[Futterama]

Hi,

I'm doing an electronics project where I need to fasten two conductors together (busbars). I think I need to use regular steel fasteners since the whole construction needs to be assembled before soldering, so everything will be heated to something like 250-300°C and nylon bolts won't do. So I'm trying to make some heat resistant spacers that will electrically isolate the steel bolt from one of the busbars. So I searched for heat resistant plastics, and stumbled upon bakelite. Bakelike is used for the handles on frying pans, and the kids happen to have an old frying pan in the playhouse, so I confiscated that (no kids were harmed during the confiscation) and started turning using a carbide insert normally used for steel.

The bakelite usually contains a filler, which may be anything from clay to glass fibre according to what I have read. It still turnes fine though but it is a bit rough on the HSS twist drills.

Anyway, I need to turn a tube ID 3mm for the bolt, OD 4mm to keep the hole in the busbar small. But the bakelite is a really brittle material and it breaks real easy. I tried turning the piece to OD 8mm and then drill the 3mm hole, and then turn it down to OD 4mm, that last 2 cuts were 0.5mm at a time. But the bakelite broke on the last cut.

So, either a recommendation of some other easy-to-get material that is an electrical isolator, heat resistant and machinable OR any recommendation on how to turn this small piece without breaking it.

Thanks.
And sorry for the really long post, but I like to include some background information ;-)

Best regards,
Martin

[keebler303]

Just buy the closest thing you can find and make it work.

Ceramic Standoffs would be the normal approach.

Standoffs | Farnell Danmark | Results

[Futterama]

keebler303, Ceramic Standoffs, good idea, but where can I get those in such small dimensions? Farnell only has down to 9.5mm and that is way too much :-/

[keebler303]

You could also use polyimide rod but it is extremely expensive.
Machines excellent though.

Vespel is one trademark. You can also get screws made from the stuff.
If you just need it to survive a reflow soldering cycle, teflon or peek might work alright.

[ChrisW]

You can sandwich that material between two pieces of material drill it screw together and turn od.

Lmk if it works.

[Futterama]

Polyimide is way too expensive, holy crap. PEEK is ok, OD 16mm x 300mm rod is $68 inkluding danish tax at farnell. Maybe I should contact a plastics supplier and ask for leftovers, I might get lucky.

Another option I was thinking about, was to cast the tube from concrete or ceramic glue. The tube is essentially only there to keep the steel bolt from touching the busbar in the hole, so no real stress is applied. I can use a fibre washer to take up the pressure from the bolt.

ChrisW, I don't really see what you mean. I will have to experiment a bit on the lathe, maybe drill a small 2mm hole at first, turn the OD down, and then drill the rest of the hole. Or maybe see if I can fit a OD of 4.5 or 5mm. Maybe use a carbide insert for aluminium, those I've got for aluminium is much sharper than the ones for steel, maybe they will put less stress at the thin bakelite tube at the last turning pass, or maybe they will just get dull real quick.

I have some things to try now, some new ideas, thanks :-)

[keebler303]

I told you it was extremely expensive.

You can buy ceramic tubing in small diameters, made for thermocouples. Not sure if that would help since they aren't threaded. Around here, I'd buy it from Omega Engineering, not sure if that is an option for you.

Ceramic Thermocouple Insulators

[Futterama]

Oh, I don't need threads in the tube. The threads goes into one of the busbars, so the bolt will be electrically connected to one busbar, and then I need to isolate it from the other busbar while the bolt holds the busbars together (the busbars are seperated by an isolator sheet).

[ChrisW]

This is what I was taking about making a sandwich.
:sly::banghead:

Attachment 220752

[Futterama]

Chris, ahh ok. I have been thinking about something similar but my thought was to drill the hole and put a metal rod inside while turning down the OD. The metal rod would have to have a nice slightly tight fit.

[Futterama]

Ok, so I read somewhere that the dust from Bakelite is carcinogenic so I really don't want to expose myself to that.

Someone mentioned silicone wire and if I use a fiber washer to take the pressure from the bolt, I only need to isolate the bolt threads from the ID of the hole in the busbar. Sure, this can be done with an arigap, but I want something in there to make absolutely sure there is no short circuit, even is the bolt loosens. So using the silicone tube from a silicone wire seems like a solution. Fiber washers and silicone wire has been ordered so I can try it out.

Thanks for all your suggestions, I consider this case "solved".