Threading dial question

[SmoggyTurnip]

Yours is definitely an Imperial lathe?
What is the pitch of the leadscrew?
Does your lathe manual have anything to say about using the threading dial?

Yep it is imperial - no question about that.
The leadscrew is 12 threads per inch.
The only thing the manual says about threading is how to change the gears.

 

[GailInNM]

Is it possible that the thread dial is not fully engaged? On the old Southbends the thread dial could be pivoted to disengage from the lead screw to save wear and tear on both the lead screw and the dial gear. A bolt was loosened to move the dial ant then tightened after engagement When engaging the gear you had to make sure that the gear was fully engaged or this sort of funny thing could happen. I don't know if yours is built the same way or not.
Gail in NM,USA

 

[Lew Hartswick]

I'm curious. If you (with the lathe stoped) move the carriage exactly 1 inch, how much does the
threading indicator wheel turn?
...lew...

 

[mklotz]

Lew,

He's already said that one dial revolution corresponds to a measured travel of four inches.

So, one inch of travel should correspond to three marks on the dial.

That's what is hard to understand. I can't see why anyone would want such an unusual relationship on an Imperial lathe. Where's the benefit? Four inch travel per revolution is normal for most thread dials but they're usually marked with only eight (four numbered and four unnumbered) lines.

 

[GailInNM]

Marv,
I think it has a 12 pitch lead screw, therefore 12 numbers on the dial. Look a the lead screw in the photos. I think this was done so a lower gear ratio could be used in the change gears and therefore make the gears smaller in diameter on light duty lathes. The maker probably assumed that no one would try to cut a coarse pitch thread with a smaller lathe, so the gears would not get overstressed.
Gail in NM,USA

Edit: I missed that he stated that it has a 12 pitch lead screw.

 

[GailInNM]

Looking at photo 2, I don't think you are missing by one tooth. I think all you are seeing is backlash in the thread dial gear to the lead screw and maybe some contributed to the backlash in the half nuts. One index line on the thread dial will equal one tooth on the thread dial gear. If you disengage the half nut and then move the carriage while putting a little pressure on the halfnut engaging lever, you will see the dial move one division before it will drop in. If you do the same thing in moving the carriage in the other direction you will see the same thing, but because of backlash in the system the indexes will not line up perfectly with the pointer. Same sort of thing that happens with the cross slide lead screw where the knob has to be turned a bit before the slide moves when reversing the travel direction.

Engage the half nuts, and turn the spindle forward a couple of turns and note the thread dial position. Now without changing anything, rotate the spindle in reverse a few turns. The dial will have changed because of the backlash, but the half nuts are still engaged in the same thread on the lead screw.
Gail in NM,USA

 

[SmoggyTurnip]

Looking at photo 2, I don't think you are missing by one tooth. I think all you are seeing is backlash in the thread dial gear to the lead screw and maybe some contributed to the backlash in the half nuts. One index line on the thread dial will equal one tooth on the thread dial gear.

Nope - there is 48 teeth on the dial gear so 1 tooth is 1/48 of a revolution or 1/4 of a mark.

Last night I actually used the threading dial successfully for the first time making 8 threads per inch.

At first I just stoped the motor after I returned the carrage and then aligned the dial to 1 by moving the carrage while the motor was stoped. Once it was aligned I started the motor and it cut in sync.

After I gained some confidence with that I started engaging the half nuts with the motor running - it wasn't half as bad as I thought it would be. I never missed once. Tonight I will try it again using 1, 4, 7, 10 and see what happens.

On the the lathe there is a table that gives the numbers that can be used for different pitches.

For 9, 12, 18, 24, 36 48, and 72 tpi you can only use the number 1.
for 9.5, 11.5, 13.5 tpi you can use 1 or 7
All other threads you can use 1, 4, 7, or 10.

 

[SmoggyTurnip]

Is it possible that the thread dial is not fully engaged? On the old Southbends the thread dial could be pivoted to disengage from the lead screw to save wear and tear on both the lead screw and the dial gear. A bolt was loosened to move the dial ant then tightened after engagement When engaging the gear you had to make sure that the gear was fully engaged or this sort of funny thing could happen. I don't know if yours is built the same way or not.
Gail in NM,USA

Miine is the same way but it is defininitly engaged all the way - I checked this several times.

