Field Coil Winder using a Sewing Machine Motor.

[pwdrive]

It was just a thought Gary, I am not that well up on speaker magnetics.
Good to hear you again.

Lawrence.

[Yorkie]

I don't think its necessary to use solder tags David, or desirable. The tags will have to be right near the edge and likely to break out the cheek material (I assume you were thinking of riveting them on).

BTW is the voice coil OK, sometimes EKCO for no reason I can see used ali wire that is very prone to being open.

Gary

I hadn't intended to rivet the tags - I thought I might do something along the lines of how the coil in this youtube video are terminated, though I must admit it looks a bit 'hammy'. Interesting to see how the wider traverses back and forth on a rack and pinion mechanism.
Another forum member - can't recall who - originally posted this video on the forum:

http://www.youtube.com/watch?v=1SwWuILlH4Q

I did check the speech coil a while back Gary and it was fine, so I'm hoping it still is. I noted the ta-ta you had with the AC97 speech coil in the Bulletin!!

I've finished the wire guide - basically two pulleys with deep 'V' grooves so the wire won't ride up the groove. (hopefully!). As I'm left-handed I've fitted a handle to the left of the pulleys so my hand doesn't get in the way of the wire as it passes from the feed bobbin over the two pulleys and onto the coil bobbin. The idea is to move the pulleys back and forth to try to get some semblance of control as to where the turns lay - too much to hope that they come anywhere near being side by side of course, but I'm hoping it will give me a bit more control than just guiding it through my fingers. I've made an end cheek from Paxolin, and I'm ready to roll, so when I get back from exile in London, it will be time for the 'MOT' - moment of truth. If I don't do a brilliant job of winding the coil, I'll wrap the finished coil with gaffa tape and just post a pic of that rather than to show it undressed!!

[Yorkie]

Another high quality project in the making, David. Very smart idea to use a calculator as the counter and an equally clever idea to use a sewing machine motor an control as the driving force. You are definitely showing the way here. Inspiring stuff.
Tony

Thanks for your kind words Tony, but I'm not so much leading the way, as plundering everyone else's good ideas which I've shamelessly scavenged from here and there and cobbled together with what bits and pieces I had to hand. (The woodwork has more than a passing resemblance to the winder that Gary made). The only aspect that I can claim originality for is the feed wire bobbin tensioning arrangement, using two weak compression springs to stop the feed coil from 'free-wheeling' when the motor is stopped, risking unravelling more wire off the feed wire bobbin, or if the wire were to go slack, risking 'snatching' it and snapping the wire. By moving the outer cups on the springs inwards or outwards along the shaft, the tension can be made weaker or stronger. The inner cups with the large brass discs aren't fixed in any way - they are just slid along the bobbin shaft till they make contact with the feed bobbin, then the outer cups are positioned to apply the desired level on tension and the grub screw tightened on the cups.

I think that for wire much finer than this, I wouldn't mount the wire bobbin on a shaft - I'd lay the bobbin flat so that the wire comes off from the end of the bobbin, then have some sort of overhead feed arrangement through say a fine tube with very slight friction somewhere along the way - a bit like the Alabama Antique Radio Soc'y jobby. Just thinking out loud really - I can't see how - especially when the wire is on a large bobbin, that it can be pulled of the bobbin and onto a motor driven bobbin without risking breaking the wire. That said, I have on experience at all in the black arts of winding thousands of turns of fine wire onto coils or transformers.

For now, I'll be happy if I can successfully wind one field coil. (The only time I ever feel like doing anything twice is just before I've done it once!).

[Radio Fixer]

I should have said David that if you go the way of making up the In and Out wires directly to the winding then there are a couple of tips.

For the In, with the hole thru the end cheek so it lays flat along the bobbin, a groove needs to be made, on the outside, to lay the wire in so it clears the metal of the pole piece. If the cheek material is thin then this can become a slot. Depends upon wire size as well. In either case the groove or slot doesn't need to extend all the way across the cheek. The wire can be glued into this space.

For the Out wire, having attached this to the coil wire and secured it as previously described, then its a good idea to make a simple loop of it around the bobbin, with a half knot, before the final covering of the coil.

Good luck with the winding.

Gary

[Radio Fixer]

I cant help wondering David whether the micro switch is best for the job as I/P to the turns counter? Its surprising how many RPM you can get up to once underway. Don't know what mine was but the 40,000 turns didn't take that long to wind and even at 100 rpm that would be 6.5 hours, so could at times have been more? Your switch could certainly be clattering round at a fair rate of knots. Actually a reasonable speed is OK as you see the trend in the build up of wire and can adjust to fill the hollows easily.

A better choice might have been a reed switch actuated by a magnet sunk into a disk of mdf say, possibly with a matching weight the other side.

Be interesting to see what happens.

Gary

[AlanBeckett]

The Microswitch should be good for 1mS and a Reed Switch is unlikely to be substantially better. However, there may be bounce problems. Whether that causes a problem depends on the Calculator. However, I expect that's pretty well debounced.
I've got a couple of Opto Switches - slotted sort - which would be much quicker but you'd need a bit of circuitry.

Alan

[Yorkie]

Thanks for your observations Gary and Alan.

1n fact after about 200 RPM, the calculator ceases to count - I guess because it can't cope with keystrokes above that speed, and wasn't designed for such a need,. but I don't envisage the winder going at anything like that speed (12,000 turns per hour). True, a reed switch could be used and would only need a small neodymium magnet to operate it, but in terms of reliability, a 'tally counter' operated by a cam as shown in the first video above would be the simplest method, albeit it would make a clatter at high revs, as evidenced in the video. I tried a tally counter on my hand operated coil winder, but the spring was too strong so I had to dismantle the counter and take turns off the spring. ((Tally counters only about a fiver).

