Springback of presswork......a tough question.

[Sirius2]

Hi, can anyone help? Or point any refereces?.

We may be getting a press tool to make for some sort of parabolic dish. It is heavily drawn in ratio (say 350mm dia by 225 deep form). This will be from flat cut sheets, and the final dish has four holes 'normal' to the curvature to be pressed in it.

We have been sent an Iges file of the true parabolic curve, which is very handy, but the final dish has to be true to true parabolic form geometrically within 0.25mm all over.

I forsee the shape being pressed springing back somewhat due to the depth of the pressing, at least upto tolerance. Initially, we want a perfect pressing (well, within 0.1) so that the tool can wear a bit etc.

** I have a drawing package that can draw parabolic curves, and was wondering if there was any formula for springback on this type of dish/form so I can construct a slightly different form to take account.
It will be a costly mistake if its wrong, because a whole range of tools is on the cards.

I suppose the material type comes into it, but is there any basis to work this out?.

Thanks

Sirius2.

 

[Kapitan]

The material will certainly make a difference, so will the design of the press tool(s) and the component. Forming a part like this is not going to be easy, there will be problems with thinning and wrinkling of the material. If a formula could be found to predict the results some empirical work should be done first, make up a soft punch and press a part to see what happens. There are too many unknown factors here at the moment.

1) What is the material to be formed ?
2) What is the material thckness ?
3) How many parts are to be made ( a hour, week, year ) ?
4) What type of blankholder does the tool have ?
5) Is there any form of flange on the periphery of the finished component ?
6) Is a centre hole allowed in the component design ?
7) Are the 4 holes to be drilled after forming ?
8) Is a developed blank being used or is there a trimming operation ?

When a part has been formed it will have to be measured, if it is scanned or digitised then some sense has to be made of the coordinate data. This can be a complicated business. Or are you going to use an inspection fixture ? Will there be any datum features on the dish to relate any measurements to ?

If springback is the biggest problem then any compensated form should remain parabolic. The punch being of a slightly shorter focal length paraboloid. Any deviation can only be found by measuring a development model and inputing it into the parabola equation;

x = y^2 / 4*f

Where
i) x is the axis of revolution of dish.
ii) y is the radius of the dish.
iii) f is focal length of parabola.

Using the dimensions given the tangent angle at the rim of the dish will be approximately 69 degrees to the blank plane, so any relaxing of the shape will be really minimal as the dish is approaching a cylinder, at its maximum diameter but without some type of flange on the rim it will not be very stiff. At the bottom of the dish the radius of curvature will be about 68 mm., so thinning of material will be at its extreme here.

Have you considered spinning this part, it's a natural process for forming a surface of revolution such as a parabolic dish. The tooling costs are significantly less.

 

[Sirius2]

Wow.

What a response! .

You have made some good points here. Id forgot all about soft tooling, you can make punch form dies from stacks of glued marine plywood cant you (from what I remember)?. A lot less expensive than a chunk of tool steel that size.

I havent all the details at hand, but I'll guestimate that its either 1.6mm stainless steel, coated mild steel or somekind of special 'hard' alloy aluminium.

We are making the tooling for a customers dish requirements, so the tool can look any way we want (within budget). They must think a presstool is more cost efficient than spinning I suppose. I currently dont know the quantities involved.

There is a lip protruding about 10mm around the outside with a few radial notches out. The four holes are through a deformed 'pad' which will have to be pressed into the parabolic part...(the formed pads must make it screw back to a flat face I gather), so they will need to be right on the initial blank. This is going to be intresting to try and work out the flat development of! lol.

Im guessing there will be a trimming op, but only around the lip. There can not be a center hole. We would have to develop a pretty accurate flat blank to save as much waste as possible or if we are good enough - no final trimming op.(within 5mm or 10mm if it is needed).

We have a contact probe scanner that can output DXF polyline sections, so we could measure the difference (good idea) with that.

I need to study your equations when I get to work and see if I can do it. If my sizes are anywhere in proportion you give the impression that because there is a flange/lip around the top and becuase of the incline of the shape it may not spring back much - but just thin out at the center.

If I can talk them into taking the time (you know what its like with deadlines these days) to make a test form/die block, we could use the exact parabola and go from there I suppose. Otherwise a bit more trial and error and application of the above mathamatics may be in order.

I dont know much about working out 'springback' on general presswork, but springback on a parabolic dish was just blowing my mind on how you calculate that.

Thanks for your response! Im gonna save it to disk and take it to work .

Cheers

Sirius.