1. Plastic
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How to measure TPI on lathe spindle. IF measured the same way as acme thread I have a 7-1.5" Do I count the half thread? Thanks

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If the spindle in question is on a Logan-South Bend-Atlas or similar and a 1.5 diameter 99% it is 8 thread as that size is very common on those machines. Get a decent thread gauge solve the whole issue.

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Originally Posted by dceg
How to measure TPI on lathe spindle. IF measured the same way as acme thread I have a 7-1.5" Do I count the half thread? Thanks
I'm clearly missing something since I'm not following your question. There should be a certain number of threads in one inch, most often a whole number but there are occasional outliers like an 11-1/2 threads per inch spec but I can't figure out how many "7-1.5" is. Seven threads per inch is distinctly odd but what's the 1.5 add on?

A complete thread specification will have a nominal outside diameter followed by the threads per inch. I'd be absolutely astonished if your spindle was 7" OD with 1-1/5 threads per inch. 1-1/2 x 8 or one and one half inch outside diameter with 8 threads per inch is fairly common on small lathes.

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Originally Posted by JohnEvans
If the spindle in question is on a Logan-South Bend-Atlas or similar and a 1.5 diameter 99% it is 8 thread as that size is very common on those machines. Get a decent thread gauge solve the whole issue.
Any yardstick will have marks at 1/8 inch intervals, perfectly adequate for determining if the thread is 8 TPI.

I wondered if the OP has a tiny spindle with M7 x 1.5 threads.

Larry
Last edited by L Vanice; 12-02-2019 at 05:21 PM.

5. Diamond
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One can take a paint stick or any piece of wood a little bigger OD than the ID of the lathe spindle hole/thread and with holding that wood from turning, hand turn the spindle so it is drawn into the thread.
One can put a wrench on such a piece of wood and crank it into the lathe spindle thread, perhaps with the lathe in back gear so the spindle does not turn. Good to turn it in for a full inch or more.
Soft wood is best. might put it (wood) in a vise and with saw and file make it the desired size.
About .005 to .010 would be plenty of thread marking to see the number of threads per inch.
Yes a USA machine likely to have a threads per inch. If not a USA lathe then consider metric threads.
With knowing the threads per inch and measuring the hole often it is easy to identify the thread.
some sizes:
https://www.woodnwhimsies.com/media/...te07132012.pdf

Spindle Data Summary

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Possibly the original poster only has 7-1/2 threads on his short spindle. Yes, a thread gauge or a ruler with 1/8" markings would give him the answer.

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Very likely the OP is looking at a female thread. Both a thread gauge and a ruler would be of little use if that was the case.

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9. Need to get you a thread pitch gauge or thread profile thread gauge to check the number of threads per inch. Measuring with your calipers, start at the edge of the tip of the thread and count the number of tips after the first tip, in 1"
Did I explain it a little better? Ken

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I just checked it again. I have 8 thread tips with in 1" span. The 8 th is not entirely there because the threads run out. The diameter is not exactly 1.5" but more like 1.60" with calipers. I'm hoping this is a nominal size 1.5" spindle thread aka 1.5" x 8 TPI. Never know for sure until I buy a back plate.

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If your lathe is on this site Spindle Data Summary you should be good to go with the stated. If you lathe is an Emco, Schaubin or Habagger be careful and double check the thread before spending your dollars on a back plate that might not fit..

Listed on machinest.com for class 2 external for 1 ½-8.

Major diameter: 1.4978 / 1.4828
Pitch diameter: 1.4166 / 1.4093
Minor diameter: 1.3490 / 1.3204
Over wires: 1.5249 / 1.5176 (,072)

Guess I would check it with a 1 3/8 inch wood dowel for an ID thread, if I had no other method.

OD thread: a thread gauge, micrometer, wires check, scale, hold a .125 JoBlock close to look at pitch with a loop, set lathe at 8 TPI thread and run a 60* tool bit missing the diameter and look for following the thread at crest (OD) with a loop, actually any tool bit. (* lathe travel going away from hitting the spindle face.)

You could state your location in Iowa and perhaps a PM guy might let you try his back plate or chuck.
I have a 1 1/2 -8 chuck loose but it would not be worth shipping just to try.

