This shows the place in question, .175 slot to .276. The outside of the threads go from 1.107 to 1.124 with some threads pretty sloppy in new half nuts. I can mill a new slot in the opposite side but worry about the threads. The threaded part is about 79 inches long and have not seen anything that long on Ebay. Thanks for any thoughts
From what I can see in the picutres, I'd say you should run it.
That is definitely a tangible amount of wear, but I would NOT worry about the keyway dimension. If the power feed (clutch) feeds smoothly from start to finish, it'll do fine. Technically, on paper, you could calculate the difference in the feed per rev when you're power feeding, but in practice the "angle" of the worn side of the slot- A little trig on that angle is going adjust the power feed rate by one or two orders of magnitude less than what the lathe is working to. Look at ALL the optioins for the power feed rate, and ask yourself how much more accurate your work would be if the lathe were doing that to two more decimal points of precision than what's listed.... If it's smooth in it's operation, you're good.
As for the threads themselves, the outer diameter is not relevant. They don't engage on that, they engage on the flanks, the angled sides of the groove. By the apperance of the threads in your two pictures (which isn't ideal for that, and I couldn't measure from here if they were ideal for that...), but by the appearance, I'd bet it still makes threads that are still well within recognized tolerances. Maybe not as tight a tolerance as NASA might use to bolt a door plug on a space station
but at least equal to hardware grade stuff you could buy off the shelf, and probably better. Further, even if they are worn enough to be "way out"... It will be a change over some distance. That is, it doesn't go from "like new" to "worn out" in an instant. Worn out threads will still keep their pitch. The only time you loose tolerance is when moving from a worn to an unworn section, which is a gradual thing. so within any reasonable threading distance, you'll only see a small small portion of that error, thus a completely out off spec lead screw can and will still cut perfectly in spec threads of typical lengths. And if you want to make your own threaded rod..... You're doing it wrong. Tool wear, electricity, and materials are gonna cost you more than just buying threaded rod. It's a commodity that long ago got mechanized and specialized. An inch or two of threads will have very little error.
Neither of these methods of feeding (the keyed clutch for power feeding, and the half nuts for threading) will ever change direction in use. Nor do they havve micrometer dials in place. Nor do you use them for precise measuring. They're not graduated, and if they were, they're not suitable for precise starts and stops...
Personally, given the (relative) simplicity of getting that lead screw removed at a later date, I would use the machine, make what I wanted to make, and never look back. Get the idea out of your head that the lead screw is worn at all, so you don't jump to conclusions that all errors are related to that, and address it if, and only if there is an actual problem that can be reasonably associated with that wear. My bet is that even though at some rediculously minute level, there is technically "some" error in a thrad... It's probably got more decimal points than you will ever care about.
