I have a 1991 Shoptask 1720 XTMC manual lathe. I am trying to thread and I am finding that my tpi is coming out different than the chart when I set up my gear combination according to the chart. For instance; I am trying to thread 24 tpi, so I set up my gears A=60, B=36, C=50, D=27. With this setup I should get 24 tpi, but after cutting I measure about 19 tpi. I tried changing B to 39 which shows 26 tpi on the chart and it results in 20 tpi on the material.

After this, I decided to run the math on the gear ratios. If I start at the spindle and record all the gear teeth I get:
25
50
A
B
C
D
27
30

This gives me the following ratios:
25/50
A/B
C/D
27/30
Which then becomes
1/2 x A/B x C/D x .9

If I want to achieve 24 tpi, that comes out to a distance on the X-axis of 1/24 = 0.041667 which would be a .41667 turn on my X-axis handle.

If I am set to course threading, the first gear out of my spindle turns 19.25 teeth for 1 revolution of the chuck, this gives me 9.625 teeth for a fine thread setting (double that of a course), which then equates to a rotation on the gear of 38.5% around for 1 full rotation of the chuck.

If I set C and D to the chart, so 50 and 27 respectively, then the only unknown I have is the ratio of A/B. Leaving A=60, the math comes out that B must be a 46 tooth gear. The chart shows that it should be 36, not 46. To check my math, I ran the calculation for 24 tpi using the gear list on the chart and I get approximately 19 tpi as my answer, which matches the tpi that I am getting on my material when I perform the actual cut.

So my question is if anybody else has had this issue and would know what I am doing wrong? Or, can somebody with a Shoptask 1720 XTMC check if you turn 19.25 teeth on the first gear out of the spindle box for 1 full rotation of the chuck? I bought this used and can't rule out the possibility that the previous owner may have replaced the gear inside with an incorrect aftermarket gear. Outside of that, I am unsure why my threading doesn't line up with the chart. I have looked at other pictures online and counted gear teeth and it appears the ones I can see and count in my gear box are correct.

There were a number of customers who modified those early machines with a Forward-Reverse shifter before it became a standard feature. It's possible you have a machine with a DIY gear set that changed the ratios. Post a picture of your gear train. In the meantime here is a file with some good information on the early units.

https://www.shopmasterusa.com/wp-con...-1991-2008.pdf

3. I do have one with a custom forward/reverse plate. However this is a single gear between the last 2 set of 27 and 30, so mathematically it shouldn't change the final gear ratio of 27/30. I have the original plate as well, but it looks like the previous owner may have used those gears on this plate.

Maybe I'm not understanding fully if this is what you're referring to. The custom plate is the aluminum one here in one of the attached pics.

Also, I do have the course thread option and there are 2 additional small gears that I got with it that marked with tape as "not sure what these are for".

4. Attaching a close up of the shifter plate mounted on the machine.

The gears all look correct- I see you have the coarse thread kit as well. Be sure you have them in the correct order- A meshes with B and C meshes with D- B and C are together on a common shaft.

6. Originally Posted by JTJT
The gears all look correct- I see you have the coarse thread kit as well. Be sure you have them in the correct order- A meshes with B and C meshes with D- B and C are together on a common shaft.
Yes, I have them set up that way. A and B are towards you when looking into the gear box, and C and D are towards the lathe bed. All in order top left to bottom right.

Any other ideas of why this would not produce the correct tpi, or things I could try?

Is it possible that somebody could have taken apart the headstock and put the duplex gear or triple gear in wrong, causing the incorrect rotation coming out of the headstock?

7. Adding another picture that shows where I have A, B, C, D gears.
A=60
B=36
C=50
D=27

Originally Posted by buildabikews
Adding another picture that shows where I have A, B, C, D gears.
A=60
B=36
C=50
D=27
OK- I can see it now- you have a very rare early machine which had fine tooth gears in the gearbox and also on the output- The very first gear coming out of the head casting is normally a coarse tooth, but a few early production units had fine tooth gears, so the primary drive ratio was different. Those machines came with a small white manual, about 5" X 7" , if you are getting your charts from one of the later yellow manuals, they will not be correct. Because there were only about 50 of those machines out of over 12000 sold, I'm not sure if we even have one of those original manuals. The only solution may be to mathematically calculate the correct gear set.

9. Consider me a lucky man, well, at least partially lucky as I do still have a gear ratio issue. Attaching pics of my small white manual, and this machine also has a chart plaque on the machine that matches the manual. I had considered just calculating the gears I would need for each thread pitch, but the issue I have is that they end up being gear combos that I don't have, or worse, can't purchase.

I did notice in some other pics people put online that the gear coming out of the headstock is more course and has around 18 teeth. You mentioning that this machine was one of a few that had a fine tooth output made me think about the fact that the ST86 shift lever attachment that was added probably has a gear ratio set up for a machine with a course thread gear at the headstock and that may be incorrect for this machine.

My lead screw has a course 30 tooth gear and connects to the gear arm via a course 27 tooth gear (you can see this in the recent pic). Did this "rare" early build have all fine tooth gears and the course one that are in it now would be from the aftermarket purchase? Remember earlier in my post that I said I have a couple of gears with tape on them saying "not sure what these are for"? Both of those are similar in size to the forward/reverse gears on the new plate, but they are fine threads, not course. Could those be the originals that came with the machine? Did the lead screw gear have fine threads as well, the one I have currently is course and wouldn't match up with those fine gears.

If I recalculate the gear ratios, replacing this course 27 tooth gear with a 21 tooth will give me the proper ratio. If Shopmaster sells this, then that would solve my problem.

Originally Posted by buildabikews
Consider me a lucky man, well, at least partially lucky as I do still have a gear ratio issue. Attaching pics of my small white manual, and this machine also has a chart plaque on the machine that matches the manual. I had considered just calculating the gears I would need for each thread pitch, but the issue I have is that they end up being gear combos that I don't have, or worse, can't purchase.

I did notice in some other pics people put online that the gear coming out of the headstock is more course and has around 18 teeth. You mentioning that this machine was one of a few that had a fine tooth output made me think about the fact that the ST86 shift lever attachment that was added probably has a gear ratio set up for a machine with a course thread gear at the headstock and that may be incorrect for this machine.

My lead screw has a course 30 tooth gear and connects to the gear arm via a course 27 tooth gear (you can see this in the recent pic). Did this "rare" early build have all fine tooth gears and the course one that are in it now would be from the aftermarket purchase? Remember earlier in my post that I said I have a couple of gears with tape on them saying "not sure what these are for"? Both of those are similar in size to the forward/reverse gears on the new plate, but they are fine threads, not course. Could those be the originals that came with the machine? Did the lead screw gear have fine threads as well, the one I have currently is course and wouldn't match up with those fine gears.

If I recalculate the gear ratios, replacing this course 27 tooth gear with a 21 tooth will give me the proper ratio. If Shopmaster sells this, then that would solve my problem.
Yes, the original owner must have changed the final 2 gears to coarse threads, and that is the ratio problem. In those early machines the dual gear on the leadscrew had a fine tooth gear and the timing belt pulley together in a single unit. Maybe the original owner still has these gears somewhere. If you have the 2 fine tooth idlers, you will need a third for the set, and I found one in the parts bin. I also found 2 change gears, 24 and 35 tooth which were part of the original set as well. Give me a call at 800 343 5775 and we may be able to get it sorted out.

11. Can you give me the name of the company again that I can contact about a 21 tooth Mod 1.5 gear?

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