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Edumacate me on thread dials...

DocsMachine

Titanium
Joined
Jan 8, 2005
Location
Southcentral, AK
Okay, I've got a big 16" Springfield lathe (currently still in parts) with a leadscrew reversing mechanism, so I don't think it came with a threading dial.

I'm used to threading with a dial, however, and so at least for the time being wanted to have a dial available and usable. Original Springfield parts are basically unobtainium, so I'd been keeping my eyes open for one that might fit. That is, a fairly big one.

Another PM'er recently had one, of unknown make or origin, but his pic showed it was pretty substantially sized, and the asking price was right. I picked it up and just got it in today.

threaddial01.jpg


Now, I expected I might have to replace the gear- I figured I was just buying a "kit" that I would have to adapt to my machine. However, the gear proved to mesh quite nicely with my leadscrew (shown here obviously not attached to the machine.)

threaddial02.jpg


The leadscrew is 6 TPI...

threaddial03.jpg
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And the gear has 36 teeth, which is evenly divisible by 6. The dial face has 6 divisions as well...

threaddial04.jpg


Although badly beat and in dire need of a refacing and restamping.

I am not exactly up to speed on how thread dials work, but I was given to understand the gear needed to be evenly divisible by the leadscrew TPI pitch, and that the dial needed to be that number (IE, the number of times the gear tooth count is divisible by the leadscrew pitch.

If that's wrong, please keep in mind that's all knowledge gained in passing, with a heavy dose of assumption to boot. :D

So, did I luck out and get a dial I can use more or less as-is, albeit with some cleaning, or will I need to rework it, and if so, how do I do that? What gear pitch should I have, if that's the case?

Thanks.
Doc.
 
Thanks Hodge. A bit to digest in that thread. (I admit I was kind of looking for a yes-or-no answer on this particular dial. :D )

Also, anyone know how to get these apart? I'd read a rumor that there's a setscrew down at the bottom of the angled hole blow the dial, which I'd always taken for an oil hole. This one's pretty grungy right now- IS there a setscrew down there, or do I need to look elsewhere?

Doc.
 
That thread also brings up a point, which I didn't previously know: According to the plate on the QCGB, this machine is capable of several fractional threads, like 1-1/4, 2-1/4, 3-1/2, etc.

Seems to me that, according to the explanation given in that thread, I can do 1/3rd and 2/3rds thread with this dial, as-is, but not 1/4 and 1/2 threads.

Is that correct?

Again, the lathe has a leadscrew reverse, with a single-tooth dog, and so can thread without a dial, so I suppose this isn't a huge issue. I'm just more familiar with a dial, and would like to outfit this machine with one.

Doc.
 
I always find it helpful to think of the threading dia s being a very coarse dial type vernier. So if the screw is stationary it measures the saddle movement along the bed. If its rotating a correction is applied for the thread pitch so the same position on the dial effectively gives the same distance from the start of the thread for whole turns of the dial. Things may line up for less than a whole turn of the dial. Depends on how many teeth there are on the pinion. Your 36 teeth means 36 turns of the screw per turn of the dial.

My P&W B also has a 6 tpi screw but only 24 teeth on the threading dial pinion as opposed to your 36. The book says :-

1 turn of the dial indicates 4" advance on leadscrew (your 36 teeth give 6")

For even threads that are multiples of 6 :- half nuts may be engaged at any point.

For even threads that are not multiples of 6 :- half nuts may be engaged at any graduation.

For odd threads:- half nuts may be engaged at alternate graduations.

For fractional threads in halves:- half nuts may be engaged at opposite graduations.

For fractional threads in quarters use same graduation each time.

For other fractional threads use of dial is not recommended. Readings being either fractionally more or fractionally less than a full turn of the dial.

The P&W dial is engraved 1 - A - 2 - B - 3 - C - 4 - D. So 1" of leadscrew (or saddle when the screw is stationary) travel between adjacent number or letter pairs and 1/2" between numbers and letters.

I suggest that you follow the same scheme with your dial viz 1 - A - 2 - B - 3 - C - 4 - D - 5 - E - 6 - F

Far as I can see even and odd threads are same as for my P&W.

For 1/4 threads you need the dial to measure 2" of travel so successive engagements of 1 - 3 - 1 or equivalent on the P&W and 1 -3 - 5 - 1 with your dial if you number it as I suggest.

For 1/2 threads you need dial to measure 4" of travel so successive engagements on same number on the P&W and 1 - 5 - 3 - 1 with yours.

For 1/3 threads you need 3" of travel which the P&W can't sensibly handle but you could use 1 - 4 - 1 (I think)

For 2/3 threads you need 6" which again the P&W can't sensibly handle but you could use same number each time.

Realistically 1/4 and 1/2 threads are tricky.

Clive
 
Hi Doc,

I don't see why you can't do 1/2 threads with your dial. You just have to think of the dial as indications of 1 inch. The 6TPI leadscrew causes the dial to travel 1 division per inch. So if you need to do 1/2 threads you have to find where they intersect perfectly with inches. They don't at 1 but do at 2 inches. So that means you can use every other numbered mark on your dial to engage for 1/2 sizes. 1, 3, & 5 or 2,4, & 6.

Edit: For 1/4 sizes, do not use this thread dial. Use a dial with 24T worm wheel, numbered 1-4. Hope this helps.

Best Regards,
Bob
 
Have to respectfully disagree with Clive's recommendation. Your thread dial measures inches traveled for each division. 1/2 threads intersect perfectly at 2 inches. Therefore your dial can be used without weird calculations.

