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Old 6th May 2014, 8:19 am   #1
johnyjf
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Question Paxolin Coil Former cheeks how to cut?

I'm attempting to make a large radio coil of the Gilbrt Davey type, former about 2" in diameter and 3 paxolin cheeks to fit on it outer diameter about 2 3/4". My question is what is the neatest way to cut the paxolin cheels froma 2mm thick sheet. I thought of sandwiching the sheet between two layes of scrap ply and clamping them rather tightly, then use a bosch hole saw in a drill stand but I'm worreid the paxolin will chip and split at the edges. Does anyone know of a better technique please??

Regards

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Old 6th May 2014, 9:12 am   #2
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Default Re: Paxolin Coil Former cheeks how to cut?

Cut big and then sand to final shape.

If the outer is round, then you can mount it on a temporary central hole chucked up on a mandrel in a lathe. gently cut large then turn down with very slow feeds, finish with emery, and if the centre hole is to be round, you can scribe it while still spinning.

A big hole saw is going to grab at the last bit and take a chuk out unless the paxolin is well supported. So stick it down to a piece of flat timber with double-sided tape and use the hole saw to cut into the timber as well. A pillar drill will help.

Paxolin (and formica) cut well with routers. circle cutting attachments are usually part of the kit. Mount on sacrificial flat timber with double sided tape. You may need to make a circular ring as an outer guide to rout small holes.

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Old 6th May 2014, 4:46 pm   #3
David G4EBT
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Default Re: Paxolin Coil Former cheeks how to cut?

Coincidentally, I recently made some end cheeks for a coil former for the field coil on a pre-war Ekco AC77 to wind a new field coil. (2.3km of wire - 18,000 turns!). The original former only had cardboard end cheeks which I didn't think were up to the task. I've attached a pic of the former. (The end that protrudes from the cheek is to mount the hum bucking coil). I didn't have a problem cutting the paxolin discs in a matter of minutes.

I cut the holes with a holesaw in a pillar drill, with a square of paxolin taped to a piece of scrap wood with gaffa tape to avoid the risk of the paxolin spinning around if held by the fingers had the drill 'snatched'. Don't use a pencil to mark the circles on the disc - drill a small hole in the centre of the square to accept the point of a pair of dividers then use the dividers to scribe the circles so you can see them clearly. That hole can be the pilot hole to centre the holesaw centre bit. I have a bandsaw which I used to cut the circle, but I assume that you don't have a bandsaw, in which case I'd have used a junior hacksaw to cut the corners off the square to form an octagon, which will leave little waste to be cleaned up with a file and sanding block with say 120G sandpaper.

The holesaw shouldn't chip the paxolin, and neither will a junior hacksaw if you're careful.

I've attached a sketch to make it a bit clearer.

Hope that helps Johnny, but if you struggle to make the discs, do drop me a PM and I'll happily do them for you.
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Old 6th May 2014, 8:46 pm   #4
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Wink Re: Paxolin Coil Former cheeks how to cut?

Thanks very much for your advice, both of you. I'm goings experiment a bit, will publish the results (pics).
Regards
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Old 14th May 2014, 5:21 pm   #5
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Default Re: Paxolin Coil Former cheeks how to cut?

The work progresses, paxolin is a hard material. Have got another half hour's sanding to do but it does look promising! The inner holes will be fun to make as well!!!!
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Old 14th May 2014, 8:55 pm   #6
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Default Re: Paxolin Coil Former cheeks how to cut?

There are some CNC routing services on eBay that will do the job (£20 I believe?)

http://www.ebay.co.uk/itm/CNC-Routin...item20e459024e

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Old 15th May 2014, 6:55 pm   #7
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Default Re: Paxolin Coil Former cheeks how to cut?

Thanks for the info, I have now got the outside edge of cheeks as I want them, the inside holes need a 2 mm enlargement that I am working on with a electric drill and a hole saw wrapped with abrasive paper but it is take a long time to gain a couple of mm. I want to make these myself though if it proves too masochistic the next time I may go the CNC route.
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Old 16th May 2014, 5:22 pm   #8
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Default Re: Paxolin Coil Former cheeks how to cut?

WEll I have completed the cheeks and here are the photos as promised:
I will publish another one when contruction is finished. I plan to use the coil in a one valve semi (or possibly pseudo) vintage type design with a PM2 or similar!

