Spark Plug feedthroughs 101

Things at the limits.

Spark Plug feedthroughs 101

Postby johnf » Fri Sep 24, 2010 2:57 am

Heres how to make usable feddthroughs out of spark plugs good to around 20kV with out any further measures

photo 1 choose a non resistor plug ie no resistor in the centre leg with as long a reach as possible (plug thread length)
sp1.jpg




Now you have to part off the bottom part of the threaded portion so that more of the centre is exposed
careful you don't want to hit the ceramic insulator inside.
using a thin diamond coated disc in a dremel cut the insulator almost all the way through all the way around
Remove sparkplug from lathe and holding the exposed inner in a vice gently bend the inner a little, the ceramic will crack where you sawed it all around with the dremel

things should look like this
sp2.jpg


Now the tricky part you will have to make a custom lathe tool to face an O-ring groove into the shoulder on the sparkplug.
Its not that easy as there is not much room and the toll tip is quite small and is easy to break. But at least the plug is made of steel not stainless which would make this operation nigh on impossible

The finished beast should look a bit like this
sp3.jpg
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Re: Spark Plug feedthroughs 101

Postby Doug Coulter » Fri Sep 24, 2010 9:47 am

Excellent! So that's how you got all that extra center conductor available :) To keep the good info in one place, the plug type is NGK D7EA.

I can and have made that lathe tool for O ring grooves -- it's a pain, and I have to go slow as I use A2 tool steel (not HS), or even W1, but it works -- needs a sharpening and re-heat treat sometimes. You just go slow and use a lot of lube/coolant. Which as John points out you can't do with SS.

I have also used a slightly simpler sealing scheme there, where you just cut the flat, flat, and turn off a couple of threads at that end, then use a tight fitting O ring that has to stretch to get over the threads. You can't tighten it quite as much without smushing out the O ring, but that isn't usually a problem.

Seems to seal about as well, and it's a little easier to get the flat to the smoothness required for a good seal. Nice that plugs have a hex head that makes them chuck up easy in the lathe.

If it's not going to get hot, Hysol epoxy or maybe even loctite on the threads should seal well enough, but I've baked a couple of these with a little too much enthusiasm, so making it easy to swap out seems a better idea.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Spark Plug feedthroughs 101

Postby chrismb » Fri Sep 24, 2010 1:00 pm

Presumably, you are talking here of the cut-off end being vacuum-facing?

I have been debating the pros/cons of a spark-plug setup for a version 2 of my current feedthrough arrangement. I was, rather, thinking that I might fit it so that the 'sparky' end of the plug was the outside connector, as I'll need a good insulator and threaded part inside the vacuum vessel. Then to connect, snap off the outer electrode, spot weld to the inner, then cut/drill a teflon rod, or more simply form a loop of insulating tape, that'll then slide down the wire and sit between the 'sparky' insulator and conductor to shield the wire.

My hesitation was that I need 8 feedthroughs for my next version, and I do not feel confident that 8 threaded spark plugs would seal up so good. ANy opinion on that welcome, as it could be a pretty neat solution if I am likely to seal the threads well.
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Re: Spark Plug feedthroughs 101

Postby Doug Coulter » Fri Sep 24, 2010 1:40 pm

Yes, he is having that end vacuum facing, see here. He's driving deflection electrodes with them, in fairly high vacuum -- similar to what I think you might be about to be doing.

I've seen it done both ways, it kind of depends on which way Paschen's law is working for/against you. You'd maybe not want to do this the other way, due to the O ring placement,
and might have to seal up the threads with something else to make it work as well. To the extent I've had them leak, it's been on the ceramic/metal seal around the base, and a little epoxy fixed that; nice to have a He leak detector, the leaks are fairly tiny, just enough to mess up base pressure but resistant to finding the normal ways.
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Re: Spark Plug feedthroughs 101

Postby Starfire » Fri Sep 24, 2010 2:21 pm

John - which plug are you using?
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Re: Spark Plug feedthroughs 101

Postby Doug Coulter » Fri Sep 24, 2010 3:13 pm

Read the whole thread, second post in this case.
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Re: Spark Plug feedthroughs 101

Postby Jerry » Sat Sep 25, 2010 1:36 am

Face grooves are not that bad to do. But you need the right tool geometry. Assuming standard CCW rotation of the chuck you want a tool that looks somewhat like a mirrored comma. The outside radius should be slightly tighter than the OD of the groove and the inside should be slightly greater than the ID. The finished tip should be full width at the top tapering to the side with a curve. Use as thick as a blank as you can as usually there is a bit of stickout to get to the groove.

With that even stainless is easy, here is an adapter I turned out of some mystery metal stainless. Probably 316.

IMGP4941.jpg
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Re: Spark Plug feedthroughs 101

Postby Doug Coulter » Sat Sep 25, 2010 10:31 pm

I think I'm visualizing what you describe here, but a picture or drawing of one sure would be nice. The one I made kinda stank -- it worked, but... It was just a piece of tool steel rod, with the end ground to the O ring size, more or less, and a flat cut in the top (a little rake) and relieved below. The trouble with that one -- rigidity was awful, as for say, a 1/8" thick O ring it was a 1/8" rod held in the toolpost adapter I made. I was coming at the work end on. Sounds like what you are describing is coming from the side?
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Re: Spark Plug feedthroughs 101

Postby johnf » Sat Sep 25, 2010 11:35 pm

Doug

you have it right
In my opinion if you are not grinding these tools as Jerry describes fairly regularily then this is a pain, with much work on the grinder as you break / blunt your tool

Hence my warnings in the post for a newbie for creating a tool like this. Its easy if you know how, but near impossible if you don't / or need the practice if you haven't done it for 10 - 20 years

and Jerry thanks for a description of how to create the tool --I forgot to mention this in the post.

I had to do this with a triple start thread for one of my exams thirty four years ago as a fitter turner apprentice with no indexer on the lathe. I cheated by marking where the jaws on the chuck were positioned on the headstock with a pencil line and made sure the thread feed aligned -it worked --I saw the errors in my ways and transferred into a instrument making internship shortly after-- I had all I needed for machine shop so 2 years of electronics and a short stint 6 months of glassblowing finished it off (90% of this I have forgotten).
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Re: Spark Plug feedthroughs 101

Postby Jerry » Sun Sep 26, 2010 12:45 am

Doug Coulter wrote:I think I'm visualizing what you describe here, but a picture or drawing of one sure would be nice. The one I made kinda stank -- it worked, but... It was just a piece of tool steel rod, with the end ground to the O ring size, more or less, and a flat cut in the top (a little rake) and relieved below. The trouble with that one -- rigidity was awful, as for say, a 1/8" thick O ring it was a 1/8" rod held in the toolpost adapter I made. I was coming at the work end on. Sounds like what you are describing is coming from the side?



When I cut that groove I used a 1/2" square HSS blank. Took probably about 20 minutes to grind it out. Something like this but out of a solid piece and you dont need to extend so far out. Ill dig to see if I have one ground. Kind of doubt it though.

Machinery's Handbook has specs on proper geometry for o-ring grooves. If you dont have a copy you need to buy one. This is not optional! Just about any edition will do. I have about 3 or 4 of them from 5th on up.
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