[How do I?] Need help figuring out my thread chasing dial

[eric.grulke.79]

I want to cut an 11 1/2 TPI thread for a pipe fitting but my thread chasing dial has me bamboozled.

It comes with two different sized drive gears, 14 and 24 tooth, with corresponding reference dials.

The body of it has a label telling you which Setup to use for the TPI you want to cut.

Neither indicator is divided into 1/4’s so I’m kinda confused.

Has anyone used this type before or have any suggestions on how to use it.

The lead screw is 4 TPI.

I’ll post pics.

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[eric.grulke.79]

Here are some more pics

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[benmychree]

With a quarter thread, you probably will have to close the half nut in one place only on the dial, that is generally how it works on any lathe with an english lead screw such as 4 TPI. I posted an article way in the past regarding the subject and also on metric transposing gearing.
You should just chuck up a piece of stock and take a trace with the thread tool and see if it tracks the same trace when you go to the same number as I suggest.
Generally, the answer is:
thread is a factor of the lead screw (4, 8, 16, 32,64 ---) close half nuts anywhere
thread is an even number; close on any line
thread is an odd number, close on any number
thread is a half number, close on opposite numbers on the dial
quarter threads, close on one number only
This is for the common dial with 4 numbers and 4 lines (8 lines, 4 with numbers)
The numbers on this style of thread dial each indicate travel of one inch on the lead screw,#1 - #4 = 4" of travel

 

[eric.grulke.79]

Thanks Ben, do you have any insight into the style of dials that came with my lathe?


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[T Bredehoft]

Better than measuring the trace of a tool, set up an indicator and manually turn the chuck one revolution by hand with the indicator set at zero to start. One rev will move the indicator one thread, 8 pitch = .125, 24 pitch = .041, etc.

 

[benmychree]

Better than measuring the trace of a tool, set up an indicator and manually turn the chuck one revolution by hand with the indicator set at zero to start. One rev will move the indicator one thread, 8 pitch = .125, 24 pitch = .041, etc.

That is fine for determining the lead, but my suggestion was along the lines of proving that using a particular line on the dial will result in the tool tracking back in the same cut with each subsequent pass to finish the thread.

 

[benmychree]

Thanks Ben, do you have any insight into the style of dials that came with my lathe?


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Insight into the thread dial for your lathe is that it seems pretty bizarre, at least to me; the only times that I have seen thread dials with more than one gear is on machines that have metric lead screws, are you sure that yours is imperial? It might be well to try to find an online manual for your particular machine to make some sense of the situation. My lathe, a 19" Regal Leblond has a 16 tooth gear on the thread dial, and a 4TPI lead screw, and gearing that is typical to most all american built machines, except I modified it to allow metric threading and special and diametral pitch leads and threads also.

 

[Winegrower]

John, could you explain a bit about your modifications for extra functions? Sounds interesting.

 

[eric.grulke.79]

Hey John, I measured the lead screw, 4 TPI, it’s an older Magna-Cut lathe, Chinese manufacture, I have the manual but it doesn’t indicate how this dual is used and the grammar/translation is terrible.

Perhaps I’ll just flip one of the discs and mark it myself into 1/4’s.

Eric


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[benmychree]

All that I did for the conversion was to make a 127 tooth gear and fasten it together with a 120 tooth gear in place of the original idler gear on the end train, to supplement this, I made a slew of change gears that fit either on the stud gear or the change box gear; I was lucky to find a chart from a Leblond lathe of the heavy duty line that showed it all, most all lathes have about the same series of gearing on the lead screw train. One interesting thing I found is that say, if you have a 32 tooth gear on the box and need to cut 27 (which most lathes cannot normally cut) you just set the QC box for 32, and substitute a 27 tooth change gear for the 32 on the box., same for any other odd thread.