I came up with a bobbin assembly jig over the past several months of winding pickups and finally have a system down. The basic idea is this:
-Small piece of maple, magnet hole-sized holes drilled. This'll be used to put your two outside magnets in, which will let you put this jig on.
-Piece of plywood. 4" x 1.75" is a good place to start. I use a 4" x 1.75" template with a nice shape to start. I just tape it to the plywood, band saw around it, then finish it with a guide bearing bit on the router table.
-Desired flatwork. I just tape the flatwork down about as close to the middle of the block as I can get it. Then I use an undersized bit on the drill press (maybe .170" drill bit for .187" mag holes, or .185" drill bit for .195" mag holes), and drill it out. Use a piece of wood under it so the bit doesn't tear out the plywood when it gets to the bottom.
-Remove the flatwork, then use the proper sized drill bit (as close to .187" / .195", etc., as you can get). Just go through the holes you already made, widening them out.
-Here's where you sand the wood to the desired thickness, or glue/epoxy on wood veneer, pickguard material, etc., and then sand, to get desired thickness. I put the bottom and top flatwork pieces on either side of my wooden block, and measure it with calipers. If you want flush pole pieces, for example, and you're using .710 mags, from bottom flatwork to top flatwork with the wooden block inbetween, you'll want it to measure .710".
-Cut it in half freehand on a band saw. I tried to get it perfectly straight until I realized it doesn't matter. The two pieces will fit together like puzzle pieces no matter how sloppily you cut them. Just do your best to cut straight through the center of the holes.
-If needed, adjust the thickness further. This is best done sanding the two pieces down individually, and testing with calipers until they match within a few thousandths and are the desired thickness for your pickup.
-Wood Hardener. This stuff is great. It seals the plywood and toughens it up. I put some on with a glue brush and make sure the semi-holes on each piece get wetted out very well. I have never had an issue with these blocks cracking or breaking using this method, no matter how hard I bang top flatwork on with a hammer, or press it on with the Arbor Press. Which brings me to...
-Piece of plywood, same size as in Part 1. 1/2" thick seems to be plenty good for this; a 2'x2' of 1/2" thick plywood will go a long way with making these assemblies.
-Place your *top flatwork piece* in the center with double-stick tape. Take it to the drill press, and drill through the holes using a slightly undersized bit.
-Trace around the flatwork, still taped to the wood, with a mechanical pencil. Now you can take the flatwork off.
-Drill out the holes *OVERSIZED.* For .195" magnets, for example, I'll drill it out with at least a .221" drill bit on the press. You want plenty of room here so your magnets have somewhere to go when they push up through the top flatwork, and don't get stuck.
-Get a Dremel with one of those Stewmac Router Bases and any standard Home Depot/Lowe's Dremel router/cutter bit. I've had especially good luck with the Tungsten Carbide cutting bits. Adjust the height of your Precision Router Base by placing the top flatwork piece on the router base, and compare the height of your bit with the thickness of the flatwork. They should match.
-Cut out the inside of your lines that you traced. For the Precision Router Base, I recommend using Teflon Tape on the threads of the height adjustment screws. They tend to loosen while the tool is running if you don't do this, and that'll mess with the depth of the cut.
-This is the most time consuming part. However, I think the time that goes into making this part is well worth it. It allows you to use either a hammer or an arbor press when putting the top flatwork on, and also ensures that it'll look neat and you won't damage your flatwork or get any dents/scratches/imperfections in it.
-Wood Hardener, as before. Let everything set up, and you're ready to go. Only thing you need is a rubber band to hold the bobbin assembly jig together. I use no. 64 rubber bands.
-You can also use this piece to tap out rod magnets if you need to. Most often though I flip it upside down, and hammer the mags on the bottom flatwork side, just in case they aren't flush with the flatwork. That usually gets them nice and even. It's very good for this although it isn't something I need to do often.
Now I'll provide some pics to clarify. Easy process.
First, putting in the outside magnets.
The outside magnets act as a guide so we can attach our bobbin assembly block. Wrap the rubber band around the two pieces to hold them together. This particular block has a P-Bass indentation on it because I hadn't come up with the "Part 2" assembly yet. You can see the two middle magnets are sticking up, obviously...these get pressed down with the Arbor Press. I use the corners of the ram so the other mags don't get in the way.
Now the magnets are pressed in.
Here's where I custom-set polepiece heights. If I want one a tiny bit higher, I'll flip the jig over, use a punch and a hammer, and lightly tap the magnet so it's slightly recessed in the bottom flatwork, so it'll stick a little higher out of the top flatwork. This is especially helpful as rod magnets are only available in particular sizes. I've experimented a bit with reaming out the top flatwork, too, in lieu of beveled pole pieces. I've found it's worked fine and helps the top piece to go on a bit easier. I superglue all my magnets and flatwork when assembly is done anyway, and have never had an issue with bobbin integrity doing this.
This is where I tap the top flatwork on, but only so much to get it started. Beveled magnets help, *slightly* reaming the top flatwork helps. The goal here is just to get it lined up, and staying in place, so we can put our top piece of our jig on.
You can either tap it down with a hammer, or better yet, put the whole thing on the Arbor Press. The top piece will act as a caul and you can simply press it on. You will likely have to press one side, then move it and press the other side, to get the whole piece on. The rout you made will ensure it doesn't wander and keeps everything trued up.
Pressing it down...
Top piece removed. Now you can remove the rubber band, pull out your two half-pieces (they can be a little tough to get out, which is a good sign you've gotten the flatwork pressed on well...but I've never had a real issue taking them out).
Done! Ready for prep before winding. For me, that's superglue for the mags/flatwork, and taping the mags.
I have a method using the Stewmac Soundhole Routing Jig for making a wooden disc with a 4-40 screw in the center to attach to a mini lathe with a 3/4"-16 faceplate. IMO the absolute best for pickup winding. And recently I made an "add-on" disc that slips on top of the initial one, so you can wind humbucker bobbins on it as well. But I'll save that for another thread if I have time.
Hopefully this is helpful to someone here! I spent a fair amount of time working on this idea and went through a lot of testing before I got into a rhythm doing this. But it works very well, and I do think the initial work is worth it, as it really streamlines the process.
It really shines on things like Jazz Bass pickups, where you have 8 rod mags. Having the jig rubber-banded around the bobbin, and simply dropping the magnets in with your fingers and pressing them in on the Arbor Press goes *very* quickly.
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