Folks,
This is kind of boring...cutting Pyrex glass tubing (cleanly). This is not as easy as it sounds and is more difficult than cutting soft glass. There are different ways of cuting hard (Pyrex) glass, but the method described here works well.
This glass tubing forms the ion gun's envelope and must have square (and clean) ends to press against the Viton o-rings for adequate sealing. OK now, don't fall asleep reading this! Later.
Regards,
TIM
2 March 2003. Experimental Ion Gun Construction Notes. These notes specifically address cutting Pyrex glass (#7740) tubing for the ion gun’s vacuum envelope. I fabricated the device for cutting large diameter (25-mm & above) shown on page 64 in “Building Scientific Apparatus” (J.H. Moore, et al). This was an easy way of mounting an electrically heated wire to induce thermal stresses in the glass tubing along a pre-scored line. I used phenolic stock for the base and v-block for the glass-cutting device. The v-block supports the glass tube when it is rotated (manually) and simultaneously pressed down against the heated wire. The power supply for heating the wire consisted of a 10-ampere variable transformer, an 8-volt output step-down or filament transformer, an AC ammeter and 12 gauge wire jumpers. The wire current was 15-amperes, with a resulting “bright orange” glow.
I pre-scored the 25-mm diameter glass tubing by first marking where the cut would be made and then masking each side of the cut with masking tape. I scored the glass all the way around with a diamond-coated three-cornered file. Conventional files or a glass knife can also be used for scoring. After that, I removed the tape and pressed the glass against the wire (stainless steel lacing wire, 20 gauge or 0.032” diameter) and rotated the tubing. Since the glass is thermally conductive, the wire’s temperature drops at the point where the glass makes contact; that’s one reason for rotating the glass, besides ensuring the thermally induced stresses occur along the entire scored circumference.
After slowly rotating the tubing for about 30-seconds, I then quickly turned the tubing on a very damp wad of paper towels. I didn’t notice any visible cracking from the resulting thermal shock. However, I was able to cleanly break the glass by gently grabbing it with pliers and trying to “bend” the glass; it snapped-off cleanly at the scored line. I made several cuts like this and cut one glass section that was only 1.125-inches long; this is the section between the intermediate electrode and anode. I cut this piece using a modified technique than described above. Instead of pliers, I used a diamond disc in a Dremel tool to cut the glass along the scored line. Once the glass was cut, I squared the ends (if required) with the diamond disc. This was followed by a quick manual grinding on medium grit emery cloth by rotating the tube ends against the emery, using a circular motion.
UPDATE: HULL-RANEY EXPERIMENTAL ION GUN
Re: UPDATE: HULL-RANEY EXPERIMENTAL ION GUN
Another method that I use is flame cut it in a lathe. That makes a very nice smooth square cut. I use a small minitorch and Hydrogen / Oxygen flame. I used this method when I made my linac tube.
Re: UPDATE: HULL-RANEY EXPERIMENTAL ION GUN
Phil,
That's a good idea. I may try a modification of that idea since it would be a pain to drag my oxyacetylene outfit over to the lathe. I'll have to think of a solution that befits my laziness.
Regards,
TIM
That's a good idea. I may try a modification of that idea since it would be a pain to drag my oxyacetylene outfit over to the lathe. I'll have to think of a solution that befits my laziness.
Regards,
TIM
Re: UPDATE: HULL-RANEY EXPERIMENTAL ION GUN
Tim - re: glass tubing cutting with a hot wire, and boring glass plates, etc..
I use the hot wire technique with a minor variation: I score all the way around (3 cornered file works well, too) and then use a circumferentially wrapped hot wire . I have a simple little fixture with two bolted electrical connections for the hot wire. They are as close together as possible so as to leave a very small unheated region.
I use nichrome wire for the heating element, (salvaged from hair curling irons, the ladies in my household throw away), since it retains more of its strength at red-orange heat.
Otherwise our methods are the same. I usually hear the tubing thermally crack part way around with the first heating. Then rotate the tubing about 30 degrees to bring the uncracked part under the wire, and heat once more. Most of the time the pieces separate cleanly by themselves, leaving an edge that needs only a light touch up with a diamond hone (the best) or some SiC wet dry paper 400 or 600 grit.
