One of my theories is that the face that is shown isn't completely flat and that is causing the offset in the material. This could cause the offset of the adapter that we see in the video. I held a straight edge up there and couldn't get it to rock back and forth, which would indicate a ridge of material... so I don't know.
If there's a crown on there, absolutely. You might be off-angle. Though the degree to which you are off-angle, to cause so significant a wobble, I'm not sure.
My other theory is that the material I grinded off wasn't even - aka there is imbalance in the taper lock because of how I grinded away the old female spline. That could be causing vibration without adapter or driveshaft.
This I highly doubt. The minuscule difference in balance among the grinding imperfections I can't see causing that amount of shake, and, definitely it is not the result of the very visible wobble which is probably 2 orders of magnitude more off-center material than could be possible with those grinding gouges.
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From the looks of things you have 2 problems:
1 - You have weight that is off-center being flung around. That's definitely causing wobble. You could fix this at home by simply spinning the motor up, and holding a grinder, file, chisel, etc near the surface and grind down all the high spots, in effect using the motor as its own lathe.
2 - Your coupler is almost certainly mounted off-angle, and thus the driveshaft that connects to it is being spun off-angle or off-center or both. So now instead of a few dozen, maybe hundred grams of off-center material from the coupler, you have the entire mass of the driveshaft being spun that way. This will also destroy your transmission in, oh, 10-100 hours of driving.
A recap of what I went over during your spring break:
In your case, the off-center is possible, but the off-angle is almost a certainty (not because the male protrusion is crooked to the coupler body, but because the two halves of the coupler body are off-angle to each other.
Imagine spinning either of those situations and measuring the wobble, you'd certainly detect it.
You don't need a dial indicator if you want a head start. IIRC, couplers need to be centered and parallel within 0.004". At least, this is factory spec. They may vary more than that, even OEM, but even more reason to align them accurately.
A book is accurate to 0.004" (4 thou), a bible is accurate to 0.0015" (1.5 thou). Stack books (not great, as they turn into parallelegrams when you compress them) or offcuts of 2x4s to raise the book to the height that it'll be partially interfering with the coupler shaft. Then, while making sure the spine isn't arching as you do so (keep the measuring half flat), turn pages until you're just barely not able to fit one more page between the existing pages and the coupler shaft. Then rotate the shaft 90 degrees at a time and see whether it jams up or loosens, or, see what pages # you're on if you're doing a full reset. You can eyeball an air gap of 0.001", or just flip pages.
That said, accurately measuring it doesn't tell you much, we can see from your video that you're off by amounts that could be measured with a tape measure, let alone dial indicator. The challenge is how to fix it.
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There's no way around it, you're going to have to at minimum split the coupler, if not remove it. I'd start with lapping it flat. You could try spinning up the half still attached to the motor, and using a wood chisel or file to face it. The transmission half of the face you'll have to get more creative. If you have access to a lathe, slap the output splines into a lathe and just face the other side. At least that'll get you square and perpendicular. Centered... hopefully is also fixed by facing. Presumably the coupler was machined properly in the first place, it's just your grinding that upset it.
In a nightmare scenario you could shim the low side with a sliver of pop can or even aluminum foil, so that when clamped it squared it up. A little trig based on your bible-measured runout and the distance of that measuring point to the coupler face would tell you how many thou of shimming you'd need.
