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QSIMDO
04-27-2019, 05:13 PM
I need a 90 degree air bend using a V-block die in a piece of .1875 mild steel sheet.
Where would I mark the bend line relative to the desired length of the legs?
Would it be .094 away from the line?

Forgot to say the radius will be essentially 0.

Bob La Londe
04-27-2019, 06:05 PM
Inside length or outside length? Once you answer that you should answer your own question.

metalmagpie
04-27-2019, 06:27 PM
Ha! Right to the crux of it!

What I do is to make the bend, then trim the ends. Dumb and wasteful, I know, but the parts come out right.

metalmagpie

J Tiers
04-27-2019, 06:42 PM
You get that from bend allowance tables. Like this one. Gives the amount to subtract from the OD to get the proper bend locations

http://i.imgur.com/71gyTH4.jpg (https://imgur.com/71gyTH4)

deltap
04-27-2019, 07:08 PM
Good luck with the 0 radius. Maybe if you made a miter cut for most of the thickness and bent the last little bit you could come close. 3/16 would crack trying to bend a 0 radius. Heat it red hot and then bend it?

Bob La Londe
04-27-2019, 07:49 PM
Good luck with the 0 radius. Maybe if you made a miter cut for most of the thickness and bent the last little bit you could come close. 3/16 would crack trying to bend a 0 radius. Heat it red hot and then bend it?

Actually I did something like that a few times in aluminum. Make a saw cut 3/4 of the way through, bend it, and then weld the inside of the bend. If you don't get to crazy with the weld you get some amazing looking boxes that way.

garyhlucas
04-27-2019, 07:54 PM
Bend a sample say 2” wide right at the middle and measure the result. If you use the same material it will give you the exact bend allowance.

vpt
04-27-2019, 08:32 PM
Bend a sample say 2” wide right at the middle and measure the result. If you use the same material it will give you the exact bend allowance.

This is what I do when I care.

J Tiers
04-27-2019, 11:16 PM
I have used the information in the table I attached for laying out sheet metal since 1979. It has been very reliable, and has provided good alignment of tabs that had matching holes, etc, even for one-off prototypes where trial and error were not used. Recommended.

Paul Alciatore
04-28-2019, 01:44 AM
J, Thanks for that chart.




You get that from bend allowance tables. Like this one. Gives the amount to subtract from the OD to get the proper bend locations

http://i.imgur.com/71gyTH4.jpg (https://imgur.com/71gyTH4)

darryl
04-28-2019, 02:21 AM
To me it makes sense to allow about 20 thou on top of the material thickness for the bend- if you can make it that tight. Chances are that steel will allow that before aluminum would- unless you're dealing with an al alloy that is soft enough- which I don't encounter in my world of 6061 and other readily available alloys. You do have to be careful with aluminum because if you encounter an alloy that doesn't like to bend, then it's game over. You can make two pieces out of one in no time if you use something like semi trailer skin or similar.


But if it's bendable aluminum I would still not try a bend any tighter than a radius equal to the material thickness- in which case I allow between half the material thickness and the full material thickness. Most sheet steel can bend tighter, but you do need a lot of force to get it there- plus it's handy if the machine can do coining. That's the ability to bring the material to a full 90 with bending happening only at the corner and no curve being found on either 'flat' side. A material at 3/16 thickness will not like anything tighter than about 1/8 radius, so add that plus the material thickness to your figure for bend allowance.

If you don't already know the allowance by experience, then definitely do a test piece- that will really be the only way you can know for sure, given your bending machine and the particular material.

DR
04-28-2019, 05:36 AM
How do you "air bend" to zero radius?

From memory I believe a bend allowance chart with a better explanation is in my Chicago press brake manual.

QSIMDO
04-28-2019, 11:32 AM
How do you "air bend" to zero radius?

From memory I believe a bend allowance chart with a better explanation is in my Chicago press brake manual.

Apparently I can't.
Even with a relatively sharp tool profile only 20 tons isn't enough to form a sharp crease on the inside of the bend.
No problem, I just changed the design of the part.
Thanks everyone for the input.

J Tiers
04-28-2019, 11:56 AM
Apparently I can't.
Even with a relatively sharp tool profile only 20 tons isn't enough to form a sharp crease on the inside of the bend.
No problem, I just changed the design of the part.
Thanks everyone for the input.

You normally cannot do that type of bend, unless you "coin" it into a "v" lower die. Air bends will naturally form a larger radius, since there is nothing to force the metal to bend sharply. You tend to get a "bulbous" corner.

Sharp bands are not desirable if they can be avoided, and also are best done across the "grain" of the metal, which runs in the direction the sheet was rolled out. Cracking is likely to occur, although for lower stress applications one can usually get away with it.

QSIMDO
04-28-2019, 11:42 PM
I got to maybe 80 degrees and that's all it was having so I lit off the hot bender and finished it.
Ended up not liking the design but thanks for the chart I made a copy.