Straightness of a bend on sheet metal parts

[guyst]

Hi, I'm designing a frame which requires relatively high straightness, long bends in 3mm thick plain steel sheet metal. Talking to manufacturers and searching online, I'm having difficulty finding the achievable straightness of the bends, and how to achieve them. Regularly, when manufacturing this type of parts, most times we get quite bent long sections (with more than one manufacturer).
Any advice will be appreciated..
See drawing example attached for reference

 

[Jboggs]

I think you will find that bends created on press brakes will be much straighter than bends created in a rolling action (like on continuous gutters). If the entire bend can fit within the physical limits of a press brake, it should produce "straighter" results than rolling.

 

[ctopher]

I only had to do this once several years ago. We made a fixture from tool steel, sandwiched the sheetmetal with it, stressed relieved in oven. I don't remember temp/time.

 

[rb1957]

this looks like a cable tray ... why a small tolerance for straightness ? If you're saying the two standing legs need to be straight, parallel, and a set distance apart ... then make in two pieces, 2 Ls, and rivet together after installation.

 

[guyst]

I think you will find that bends created on press brakes will be much straighter than bends created in a rolling action (like on continuous gutters). If the entire bend can fit within the physical limits of a press brake, it should produce "straighter" results than rolling.

Thanks, but we normally send these kind of parts to be bent in a press brake, it doesn't come out straight. ever..
(But usually we make them from 304)

 

[guyst]

I only had to do this once several years ago. We made a fixture from tool steel, sandwiched the sheetmetal with it, stressed relieved in oven. I don't remember temp/time.

Oh my.. do you remember what kind of straightness figures you aimed for?

 

[guyst]

this looks like a cable tray ... why a small tolerance for straightness ? If you're saying the two standing legs need to be straight, parallel, and a set distance apart ... then make in two pieces, 2 Ls, and rivet together after installation.

Hi, its just an example drawing I made to help explain my question

 

[ctopher]

Oh my.. do you remember what kind of straightness figures you aimed for?

No. I only remember that we did that process.

 

[btrueblood]

"we normally send these kind of parts to be bent in a press brake, it doesn't come out straight. ever.."

What part deviates from straight, the material between the two bends or the edges of the two bent legs? Are the two legs parallel or deviating?

I find it hard to believe that a punch press forming the bends with typical tooling cannot hold the two legs parallel within 1/2 mm over 750 mm length. But then, we have some pretty sharp sheet metal people up here around us.

 

[guyst]

"we normally send these kind of parts to be bent in a press brake, it doesn't come out straight. ever.."

What part deviates from straight, the material between the two bends or the edges of the two bent legs? Are the two legs parallel or deviating?

I find it hard to believe that a punch press forming the bends with typical tooling cannot hold the two legs parallel within 1/2 mm over 750 mm length. But then, we have some pretty sharp sheet metal people up here around us.

Hi.. the bent legs deviate - see the drawing for indication. As opposed to the drawing, the straightness we get is around 3mm in this type of parts and length.

 

[CWB1]

Tolerances vary depending on the material and specific process used. You could stress relieve or not, cold or hot work, there are multiple forming processes to consider (folding, stamping, rolling, etc), and you can also have secondary straightening, heat treat, and other operations to consider.

Rather than asking for a bad rule of thumb 1. tell a few suppliers what you need quaNtitatively (not quaLitatively), 2. ask them to describe the manufacturing process, and 3. discuss various alternatives. If your proposed tolerance drives too much cost then loosen it.
Engineers and tradesmen alike generally hate rules of thumb bc they create issues - needless cost, wasted time, complexity, questions, etc so if you ask for them, dont expect a good answer.

 

[mfgenggear]

OP
For a small lot, hydro press, if it needs closer tolerance secondary two PC die , with a press. This type work because od the short length of the flanges could cause issues.
Not saying it can't be done with a press brake. But it will depend on the precision and experience of the operator.
For a large quantity stamp and die no problem.

 

[btrueblood]

You could look at a secondary process, either grinding or milling the two legs to achieve the straightness/flatness you desire. Of course, that costs money. I do agree with CWB, that you should approach several suppliers to see what they think about achieving that spec.

Does the part need to be straight in the opposite (perpendicular) direction, and if so, to what precision?

edit: one more question to clarify:
Does the height of the two legs vary, i.e. one leg sticks out a bit more than the other and that issue causes the out of spec condition? Or does the height of either leg vary along its length?

 

[ShataCastings]

Cant see the thickness, but its a really challenging project for only bending, as I can see thickness according to drawing scale.

Use thicker material and grinding after bending is a not bad idea.