bend allowance for odd shapes

[TheTick]

Any bending mavens out there know how to get the bend allowance for bending odd shapes like C-channel or angle iron?

 

[dgowans]

Tick,

I've found that if you ask 10 different fab shops for their standard bend allowances for different materials you will get 12 different answers. Is this something you can come up with by trial and error?

 

[TheTick]

I have some legacy work to fall back on. I'm trying to find a way to at least come close by calculation.

 

[MikeHalloran]

I cut a section and run a polyline along the neutral axis in AutoCAD. It's usually close enough even for extreme precision sheet metal work.

The only problem is that when you ask a shop what bend radius they use, you'll _always_ get a wrong answer. You have to go out and measure the actual radius they achieve on a representative part.

I hate that whole 'bend allowance' fantasy.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

TheTick (Mechanical)
If you have access to the shop where the work is to be done,
Shear a sample of metal, take it to the brake and bend it,
Measure the result and correct accordingly. This way you will be sure you have the correct radius and bend allowance
B.E.

 

[jamfu]

Tick,
A good rule of thumb for bend allowances on a 90 deg bend are as follows:
thk<14GA BA=1.57(r+1/3t)r=male die rad
thk>15Ga BA=1.57(+.4t)

 

[ajack1]

I would agree with jamfu if you are coining the outside rad if not I would use
thk<14GA BA=1.57(r+2/5t)

 

[deanc]

The calcs. are good to start. But are you bending with a press brake or a leaf? Hot or cold? Free form or die? With the grain or across?

These all make a difference. Go to the Fab Shop and ask. If you need something special, pay for a few samples.

 

[jistre]

Yeah, go to the shop. Theoretical bend allowances are good for figuring out roughly how much material you'll need as a first approximation, but I've found they're utterly useless for production. Each shop has figured out their own bend radii on each of their machines and workers using their particular source of steel. One time, I had to send a shipment of parts back that were out of tolerance because they'd gotten new dies for their brakes and had forgotten to readjust their allowances. Another time, they switched metal suppliers and had to delay shipment by a day so they could adjust their radii. There are a million factors that go into a job shop's actual bend radii.

 

[CorBlimeyLimey]

TheTick ...

Am I right in assuming you are wanting to bend, extruded/pultruded/rolled channels and angles?
What material?
Are they structural or decorative?
What is the min radius required?
Are they being formed the "right" way or the "wrong" way

All these things will determine which processes will be used (Rolled or Draw-formed, hot or cold) and the allowances needed.

 

[SnTMan]

Tick, for small quantities and/or "normal" accuracy (say, R&D not practical) I agree with MikeHalloran, use the neutral axis length.

Regards,

Mike

 

[ajack1]

The neutral axis is not an accurate way to develop a bend, never has been never will be.

 

[SnTMan]

ajack1, ref the eqns above, the difference between these and an eqn based on the N.A is only the factor of t, 1/2t based on the N.A, and factors from 1/3 to 2/5 per the above. As the material becomes thinner, the differences become smaller as well.

Depends on how "accurate" you need to be. If I needed to slam out a piece or two and was measuring with steel rules, etc., I'd feel pretty good about using the N.A allowance. You'd have to measure pretty carefuly to see any difference.

Regards,

Mike

 

[GoSabres]

The polyline method in ACAD has always worked for me. I have used it many times and have had very few times where it wasn't close (+/-.010") using CS (hot or cold rolled) or SS. Aluminum can be slightly more (+/-0.015"), but as long as you are not looking for precision then you should be allright. But I am also in agreement with many of the others, in that you should just have some samples made and average the results. **Remember, the accuracy not only depends on the calculations; it also depends on the machine and the operator.**

 

[presstoolman]

as a press toolmaker i use the formula 1/3 material thickness + the inside bend radius x 1.598.
this is a very accurate way of doing it.

 

[smcadman]

Any bending mavens out there know how to get the bend allowance for bending odd shapes like C-channel or angle iron?

If I understand your post correctly, you do not want to bend a flat piece of sheet metal into a C-channel, you want to bend a straight C-channel instead similar to this:

bent c-channel

We used a section bender to bend angle and c-channels. Typically these are used to roll them into a circle but we bent the material by inserting it and moving the rolls up until we reached the desired angle.

Section bender sample

Sorry I cannot tell you the bend deductions because it will depend on the machine you use to bend it.


Flores

 

[TheTick]

Correct. FYI we have solved this by experiment. Still, I would like to hear about calculating & predicting.

 

[Gymmeh]

Could someone elaborate on the ACAD method?

"polyline along the neutral axis in AutoCAD"

Thank you

 

[CorBlimeyLimey]

A Polyline is a collection or group of sketch entities which have been combined to behave as if it were one entity.

If a polyline is created on the neutral axis (often along the mid-thickness of the material) it can be measured to give a close approximation of the flattened length.

 

[KENAT]

It's the digital version of running a piece of string or thread along the CL/Bend line.

The subtle variation in some cad packages is to calculate the profile length for a section. Subtract twice the thickness, then divide by 2 and hey presto!

KENAT, probably the least qualified checker you'll ever meet...