About half the time, with thin wall tubing, it will crack all the around the first time. With a little practice, one can cut off quite narrow rings. I can do about 3/8" rings quite regularly, havent tried to see how narrow they could go.
If you fo have a lathe, an ordinary carbide bit will make short work of cutting off the tubing.. or.. for boring a hole in a glass plate. A simple fixture for boring the plate, is to use a wood block and mount the glass to it with some self tapping screws at the perimeter. You can use the standard carbide glass boring drills, or just cut in with a sharp carbide bit, aligned so the outer edge cuts parallel to the lathe axis, or at whatever angle you want.
To do round glass plates, I just mount as if I were going to bore a hole, but instead, mount a Glass cutter in the tool post.
With the headstock drive disconnected just bring the glass cutter in to press fimly on the glass plate. Rotate the chuck once (only once) and you will have a circular score. Remove the plate quickly and tap along the score. Done correctly, the glass disk will drop out.
To clean it up, I have a small vacuum chuck, made from a pastic funnel that is connected by a hollow bolt to make a rotating joint to a hose through the headstock to the vacuum pump This holds the rough round plate quite nicely leaving the edge open for finish grinding. I usually just eyeball the plate into concentricity, and then hand grinad the edge with a stone or a diamond hone. Since there's a bit of setup involved, I usually make a half dozen plates at a time. Poor man's window fabrication method.
Dave Cooper
I use the hot wire technique with a minor variation: I score all the way around (3 cornered file works well, too) and then use a circumferentially wrapped hot wire . I have a simple little fixture with two bolted electrical connections for the hot wire. They are as close together as possible so as to leave a very small unheated region.
I use nichrome wire for the heating element, (salvaged from hair curling irons, the ladies in my household throw away), since it retains more of its strength at red-orange heat.
Otherwise our methods are the same. I usually hear the tubing thermally crack part way around with the first heating. Then rotate the tubing about 30 degrees to bring the uncracked part under the wire, and heat once more. Most of the time the pieces separate cleanly by themselves, leaving an edge that needs only a light touch up with a diamond hone (the best) or some SiC wet dry paper 400 or 600 grit.
About half the time, with thin wall tubing, it will crack all the around the first time. With a little practice, one can cut off quite narrow rings. I can do about 3/8" rings quite regularly, havent tried to see how narrow they could go.
If you fo have a lathe, an ordinary carbide bit will make short work of cutting off the tubing.. or.. for boring a hole in a glass plate. A simple fixture for boring the plate, is to use a wood block and mount the glass to it with some self tapping screws at the perimeter. You can use the standard carbide glass boring drills, or just cut in with a sharp carbide bit, aligned so the outer edge cuts parallel to the lathe axis, or at whatever angle you want.
To do round glass plates, I just mount as if I were going to bore a hole, but instead, mount a Glass cutter in the tool post.
With the headstock drive disconnected just bring the glass cutter in to press fimly on the glass plate. Rotate the chuck once (only once) and you will have a circular score. Remove the plate quickly and tap along the score. Done correctly, the glass disk will drop out.
To clean it up, I have a small vacuum chuck, made from a pastic funnel that is connected by a hollow bolt to make a rotating joint to a hose through the headstock to the vacuum pump This holds the rough round plate quite nicely leaving the edge open for finish grinding. I usually just eyeball the plate into concentricity, and then hand grinad the edge with a stone or a diamond hone. Since there's a bit of setup involved, I usually make a half dozen plates at a time. Poor man's window fabrication method.
Dave Cooper
Re: UPDATE: HULL-RANEY EXPERIMENTAL ION GUN
Dave,
I hadn't thought of using the lathe and carbide tooling to cut glass tubing. I'd have to cushion the tubing, but otherwise, sounds like less trouble than using the diamond discs, etc. I'll have to try it. Thanks!
Regards,
TIM
I hadn't thought of using the lathe and carbide tooling to cut glass tubing. I'd have to cushion the tubing, but otherwise, sounds like less trouble than using the diamond discs, etc. I'll have to try it. Thanks!
Regards,
TIM