Sacrificial arbor for coil winding? - Page 1
Forum section: electronics / projects

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 08, 2016, 05:27:43 pm
content: I need to wind a small coil that's bonded. No room for a bobbin. I was thinking to machine an arbor out of either machinable wax or aluminum, wind the coil with some low viscosity epoxy, cure it, then melt out or dissolve (sodium hydroxide) the arbor. The ID of the coil is only 0.098", with about 150 turns of fine wire. I don't think I could slide it off a regular arbor no matter how much mold release I used. Is this idea crazy or has anybody done anything similar? I do know they make metal bellows in a similar manner- plate the bellows on an aluminum arbor and then dissolve it out.

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2016, 05:48:47 pm
content: What about a drinking straw?Will it be on ferrite?Voltage?OD and length restraints?

author: pelud
author_rank: Regular Contributor
timestamp: July 08, 2016, 06:09:00 pm
content: I can't really comment on how it would work, but perhaps using a crayon might be a good source of wax that is easily chuck-able?

author: LaserSteve
author_rank: Super Contributor
timestamp: July 08, 2016, 06:20:06 pm
content: Machinists Wax is a mixture of paraffin and usually LDPE with some additives. The cheap versions shrink about 4% when cast. Usually you would design a manual pre-form in such a way that the coil slides off it. Ie, Mill one out of Delrin or Teflon...Steve

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2016, 06:35:51 pm
content: Yep that should work, wax former, transformer shellac over the top of the windings then melt the wax away.

author: mjkuwp
author_rank: Unknown
timestamp: July 08, 2016, 06:36:15 pm
content: for 3D printers of the FDM type there are filaments that are soluble. These are used for support materials in 2-material printers and the support material is dissolved in some kind of bath afterward. maybe just water but I am not totally sure.

author: calexanian
author_rank: Super Contributor
timestamp: July 08, 2016, 07:06:37 pm
content: Bismuth casting alloy is used for this sort of thing. Also polystyrene as well. Both can be melted out at relatively low temperatures.https://www.amazon.com/Low-Melting-Point-Bismuth-Based-Alloy/dp/B001QUVQGK

author: Gyro
author_rank: Super Contributor
timestamp: July 08, 2016, 07:23:43 pm
content: QuoteNot even room for a strip of waxed paper or plastic film? This would tend to tighten and grip an arbor when rotating in one direction and slip in the other. Hopefully enough to slide it off the arbor afterwards.

author: Vgkid
author_rank: Super Contributor
timestamp: July 08, 2016, 07:33:16 pm
content: I know that sensitive research used salt as a coil form. Then disdolved it, not sure how easy that would be at your size.

author: C
author_rank: Super Contributor
timestamp: July 09, 2016, 01:29:30 am
content: Quote from: Gyro on July 08, 2016, 07:23:43 pmHow about a pipe as the basic arbor for above with a slot on both sides across the diameter of the pipe..Then when rotating backwards to remove, the strip can collect inside the pipe making arbor removal easier.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 09, 2016, 04:21:09 am
content: Good thoughts all, thanks! I've got some machinist's wax coming and will try that first. It may just break off, as the arbor is so small. Then I'll try the aluminum and maybe order some bismuth alloy. My guess is the combined force of 100+ turns in a small space, plus the cured epoxy, will make it impossible to slide off any sort of wax paper layer. I also considered a Teflon arbor, but the wire will dig in enough to have the same problem. My tolerances on the finished dimension are only a couple thousandths of an inch, so a decent machined arbor is essential. I'll report back on what happens.

author: ConKbot
author_rank: Super Contributor
timestamp: July 09, 2016, 04:30:02 am
content: Could you cast the machinist wax over a sandblasted (or roughed up in other ways) aluminum arbor, then turn the OD? Precision, strong, easy to chuck up, melt the wax out and the coil is free.

author: albert22
author_rank: Regular Contributor
timestamp: July 09, 2016, 04:30:29 pm
content: I think it would be easier to build an expandable/contractible arbor or one that can be disassembled to take the coil out.Like this>https://www.amazon.co.uk/Merry-Tools-HK-Universal-Transformers/dp/B00BKXWJGEAlthough much more smaller in your case.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 09, 2016, 09:52:26 pm
content: Never seen that Merry Tools winder. Nice. For some reason they don't seem to be sold here. It's very similar to my DIY winder. Ah, for the days of DIY ham radio, when people actually built things. My coil is going to be about 150 turns of #36 AWG wire. Though I could probably make an expanding arbor, remember that the coil will be sopping wet with epoxy that will have to be cured before the coil can be removed from the arbor, Thus I'm still leaning towards a sacrificial arbor. Coating a metal arbor with wax to gain some strength isn't a bad idea.

author: Back2Volts
author_rank: Unknown
timestamp: July 10, 2016, 07:36:10 am
content: Quote from: ConKbot on July 09, 2016, 04:30:02 amAlong the same lines, cool the arbor and then dip it for a second in melted wax to coat it.

author: ConKbot
author_rank: Super Contributor
timestamp: July 10, 2016, 05:27:57 pm
content: Quote from: Back2Volts on July 10, 2016, 07:36:10 amAhh, yes, build it like a candle, definitely easier than casting.

author: kreutz
author_rank: Contributor
timestamp: July 10, 2016, 08:38:31 pm
content: Along the lines of message #9 by C. You can use a "wrap sleeve anchor", like the one in the photo (http://www.indiamart.com/proddetail/wrap-sleeve-anchor-bolt-7220313297.html), as the form core. Compress it until you get the right diameter, Wrap some waxed paper on it and wind the copper wire on top. It will be easier to release the metal core form. You don't want the copper wire wound directly on the transformer core material, it might chip the enamel and give you headaches afterwards, just transfer the whole winding, wax paper included, into the transformer core.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 11, 2016, 03:51:26 am
content: This isn't a transformer, so no danger of hitting an inner core- I have a few thousandth of an inch clearance, a mile in my line of work!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 20, 2016, 07:48:09 pm
content: Just an update- machinable wax makes a pretty good arbor. Wind with epoxy, cure and melt out the wax. I'll use a solvent dewaxing step at the end to really clean it up. Biggest problem is I should have used magnification to really see the tiny coils while winding- I could have done a neater job.

author: Back2Volts
author_rank: Unknown
timestamp: July 20, 2016, 08:00:55 pm
content: Did you actually run the wax through a lathe to make the arbor?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 21, 2016, 06:08:08 pm
content: Yes, chucked it up and turned the necessary diameter and flanges. It worked well, probably why they call it machinable wax! It's quite a bit stronger than plain paraffin wax like for canning. It only comes in larger bars and blocks, so we recast some as 1/4" rods.The only problem is getting the residual wax off the coils, now solid and self supporting because I wound them with some low viscosity epoxy on the wire. None of the common solvents is effective at dissolving the wax, but apparently the trick is to dissolve it in hot mineral oil. Just heating up the beaker now...