Home sheet metal stamping help needed!!! - rec.crafts.metalworking

Message from discussion Home sheet metal stamping help needed!!!

Robin S.

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More options Sep 27 2006, 5:14 pm

Newsgroups: rec.crafts.metalworking, alt.machines.cnc

From: "Robin S." <lasern...@hotmail.com>

Date: 27 Sep 2006 00:14:05 -0700

Local: Wed, Sep 27 2006 5:14 pm

Subject: Re: Home sheet metal stamping help needed!!!

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F. George McDuffee wrote:
> On 26 Sep 2006 17:24:53 -0700, "super88" <tdebo...@gatecom.com>
> wrote:

> >I'm going to post this message to all groups that I feel may be able to
> >help. Bear with me, I'm ignorant, but ambitous. I'm tryng to make
> >something similar to a cookie sheet. A little larger radius (2 1/2")on
> >the corners and about 1 1/4" deep out of 22 ga. cold rolled steel.

I build automotive body panel dies. The most important part of this
affair is to understand what kind of part you expect out of the die. If
you're expecting a cosmetically perfect piece of sheet metal that, if
painted, looks like a mirror, you're expecting way too much. It sounds
like you have never built a die at all, so you're missing some basic
knowledge and a whole lot of experience. But since you asked...

> I've
> >built a "press" using I beams and 25 ton bottle jacks to do the
> >job.(Keep in mind if I could afford to job this out, or pruchase a
> >press, things would be different.) Anyways, I've managed to make the
> >part except for the corners wrinkle badly. I'm in the process of
> >casting the female to allow me to use binders. The question is where is
> >the best place to bind the blank?

Typically the entire blank is held in a binder. This may not be
absolutely necessary, however. Indeed, it takes quite a bit of work to
make a binder actually do its job. It should be understood that it is
not the job of the binder to squeeze the snot out of the panel. The
binder is only there to prevent wrinkling. It does this by being
*exactly* one sheet metal thickness away from the draw cavity which it
faces. This is achieved by the long and tedious process of spotting.
Additionally, binders have balance blocks which actually prevent the
binder from squeezing the material.

Beads, on the other hand, are a bit different. Draw beads increase the
resistance to the metal flow into the die cavity. A bottom line rule is
that when you get wrinkling, you're getting excessive flow and you need
to reduce it. When you get thinning/tearing, you're getting
insufficient flow and you need to increase it. Beads are used by the
toolmaker to adjust this flow. Beads which are rads will cause
resistance where as lock beads which have a square profile (with small
rads) will actually prevent virtually any metal from flowing (causing
stretching instead of drawing). Typically you start out with a rather
excessive bead which is sure to tear your panel. Then, you grind away
the bead until the tearing disappears, and you ideally get what you
want (much easier said than done).

Note: the binder force must ensure that the panel's tendancy to wrinkle
does not push the binder away from the panel itself. Remember that the
force available from the press will have to oppose the binder force
while forming the panel as well. Nitro springs are good if you don't
have a draw cushion (you don't) but in your case, die springs may work.
It's difficult to tell.

Since your corners are wrinkling, you need to add beads around them.
The beads should taper into nothing (or at least be reduced) as they
enter the straights. Running a bead around your entire profile will
allow you more control over the flow into your die, but adds
complexity.

> In the corners, or along the
> >straights? Any help, opinions, suggestions, ideas are appreciated! I'm
> >doing this with determination and junk layin around the yard. Buy a 10K
> >press, hire a die maker...etc. are not the answers I'm looking for.
> >Unless someone is willing to produce this for less than a grand. Thanks
> >again!!

You just need to understand that if this doesn't work out the way you
wanted, it is unlikely you will ever find out why. This type of skill
cannot be learned from printed words. Only experience under someone
more capable can give you the ability to create excellent results.
Indeed, making a toolmaker who can build and tryout a class A draw die
(outer sheet metal, cosmetically "perfect") usually takes 5+ years
after apprenticeship. To give you an idea, complex body panel draw dies
can cost in excess of $500k each.

Other important points:

Any rad which the metal is flowing over *needs* to be polished to a
mirror. This typically requires polishing stones (up to 100 grit) and
then emery cloth up to 600 grit (120, 240, 400, 600).

Any rad which the metal is flowing over *needs* to be a true rad. If
you can feel edges around any part of the rad, you are increasing the
resistance to flow and you're likely to get a tear or at least thinning
in that area.

Spotting blue is nearly a requirement unless you have virtually no
specifications beyond the part not splitting in half (and maybe even
so). Again, the requirements of the panel need to be specified.

Smaller femal rads shall not spot into the panel. Metal flows over male
rads only. Female rads are to be cleared out.

If this die has to make more than about 1000 parts, flame hardening the
rads is virtually a necessity. The rads will get scored with any debris
on the panels and will eventually wear away if left soft.

> ==============

> The ingenuity and dynamism of participants of these groups
> continues to impress me.

> I think I understand your product/problem (and then again maybe
> not.)

> The sides and ends form well because these are flat curves. The
> corners however are compound curves that bend two directions at
> the same time with the result there is an excess of material and
> it wrinkles. This is the same problem that you encounter when
> you wrap a package, and have to fold the corners over to make
> everything fit. Indeed, many cookie sheets have "envelope" or
> folded ends, or the corners are "fluted" for just this reason.
> Therefore, it is doubtful that binders will help the problem,
> because the [extra] material is not moving in from the sides/ends
> during forming, but is preexisting in the flat sheet.

Draw beads will solve the problem, assuming the OP is capable of
adjusting them correctly (very unlikely, but that depends on the
requirements of the panel). A binder by itself is indeed very unlikely
to solve the problem.

> Good luck on your product, what ever it may be.

> One final thought - have you contacted Elko or other cookie sheet
> manufacturers? They may have something off the shelf.

By *far* the best advice available.

Regards,

Robin

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