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View Full Version : OT? Glass forming over a machined mandrel



JimA
02-09-2018, 10:52 AM
Anyone here work with glass? I have some pyrex tubing that I need to enlarge slightly. Tubing is .750 OD x .550 ID. I need to enlarge ID to .566 + - .002. Last time I played with glass and a Bunsen burner was in chemistry class way into last century. My thought is to machine a mandrel to 0.566", heat the glass and carefully slide the glass tube onto the mandrel to enlarge. Let the glass cool and hopefully it will slide off. Only sliding tubing onto mandrel about 2.250 inches. Cut that section off and do another. Maybe mandrel can be solid rod or hollow to be able to cool quicker if glass will not slide off mandrel when cools to a solid. Researching mandrel material taking into account coefficient of expansion between mandrel and pyrex. Anyone done this? Thoughts and input?

Dave C
02-09-2018, 11:08 AM
I did something similar years ago for the testing lab, but the tube was smaller. Warm your mandrel and slide the tube over it. Pull the tube off before it is completely solid, but rigid enough to hold it's shape. Cutting it off was the hard part. They slid off the mandrel ok, but several tubes broke while cutting them off.

JimA
02-09-2018, 11:46 AM
Any special considerations to the mandrel material? To me aluminum and stainless have too much thermal expansion. 12L14 or 4130?

JohnMartin
02-09-2018, 01:28 PM
Any special considerations to the mandrel material? To me aluminum and stainless have too much thermal expansion. 12L14 or 4130?

That thermal expansion might be a good thing, if you think of it as thermal contraction instead. Assuming you insert the mandrel hot.....

lbender
02-09-2018, 01:33 PM
Graphite makes suitable tooling for handling hot glass.

Paul Alciatore
02-09-2018, 01:47 PM
Perhaps the tubes could be cut a bit over length first. Then, after forming them use a sanding belt to bring them to the final size. Glass shops use belt sanders all the time on the edges.




I did something similar years ago for the testing lab, but the tube was smaller. Warm your mandrel and slide the tube over it. Pull the tube off before it is completely solid, but rigid enough to hold it's shape. Cutting it off was the hard part. They slid off the mandrel ok, but several tubes broke while cutting them off.

BobinOK
02-09-2018, 02:19 PM
It has to cool slowly or it will shatter, at least that was my experience when playing with a torch and glass.

Ggerg1186
02-09-2018, 04:26 PM
Seems like you would want to use hardwood for your mandrel and not steel. Something with heat insulation properties.

Tim in D
02-09-2018, 06:39 PM
OP didn't say length but there are two ways to approach this.

For short pcs like 6" or so you can make a SS mandrel and shrink a slightly larger pc of tube onto it using vacuum. I made some elliptical LASER cavities using this technique. The mandrel was polished 316SS. You kinda have to hold your mouth just right to get some good pcs!. Always anneal the glass before you try to do any cutting on it.

Another way is to slip the tubing over a graphite mandrel. The set-up is all vertical. Inert gas to prevent the graphite from burning.An electric tube furnace surrounds the working area. Again, not something easy. As is often the case, "the learning curve is quite steep ".

Cheers,
Tim in D