Vacuum Chamber Construction

For posts specifically relating to fusor design, construction, and operation.
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Richard Hull
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Re: Vacuum Chamber Construction

Post by Richard Hull »

It is obvious that the pump was in a place or position where the pump would not allow for easy draining to replace oil. The tube will go to a drain pan at a lower, convenient location. The hose would have a petcock that could be opened to drain the fouled or old oil.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the explanation Richard, I do have another question. I'm missing a small number of vacuum parts for the hookup to my chamber, a couple clamps etc. that I plan on ordering soon. Once those parts arrive, if I put my turbopump in series with my backing pump, and leave the turbopump off, can I just turn on the backing pump without causing any problems? Having read a number of other threads I know that if I turn on the turbopump without the backing pump operating I can do some serious damage, but if the turbopump is off will it just act as a bit of additional tubing?
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Richard Hull
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Re: Vacuum Chamber Construction

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Absolutely! Same goes for a diff pump. It is just another "hanger-on" in any vacuum system if it is not turned on. The proper way to work a system is to open all valves and pump the entire vacuum system down as low as the mechanical pump can go. (hopefully at or below 50 microns) Once at fore-line vacuum, the secondary pump (turbo or diff pump), is turned on and you are off to the races from there.

All of the above assumes that your system is sealed and tested. During testing, a stepped assembly process is demanded to be sure you are sealed at each step. This can be a tedious process. Newbies tend to assemble the entire system and then wonder why they can't get a good vacuum as they now have to hunt for issues and leaks.

System assembly absolutely demands a TC gauge at minimum!!! In the last stage a deeper reading gauge on the reactor vessel would be nice, leaving the TC gauge in the fore-line permanently. This means you need a lot of blank offs, clamps and adapters if seeking to do a professional level job of it.

Note: Valves in a vacuum system are more for isolation, shut-down and compartmentalized tear down or modification of the system without fouling the entire business if one component needs replacement or a new one added in the middle of the system. There is no such thing as too many valves.....(if you can afford good ones.)

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Earlier this week, my 8020 frame arrived. I had to wait a few days for my M8 Tap to arrive, but on Friday it did and I've completed tapping the ends of the extrusion so it can all be put together. I plan on mounting the vacuum chamber in the center of the frame.
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John Futter
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Re: Vacuum Chamber Construction

Post by John Futter »

Nicolas
I would srongly suggest you do a pressure test on your chamber first.
Ie blank off everthing and feed shop air at 15psi (no more)into your chamber through your gas feed line
Use 50:50 detergent and water and go round every weld and vacuum joint with a small paint brush dipped in the detergent water mix and look for foaming or bubble formation
this method far quicker than trying to use vacuum in the first instance
only when you pass this stage put the chamber under vacuum
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Richard Hull
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Re: Vacuum Chamber Construction

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As the whole device is SS, I would pressurize it to 30 psi and put it in a 5 gallon bucket of water. Have a grease pencil handy.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the suggestion John, I'll definitely look at doing that. Currently in an apartment in the great white north, so I'll have to poke around and see if I can find a compressor of some sorts.
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

Add a Schrader or Presta valve stem adapter to one port, e,g. gas inlet hose.
Then use a bicycle tire pump, or haul the chamber over to some gas station or fabrication shop.
All models are wrong; some models are useful. -- George Box
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

I'm currently trying to tap a blank for my vacuum chamber, to use as a port for my thermocouple gauge and also for the pressure test. Based on Rich's suggestion I found a 1/8 NPT to schraeder valve fitting that will allow me to hook up my bicycle pump and look for leaks. I used my Machinery's Handbook and grabbed a 5/16 drill bit for the initial hole which went fine. However trying to tap the hole was a no go. My tap would not bite, and looking at the tap itself it says a "Q" drill bit must be used to drill the initial hole. I assume I was unable to start the tap because the hole I drilled was too small, but I believe I've correctly confirmed for my thermocouple gauge that is a 1/8 NPT male fitting. Have I made a mistake somewhere? Did I select an incorrect tap?
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Rich Feldman
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Re: Vacuum Chamber Construction

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I've tapped plenty of 1/8 NPT (27 tpi) threaded holes, but don't remember the starting size. 5/16 seems kind of small. Isn't the OD of the pipe, at big end of taper, 0.405"? Consider a tapered reamer with T-handle to adjust your starter hole. Could always practice in a scrap of aluminum or brass.