Regards
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Old 16th May 2014, 6:13 pm   #9
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Default Re: Paxolin Coil Former cheeks how to cut?

Impressive! I'm glad I own a lathe, though

Richard
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Old 19th May 2014, 9:53 am   #10
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Default Re: Paxolin Coil Former cheeks how to cut?

Members might like to see details of this coil at:

http://www.daveysradios.org.uk/conresdaveycoil.pdf

Les.
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Old 20th May 2014, 6:23 pm   #11
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Smile Re: Paxolin Coil Former cheeks how to cut?

I wish I still owned a lathe, makes the job one whole lot easier. The more observant among you will have noticed that the cheeks are half the height specified in the Gilbert Davey article, I said whoops (or something similar) when I realised it. Am going to have to lengthen the 200 turn section to compensate. Also have (somewhere on line) read that the reaction section could do with a few more turns so will install 7 solder tags and add 10 turns to the original leaving the 30/10 join as a tap. I start Winding Friday.
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Old 27th May 2014, 7:54 pm   #12
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Default Re: Paxolin Coil Former cheeks how to cut?

Some further progress though still not wound!
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Old 27th May 2014, 9:06 pm   #13
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Question Re: Paxolin Coil Former cheeks how to cut?

TPI Mystery: I have just wound the 65 Turn Medium Wave winding and it takes up about 2" of the former's length. According to the tables in the back of my Babani Coil Design and Construction manual it should only take about an inch. Does anyone have any idea what the difference is due to? I happy enough with the winding just curious.

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Old 28th May 2014, 7:29 am   #14
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Default Re: Paxolin Coil Former cheeks how to cut?

The original coil used 28SWG double cotton covered wire ('DCC') which for a given SWG gauge will have fewer turns per inch. The alternative coil - which I'm assuming that you are making - used enamelled copper wire of 26SWG - slightly thicker. That's .0148" (0.45mm). By comparison with DCC, the insulation thickness of enamelled wire is negligible. .0148" equates to 67 TPI so the result you have equates to that. To what extent the use enamelled copper wire as compared to DCC wire affects the inductance I've no idea as I don't hace TPI tables to hand, but obviously there will be an effect as the spacing of the turns with DCC is, I would assume, twice that of ECC. The only way to find that out would be to calculate the original inductance of that winding from the circuit (using an online calculator), then measure the inductance of your coil with an inductance meter. The proof of the pudding will be when you find out what coverage you actually get with that coil.

Hope that helps a bit.

Best of luck with the project - the coil former looks neat!
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Old 28th May 2014, 11:08 am   #15
johnyjf
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Wink Re: Paxolin Coil Former cheeks how to cut?

Thanks for the info, I have played with on-line inductance calculators quite a bit and find them very inconsistent between different calculators not helped by the fact that many of them are for Short Wave and VHF air cored coils only, Not sure what the permeability of a 2" diameter Paxolin former is. I am using 28 swg enamaled wire which is yet another departure from the original specification so we will see what the practical inductance and tuning range turns out to me. Its proving an interesting project and much cheaper than buying Denco, Teletron or Repanco coils on EBay if quite a lot more time consuming.

Regards
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Old 30th May 2014, 9:12 am   #16
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Smile Re: Paxolin Coil Former cheeks how to cut?

Here is the finished article. I have made a couple of further mods including adding another 10 turns to the reaction winding with a tap at the original 30 and I have used super glue to hold the end of the solenoid style windings in place. This is not the smartest solution but is effective, I should have varnished the windings but as this is the first coil I have wound in over 25 years I don't feel too bad about that. Now need to get on with the rest of the construction though that might take a few weeks.
Thanks for all the help.
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Old 31st May 2014, 1:46 am   #17
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Default Re: Paxolin Coil Former cheeks how to cut?

You've done a brilliant job on that coil former and the finished article looks great too. I made a rig a few years back to rewind coils and transformers. It's a right pain doing it all by hand.
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Old 3rd Jun 2014, 10:28 am   #18
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Default Re: Paxolin Coil Former cheeks how to cut?

Thanks for the compliment. I have ordered some bits to make a valve psu before I go any further. EZ41 rectifier (lowest filament current demand I could find and a 125 0 125 volt transformer from VVT on the isle of wight plus a largish Eddystone box. Will publish some pictures when done since it will be multi purpose.
Regards
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