Rewinding and potting a Sag 12 Graziano clutch

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Harold_V
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Rewinding and potting a Sag 12 Graziano clutch

Post by Harold_V »

Earlier this year I had posted on this project, seeking guidance on the possible rewind of the clutch. I had already removed the coil and determined the number of turns, the wire size, and the coil dimensions, but I have no experience in this arena, so I had hoped to find someone who did. I received some excellent guidance, and kept it in mind as I came to the conclusion that it would be in my best interest to pursue the project myself.

At one point I was willing to have the coil rewound by others. What changed that was that the only source I had discovered had given me enough information that I didn't want to risk them doing the job. For starters, they talked about burning the coil out of the housing, something that sent up a red flag for me. The clutch has some precision diameters and surfaces, so it would not respond well to being subjected to a burn. If they didn't have that understanding, I was not willing to risk using them. Besides, they wanted me to send a complete clutch assembly, so, upon receipt, they could provide me with a quote. I wasn't willing to risk sending a clutch that I may never see again.

Fast forward to a few weeks ago. For what ever reason, I was motivated to start working on the rewind and potting project. It had been placed on the back burner, as I have more than enough projects to dominate my time. I had already determined the wire size (turns out to be #25 AWG, enameled magnet wire), and I knew the number of turns (509). I started the process by searching eBay for wire. I found a source for 2,000' and placed my order. I wanted to know the actual wire size before designing the bobbin. Once received, I determined that I could wind 30 turns per layer, so I sized the width of the bobbin accordingly.

I designed a bobbin that permitted winding the coil to the desired diameter and width. The bobbin was made to be dismantled with the coil intact, as well as permitting wrapping the coil with fiber glass tape @ 45° intervals, which would restrain the windings until the coil could be potted. I wanted to be able to remove the coil from the bobbin without doing any damage to the insulation, so I made the inner form with a slight taper (about four thou), so the coil would come off easily. The inserts that were to be removable to allow taping were machined straight after being inserted in the bobbin and sized to the larger diameter of the tapered form. Once the coil had been taped and the side plate removed, the coil was easy to remove.

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I liked the idea of a turns counter, so I did a search on eBay and found one for $15, delivered. It was simple to mount---I made a small angle bracket and mounted it beneath the spindle, using one of the screws that hold the way cover in place. A magnet, which came with the unit, was simply placed in one of the sockets for the D spindle, then covered with a small piece of red tape to prevent dropping out under some strange situation. Luckily, the magnet was a perfect size, so it rested at the very top of the hole, almost flush with the surface. That assured a reliable gap between the magnet and sensor. The setup was fast to make, and worked flawlessly.

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In order for the wire to be dispensed, I wanted to have a slight tension on the reel, so I took advantage of a large piece of angle iron I had, which already had a slot and mounting hole. I simply removed the tool block from the compound of the lathe, and replaced it with the angle bracket. Using some washers, a Nyloc nut and a bolt, I installed a pair of silicon rubber washers, each of which was cut from closed cell rubber. The rubber washers provided the required friction to create the desired tension, with the reel turning easily, yet providing enough tension to eliminate any slack while turning.

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With the setup made, I did a little experimenting and concluded that it would be in my best interest to wind the coil by hand turning the spindle. It was easy to turn, and allowed for control of how the wire wrapped. I was not concerned if it was slower, as it has to be done only once, and the control gained by hand winding was worth the little time lost.
I was pleased with the results. It turned out quite nicely and yielded an acceptable resistance, so the wire size selected was correct.

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Potting was a concern. I made a couple inquiries to try to determine a good choice of compounds, but hit a wall almost instantly in my research. I found it strange, but virtually none of the makers talk about compatibility of their potting compounds with oil. I even talked with one firm which was a custom blender. They implied that they could provide a functional potting compound, but the cost may not be palatable. Thinking I had nothing to lose by asking, I requested a quote. I nearly fell off my chair. They would provide a two fluid ounce compound for the paltry sum of only $500.
I thanked them for their time and effort, and explained that such a price was not in the cards. They were exceptionally understanding and remained polite to the end.