Even or odd threads: engage on any number (On many thread dials there are hash marks between numbers. They would also work for even threads because even threads also intersect at 1/2" - always).

1/2 sizes: engage on every other number.

Edit: 1/4 sizes: do not use this thread dial. Use a dial with 24T worm wheel, numbered 1-4. Hope this helps.

Best Regards,
Bob
 
Hi Clive,

If you have time, put a pencil/sharpy in your tool post and set up your thread dial. I assume your dial has 4 numbers and 4 hash marks. Set up to do a half thread. I think you'll find that you can engage successfully at every other number (1 & 3) or (2 & 4).

I also think that some machine makers did not understand how the thread dial works either. I've seen similar documentation regarding multiples of numbers. That would make sense if your thread dial did not evenly divide into the worm wheel or some weird gearing strategy. Anyway, once I started thinking of the dial as linear measuring device. It became easy to understand for me. Hope this helps.

Best Regards,
Bob
 
Doc,

Clive got me further thinking. It is possible with 1/4 threads for your thread dial to get out of synch at 3 turns (travel would be 18" - not evenly divisible by 4"). So I'll amend my recommendation for you. Do not use this thread dial for 1/4 threads. All others we discussed are fine.

For 1/4 threads, you can use a dial that has 4:1 (24T) worm wheel teeth to lead-screw pitch ratio and numbered 1-4. Then it cannot get out of synch. Hope this helps.

I updated my previous posts to reflect this recommendation.

Best Regards,
Bob
 
I'm not sure it matters what the thread dial does. Rather the 1/2 or 1/4 thread fractions come from your gearset (either loose gears or quick change) Once these are in place, all you need to cut a repeated thread is to engage the lead screw at the SAME number shown on the thread dial for each successive cut, the same number indicating the lead screw is in the same start position.

By examination of the thread dial and its engagement relative to the lead screw contacting gear, you may establish a correlation between "opposite" number positions, or possibly quarter number positions - although the proper engagement may happen at "thirds" or any other repetitive position. This would be a case where the contacting gear is not original to the lead screw and may have a different diametral pitch than the lead screw requires.

But if you engage on that SAME number, the lead screw MUST be in the same relative position.

Of course you could always "leave the lead screw engaged" and back down the lathe carriage with the tool pulled back for the next thread pass without disengagement. In which case you put yourself back to about 1840 in threading technology. But other than the loss of time it is a method to recommend for assurance.

Joe K
 
But if you engage on that SAME number, the lead screw MUST be in the same relative position.
Joe K

Sorry Joe thats not explicitly true although standard imperial practice of defining threads as Turns per Inch and using 1" as the travel between divisions effectively makes it so for whole numbers with any sensible pitch feed-screw and any sensible number of teeth on the dial drive pinion. Fundamentally the system is engineered to make it work for all except edge cases.

The problem occurs when trying to do fractional pitches. The denominator of the fraction has to be a whole number factor of the number of divisions on the dial or, in different terms, a whole number factor of the advance per turn of the dial.

So my P&W has 4" advance per turn and can do threads with 1/2 or 1/4 fractional TPI as both 2 and 4 divide into 4" without remainder. Either by engaging at an appropriate number or by waiting until the same number comes round each time. It can't (simply) do 1/3 or 2/3 because 3 won't divide into 4 without remainder.

Docs Springfield has 6" advance per turn so it can do 1/3, 2/3 and 1/2 fractional TPI as both 3 and 2 divide into 6" inches without remainder. It can't simply do 1/4 as 4 won't divide into 6" without remainder.

If you want to do TPI including all of 1/4, 1/2, 1/3 and 2/3 fractions you need a 12 division dial reading 12" of travel. 72 teeth on the pinion will do it. But it will be slow turning and have a confusing range of divisions between various threads.

You get used to that sort of thing if you have metric machine. My metric Smart & Brown 1024 has a 12 division dial and 4 pinions to cover the metric pitch range. Choice of 1, 3, 4, and 6 intervals between engagement points. I can use it properly but most of the time I get lazy and wait for the same number to come round each time. Usually tweaking the speed so I don't have to wait very long after getting back down the bed and setting the new cut.

Clive
 
24 teeth would be correct for a 6TPI leadscrew tracking 4 inches length, the 32T is correct for an 8TPI screw at that length, I’ve never seen a thread dial track 6” length.

I did 2 vids years back & go through the calculations (fractions) for setting gears or using the dial. I did show one that came off from a P&W 6TPI machine, 2,4 & 8TPI are more common leadscrews IME.


Good luck,
Matt
 
Well, I can certainly make a 24T replacement gear for this thing- as I said, I kind of expected I may have to, since I'm adapting an unknown to an obscure. :D

Wouldn't take anything to reface the dial either- it kind of needs it no matter which way I go.

If I'm understanding you all correctly, it sounds like the 6-mark dial could be used and would be functional, but would be tricky to use or possibly not usable at all, for a few selections like 2-1/4 TPI.

Now, again, not too overly worried about it- first, I think the chances I'll ever have to turn a fractional thread are pretty dang low. :D I think the coarsest I've ever turned was 13 TPI.

And second, the machine has a leadscrew reverse with a single-tooth dog- once I learn how to use that, technically I'll need no dial at all. It's just that I learned with a dial, and for the moment am most comfortable with it- and have two other machines with dials and no LSR.

Doc.
 
The dial goes with the gear, and the major marks are generally inches. So the 24 tooth should go with a dial having 8 marks, 4 major and 4 minor. Then the usual rules for the dial will work.

The major marks will be inches, the minor give half inches. Should do to 1/4 threads.
 








 
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