Vacuum Chamber Construction

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John Myers
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Re: Vacuum Chamber Construction

Post by John Myers » Wed Mar 18, 2020 3:57 am

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 » Wed Mar 18, 2020 6:36 am

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 » Wed Mar 18, 2020 2:50 pm

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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Nicolas Krause
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Re: Vacuum Chamber Construction

Post by Nicolas Krause » Wed May 13, 2020 5:30 pm

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?
IMG_1063[2].JPG
IMG_1062[1].JPG
Secondly I've successfully tapped a blank and put in a bike valve. It took a bit of work but pleased with that effort.
IMG_1057[1].JPG
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?
IMG_1061[1].JPG

John Futter
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Re: Vacuum Chamber Construction

Post by John Futter » Wed May 13, 2020 7:04 pm

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

Post by Richard Hull » Wed May 13, 2020 8:27 pm

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Vacuum Chamber Construction

Post by Dennis P Brown » Mon May 18, 2020 8:49 am

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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Rich Feldman
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Re: Vacuum Chamber Construction

Post by Rich Feldman » Mon May 18, 2020 3:09 pm

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 » Wed May 27, 2020 2:10 am

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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Dennis P Brown
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Re: Vacuum Chamber Construction

Post by Dennis P Brown » Wed May 27, 2020 7:22 pm

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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