Stretch Forming of Aluminium Sheets - Custom made machinery

[Leuf]

Hi,

I'm currently trying to get a better understanding of the requirements of the stretch forming process so I can properly assess the feasability of an inhouse project. I've seen a few threads here breaking down the maths behind the process but I do have a few more questions on the topic. I targeted stretch forming because of the tolerances required by the customer, but I'm opened to other ideas.

My customer required that we 'form' sheets of aluminium with a decorative top layer ALREADY INSTALLED prior to forming. We are planning to use a product fairly similar
to this : Link. We must not damage the decorative layer in the process. The sheet's overall dimensions are L:99.375in W:22.750in and the sheet will be bent on the short side.The desired shape is pretty close to the projection of a comma/trigger along the hole lenght of the sheet.

Anyone ever had success with similar products in the past?

Also, I am planning to create a custom machine to do the forming instead of relying on buying an 'off-the-shelf' solution.

Here are a few notes:

1) Sheet thickness is .076in (.063 aluminium 5052-h32 + .013 decorative layer)
2) Material can't be heated because of the decorative layer's glue
3) Space is not a problem.

Cheers,
Leuf
Mechanical Designer

 

[MikeHalloran]

Stretch forming, as I understand it, involves gripping all four edges of a sheet and pulling the grippers away from each other while at the same time forcing a convex 'punch' against the center of the sheet, forming something like the topography of a bathtub, with less curvature. ... and some wrinkled corners. I.e., it's a 3D forming process, great for making double curved aircraft parts.

Your description makes it sound like your part could be made in a large press brake, perhaps with some custom tooling, perhaps even with commercial tooling.

Prefinished/coated metals are successfully formed all the time. It requires some care in handling, and accommodations like carpeted tables, and is only limited by local curvature of the workpiece.






Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

The unknown here is how does the decorative surface react to bending or stretching,
First does the sheet have a double curve? If yes then stretch or stretch wrap forming is required . Since this involves stretching the material to just at, or just beyond its first yield, will the coating stay put, or will it tear? This stuff sounds similar to a Formica covered aluminum, also do you have enough spare material on the finished part to be able to trim off the clamp/gripper marks?
If the sheet has a single curve then this can be done on a press brake by bumping, or by sheet metal roller, if the curve in the tail of the comma is particularly tight ,then a Farnham roller can be used. Protective coatings can prevent damage to the sheets.
B.E.

You are judged not by what you know, but by what you can do.

 

[Leuf]

First of all,

Thank you for your responses; I was doubtful about the possibility of having a response and I am happily surprised by the fact that I did.

So here are a few updates on my situation

After seeing your responses, I decided to take the initiative of ordering a few samples from the supplier and go forward with some destructives tests i.e. pull tests,scratch test, etc so I will be able to assess the surface finish of the sheet metal under stress.

Your description makes it sound like your part could be made in a large press brake, perhaps with some custom tooling, perhaps even with commercial tooling.

Well I thought that it would be more cost efficient to go with the stretch forming method mainly because that a press brake requires 2 separate tooling with the final component shape machined on it. For stretch forming, only one is needed. Is the trade-off really that significant? I will have to dig deeper before being 100% sure.

Attached is the desired shape I'm planning to produce.

If the sheet has a single curve then this can be done on a press brake by bumping, or by sheet metal roller, if the curve in the tail of the comma is particularly tight ,then a Farnham roller can be used. Protective coatings can prevent damage to the sheets.

Rolling the shape is not off limit but is it possible to create a variable radius with this process? It seems that you can't really avoid having a few scrapedcompo

Thank you for your time guys!

Cheers,
Leuf
Mechanical Designer

 

[MikeHalloran]

It's eminently possible to make that shape in a press brake with ordinary air-bend tooling, especially if you can accept some subtle axial creases. You will get a better part with an experienced operator or a CNC machine. Take a look at large tapered poles used for traffic lights and electrical distribution. More polygonal faces will cost more in cycle time, but give a smoother part.

You can probably also make it, repeatably and smoothly curved, with a custom punch working against a semi-hard urethane pad in a press brake, and you may be able to make it with a semi-custom crowned punch just a little smaller than the tightest radius.

It's also possible to make it in a three-roll forming machine, given some operator skill. I'm not sure if CNC would help, or is available on such a primitive machine.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

Leuf (Mechanical) (OP)
It is quite easy to make a variable radius using either the bumping process or the Farnham roll. The Farnham roll was originally invented to form the leading edge skins for aircraft wings, where you have considerable variation in the radius of the formed part.
I regularly used to form oval columns for hotels and restaurants on a press brake, with these, we made the columns in two halves, bumped the minor axis first, then came back and bumped the major axis.
B.E.

You are judged not by what you know, but by what you can do.

 

[dhengr]

Leuf:
I agree with MikeH about using either/or roll forming or bump forming in a break press, as reasonable methods. However elaborate or exact we think we can engineer and describe something, we used to furnish our suppliers a plywood form/template of the shape we wanted. This was for them to use against (to confirm) the shape he was forming.