Schrader valves are held shut by a spring, of course, and open when filling attachment depresses the pin. I think the spring is plenty strong to resist suction when vacuum is inside. But consider the reason metal valve caps with o-ring seals have been available since 19th century.
All models are wrong; some models are useful. -- George Box
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Re: Vacuum Chamber Construction

Post by John Myers »

An online tap & drill calculator shows it should be 21/64 (.3281) which is close to the Q drill bit size (.3320)
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

Yes, drill Q seems to be what they say, except McMaster-Carr's economy carbon steel tap calls for drill R (0.339). If you go a little large and don't get full height internal threads at the small end of taper, would a bad thing happen?

Machinists argue about the value of tapering the hole before tapping pipe threads.
https://www.practicalmachinist.com/vb/g ... ap-151341/
With a straight hole, the big end of the tap does a lot more work than the little end.
I like to use a tapered reamer from the hardware store, with a T handle.
All models are wrong; some models are useful. -- George Box
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the clarification! I'd seen Q and R mentioned as well online, but had decided to go with the 5/16 since if it was a mistake I could always make the hole bigger!
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Re: Vacuum Chamber Construction

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I have some successes and failures to report by way of an update. I've successfully sourced some valves via eBay for my fusor, a ball valve is now attached to my roughing pump and I've obtained a manually operated butterfly valve. With regards to the butterfly valve, the lip for the seal seems different from the knife edge I've got on all my other conflats. Is this just due to a manufacturing difference or does it mean it can't take a copper gasket?
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Secondly I've successfully tapped a blank and put in a bike valve. It took a bit of work but pleased with that effort.
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Finally the failure, while assembling the chamber for a leak test, I accidentally cross-threaded two bolts on one of my half nipples. This is the only nipple on the chamber that has threaded holes, I purchased it at a cheap price off of eBay, all others were ordered new and didn't have threaded holes. While trying to remove the cross-threaded bolts both snapped. I'm now wondering what the best course of action for removing them is and repairing the damage. Can I just drill out the holes with a slightly larger drill bit and remove the threads? Or is it better to get a tap and re-thread them?
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Re: Vacuum Chamber Construction

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Take it to a tool making house that has a spark eroder
my local one charged $25 per bolt removal and they did such a good job that putting a tap back down the hole removed no material and the correct bolts worked fine mixup between 5/16 and 8mm bolts
you now need to buy a tub of no sieze grease i prefer the nickel based one as the copper one is a nasty color
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Richard Hull
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Re: Vacuum Chamber Construction

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I'm cheap and opt for Q&D, (quick and dirty)....If it were mine I would flush cut the bolts and bore 5/16 holes through the old bolts, then use mixed proper threaded bolts on the other 4 holes and use 1/4" bolts and nuts in the two bored holes. You just have to be very careful flush cutting the bolts against the flange face. While scratching the flange face is no big deal, the knife edges must be kept pristine. Pretty and proper is nice and fine if you have the bucks and people who can do the work to keep pretty in place. I tend to just want stuff to work as planned when sealed up.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Vacuum Chamber Construction

Post by Dennis P Brown »

Have you considered differential thermal expansion heating/cooling? That is, heat the assembly in a regular oven to four hundred Fahrenheit or so, then use ice to chill the bolts. Next, using vice grips locked on to the bolts try and rotate them?

Failing that, I've had luck drilling such bolts nearly out and then extracting.
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

We know you enjoy cutting metal & getting hands dirty, but you don't have to for this.
Broken-off bolts have been dealt with millions of times in the last 150 or 200 years.
There are people not far from you, with proper tools and experience, who do it all the time.

