Box and Pan Brake

[Pitts2b]

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

 

[Pilot-34]

You might think about using a press and then you can use pieces of pipe to create any radius you want

 

[TFF]

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.

 

[Victor Bravo]

Have the fingers re-milled to 1/8" radius, and you will likely be able to do 75-85% of all bending work on the majority of homebuilt aircraft. I can't think of too many places where a sharper radius is needed, and/or the larger 1/8" radius would cause a problem.

 

[proppastie]

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]

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]

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.

 

[pfarber]

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]

You are right I did not show you how I did it.......

 

[proppastie]

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.

 

[reo12]

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.

 

[proppastie]

I have learned to like z bent items (see latest post my build log)...they end up straight.

 

[pfarber]

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]

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]

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]

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.

 

[reo12]

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

Thanks.
smt

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

Thanks.
smt

A good quality press brake would probably be my choice for some parts. I've only used one once though - a fancy computer controlled unit with 12ft bending capacity. I was having a shop bend a replacement aileron for a Falcon UL. The shop foreman walked over and cycled the press while the machine operator and I were confirming the accuracy of the punch in relation to the reference lines. The punch was off location - offset the same amount but to opposed sides of the required bend location. Ruined the part. To this day I can't believe the foreman just walked over and without a word simply cycled the press. The same idiot insisted that they could plasma cut the skin for me rather than my cutting it with offset shears back at my shop where I had padded a work surface to work on. That failed also. That was before I had worked with a plasma cutter and knew the limitations.
I bought an older Verson 12ton press brake a bit over a decade ago. Business slowed. I never got the dies for it. Sold it to a friend last year.
All brake designs have advantages and disadvantages. Urethane bottom dies for press brakes look nice for delicate parts. Depth control or proper upper die choice is imperative for accuracy of bends.
I've used packing tape on my pan brake to limit marring of parts.

 

[reo12]

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.

I have some 3ft long press brake dies that were made that way. I consider them to be functional precision as they were for bending hot rolled steel.

 

[PMD]

In my aluminum boat building days, I had the luxury of using a friend's shop, equipment and staff to do a lot of forming of large sheet (and rolling of tube). IMHO, the ultimate tool for many things is indeed a press brake, but the main problem is most shops that have one have very limited tooling aimed at making sharp radius bends in steel. From those days (long ago) my shop foreman's nephew learned the trade and went on to have an aluminum fab business of his own, so I can get limited larger radius bending done with a reasonable selection of tooling.

BUT: NONE of the above is anywhere near what CAN be done to precisely and safely form aluminum sheet. Press brakes can be equipped with lower die supports that can be squared (x axis) or crowned (z axis) to make very precise bend centerlines. Lower urethane dies have been mentioned, but there are also rotary dies that allow the upper die to form its radius against a pair of lower dies that rotate around it as the stroke progresses. You can also get rolls of plastic protective film from press brake machinery companies. If I ever become stunningly wealthy, a lot of this kind of stuff will land in my shop.

I guess what I am trying to convey is making things is all about the tooling (and toolmaker). If you have a particularly difficult or accurate thing to do, look around for someone who is already doing that and you should be able to find the tooling and skills you need. If you just have a pocket full of cash you want to shrink to a more convenient size to fit your wallet, go into the boat business and enjoy a lot less critical forming experience. Hard to get really good at it making just one part or even one set of parts.

 

[Aviacs]

Thanks, Reo.
A long time ago I ran them off and on for about a year, mostly D & K, little 4 footers up to 10 foot, in a sheet metal cooler box factory (industrial refrigerators & freezers up to several acres). OTOH, my primary duties were in the wood shop & I had no set up or tooling responsibility for the press brakes. Just a warm body to place and remove sheets of galvanized or sometimes stainless, and push the cycle start button.

I never learned if they were acceptable for aluminum airplane parts. Maybe litte used by homebuilders due to costs compared with leaf break?

That is an awful experience that you had - and of course no one ever offers or even can be induced, to make it right by at least compensating for the destroyed material.

Urethane bottom dies for press brakes look nice for delicate parts.

Ooh, yes! I had fogotton the rubberform pressing options. or small dies.

If I ever become stunningly wealthy, a lot of this kind of stuff will land in my shop.

My sentiments.
But it is probably too late for me.

there are also rotary dies that allow the upper die to form its radius against a pair of lower dies that rotate around it as the stroke progresses.

This part is fascinating! Never knew that.

I guess what I am trying to convey is making things is all about the tooling

That i can handle. Planer here can cut up to about 80" long dies. Though it will automatically put about .006 crown in that length. (due to way wear). OTOH, that rolling tooling suggests the planer could be contrived to run similar so long as sheets are less than 24" wide. Hmmm......
During late teens, i spent time in an old Danish machinist's shop. (Same man who introduced me to EAA)
I was oohing and aahing over the machines one day and he said WTTE "Son, the machines don't mean anything without tooling. Best machine in the world is useless without good accessories and tooling to facilitate the work. Always buy the tooling!" He even had an enormous old safe, that sparkled inside like a jewelry store, to keep the best stuff.

Sounds like overall, this is one of those situations where the ideal solution is to convince a neighbor to set one up and house it warm and dry, and you get visitation rights and make the tooling.

smt