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Pete W.:
Hi there, Jonny,

Thank you for your post. :mmr:

I've now had a look at the spindle through a pocket magnifier and tried the fit of some of the threads. Despite my run-out groove being quite a generous width, the starting lead of the die still results in the very end thread preventing full engagement. I think one way round that, as I wrote earlier, is to screw-cut the threads using my chaser. (I have three ¼" x 40 tpi ME dies and they all have a significant lead on both sides so I don't think reversing the die in the holder is going to work.) :bang: :bang: :bang:

Also, Jonny, I'll machine the next one as you suggested, without removing it from the chuck until the very end. That means that I shall have to screw-cut the threaded section nearest the chuck so I might as well screw-cut them both. I don't want to alter the set-over of the top-slide so I shall have to machine the part with the short spigot towards the tail-stock. Also, in order to be able to try the polariser unit on the taper as I machine it, the taper has to 'point' towards the tail-stock.

I'll machine the short spigot first and then reduce the diameter of the two threaded sections (and what will later become the long spigot) to ¼" and screw-cut them before machining the taper. Only then will I plunge cut to reduce the diameter for the long spigot (to 0.150") so as to maintain the support for the part for those outer operations. I'll use my tail-stock steady for all those operations that permit it. I'll need to be careful to note my cross-slide dial readings while screw-cutting because at that stage only one threaded section will be accessible for checking the fit.

So, what to do next? :scratch: :scratch: :scratch:

I think the next thing will be to turn another dummy taper or two to check that the set-over of the top-slide hasn't shifted. Then I think I'll have a practice at screw-cutting with the chaser.
I bought 4 feet of ⅜" mild steel rod so I think I have enough material! :lol: :ddb: :lol: :ddb: :lol: :ddb:

For the record, Jonny, I only removed the job from the chuck once, to saw off the part-machined part and reverse it in the chuck. I did, at one earlier stage, draw it further out of the chuck but I took care not to turn it wrt the chuck jaws. Also, I did as stated try the threaded portion in the microscope sub-stage bracket and it did fit then - but it doesn't now! :bang: :bang: :bang:

vtsteam:
Pete, did you miss my earlier suggestion to cut the threads once the proper way with your die, then re-cut the same threads with the die reversed so you can cut full depth threads all the way to a shoulder? I did that many times in the construction of my lathe.

Pete W.:
Hi there, Steve,

Thank you for your post - yes, I did see your suggestion. I responded to it in my first main paragraph, the sentence in brackets.

vtsteam:
Oh, okay, I finally found it (after reading three times through the thread!) I guess I don't understand what you meant by lead in, because all my dies cut to the very edge of the die, and the start end is tapered, while the opposite end is full depth. so if you reverse them they cut full depth to an edge. But I'm guessing that what you have is a die where the cutting edges don't extend to the end of the die -- and maybe there is some kind of coutersunk hole there. Anyway, I trust you know what you have won't work, so apologies for the repetition!

Pete W.:
Hi there, all,

Well, after a lot of distractions from workshop time, I managed to get to grips with a second attempt.

I'd previously had difficulty getting a good thread right up to the shoulder (see earlier posts). Thinking this was because of the lead (aka taper) in the mouth of the die, I'd bought a 40 tpi chaser. So, before starting on a spindle for real, I'd had a few practices with the chaser on some 1/4" free-cutting mild steel rod.

The camera didn't do that thread justice! Some of my attempts weren't bad.

I started on the spindle by turning the stock material (3/8" free-cutting mild steel) down to the maximum diameter of the spindle, namely 0.350". That very ordinary process looks like this:

The end of the material shows an earlier 'dummy run' at turning the taper. That gets removed in subsequent operations.

Then I used the top-slide and the mini-thin grooving tool to define the lengths of each section of the spindle. I dialled the required dimensions after correcting for the angular offset of the top-slide (1/Cos 4°), pretty small but I did it anyway. I turned each groove to the appropriate depth. The tresult looked like this:

(There's a very thin 'extra' section right at the outer end of the job - this is to allow for the centre drilled hole in the end of the stock, to be removed later.)

Then I turned each of the outer sections to the outside diameter of the thread, 1/4" (intending to further reduce the spigot diameter as a later operation), resulting in this:

Then, with the lathe gear-box set to 40 tpi, I presented the chaser to the 1/4" section. I had the chaser mounted in a rear tool-post, NOT inverted, with the feed set to move the carriage away from the chuck. Because there wasn't enough room to clear the chaser, I removed the tail-stock and centre.

This, and cutting the grooves at the chuck end of the job, caused my downfall! The job rode up over the chaser and snapped, resulting in this:

:bang: :bang: :bang: :bang: :bang: :bang:

Oh, well, as my dear old mother used to say, 'If at first (and second) you don't succeed, try, try, try again!'. :nrocks: :nrocks: :nrocks:

I should have left those grooves at the tail-stock end of the job until later. It would also have helped if I'd had a piece of 0.218" hardened steel rod with a true 60° point ground on one end and its other end set in a 2MT shank - Set in the tail-stock, it would have supported the job against light cuts.

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