I continued my search, but had concluded that a 3M product was most likely to be my choice, as their rep had suggested that it was the most inert of all their offerings. It, too, was not cheap, but far cheaper at approximately $60 for two ounces (which would be enough for one clutch) than the previous experience. With my search continued, I found a compound that mentioned its resistance to mineral spirits. After two days, it would experience a minor change of only .05%, but the percentage remained unchanged after 45 days. I figured that would be a good choice, and I could procure eight fluid ounces for about $35. It was ordered and used.

The potting compound has about an hour of pot life before it begins the slow process of hardening, which takes 24 hours at ambient temperature. Curing could be accelerated by heating, but I was in no hurry. With the compound poured in the coil cavity, the clutch was subjected to a vacuum. That proved to be a good decision, as it was obvious that a great deal of air was removed from the coil, and replaced by potting compound. So much so that I had to mix a small amount of new compound to replace the amount consumed by the coil. Ultra fine air bubbles were slowly eliminated as the compound cured, leaving a transparent solid with a coil contained within.

With the coil now potted, all that remains is for the transmission to be removed from the lathe, and dismantled. I'll replace the original transmission wiring with new wire, insulated with Teflon, as the insulation on the original wires has hardened and shed in several areas.

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Assuming the rewound clutch is reliable, and functions as intended, the decision to rewind in my own shop was sound. The Graziano is equipped wth six identical clutches and the forward clutch is suffering with lower resistance, although not low enough to blow a fuse. Once I know that my work has been acceptable, it, too, will be rewound.

Thanks for taking a look at my project.

H
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Steggy
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Re: Rewinding and potting a Sag 12 Graziano clutch

Post by Steggy »

Harold_V wrote: Mon Dec 07, 2020 1:14 amEarlier this year I had posted on this project, seeking guidance on the possible rewind of the clutch...Thanks for taking a look at my project.
Nice job, Harold! Me being me, I would have been figuring out how to get rid of that pesky coil and use pneumatics to actuate the clutch. Winding 50 gazillion turns of wire on a bobbin is too much work for me. :D
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Patio
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Re: Rewinding and potting a Sag 12 Graziano clutch

Post by Patio »

Harold, that clutch came out real nice! :)
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liveaboard
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Re: Rewinding and potting a Sag 12 Graziano clutch

Post by liveaboard »

Thanks for sharing your work and your research.

My new machine has electric clutches, and with no parts available I was a bit worried.
Now I'm not.

I once needed epoxy to repair a fiberglass fuel tank of a motorcycle I had.
The tank was custom made, and the polyester resin was a chemical resistant type but I don't remember what it was called.
It was ok for a few years, then it was splitting at the join. The shop that built it had disappeared.

In Amsterdam there's a specialty shop for fiberglass and epoxy, they have their own product line.
"Polyservice" if anyone ever wants to know. They can't send international due to flammability of the products, I always go to their shop when I'm in the city.
Their epoxy system is several types of resin and 2 or 3 different hardeners.
They told me their standard resin is fine, but I have to use the special chemical resistant hardener.

It worked.
I don't know if the chemical resistant polyester or the chemical resistant epoxy would be suitable for potting a coil, but thought the info might be useful to someone.
shootnride
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Re: Rewinding and potting a Sag 12 Graziano clutch

Post by shootnride »

Beautiful work Harold. And, a very educational thread. From what I've read (mostly on the PM forum), I'd bet you could find quite a few buyers for those coils. Of course, it may be one of those situations where you'd be working for 10 cents and hour in order to sell them :-)

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NP317
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Re: Rewinding and potting a Sag 12 Graziano clutch

Post by NP317 »

Harold:
Thank you for sharing your clutch coil build.
I particularly like the bobbin you created. Awesome!
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SteveM
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Re: Rewinding and potting a Sag 12 Graziano clutch

Post by SteveM »

Nice,

There are winding counting machines for winding guitar pickups:
Image

but at over $500, if I have to wind pickups, I'll go with your approach.

Seeing the finished product, and knowing how well you make things (think about the speaker switch), I'd have to think that there is a market out there for this. There has to be a lot of other Sag 12 Graziano owners that have clutch problems.

Steve
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