come from? First time getting into accurate sheetmetal work, and I hate not knowing how to do something.
Also if you know of any books/websties that would be a help on this subject, that would be awesome!
11-15-2007, 05:51 PM #7
.182 comes from .152 radius and half of stock thickness. We are working along the neutral axis of the sheet metal
11-15-2007, 06:03 PM #8
Exactly what type of bending equipment are you using, apron or press brake (even what model number)? What nose radius? It makes a big difference.
It would also help to know how long the bend is and what hardness and alloy the SS is (this will relate to the suitability of the equipment).
Most sheet metal working texts will have a chapter devoted to bend allowance calculations (with plenty of examples, charts, and illustrations) so getting the whole story here may take a while.
11-15-2007, 06:06 PM #9
Hello ... (new here)
... sorry to jump right in with both feet and combat boots on but ...
There is no need to make bend allowances for sheet metal ... (or even steel plate up to 1/4 inch thick .. even 3/8ths).
... what I mean is that ... if you bend a U shape ... with an "inside the U (upright legs) " at 10 inches ... and (2) 5" long legs measured from the inside of the bottom of the U ... then the total length of the piece of sheet metal would be be ....
an even 20"
... been there done that - - many many many times ... and that IZ exactly how a fab shop will calc the piece length and bend layout also.
11-15-2007, 06:09 PM #10
Did you notice where he said +-.010?
11-15-2007, 06:11 PM #11
Ok that makes sense! Our machine is a 10 h.p. Atlantic hydraulic press break. The matl. is 304 SS. Not sure the nose R right now, I just measured the radius on the part after it was bent to 90*.
11-15-2007, 06:11 PM #12
It depends a lot what you are bending, in what manner, and what has to come out the correct height. The third- half rule generally applies. Third half rule = the neutral axis will be 1/3 material thickness from the inside of the bend at up to 2 Material thickness bend radii, past that the neutral axis slides further out to 1/2 material thickness. Basically you dar the part " on the neutral axis". Figure bend lengths by finding arc lengths at neutral axis for bend allowance. add in adjusted straights for correct overall dim's, don't forget to adjust the holes in bent flanges if required. Cheating the bend radius is a quick way of final adjustment after trial bends.
11-15-2007, 06:24 PM #13
What are the dimensions of the flat and flanges?
11-15-2007, 10:48 PM #14
wow, all these answers & not much talk of flat length, mold length, set back & sight lines etc....
How about letting us know the actual Rad of the bend?
At least Greg has the correct math
11-15-2007, 11:04 PM #15
I think John covered all that, and the third post mentions actual measured bend radius.
11-15-2007, 11:16 PM #16
oops, missed a line in the 3rd post [img]smile.gif[/img]
11-15-2007, 11:35 PM #17
I read the whole post three times before I noticed it
11-16-2007, 12:03 AM #18
There is an actual formula for it, although in 28 years of dealing with sheet metal as part of product design, we basically did it the same as WillieO. That for the same 0.010 tolerances, where we had several tabs with holes that needed to line up with other holes on relatively complex bend-ups.
Most all of our stuff was 16 and 18 ga with sharp inside bends.
Now, I have a nice cheat sheet with the formula, as well as a bunch of tabulated examples. And I get into more large radius bending on 12 ga and larger. And ProE , Solidworks etc generally will figure the unfold for you.
11-16-2007, 12:07 AM #19
Where I work we do a lot of fabricating with 304 SS. Our shop tolerance is +/- .030 so our methods may or may not work for you.
For bend deduction we use material thickness x2. So .060 mat'l would have a deduction of .120 We usually round that to .125 for easier figuring.
If you have a U shaped part for example say 2"x4"x2" with the 2" being the flanges then add it all together 8". Now subtract .120 for each bend. 8" -.25 the flat pattern will be 7.75" Yes this is a very simple and quick method but we can easily hold a .030 tolerance.
Another problem you might have is the wrong sized punch and die in your brake. For 16ga (.060) the V on the bottom die should be .5" wide at the top of the V. Quick and dirty method again, Thickness x8 for V opening on .25" thick or less metal. x10 for any metal thicker than .25"
Using the formula that Greg posted I got .136 using a .060 radius. Where I rough figured .125
I had never seen the formula Greg posted, it is very interesting. Holding a tolerance to .010 I can see where this formula would be more accurate. The next time we get a part thru the shop calling for precise hole locations, I will have to try this formula. Learn something new every day.
Edited the post becuase I can't do basic math.
11-16-2007, 09:27 AM #20