I like Dennis's shrink idea, but am skeptical about this particular geometry.
Because of the time for temperature changes to diffuse into and along the bolt, whose thermal contact with internal thread was increased by the mechanical abuse, and then the time to switch from ice to vise-grip connection. Can't hurt to try.
All models are wrong; some models are useful. -- George Box
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Hi Dennis,

I've given the heating/cooling idea the old college try and haven't had any luck. I'm going to look around town for some people with bolt removal experience
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Re: Vacuum Chamber Construction

Post by Dennis P Brown »

One method (used on rusted and/or seized bolts) is a torch. Heat the bolt to red heat, then vice grips (rapid cooling of bolt) and a powerful twist. The extreme thermal expansion/contraction can breack the bond between metals. This often allows removal. But it is drastic and not something I'd attempt except as an extreme measure.
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Thanks for the suggestion! I might try that if I get really desperate, but after a quote from a local tool and die maker of $263 to remove the two bolts, I've ordered some flush bolt cutters and purchased a cobalt drill bit and and am going to try and solve the problem myself.
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Re: Vacuum Chamber Construction

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As I noted drill 'em out you may save the fine threads if very careful and have the thinned remnant threaded sections separate using a pick. If not, just slightly over drill and tap to the next size bolt. (consulting the bolt and tap tables for drill size and tap, of course). I do not know your clearance below for a nut, but as I originally suggested you can just clear-hole bore for a suitable pass through bolt and nut. So many ways to skin this cat and no big deal at all, of course.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman »

Thanks for sharing the quote. More money than I had guessed. For that, can we expect absolutely no new damage to the threads, and no new scratches, scuffs, or dings on the flange faces? Does the work include fixing of presently damaged threads?

Bolt cutters will leave the stub distorted more than cut-off with a hacksaw, angle grinder, or any kind of abrasive cut-off wheel.
At our local self-storage facility, the office staff has lots of practice cutting padlocks. Primary tools are long-handled bolt cutters and battery-powered cutoff wheel.

Can your milling machine help to hold the workpiece and start the drill at right spot and angle?
Other drill-jig ideas that come to mind:

1. A thick nut, screwed onto a couple of threads deliberately left on broken bolt. We know those are not co-axial with threads in flange. :-)

2. Spare Conflat flange with matching circle of through holes, and a short tubular bushing with right OD and ID.
Drill jig bushings are available in many sizes, for example 5/8 outside and drill R inside. https://www.mcmaster.com/drill-jig-bushings/

3. Any scrap of flat steel, say 1/4 to 1/2 inch thick, with guide hole (and optional counterbore) made on drill press. If made on milling machine, it could have properly placed holes for a couple of other holes in bolt circle.
All models are wrong; some models are useful. -- George Box
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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause »

Hi everyone,

I've managed to drill out the bolts to a standard that I hope will allow me to seal the chamber. The process was a bit finicky but it went as follows, flatten the top of the bolt with a mill, drill a starter hole with a carbide spot drill, then drill through with successively larger cobalt drills. One hole went quite well, and the other not so well, part of the bolt is still stuck in the hole and almost appears welded. I'd continue to attempt to drill it out, but the 250$ drill press I bought off of Amazon is really not very rigid. I'll hold off on trying to improve things until I have access to a better workholding setup.
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Dennis P Brown
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Re: Vacuum Chamber Construction

Post by Dennis P Brown »

If you have a small circular file, carefully remove the remaining bolt threads till their almost flush (the abrasive force might also help loosen the remaining threads); then lots of tap oil and re-tap the thread carefully (use a tapered end tap only!) I've cleared remaining threads that way. Just don't over force it. Running a well oiled tap through a 'rough' threaded hole is always a good idea to be certain threads are clear and uniform again. Remember, very small partial turns (1/16) and back off and repeat as needed.

Anti-seize compound (never oil) is one's friend with steel on steel bolts into threaded holes.
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