How does material thickness variation affect press brake bend angle?

[DrPipper]

I am researching purchasing a precision press brake, like a Trumpf 3066 or Amada HDS. We bend 14 gauge - 20 gauge stainless steel, and the thickness of the material varies by about 0.003". How does this variation in thickness affect bend angle? For example, does 0.002" thicker material result in 2 degrees underbending?

 

[btrueblood]

Well, do you need to reset the brake to setup for different material thicknesses, or do you set a force limit? Generally, I've only heard the latter done, though I have not used a press brake personally, only pan and box brakes. My understanding is that thickness variations don't bother press brakes.

 

[ajack1]

Brake presses are generally used to form a “V” at various angles and material thickness doesn’t really matter as you just adjust the shut height and the clearance still remains equal.

What becomes more of a problem is when you form something like a “U” in one tool where adjusting the shut height will make the bottom right but the sides will either pinch the material or have too much clearance.

 

[Compositepro]

My understanding is that material thickness variation is a major problem in press-brake operation. The bend angle will change. There have been detailed studies done on this which should be found by Googling.

 

[ornerynorsk]

The short answer is that it will affect your bend angle. You will have to make adjustments to your depth control, depending on your allowable tolerances.

Material thickness variations will cause variations in the finished part based primarily relative to your bottom die width of opening. The wider the die opening, the less variation. This is true for air bending, which is the most common. Dies which are specially constructed to bottom out will tend to peen or set the corner, regardless of slight material thickness variations, but this takes an operator with a little more finesse to avoid putting excessive tonnage on the press and tooling that may not be necessary.

How precise of bends are proposing to make (+/- what fraction of a degree) ?

Now, there are press brakes available with bend angle sensing that will correct for anomolies. If your requirements are very tight, this is definitely worth the investment.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[BillPSU]

On thicker materials typically as you go across the piece of material there can be crowning in the material thickness due to the process of making the material. The material can be thicker at the center and thinner at the edges. This can impact forming the same angle for the length of the material.

Variation in thickness say you are changing from one coil of material to another coil of material will require changing the penetration depth of the tooling. If you are talking about 20ga material which is .036" and changing to .033" I would expect a large amount of bend change. Normally in an air bending application the vee die opening is 8 x thickness or for about .300" wide but you may be using .375" wide vee die. You will have to find out what changes from experience.

In agree with ornerynorsk what is the tolerance you are attempting to maintain?

 

[btrueblood]

Ornery,

"Dies which are specially constructed to bottom out will tend to peen or set the corner, regardless of slight material thickness variations, but this takes an operator with a little more finesse to avoid putting excessive tonnage on the press and tooling that may not be necessary."

I think this is the type I've seen used - the operator would dial in a pressure/tonnage setpoint based on the nominal material thickness and part width. Or am I dreaming this?

 

[ornerynorsk]

BT, yes, some of the newer brakes I've seen can actually be set for high limit on tonnage in the control, but there are still a lot of older presses in industry where you go by sound and intuition.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[JJthompson]

I work with Air brakes almost every day and found that getting accuracy greater then +/- 1 degree is a bit of a joke. We have an acupress 700ton 24ft bed and bend metal ranging from 22ga to 1" thick. We tend to see overbent spots right under the cylinders of the punch and unless you have your crowning dialed in perfectly you will be underbent in the middle. Also hardspots and material inconsistancies will throw it off greatly. Further more what are you using to measure the angle? Most of the time a basic protractor will be used that can really only measure to the closest degree. Punch wear also throws bending procedure out over time.

I basically have given up on accuracy and design out tight tolerance parts.

As far as flange length inaccuracies that can be calculated from K value and bend radius fairly well.