Author Topic: Surf PI progress - free stem!  (Read 1032 times)

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Surf PI progress - free stem!
« on: July 09, 2016, 05:14 pm »
Hi All,

One of my colleagues hurt his back moving a load of hard core and turned up at work with one of those adjustable crutches made of tubular aluminium. It struck me that the adjustment mechanism was like that on the old Silver Sabre I'm restoring; two sprung buttons that pop through holes in the outer tube. I measured the inner tube and it's 22mm diameter, the same as plastic overflow pipe. After asking around I managed to get hold of one of these crutches for nothing. Overflow pipe is a perfect fit, and it was easy to drill for the sprung buttons. Now I have an adjustable stem with a plastic lower section. I've put a T-joint on the lower end (pushed on for now, but it will be glued) with a short length of pipe on either side going through a pipe clip of the type used to fasten the pipe to the wall, or in my case, the coil housing. I'll need to find plastic screws for this. The other end of the stem has an adjustable forearm support and a sort of upside-down pistol grip. The angle of that isn't quite as I'd like it but it will do for now. The outer tube is 25.5mm diameter so 25mm conduit clips might be close enough for fastening the electronics box to it.

My thousand feet of red Kynar wire arrived this week so I'm thinking about the coil now. I've got access to a laser cutter which I'm going to use to make the former out of 3mm plywood or MDF. It can cut intricate shapes very accurately, many times over, without getting bored and with relatively few fires, considering...

Cheers for now,



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Re: Surf PI progress - first coil
« Reply #1 on: July 11, 2016, 08:47 pm »
Hi All,

I've made the coil former and wound my first coil. I'll try to attach a photo. The former is made of two 3mm plywood plates spaced apart by "ribs" 3mm high. These are glued into slots in the plates; you can see the dark edges of the ribs. The outer ends of the ribs, where the coil windings lie, are shaped to make the wires form into a circular bundle. The spaces between the fingers around the outside of the plates allow me to lace the windings together with dental floss before I separate the plates.

I started with 24 turns expecting it to be too many, and it was. I measured the inductance at 438 microhenries using a Peak Atlas LCR meter. A quick calculation in Excel told me 21 turns should put me in the 300-350 microhenry bracket. I took off three turns, laced the coil and removed it from the former. I now have a 329 microhenry coil also shown in the photo.

I know twelve 6mm bolts is probably overkill but it's what I had. If I can find some nylon nuts and bolts I'll be able to measure the inductance without having to remove them.

Any comments, suggestions etc. welcome.



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Re: Surf PI progress - free stem!
« Reply #2 on: July 12, 2016, 09:09 am »
I admire your knowledge, patience and ingenuity sir - and I would be interested in follow up posts on how it all works out in the end plus a picture of it. I'm glad to see the Heath-Robinson spirit of inventiveness is alive and kicking!

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Re: Surf PI progress - free stem!
« Reply #3 on: July 12, 2016, 12:02 pm »
That's some skill you have there, let us know how it all performs when you are finished.
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Re: Surf PI progress - free stem!
« Reply #4 on: July 12, 2016, 01:47 pm »
Brilliant stuff! Plastic rivets might be useful, the type used for removable panels in cars.


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Re: Surf PI progress - free stem!
« Reply #5 on: July 12, 2016, 07:51 pm »
Thanks to all for the kind and encouraging comments! Are the plastic rivets of the barbed, "Christmas tree" variety? I'm not sure whether they'd wear the holes in the plywood after repeated use. I don't have any to try. I've ordered some nylon bolts and wingnuts which should be with me by Thursday.

As I've moved on from the free stem aspect I'll start a new thread about the general progress of the project, with some photos I took this evening showing the coil former more clearly. I bought some spiral wrap today for binding the coil and putting space between it and the Faraday shield to reduce capacitance. I also bought some 25mm conduit saddle clips which seem a good fit on the larger diameter tubing of the crutch. I'll use these to mount the electronics box under the forearm section of the crutch.

The former took about three hours to design using Sketchup 8 (free) and only about half an hour for the laser cutter to make all the parts. Making one of these formers by hand would be completely out of the question. Far too fiddly.

More pictures in the next thread. Thanks for looking!