Wow, this is the forum at it’s best!  Really good advice and quick.  Thank you.

On 17 November 2025 at 23:15 Dick H Said:

I think the usual term for such vacuum connectors is “Feedthrough”.

I’ll use that in future, and it gets better results from Google.

On 18 November 2025 at 00:22 duncan webster 1 Said:

… we used Oxley pins …I don’t see why your vac pipe connector has to go through the same hole. Drill through elsewhere, tap outside and use a Dowty washer under the pipe fitting. …

Oxley pins and Dowty Washers both new to me, and relevant.    Wade makes the same point about separating the exhaust pipe from the electric wiring and I’m persuaded.   I combined them to avoid making two potentially leaky holes in the base and because there’s precious little space inside for them.  But I can find room for two small holes, and they simplify the feedthroughs,  One reason for using PTFE pipe is it being flexible makes it easier to feed  through the maze of electronics under the base – also tight for space.  However, a separate feedthrough likely means PTFE can be replaced by a straight-ish copper pipe.  Good to have a CAD model to see, because the clock is doing a run test and I can’t turn it upside down and look at the real thing.

On 18 November 2025 at 05:36 Joseph Noci 1 Said:

And general epoxies from the hardware store all shrink as they cure.

Good to know what to avoid!  I didn’t know that.

On 18 November 2025 at 06:46 Diogenes Said:

..maybe there’s an injection-moulded connector available with pin conductors already…

There are, and Robert has a MIL-STD socket, too big unfortunately.  New connectors are expensive.  At the moment DIY is cheaper, and it has fun and interest too.  If DIY doesn’t work, a connector is the answer.  Until this morning, I was biased against commercial connectors because they don’t include an exhaust pipe.  Not now – Duncan and Wade persuaded me this morning to separate the exhaust and electric feedthroughs.

On 18 November 2025 at 07:06 David Senior Said:

I suspect the ptfe tube will be a problem. When you apply the vacuum it will tend to reduce in diameter, and, as the epoxy isn’t likely to have a good grip on it, the seal will have gone. Probably needs to be a metal pipe.

Point taken!!!

If you are worried about making the O-ring groove on the bolt, consider putting the recess in the mating part and just aiming for a good finish on the bolt sealing face.

Dave

The size and shape of the mating part is awkward.  The bolt sits in a recess under a cast-iron block, the base, that’s much too big for my lathe’s chucks.  It’s a faceplate job, and reaching the surface requires a fully extended cutter. As the surface is sloped and very hard, the cutters will chatter.  I’m intimidated.    In contrast, easy enough to mill the recess with a carbide cutter, but not the groove.  Simpler, I think, to groove the bolt head because the part can be held in a 3-jaw.

On 18 November 2025 at 07:10 JasonB Said:

Potting epoxy from your local electrical wholesaler will be better than any old epoxy. A vacuum is the only way to really suck bubbles out.

Good idea.  And I have a vacuum pump!

3D print something like your green spacers to hold the wires while potting, if you apply a little tension they will remain straight.

Yes!!!

Why use thread sealant when you have an O ring as a seal.

Belt and braces.  Thinking about it though, O-rings work by being pressed into the gaps.  Therefore the O-ring might work better without sealant, because a pressure differential both sides will push the rubber in more firmly.

Cut a recess around the M10 hole for the O ring as it will be a lot easier. Keep a plain 10mm shank about 2mm long so the ring is not against a thread.

That’s the plan.

Otherwide gring a tool to cut the recess in the bolt, probably a cranked one as straight would be a bit long. Or bore a 10mm clearance hole in some Silver steel and make a hollow cutter like  a Rotabroach type.

Excellent! I hadn’t thought of cranking or a Rotabroach.   Thanks for the photos too – very clear.

Your sizes are wrong as 12mm AF won’t leave room for an or ring around the M10 thread

Typo, actually 16mm AF.

Copper is soft so easy to roll it square

Why didn’t I think of that?  I have a roller.

If anyone is interested, square wire is preferred to round cos’ I is a wire-wrap fanboy.  Avoids soldering, simple tool, though not suitable for everything.

Apparently wrapping the wire around a square pin increases contact pressure enough to create tiny cold-welds on the corners, improving reliability.

Many thanks one and all.

Cheers,

Dave

On 18 November 2025 at 10:13 noel shelley Said:

What about a fitting for your vacuum gauge ?

No need, it’s an electronic sensor, an automotive Bosch BME280.  It reports temperature, pressure and humidity through the wires, whenever the microcontroller asks.

…Glass would be a good one ? Incorporate the base of an old valve ? Octal base.Good luck , Noel.

I have plenty of valves. The glass to cast-iron interface would tricky unless I got into glass-blowing, another skill I don’t have!  The idea had me wondering about potting the wires with molten glass.  Glass and Copper have very different coefficients of expansion, which breaks the glass.  I think valve pins are a special Nickel/Iron alloy.  Platinum wire works too!

Surprising how much thought and ingenuity is going into in what seems a simple part.

Ta,

Dave