Machining Polyurethane

I'm looking for advice on machining Polyurethane, 40 durometer. These are small pieces, only a few parts and will be adhered to aluminum. I have heard of freezing (with dry ice) the polyurethane before, is that going to be necessary?

I appreciate any advice.

I guess it comes in a bunch of variances, but the few I worked with were ok. Like most plastics just make sure the chips clear and don't get it hot.

I used to freeze them in the freezer (no access to liquid nitro) and it helped. We ground the material square because it worked better than milling (didn't need to freeze for grinding, just put in a machinist vice)

If there are counterbores, put them in before you drill the through holes or the material will try to grab and tear.

I hear that "cold shot" helps, but i have no experience with it.

oh yeah, if you do use liquid nitrogen, the part will change more, so watch your tolerances.

A sharp carbide burr at around 25K rpm will cut even softer grades of polyurethane and rubber at room temperature with no fear of grit contamination of the part or machine tool. Tight limits on dimension can be held as well.

You don't say what shape. We used to grind round pieces on tool grinder with a mandrel. Gives a nice matte finish. I suppose a carbide burr is similar.

Made a mess, though.

Machining 40 durometer is going to be a bitch. I've machined plenty of 60 and 80 durometer, and that is not much fun, but is a bit stiffer.

When machining, I froze the material and the collet I was planning on holding it in together in an industrial deep freeze. Transported the part to the machine in a cooler and used a cold air gun to try and keep it cold longer. I do not believe a home freezer is going to get it sufficiently cold enough to last more than a few seconds worth of machining.

Tool geometry needs to be very sharp with a lot of hook. I made tools that looked and worked a lot like a knife to cut the material. Don't expect to hold close tolerances. It's just not going to happen. Grinding is supposed to work, but I've never personally tried it. I've also heated drill rod until red hot and then pushed it through the part to produce small holes that couldn't be drilled.

You are right to say freezing doesn't give you long to work. I used to set up to do each operation so that I could do it right away in seconds and do something else for a while as they froze again.

Grinding works excellent--can take a dep cut and just pick feed over lightly. We used to take the grinding wheels to the bandsaw, slap on an old blade, and saw slots radially into the wheel all around it to help keep it cool while grinding poly-u. Never had a wheel explode and it worked well. Do at your own risk, I guess!

Regal13,

I've done a lot of scary stuff in my time, but I would draw a line at sawing into a grinding wheel. That would scare me silly.

Still, if it doesn't explode, it should work fine.






Frank

www.randolphmach.com

ha ha ha...that's about the reaction I expected :-)

the slots don't have to be more than about 1/2" deep into the grind wheel, and space them about every inch or two around the circumference of the wheel. I think the bond of the wheel is plenty strong enough as long as you don't crack the wheel in the process. Pluck it with your finger before mounting on the grinder to make sure it rings and if it just thuds, throw it away.

When you are grinding the polyurethane material, you can take up to about 3/8" depth of cut, but it seems like it wanted to melt if you try any deeper.

Schliebe said:

I'm looking for advice on machining Polyurethane, 40 durometer. These are small pieces, only a few parts and will be adhered to aluminum. I have heard of freezing (with dry ice) the polyurethane before, is that going to be necessary?

I appreciate any advice.

Click to expand...

Did you ever figure out a way to machine the Polyurethane?