Qt:[diameter is not exactly 1.5" but more like 1.60" with calipers.] *That is very odd.
Sears 1 1/2-8 OD is 1.575 inch.
Last edited by michiganbuck; 12-03-2019 at 10:08 AM.

12. Originally Posted by dceg
I'm hoping this is a nominal size 1.5" spindle thread aka 1.5" x 8 TPI. Never know for sure until I buy a back plate.
Don't buy anything until you KNOW. Plenty of lathes out there working still that the last time you could BUY something that would fit was too far back to remember

That is when those machinist folks step in and MAKE something to fit. I think we have some of those that frequent this forum.

Thumbnail is a simple cheap way to KNOW FOR CERTAIN about pitch (another you can't "buy" for - 3 1/2 - 4 on very old Leblond Heavy Duty)

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Originally Posted by johnoder
Don't buy anything until you KNOW. Plenty of lathes out there working still that the last time you could BUY something that would fit was too far back to remember

That is when those machinist folks step in and MAKE something to fit. I think we have some of those that frequent this forum.

Thumbnail is a simple cheap way to KNOW FOR CERTAIN about pitch (another you can't "buy" for - 3 1/2 - 4 on very old Leblond Heavy Duty)
That is worth posting twice.
Not the best test because it is a grade 2 but you might try a \$6.00 nut first.
1 1/2-8 A194 2H Heavy Hex Nut Plain – FMW Fasteners
Yes, buy local to avoid shipping.

14. Comes under the category of general threading info.

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17. Diamond
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Still the OD measure being 1.600 seems odd.

The nut try is an OK plan but if tight don’t force it on.
The thread gauge will/can tell if the thread pitch is an 8 tpi
The wire check with wires and a two-inch micrometer or a good caliper to measure will/can tell that is correct.
The crest (OD) should have about a .022 flat as the nut may be made for a correct thread and so not go on.

With the thread gauge and the wires checking good and the nut to tight you may need file or hone the crest to be about .020 flat.

*If you don't know how to wire check let us know what size (3) wires you have and we can give you the measure size it should be.

If not having a thread gauge you might use a .125 jo block and with looking with a loop see that from crest center to crest center, or from the thread edge to the next same thread edge the pitch shows .125. (yes the thread pitch gauge is better)

Re: 1/8 = .125" So 8 TPI should be .125"

There was a 1 1/2 - 6, a 1 1/2 -10 and a a 1 1/2 -12 thread but don't know if anyone used these for a spindle nose thread...and it is possible a metric or a oddball thread may have been used.

From one reference of the HL Shepard lathe company (a different lathe company.)Not likely your lathe.
QT:The H L ... [Of straightforward design, the headstock held a high-carbon steel spindle (with a 1.5" x 10 t.p.i nose a slender 11/16" bore) that ran in white bronze bearings of "large lead capacity".
Yes a 10 TPI thread would have .100 crest to crest measure.

Also the lathe was shipped over seas so a metric spindle nose may have been special ordered (Not very likely ?).

Note there are metric lathe sizes very close to 1 1/2-8
Last edited by michiganbuck; 12-05-2019 at 09:36 AM.

18. Diamond
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The HL Shepard lathe
H.L.Shepard "Sterling" Lathe

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Originally Posted by dceg
7 TPI 1.5" is the TPI 1.5" is the diameter. I'm looking for a chuck backing plate. My search found acme threads to start measuring at the top 1/2 thread and measure to top of 1/2 thread within 1". Another words you count full threads in between and two half threads count as one. an acme thread of 8 TPI has 7 FULL threads and 2 HALF threads. IF this is the same way to measure spindle threads then I have a 7 thread count. ODD yes.
This is overly confusing. One thread (pitch) is from one peak to the next. If it's a flat-topped thread like ACME measure from one edge of a top to the same edge of the next.
Count as many full peaks as you have in an inch for the TPI just using a small rule. If you don't have an inch of threads try half an inch. If it doesn't line up exactly measure across several peaks in thou and divide the measurement by the number of spans. To get the TPI divide 1000 by the result of that division.

For example if it's 7tpi then from peak 1 to peak 4 would measure 429 thou.

Or use a thread pitch gauge.