Repairing 55mm ID thread on my lathe spindle

[BadDog]

I have a large diameter 55m x 1.5mm ID thread that needs repair. It's on the left side of my lathe spindle and I'm trying to install the clutch collar for a lever collet closer. This should fit my lathe as it came off a virtually identical lathe. The clutch collar measures a major diameter on the money for 55mm, and thread gage confirms 1.5mm pitch for both pieces. Measuring the minor ID on the spindle is problematic due to the threads, but telescopic gages put it in the ball park. There are scars on that end of the end of the spindle as if someone whacked the end very hard with something. Telescopic gages show that it is still round within what I can verify with some fairly rough threads (exposed in use since new, never had a closer I gather). I've cleaned them up as best I can using a wire brush and then a smaller 1.5mm pitch tap as a sort of hand held chaser/cleaner, and to the eye the threads actually look pretty good, but the clutch plug/collar won't even start to pick up a thread. I even ground out the first ID thread in the spindle and used a diamond stone to taper the first thread on the (very hard surface) of the clutch collar. Still, it won't even start to pick up the thread, instead just rotating and rotating as if it were on a bearing step rather than trying to start a thread.

So far no luck on a thread repair or tap for 55x1.5 (no surprise there). It is a good industrial Taiwanese lathe, but I'm not entirely sure it's on size. This weekend I'll likely make a slightly undersize threaded stub with an even smaller step to test a bit better. But my operating theory is that the whacks on the end upset the first few spindle threads enough to be a show stopper unless I can fix them. And I have to fix them well because that collar has to be removed/installed every time I need access to the belt drive.

It's not a totally unheard of thread (very typical of this application), and I've got feelers out for a tap, but probably have a better chance buying a few lottery tickets and hoping for a pay out big enough to buy the $300+ tap. Since it may be slightly undersize over all, and it may need material removed, a good tap is by far my best option, but cost prohibitive. So this only happens if I get double lucky. Find someone who actually has such a huge odd tap, and someone that's willing to loan it out to clean these threads up.

So, one idea was to get a piece of tool steel, preferably air hardening (probably doesn't actually matter) and cut a "tap". I put that in quotes because it won't have clearance relief, and probably won't have much if any rake, just a gullet to gather any minor debris and hold oil. Something more like a thread repair or "roll tap". It's far from ideal, but I was thinking to "flame harden" the leading (not cutting, more forming) edge with OA torch, and let the bulk of the body act as a heat sink to harden the surface, then use "very light straw" color tempering to help prevent premature fracture of the threads. Avoiding a sharp cutting edge (particularly with rake) should provide more tolerance on a successful heat treat hardness/temper too. Not ideal, but that's the limits of what I can practically bring to bear. Lots of work for something likely to end in failure, and I would have to buy the tool steel since I have nothing on hand even close to that size.

Another idea was suggested by a friend. Basically take whatever I've got on hand and turn theaded stub sort of like an adjustable OD die, but undersize with threads on the outside (again with some small flutes). Then, split and use a separator (like a pipe plug) to spread it open. Insert to the bottom of the threads, which should still be intact, and back it out, hopefully repairing threads as it goes. The problem is that without hardening it seems unlikely to do much. We also discussed using some compound to help finish out the threads, but again that assumes I can get them at least "almost there" mechanically. I'm thinking unlikely, particularly given that the hardened clutch collar won't even start to pick up the first thread. And again, a lot of work for something with a low chance of success.

I've already tried every jury-rigged attempt I could come up with. I really liked my old lever collet closer and thought that for the price I got the (JFK brand) lever setup (that should have just mounted right up) this was a pretty big win. But in the end I may resell the bulk and just build a hand wheel closer using the spindle nose bushing that fits perfectly. Not what I wanted, but is easily suitable for my actual needs. The lever setup was just a sort of luxury upgrade, but unless someone can suggest something that has a better chance of success, I'm starting to feel it's not worth the trouble (frankly, probably not worth what I've already invested in effort). But I still want it dang it!

