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Cutting glass tube to length

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Tony Martyr15/04/2010 17:39:11
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226 forum posts
45 photos
I have to cut exactly to length the 3/16 OD glass tube for the boiler water level gauges. Is the correct way to do this to nick it at the break point with a sharp file and hope that it snaps straight, with the nick in tension, and in the correct place?
My record in glass pane cutting is less than glorious and I don't want to have to purchase several more lengths in order to get two 3 inch lengths.
What is the trick?
Tony M
mgj15/04/2010 17:56:24
1017 forum posts
14 photos
The way I was told to do mine was to take a sharp triangular file and score all the way round - best if you roll it on a flat surface so the man said.
 
Twice now that has split it perfectly neatly.
 
Hhowever, if it don't drop off at that stage - under the knife as it were - gob over the line and then support either side between two fingers and press with thumb. That works well too. Why spit on it I don't know - I'm told it helps and is important. It works.
 
Why have I cut so many lengths of gauge glass - because I didn't make sure that the top and bottom of the gauge were exactly aligned. I assumed that the silicone O ring would allow for any distortion. So it does, but either not for long under pressure, or, only for seconds if you tighten well. Aignment is key, and since I have taken to dropping a 1/4" rod down the top to make sure it drops ever so neatly into the socket at the bottom, the problem has gone away.
 
Best seal solution up to 250psi test pressure. MACCs 6mm o ring and a touch of Boss White, tightened finger tight. Works a charm. (not the big thick silcone rings).
 
That is the sum of my knowledge born of hard experience in recent weeks!!!!!
 
 


Phil Ashman16/04/2010 13:55:41
33 forum posts
I usually cut the tube as described, but a tad over length, then true it up square and to length on the side of the green wheel. Also, Arc - Euro sell  diamond stones which can be used to "sandpaper" glass to almost any shape. I've used these to shape cab windows for locos. They only cost about a fiver.
 
Circlip16/04/2010 16:52:37
1723 forum posts
And you should really subject the ends to a dose of a blowlamp to fuse the rough edges. Only one or two of us had Chemistry lessons where we did this as a matter of course.
 
  Regards  Ian. 
Engine Builder16/04/2010 17:55:45
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267 forum posts
For a more controled cut I use a diamond disc l in a dremel tool. I picked up a packet  of  4 discs about 20mm dia in my market for about £6. Water can be used as a coolant.
David

Edited By Engine Builder on 16/04/2010 17:56:31

Frank Dolman16/04/2010 18:38:05
106 forum posts

     When flame polishing, the flame will, unless prevented, run up the tube and
   brown you off.  Plug the 'cold' end of the tube to stop this happening.
Brian in OZ17/04/2010 01:37:11
63 forum posts
Hi Tony,
Agree with MGJ

....The way I was told to do mine was to take a sharp triangular file and score all the way round - best if you roll it on a flat surface so the man said. ....
 
After nearly 40 yrs in the refrigeration/air con industry I have cut many a sight glass, The file line also gives you a slight shamfer on the ends of the tube.
 
....However, if it don't drop off at that stage - under the knife as it were - gob over the line and then support either side between two fingers and press with thumb. That works well too. Why spit on it I don't know - I'm told it helps and is important. It works...
 
The "spittle" stops little splinters of glass flying around when you snap the glass, a more hygenic way to to use your oil can to run a line of oil around the cut line, this also allows the glass to slip easilly through the "o" rings in the fittings, it goes without saying that you should ALWAYS wear safety glasses when doing anything with glass, even just tightening the fittings.  
Also agree with the suggestion of a short length of steel rod to check the alignment of the fittings, in my truck always kept (carefully wrapped up) an 18" length of silver steel 1/2" and 3/8 (the sizes I generally used) with chamfered ends for just that, saved a lot of heartbreak as most of "my" sightglasses were on pressure vessals of large ammonia refrigeration plant.

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