# Thread: Threading confusion. A bit long.

1. Senior Member
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## Threading confusion. A bit long.

Hi guys:

I've managed to cut 4 nice threads with my lathe. The metric one required reversing the motor and keeping the half nut engaged but the rest were all cut the easy way, half nut disengaged, carriage rolled back, re-engage half nut for next pass.
I have been able to engage on any of the 4 marks on the thread wheel and have cut 13, 20, and 32 TPI.

For each new TPI I cut, I have to assume it's necessary to engage on a particular mark, they are numbered 1-4 so there must be a reason. Makes for some time wasting and frustration.

I've tried to work out the math but I'm not getting anywhere.

Some particulars:
For one chuck rotation the 1st change gear shaft rotates 90 degrees.
The lead screw is an oddball 7 TPI.
28 rotations of the leadscrew turn the thread wheel 360 degrees.
I think my thread constant is .03571428571 which I got by .25*(1/7).

Is there an easy way to determine which TPI can be engaged on any mark?
Since (1/thread constant) = 28 does that mean I can cut 14, 28, and 56 TPI by engaging anywhere without regards to the thread wheel?

I'm pretty sure I'm missing something very simple and I'd like to know what it is!

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On most lathes you would use any number for even threads. Example: 12,16,18,20 threads,you could use any of the numbers on the dial, 1 tru 4. You would use odd numbers for the uneven threads. Example: 11,13,27 threads you would use eithe 1 or 3 on the dial.

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If you are doing TPI you can start on any mark but then
each pass has to be on the same mark.

If it's a multiple of 4 you can start on any line for each pass.
It doesn't have to be the same line for the passes.

Multiple of two but not four can be on 1-2-3-4

Odd numbers can be on 1 or 3

I've only threaded on 3 lathes but that's how they all worked.

I think this is good for a TPI lead screw
Last edited by Blueskys; 01-10-2010 at 12:11 PM.

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First, a 7 TPI lead screw is really oddball. REALLY! REALLY, REALLY ODDBALL! I have to really seriously doubt that any manufacturer would use such a thread.

The first thing you should do is remeasure this very vital spec. The best way to do this would be to just lay a ruller along the screw and count threads. One inch would do, but it would be best to count them over a longer distance, like 2 or even 4 inches and divide the full count by that number to get the actual TPI. I suspect that you will have either 6 or 8 TPI. 8 TPI is probably the most common on small lathes, but there are many 6s out there also.

The idea of a thread dial is based on the distance over which the spindle and the lead screw will return to the same rotational relationship - some say the distance over which they will synchronize. This distance is the amount of travel that the lead screw will move the carriage when it is engaged. It is also the distance that the thread you are cutting would move a matching nut that is not rotating, while the thread (the spindle) rotates. It is plain to see, that if these two distances are the same, then the thread cutting tool would re-engage the thread being cut in exactly the same relationship. This is also true if they are related by whole number multiples of either or both distances. Thus, if the spindle rotates by 2, or 3, or 185 multiples of this distance and the carriage is also at some multiple (usually zero as you return to the start position) then you will get proper engagement.

The markings on a thread indicator dial that is designed for English threads are actually INCHES of travel. Thus, a 8 TPI lead screw with a 32 tooth gear on the thread dial would give you a distance indication of four inches (32 / 8 = 4). Such thread dials are normally marked with four numbers, 1, 2, 3, and 4 and these numbers are inches of travel. They are ususlly divided into halves (half inch increments) withour any numbers on those lines. There could be further divisions, perhaps quarters, etc. but these are less useful and are most often omitted.

How is this used? First please notice that English threads are numbers that work well with a lead screw that has an even number of threads - 8 TPI, 4 TPI, etc.

56 TPI = 7 X 8,
48 TPI = 6 X 8
40 TPI = 5 X 8,
32 TPI = 4 X 8,
24 TPI = 3 X 8,
16 TPI = 2 X 8,
8 TPI = 1 X 8
etc.

All of the above threads could be cut with an 8 TPI or 4 TPI lead screw by engaging at ANY point.

Not all TPI numbers fit in this simple scheme. Other whole numbers, like 18 TPI are used. And there are also fractional threads, like 13.5 TPI.

Remember that the DISTANCES must synchronize. Notice that any whole number of threads WILL synchronize over a distance of one inch. So, with an 8 TPI lead screw and any whole number of threads per inch, you can synchronize at any whole number of inches on the dial. Thus, for 13 or 18 or 45 or any whole number of TPI, you could start the threading at "1" (any numbered line) and re engage the half nut at ANY numbered line on the dial (1, 2, 3, or 4). You could engage at different numbered lines on successive passes. This is because any whole number of TPI will synchronize over a one inch distance.

For fractional TPIs, like 13.5, you must travel two inches to resynchronize. This is due to the fraction. It takes two half threads to equal a full one. So you must travel two inches to get to a whole number of threads. This means that if you are cutting a fractional thread with the 1/2 fraction, you will have to re engage on a number with the same parity (even or odd). Thus, if you start cutting a 13.5 TPI thread on the number 3 then you must re engage on either 3 or 1. If you start on the number 2 then you must re engage on either 2 or 4.

You could extend this to quarter fractions where you would have to use the exact same numbered line every time you re engage.

That's a simple explanatoin. It also illustrates why I strongly suspect that you do not have a 7 TPI lead screw. A 7 TPI lead screw would require that you re engage on the same numbered line for almost every standard English thread that you cut. Only numbers that have a factor of seven would be exempted. 56 TPI would be one such, but none of the examples you gave would be. The fact that you say you were able to cut "13, 20, and 32 TPI" with any number on the dial tells me that you do not have a 7 TPI lead screw. Measure it again.

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Paul:

Thanks very much for your lengthy explanation. I'm going to go through it a few more times but you've already helped - I now know the marks on my thread dial indicate inches.

Some of you guys already beat me up a bit about the 7 TPI but I did as you suggested and re-measured it. I marked out 10" of screw, the rule marks fell precisely on the thread leading edges. I then counted the threads and got 70.

I also recounted the turns of the screw required for each 'inch' mark on the thread dial. = 7
I also measured carriage movement for 7 screw rotations = 1.00"

So, weird as it is, I have a 7 TPI leadscrew and I did cut those threads as reported.

Mike

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That is weird...

7. gnm109 Guest
Never heard of a 7 TPI leadscrew. If it is, it's certainly unique.

(Are you quite certain that you didn't sneeze during your measurement of the leadscrew. )

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Sure it's not a 6tpi and you're counting the crest on both ends of the inch to get 7?

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Post a pic of the leadscrew with a ruler next to it.

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MotorradMike: Any chance we could get a picture with a ruler next to the leadscrew? If anything it would make an intresting historical document. :P

Usally you can successful cut a thread by engageing on the *same* mark for every pass. doesnt matter what mark, just that you remember what one it was.
its being able to engage on other marks besides your origional that math comes into play. (You do that so you don't have to wait for the proper mark to align up, thats all)

On my lathe, every mark is actualy 2 'threads' distance apart and the wheel takes 2 inchs of travel to revolve, so even odd threads like 13.5 can be cut by just using the same mark every time.
Threads that are direct multiples of the leadscrew can be engaged *anywhere* even if there isent a mark (assumeing you have space beween marks where it will engage)

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