[HELP] How to bend thin stainless

[brianmichalk]

I need to make a cone out of .010 316 stainless steel.
This cone is 100mm long, 10mm on the small end, and 18mm on the large end.

I started out using aluminum for this part, but need a material that won't wear out like my aluminum part is wearing.

I built a pair of mandrel pliers using structural stainless angle, and a 3/8" steel rod.  The mandrel fits together quite nicely, but when I bend the stainless, I'm not getting the radius I need, and in addition to that, I can't finish out the entire cone shape because the tool gets in the way.

This part is used as a guide for spent ammo brass.  9mm luger to be exact.  I am considering several approaches.  The first being that the part holds its own shape after bending.

The second option is to bend it over a mandrel and spot weld an overlapping seam.

The third option is to butt the two sides together and weld them that way.

Honestly, I don't know how to approach this.  Any ideas?

[Roadracer_Al]

I've built a lot of motorcycle exhaust systems in stainless. One task is rolling cones. I use a tool called a slip roll for this, and can usually get the ends of the cone to butt up when I'm done, but it takes some hand-skill to use: a flat cone has a long radius end and a short radius end (which, when rolled, correspond to the major diameter and the minor diameter). You have to manually cause the short end to slip (hence the name slip roll) so that the rollers are always aligned with the axis of the cone. It's the same reason a car needs a differential to turn.

A sincere question is this: does the ammo guide need to operate in a wet environment? If so, stainless is appropriate, if not, you're using a material which is known to be a drag to work with. Mild steel is significantly more agreeable to bend than stainless. And, 316 is tougher than 304.

One working solution to your sheet metal working problem is to build the cone in 2 equal parts - this will allow you to get your bending tools into the cone shape. TIG them together after the half-cones are formed.

Before I had my slip roll, I used to roll cones with a rubber mallet and a stiff bar in the bench vise. I would draw the axial lines on the blank, align them to the stiff bar, and hammer "off-dolly" to bend the parts. This would result in a slightly faceted cone. Then I'd use a flat-faced body hammer to plannish the parts into a more round shape.

Stainless has a lot of "spring back" and any bending form with a radius needs to be smaller than the desired radius to take spring back into account.

[brianmichalk]

A slip roller would make short work of this, but the radius is way too small for that, unless there's something out there that I missed.

No, this is not a wet environment. I was hoping to stay with corrosion free parts for longevity. Maybe I'll explore some 304 as well, but it's probably going to be only a little easier than the 316.

Now, the two halves idea. That may work, and might even work with my existing mandrel pliers. I cut my cones out of a sheet using a template made from solidworks, and it's very accurate.

Now you also have me thinking about the rubber mallet idea. If I have a male form of the proper bending radius, and use a rubber mallet that will conform to the form, then the resulting sheet should come out to the proper final curve. I'll Google this, but where does one find information on a bend radius required to get a desired radius?

Okay. I found a calculator, and am a little surprised by the results. My experience is that this material is springier than the calculations show. Assume a cone, 5mm on one end, and 10mm on the other, and only making a 90 degree bend:
final radius initial radius bend angle
5mm 4.7mm 96
10mm 8.8mm 101

[Roadracer_Al]

You're right, a common slip roll won't work @ 10mm. But, you could, in theory, build a small slip roll to suit this job.

Different means of bending results in different amounts of spring back. If you have the work in a sheet metal brake and bend it past the yield point and keep bending it in a single shot, you'll get less spring-back than if you bend it in a bunch of smaller bends. Beating on it with a hammer is a bunch of small bends. You'll never get a 10mm part with a 10mm mandrel by hammering it.

You may get best results if you anneal the stainless to it's softest condition first. Since it's going to involve welding, you'll need to pickle it afterwards anyway.

Another thought is that you could make halves using a male die and using a hydraulic press, push the part into a urethane block, causing the shape to form. This will be a much closer fit, but you'll probably still have to make the mandrel undersize.

Another thought is that you could probably make this part out of steel or stainless steel on a lathe with a boring bar.

Another thought is that you could do this with metal spinning by starting with a tube on a steel mandrel, then swaging the tube into your cone shape with spinning tools.

[oldgoaly]

mandrel, mallet, hose clamps, tack weld on mandrel, finish weld.
the hose clamps act like another set of hands to hold and form the metal.
I've made fuel pickup screens for old cars like this only I solder them.