cutting pyrex tube to length

I'm picking up a Stirling Engine project I abandoned 10 years ago when the ptfe displacement diffusor distorted with the heat of the hot bulb. I want to use a 24mm pyrex test tube cut to the correct (not sure what 'correct is in this case) length.

I am anticipating lots of broken glass unless I know how to made a clean cut/break so how is this done?

Tony

Diamond wheel in a Dremel?

Hi Tony

I've been experimenting with cutting gauge glass tube to length for hydrostatic lubricator sight glasses that have to seal on the end cut face. These are 10mm dia so smaller than yours.

As Baz suggests the most sucess so far has been diamond cut off wheel in a dremel mounted on the lathe toolpost, cutting very deeply into the tube wall until it's almost through and it will then break off easily. I've tried scoring the tube with a diamond file without much luck, the dremel has the best results although the cut ends still need a trim as in this application the tube needs to seal on its cut end face.

To get the end faces flat I've been using a diamond sharpening stone with a guide to keep the end square, good job I bought lots of glass tube as initially the atrition rate was very high, I only spoil about one in four now!

One tip you might find handy is once you have cut your first successful tube to the perfect length with nice smooth, square ends is not to let it roll off the bench and tread on it. Don't ask me how I know that!

Dave

Edited By Dave Wootton on 16/04/2022 17:25:16

I've cut DRO glass scales down and bottles with no problems at all using a cheap wet wheel diamond tile cutter, if you've got one or know someone who has.

I've had a suprising amount of success by just sharpening an HSS cutting tool to a knife edge point and turning the glass in the lathe. I wrap the tube in tape to protect it from the jaws, but a better way would be some sort of soft jaws as it can be a trial to get it concentric this way. I cut the end off of a pyrex test tube this way, but usually I cut normal glass.

You just gently introduce the cutting tool at a 90 degree angle and the glass cuts a vee which you can then tap gently to break once its deep enough. Playing a flame over the cut edge works for normal glass I think with pyrex you would have to use an abrasive grit paper to get the end sweet.

Gently scribe a line round the tube and hot spot it by heating the end a bit of glass rod to red heat and touching the hot end on the scribed line. Hopefully it will break along the dotted line. Briefly flame polish the cut end.

I used the diamond wheel and faux dremel - still took a lot longer than expected and couldnlt get the tool perpendicular - but it worked. Glass tube held in collet chuck.

I use a special glass cutter for shortening the glass on the water column for my steam engine. It has a small hardened cutting wheel and clamps around the tube. It does not cut thru the glass but produces a scratch around the tube. The tube is snapped at the scratch by holding in both hands with your finger and bending. (have not cut myself yet). (Google glass tube cutters. The only cost about $15)

I have also used the corner of a file to score a scratch around the glass tube. Wrapping both sides of the cut with tape will help to stop it shattering.

The cut end of the tube should be smoothed to eliminate sharp edges and stress points that could start cracking. I use an oxy torch to melt the ends but could also be ground with wet & dry paper.

I've never tried this, but I've seen it written that putting oil (I think, might be water) in a bottle to part way up, then plunging in a red hot poker will produce a good cut. Of course this possibly wouldn't work with Pyrex, which is more tolerant of heat stressing

A point about cutting glass is to score as lightly as possible. Too heavy pressure tends to cause small sideways cracks which may result in the snap not following the marked line.

Link to a Youtube video of cutting glass tube with a hot wire. (although a bit too long like a lot of amateur videos) https://www.youtube.com/watch?v=oU0ryjs-Tas

And a video of the score and snap method.https://www.youtube.com/watch?v=4ukVa0oW0xA

Edited By Paul Lousick on 17/04/2022 22:49:05