Anything?

[GlennW]

If you are going to make a threaded stepped plug, just make it a threaded stepped tap!

[BadDog]

The main issue there is tool steel and hardening. I figured the mild steel plug would get me an idea how far off the thread is to better understand what might work for a fix. But yes, that would be the ideal if I had the steel on hand and some way to reliably heat treat.

[Rich_Carlstedt]

You are on to the solution.
Make a clean fitting plug out of brass if possible ( 53.4 mm diameter)
Then bore a hole end-wise in the plug so that your 1.5mm tap will fit in the bore but leave 0.75mm sticking out
You will then have a 55 mm one flute tap.
Insert the tap into the threads, followed by slipping in the plug which will hold the tape in place at the right pitch center.
The threads of the tap should pick the thread location for you and cut the damaged threads. You will need to holed the tap in the correct orientation for the single flute, while turning the plug clockwise.
Having a couple of wrench flats on the plug should help that operation.
If the threads are really damaged, you could back off the tap rotation and make the single flute a negative cutter and then gradually bring it up to full cut

Rich

[Rich_Carlstedt]

Here is a quick sketch of what I mean

Rich

[BadDog]

Thanks, great solution. You and one of the local machinist group members proposed exactly the same solution. Sad to say, I didn't see it myself even though I attempted a much more primitive version that failed.

[refinery mike]

I know i am late on this one but i have a possible solution. I have done it before on 4" by 20 threads id. Take a standard tap say a 10mm 1.5 pitch. Any tap so long as the pitch is right. Grind the teeth off all but the one side of the tap. Then clamp that tap in a handle. )I turned a wooden handle like a file handle. Now use that tap to scrape the bad threads back into place. The row of points will follow the good threads and scrape the bad threads into shape. You can also find tools made for such a purpose, they look kind of like a thread chaser on a swivel handle. The same principle, the inside teeth follow the good threads and scrape the bad ones back into place. Look for "internal thread restorer."

[spro]

If you have a spacer sleeve in the left of the spindle and an outer collar to hold it... That sleeve is the diameter of a long bar similar to a boring bar. The cutting tool is a threading tool suspended at the inboard by soft sleeve in the chuck. So as you wind the bar from the right, it captures the thread and cuts out to the left side.
The cutting tool may be retracted for insertion and then locked by one bolt once in the thread. These tips about smaller taps have merit, yet there is the helix of the larger thread being different at any depth.

[BadDog]

Mike: Thanks for the suggestion. A very similar approach was actually one of my first attempts to fix the bore thread before I posted this. I definitely made the thread look a lot better, but can't get the collar/plug piece to start.

spro: Thanks, and I've considered a variety of options from one end or the other. But given that it looks like I may be dealing with an undersize or out or round issue, I think the "tap holding plug" is going to be the winner. I say that based on thread looking pretty good now, but still not being able to start the threaded plug. So a tap captured in a plug would allow be to work preferentially at the out-of-round condition (if it exists, t-gages over threads didn't confirm out of round), or even enlarge a tiny bit if that's what the problem is.

Since my last post some of the feedback I've gotten off-board indicates that these back spindle threads are often not what they should be. The sellers of such machines often deal with this when the "lever based collet closer option" is selected on Victor and other comparable lathes, much less the lesser quality lathes. And this is true even when the closer is provided by the same company (mine is JFK). So presumably companies that market these lathes with that option likely have the $200 tap on hand to manage this with less effort. But that's apparently why it's generally referred to "fitting a closer" as opposed to just installing the option. I always thought that was a nod to the need for drilling and tapping operations, but it seems the exception rather than the rule for simply "installing" these closers on anything less than a Mori or Monarch grade machine, and iffy even then if it's seen years of use to beat up the threads.

Unfortunately some good fortune in other areas has put this back on the back burner thus far. Hopefully back to this soon, maybe this weekend.