Box and Pan Brake

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Jul 20, 2020
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I'm planning on buying a break for the hangar and was curious if anyone has suggestions or opinions. I would like the option to buy fingers with different radius because I'm not happy with the results of using an additional piece of sheet metal to get the radius I want.
Thanks in advance,
Nelson
 

TFF

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Usually finger brakes come with a rack of different radi. If bending something long, lots of time I just bend a small strip for the length of the fingers and use that as the radius. It’s one of those tools that is versatile, but sometimes need some invention to use; that or buy three or four sets of plates, which no one does.
 

proppastie

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bending hard aircraft material requires moving the clamp/fingers (tool part with the radius) to allow for different thickness of material if you intend to bend with any accuracy.... simple bend and trim is easier.......using setback to determine the radius only works with soft material....hard material will bridge and give you a different sharper radius than you thought......I use a cheap Chinese junk and have two piece of .030 heating sheet metal permanent attached to the clamp with another two that will snap over for larger radius..... no matter what I do if the piece is long it will have a slight bow....I think the machine flexes. .....4 ft. of .025 is about the max thickness with .032 and .060 only smaller sizes......
 

pfarber

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using setback to determine the radius only works with soft material....hard material will bridge and give you a different sharper radius than you thought..
I have a book that prives you completely wrong. Its a free book, so you should read it. That one sentence has at least 3 rediculious errors in the 25 words it took to utter it.

FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volume 1
 

proppastie

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FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volume 1
That is a very informative manual......it is absolutely correct as regard theory and geometry of a bend......When I was a die designer designing progressive piercing and bending dies the theory was very important in order to produce parts accurate to +-.005 of an inch.....The same went when I was an engineer in the commercial toaster factory bending in Amada press breaks.....calculation of bending allowance was very important in both cases.....

When I got my bending brake and started to make bends with aircraft material I certainly understood the theory however no matter how far back I set the clamp on the break I got the same radius. In order to get a larger radius I needed to add material to build up the radius of the clamp.......setback alone did not do it.

setback.png
 

pfarber

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That is a very informative manual......it is absolutely correct as regard theory and geometry of a bend......When I was a die designer designing progressive piercing and bending dies the theory was very important in order to produce parts accurate to +-.005 of an inch.....The same went when I was an engineer in the commercial toaster factory bending in Amada press breaks.....calculation of bending allowance was very important in both cases.....

When I got my bending brake and started to make bends with aircraft material I certainly understood the theory however no matter how far back I set the clamp on the break I got the same radius. In order to get a larger radius I needed to add material to build up the radius of the clamp.......setback alone did not do it.

View attachment 126806

Setback does nothing for bend radius (other than booger up your flange length and give you the incorrect radius). Setback is nothing more than flange + radius because you need a sight line to put the fingers on.

You didn't mention K factor so that tells me you never did any math or used the less accurate (but still acceptable) lookup tables.

A press brake and a box and pan bend metal in completely different ways.
 

proppastie

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For others that might want to know and do not already know how here is the short version......calculate the length of the blank using the neutral bend line .....mark the center of the bend, position it under he tip of the clamp......the calculation of the neutral bend line length is simple geometry and with large radius of aircraft bends T/2 (1/2 the thickness) will be the location of the neutral bend line.
 

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reo12

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I've done a fair amount of bending with a Chicago box and pan brake and a cheaply made Carolina finger brake. One can calculate the bend offsets and factor them into the layout. High quality machinist grade tools and methods used to obtain as accurate a layout possible. Then use these layout locations as sight lines for the bending operations and end up with inaccurate results.

This is due to various reasons. There are adjustments in the leveling of the brake that can effect the accuracy of bends. The amount of tension adjustment of the top and bottom leaf can produce an arc in either the nose or bending leaf. The top leaf of the brake typically has play in the slot pivot or the inverted U guide that allows it to move closer or further way from the bending leaf. Forces upon the top leaf during clamping and performing the bend can allow the nose to move away from the bending leaf - often in an unpredictable fashion. In the case of the "Carolina" brand finger brake it was manufactured with 1/8" clearance in the inverted U guide. This effectively negates any argument of K factor calculations and much of the effort in layout accuracy. I've added sheet metal shims in these locations to reduce the clearance for more accurate bends. The inverted U and slot type top leaf adjustment types are shown in the following link. See locations labeled "N" on this site. https://www.americanmachinetools.com/how_to_use_a_hand_brake.htm

Accurate adjustment of the top leaf nose to bending leaf distance is critical. Centering of the material in the brake and tension of the truss rod effect how straight the bend is actually made. I have a Chicago 8ftx12gauge brake. If the tension is adjusted to produce straight bends in 14 gauge steel, it will bend an arc in the material if used on 20 gauge aluminum as the tension actually induces an arc in the nose of the top leaf.

I found the use of a hand brake to produce accurate bends to be dependent on accuracy of layout and a craftsman level understanding of the bending machine and the factors that effect the work that it performs. A press brake is an entirely different machine in function and requirements of the operator to produce accurate results.
 

pfarber

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The dirty secret is for most bends you can simply cheat about 1/16th for a sight line and stillbe within a 32nd.

Granted if you have a 1in radius this won't work,but most brake fingers are 3/16th or so.
 

kent Ashton

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I like to use bending dies that are pieces of bar or flat stock with rod of the appropriate diameter welded to the bar. Not too many are needed but various lengths are handy. For a 1/8"" radius, weld a 1/4" rod to 1/4" flat stock and use it for a die. Weld them a bit at a time so they don't curve, or make breaks in the rods. No setback calculation needed.
 

Aviacs

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I found the use of a hand brake to produce accurate bends to be dependent on accuracy of layout and a craftsman level understanding of the bending machine and the factors that effect the work that it performs. A press brake is an entirely different machine in function and requirements of the operator to produce accurate results.

for long straight bends, aircraft materials (high strength aluminum), do you have a preference?
Assume a back gage for the press brake.

Thanks.
smt
 

PMD

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I am a big fan of soft aluminum shoes over the tips of box and pan fingers to get desired radius - also keeping the bit of unevenness (is that even a word????) from fingers from putting little dents (i.e. stress risers) on the inside of the bend. I also like soft, sacrificial outside shims to prevent the folding bar of leaf or bottom die of press brake from scuffing the material. Some will use plastic sheet for the outside (re-useable). I will also go to great lengths to design bends to be cross grain if the part is to be stressed.
 
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