Really, a turns counter for this application is a needless non-essential distraction - the number of turns is irrelevant - what matters is that the resistance of the finished coil is 2,000 Ohms or thereabouts. I'm away from home at present, but as I recall, the 500g reel of 38SWG wire I've bought is 2,700 Ohms. When the bobbin is full, I'll check the resistance and either add or remove turns as required. I only fitted the counter as it was easy enough to do, and I thought it would be interesting to know how many turns the coil eventually takes.

As I see it, a turns counter is more relevant when winding RF coils, and the inductance, rather than the resistance, is important. In such cases, a turns counter is helpful for repeatability.

For an electronic counter, I think the most reliable option would be a digital frequency counter module (not much more than a tenner from Asia Engineer et al), triggered by a slotted disc and opto-interrupter, interfaced with the counter module. A disc with 60 slots will convert the display into RPM rather than frequency in Hz. (Thus, 1 rev would give 60 pulses). I built such an arrangement to display RPM of my woodworking lathe based on an article in Model Engineering Mag which forum member Ted, Ex-2Base, kindly passed to me. However, I couldn't devise a convenient way of mounting the slotted disc on the lathe, so the project stalled. As with the coil winder, it's a non-essential requirement as my lathe has eight fixed speeds and I know what they are and what speed the lathe needs to run at for a given size of object.

Interesting enough, but of little relevance to the task in hand - winding a field coil.

[Radio Fixer]

Yes! and apart from RF, a turns counter is essential when winding multi-winding AF coils such a interface Xformers for PP.

Gary

[Yorkie]

A little update.

I'm relieved to say that I've successfully rewound the field coil for the mains energised speaker on the Ekco AC77 radio that I'm restoring

As I said in an earlier post, the pocket calculator turns counter wasn't a success - it works OK up to about 200RPM, but anything faster than that, and the speed is too rapid for it to count. When I get a bit of time I'll see if I can fit a mechanical tally counter in place of the calculator for checking the number of turns - obviously when transformer ratios come into play, it's important to be able to count the turns. I've got a tally counter which I initially used on my 'Morris Gingery' hand coil winder (which I've never used as Litz wire is soooo expensive and I haven't needed to wave wind any RF coils, for which it was intended). A pocket calculator 'turns counter' works fine on that hand winder of course. The other thing I discovered about the calculator is that it has auto shut off, so within a few seconds of it not having any key presses, the blighter switches off so you loose the record of turns counted!

For the field coil, the number of turns was irrelevant - all that mattered was that the finished coil was as near 2,000 Ohms as could be, but I'd estimate that the 2.53 KM of wire I had to wind onto the former was in the region of 18,000 turns. The coil former is 1.25" diameter, the finished size of the coil is 2", so the average diameter is 1.625", which works out about 5" (12.5 cms) per turn on average, so that's about 8 turns per Metre. Th resistance of the wire is 0.85 Ohms/Metre, so for the 2,000 Ohms needed for the coil, that equates to 2,353 Metres x 8 turns per Metre = 18,824 turns. Whether or not I've got the sums right matters not in this instance! I just filled the bobbin with wire, then ran the soldering iron on the (solderable) wire to tin it for 5mm or so, and tested the resistance, intending to either add or subtract turns to get it about right. As luck would have it, it was 1,997 Ohms! The lead in/out wire, the silicone sleeving and 'fish paper' came in handy, so thanks for that. As the original coil was wrapped in black plastic, I put a couple of turns of black gaffa tape over the fish paper so it looked as near as possible to the original coil.

As to the two guide pulleys I fitted to the coil winder, they were not a success and were a needless complication. I had thought it might help me control the lateral movement of the wire back and forth across the former, but with wire of such a fine gauge any notion of trying to lay the turns side by side is fanciful. I therefore adopted Gary's method and fed the wire through my fingers using one hand to steer the wire as it came off the feed bobbin and the other to lay the turns of the coil former as neatly and evenly as I could, and it looked quite respectable. I had a heart stopping moment when the coil was at least half wound. Suddenly the motor ran at top speed and the speed controller failed to work. I quickly switched the motor off, and spent about half an hour untangling the wire that had spooled off the feed bobbin. Luckily, it didn't break or kink, though it it had, I'd have just had to repair the break and press on. The fault was caused by the suppressor cap on the speed controller going dead short in the foot controller. My fault really - I should have fitted a new X type cap at the outset.

The spring friction arrangement I fitted to stop the feed bobbin free-wheeling worked fine, but I don't think it was essential. The foot controller enables you to slow down the speed in such a way that the feed bobbin behaves itself. I was surprised how fast I could wind on the turns - the wire was far less fragile than I'd imagined it would be, and total winding time was well under an hour.

I've attached a pic of the speaker as found, and one of the restored speaker with the field coil fitted. I'm indebted to Gary Tempest for his advice and for supplying me with 'fish paper' to wrap the finished coil, wire for the connections to the coil, and silicone sleeving. He also tipped me off that Halfords Ford 'Dove Grey' paint is a very good match for Ekco grey as used on their speakers and chassis.

So, for my AC77 restoration, that's the power supply rebuilt and working, and the speaker too. I think the main chassis will pose more of a challenge. I'm rather miffed that through dithering, I missed out on an AC77 on ebay, untouched, neglected and in just as poor a state as mine, which went for under £30 and would have been excellent as a reference for when I strip this one down. Oh well, it's a hobby after all - I'm restoring radios in my scarce leisure time - I'm not putting rockets into space, so what the heck!

[Radio Fixer]

Nice job David.

Gary