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2024-06-13Project: Spindle Thread Protector
As part of my ongoing efforts to machine a new 72T gear to replace the one with missing teeth, I'm making a spindle thread protector so I can safely use an MT3 collect in the spindle taper for holding end mills (I need to machine some flat surfaces).
Tonight I cut the internal threads on the nut. It's made from a piece of 3" 4340 I found cheaply. I bored out the center, cut the relief for the spindle shaft at the back, and then threaded it using the SIR0500M16-1/2″ Right Hand INTERNAL THREADING TOOL insert cutter from Precision Mathews. This was my first ever internal threading operation, but I thought the tool performed admirably. No issues, and the threads came out fine, but not amazing. I've been struggling in general to get the right angle set on my compound. It's got a smear of markings; for this I tried using an angle gauge. I think it came out better than my other threads, but it's also harder to tell with all the threads internal.
Here's a photo of the finished interior bore/threads. The machined surface on top will seat against the back of the spindle nose.
And here it is on the lathe. I'm not sure that I've got a great "balanced" fit between the thread profile on the spindle and the nut, but the feel is great. It goes on snug, but easily threaded by hand. The seat against the back of the spindle nose is firm tight with no gaps.
2024-06-15 Finishing the Nut
I had some time today while the kids were napping to finish the nut. With it threaded onto the spindle I cut it down to rough diameter and length. I opted to keep it oversize (compared to the drawing), because it works well enough with the existing pin wrench I have from a chuck, it's faster to machine, and that extra bulk adds extra protection.
The back of the nut. I feel like the finish looks better in person. You can see in the relief(/counterbore?) on the back that the threading tool was scratching on the final few passes. I was wondering what I'd miscalculated while I was cutting the threads, but then noticed that the same thing is present in the back of my 4-jaw chuck (a vintage Atlas chuck). I guess that's just how the fit goes...
The front (nose) of the nut. I cut the 45deg chamfer with the compound, turning by hand. With a sharp TCGT insert (triangular, with a ground edge, sold for use on aluminum) it came out with a nice finish. I touched everything up with emery paper at the end.
On the lathe. The fit is great, and it sits just a tad proud of the spindle nose. I figure if that's ever a problem I can always just turn it down in place!
Conclusion
I think if I started all over again, I'd just do this in aluminum. The 4340 machined just fine (at least for where my skills are), but aluminum would have been faster and more forgiving. I also started with a 3" dia. piece, which just meant a lot of extra machining to do. But, given I was buying cheap, there weren't really any appropriately sized pieces of aluminum anyway.