Made some rubber toroids

Sounds good. But be careful freezing the rubber. I found that out the hard way a while back. I had to turn down some "swabs" which are basically rings of rubber with a taper on the outside. They use them to clean wells. They couldn't get the right size so brought some to me to turn down. The rubber was pretty soft so even a extremely sharp HSS tool didn't want to cut it. Thus I went down the road to the breeders clinic and got some liquid nitrogen. I stuck one in and then a couple of minutes later stuck three more of them in the nitrogen and then had to answer the phone. While I was on the phone I heard this muffled explosion. I went and checked the heavy duty styrofoam cooler I use for these jobs and found one of the rings in pieces inside. I used pliers to pull the other two out and just as I set them on the corner of the table the first one exploded as I was reaching to get the third unexploded one. I threw chunks of frozen rubber all over the shop. I'm still not sure how I didn't get hit by any of it since my head was just 18" away.

I turned out that there was a steel ring inside the bore which I couldn't see (the rubber was molded over it). When the rubber froze it shrank on the ring causing a lot of stress to build up. I finally finished the job by just putting one ring at a time in for no more than 1 minute. I had to do a quick rough down on one side and could just barely finish that before the rubber started getting to warm. I then put the ring back in for about 30 seconds and turned the other side. Even doing this I had one fly apart while I was machining it.

In the past I've ground a lot of rubber with a sanding disc. In that application we where making rubber vaned rotary air locks. These had about a 8" bore about 12" long. For the rotors we welded 5 straps to the side of a 1" SS shaft and then using 5 more straps we bolted on pieces of 3/4" thick conveyor belting. I "turned" these rotors using a DA sander I modified with a solid backing plate and lathe mount. It was a extremely messy job even with a shop vac running the whole time.

Wayne Cook Shamrock, TX

Probably the material was being cooled too rapidly - rubber has a pretty poor thermal conductivity, so the inner part was hot while the outside was trying to shrink down. This put the exterior under tension and the inside under compression. As you say, the stress was probably large and non-uniform.

If you were to cool them slowly by placing them in the bottom of the beer cooler, and adding LN2 a bit at a time over an hour or so, it would do better. You could also lift them off the bottom of the cooler with a cage or basket so they cool only by contact with the cold gas, that would be better too.

Jim

frozen as cold as liquid nitrogen. I've never machined rubber when it was molded around something else. It's always just been pure rubber. But you can be sure I'll remember what happened to you. ERS

Yep.

It might but I doubt it considering the steel sleeve on the inside. Without the steel sleeve I don't think I would of had anywhere near as much trouble though I might of needed slower cooling to prevent cracking.

Wayne Cook Shamrock, TX

I definitely will. I did not expect the shear force of the explosion. I threw rubber for 20 feet in all directions. Since you seem to work with rubber I thought you might like a little warning so you don't have to learn the way I did. :-)

Wayne Cook Shamrock, TX

the advice is appreciated. It's amazing how information will be buried until a situation arises where this info insneeded. It helps to have all this stuff on file in my brain. ERS

Yep. That is one of the great things about this group. There's a sharing of knowledge here (once you weed through the trash) that is amazing. I learn something nearly every time I read RCM. I also find it interesting how things always seem to come together at one time. It's amazing how often something will come up in the group about the same time as I run into it in the shop. I can't say how many times I've read something and within a week the same situation arrives through the door. Often even then it won't be just once but two or more times within a short period that the same type job or need for knowledge happens and then I won't see it again for a long time.

One example is a while back when there was a discussion on old telephone wires like they strung on glass insulators. The discussion hadn't been finished for a week before my buddy comes in the door talking about how he straightened out a mess of these wires on a place my dad leases. My buddy works for the local gas company and he has a well he checks on that place sometimes. A month or two earlier my dad had gotten some of the wires wrapped up on his pickup and had to call me to help (he'd just had his shoulder operated on and couldn't do it himself). Well my buddy did a bunch of cutting and straightening up to prevent it from happening again. He came in to tell me about it and got to talking how much all that copper wire would be worth. That's when I remembered the talk on here about that wire being copper plated steel. He insisted that it was solid copper and then proceeded out to his truck to prove it. That's when he finally noticed the shiny end of the cut where it showed to be steel. He was totally blown away by me being right once again (this has happened a lot in the past). Knowledge like this that we read here crops up some much it almost seems more than coincidence.

Wayne Cook Shamrock, TX

Here's another thing to remember about that wire - I read this in a different newsgroup, some years ago...

The fact that it is steel means that it is springy. One of the posters on that ng forgot and when he was doing something with a coil (unwinding it?) the end got free and punctured his eye. Now blind on one side.