Can Polyurethane Rubber Durometer 75A be machined?

[Davefr]

I need to make a rubber/plastic bushing 50mm OD, 35mm ID that's approx. 3/4" long. Mcmaster has this which would be a good start. Can Durometer 75A rubber be machined (drilled, ground, sanded, filed)? Ordinary rubber is a ***** to machine but Mcmaster claims this is hard. TIA

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[White Shadow]

I dont' know, but I can tell you that I had a tire shaved and it was far from a nice finish on the surface of the tire. I believe an average car tires has about the same hardness you mentioned.

 

[stockerwithalocker]

Could you dip it in liquid nitrogen or dry ice to harden it and it might machine better? The machining will warm it requiring multiple dunks/cool downs but might work depending on what you need to do.

 

[paulsomlo]

Drilling it, maybe. But I gave up trying to turn it in the lathe. I think it's typically done by getting the material cold, and using knife like tools to machine it.

 

[Davefr]

Thanks guys. I'm going to go a different route and use Acetal instead. My experience is that it's easy to machine.

 

[no704]

Be better off punching it even if you have to make a stack.

 

[Rsharp66]

I work in the rubber industry and we freeze to machine. Liquid nitrogen is too cold and it will shatter. That's how we salvage molded parts.

 

[paulsomlo]

I work in the rubber industry and we freeze to machine. Liquid nitrogen is too cold and it will shatter. That's how we salvage molded parts.

What sort of tooling are you using to machine it?

 

[Firebrick43]

Yes you can machine it if its thick enough to be stiff. Very sharp positive rake tools (CCGT 21.51 ground inserts) and like white shadow stated, crappy surface finish. freezing is a so low fridge helps alot. We did liquid nitrogen, just limited how long it was in it.

Much better just to 3d print something with TPU if you need a rubber piece. If you don't need the cushion, acetal is a much better choice.

 

[RoninB4]

I've worked with a fair number of rubber components but this is just my limited experience. Freezing the rubber can work IF you're very fast and have a minimal amount to remove, the rubber very quickly warms up and becomes difficult to cut CLEANLY. I capitalized that last word because that's what has always been the difficulty of bringing rubber to a targeted size/dimension. Rubber will flex and the stress raiser required to cut something will change with the inherent flex of the rubber, even at Durometer 75A. Dead sharp tooling can provide a better cut but seems somewhat abrasive and the cut still seems to be unpredictable.

There is only one method I've found for machining rubber and works quite well. Grinding. I have double side taped it to the chuck of a surface grinder and cut "V-shaped" ribs on long flat pieces, spun it in a Harig type head to make bushings, and cut angled surfaces on it. Grinding cuts clean, predictable surfaces, I've plowed 1/8" off in one pass. Do take care to have vacuum present as you don't want to breathe in rubber dust. I try other methods now and then but nothing works as well as grinding with a coarse grit wheel.

There are also liquid rubber compounds that cure overnight (Devcon Flexane) of various flavors that I used to make suspension bushings for an old sports car. Use the "lost wax" process for cheap mold construction.

 

[exmaxima1]

You might try grinding it with a sanding wheel or a coarse burr

 

[paulsomlo]

Here's some video - take a close look at the tooling:

and this:

 

[RoninB4]

Here's some video - take a close look at the tooling:

and this:

Nice video, that material looks a lot harder than Durometer 75A by the way the material is peeling off from the cutting tool. Material that hard doesn't flex very much and can be easily cut on the lathe, as amply demonstrated in the videos.

 

[paulsomlo]

Nice video, that material looks a lot harder than Durometer 75A by the way the material is peeling off from the cutting tool. Material that hard doesn't flex very much and can be easily cut on the lathe, as amply demonstrated in the videos.

With very sharp, positive rake tooling. And I think those are specialty inserts?

 

[RoninB4]

With very sharp, positive rake tooling. And I think those are specialty inserts?

Some are, (negative rake is more limited in numbers than positive) it's about the insert holder itself. If the holder is neutral then yes. If the holder already has positive/negative rake then you order inserts accordingly. Some inserts have a small "gulley" next to the cutting edge which makes them a positive rake. You can order inserts in a very wide range of geometry to suit cutting conditions. For small batch runs in urethane compounds I like just grinding up a HSS tool bit as I can make the rake as severe as the material calls for and make the nose whatever geometry the job calls for. Touch up on HSS is simple, not so on carbide inserts. JMO. Never ran urethane fast enough to burn HSS. Full production runs would be different.