 

[SmoggyTurnip]

I'm curious. If you (with the lathe stoped) move the carriage exactly 1 inch, how much does the
threading indicator wheel turn?
...lew...

3 divisions

 

[SmoggyTurnip]

Lew,

He's already said that one dial revolution corresponds to a measured travel of four inches.

So, one inch of travel should correspond to three marks on the dial.

That's what is hard to understand. I can't see why anyone would want such an unusual relationship on an Imperial lathe. Where's the benefit? Four inch travel per revolution is normal for most thread dials but they're usually marked with only eight (four numbered and four unnumbered) lines.

I was thinking about changing the faceplate on the dial. - As you can see ini the picture it is only held on by a Phillips head screw.

 

[SmoggyTurnip]

So what would this table:

For 9, 12, 18, 24, 36 48, and 72 tpi you can only use the number 1.
for 9.5, 11.5, 13.5 tpi you can use 1 or 7
All other threads you can use 1, 4, 7, or 10.

look like if I used a dial with 4 marks with lines inbetween?

 

[Lew Hartswick]

>For 9, 12, 18, 24, 36 48, and 72 tpi you can only use the number 1.
for 9.5, 11.5, 13.5 tpi you can use 1 or 7
All other threads you can use 1, 4, 7, or 10.<

If you can never use 2,3,5,6,8,9,11,12 Then what use are they? Make teh dew dial face.
...lew...

 

[mklotz]

For 9, 12, 18, 24, 36 48, and 72 tpi you can only use the number 1.
for 9.5, 11.5, 13.5 tpi you can use 1 or 7
All other threads you can use 1, 4, 7, or 10.

1,4,7,10 correspond to carriage movements of one inch. Since 9,12,18,24,36,72 tpi threads (and all other integer tpi threads) repeat every inch, I see no reason why they advise you to use only 1. Furthermore, 12 tpi corresponds to the pitch of the leadscrew. When cutting a thread with the same pitch as the leadscrew, you can engage the halfnuts anywhere.

[Aside: You can verify this by cutting a "scratch" 18 tpi thread, engaging at, succesively, 1,4,7,10. The thread should track on all four engagement points.]

The half threads (9.5, 11.5, 13.5) repeat every two inches so the advice to use 1 or 7 (carriage movement of 2") seems correct.

Like Lew, I still don't see why they have twelve marks. Since one revolution of the dial is four inches, I can see no reason to not just have the traditional eight subdivisions, used as indicated in my writeup. What is gained by having the extra marks?

Idle speculation here... The Chinese reverse engineers saw that there were 8 marks on the dial on a 8 tpi leadscrew lathe so they figured that a 12 tpi leadscrew needed 12 marks.

 

[NickG]

This can be a disadvantage with some of the modern lathes .... they have no back gear, so it's difficult to get a low enough speed for screwcutting, especially with variable speed drives. Also, unless it's got a whopping motor, they will struggle to get the torque required to screwcut harder materials, or larger diameters. You may be able to get away with taking very small cuts though. I nearly bought a C4 and know they have a powerful motor though so you should do OK with that particular lathe. My old Harrison can go down to 30rpm which is one of its advantages.

I tried (for the first time) cutting a screw thread on my lathe for the hot cap on my stirling engine. The two parts sort of screw together but not very well, and at the dimensions I originally went to, they were nowhere near assembling! It seemed like I had to take miles more off the female thread than I originally planned, but I think I messed up re-engaging the lead screw in the wrong place or something.

Was just trying to remember how I did it and to be honest, I can't remember! I think I just ground the tool to as near as I could 60 deg, with a thread gauge I think (this was probably quite bad). Have just worked out that for a flank angle of 60 deg and a 32tpi thread, my depth of cut should have been about 0.0135 to leave the 5/8" major diameter. Can anyone tell me if that is right?! Or does anybody have any information on M.E. threads, major and minor diameters etc, what clearance there should be between male and female for the correct fit?

Think I need to practice as it would be very useful to be able to cut threads properly to the size I want. It keeps everything nice and concentric and straight doing them in the lathe. Need to make use of my screwcutting gearbox too!

Thanks,

Nick

 

[mklotz]

Nick,

Download my DOT (Depth Of Thread) program and it will tell you more than you ever wanted to know.

 

[kvom]

For a 32 tpi 60 degree thread:

pitch is 1/32 or .031"
slant depth = .75 x pitch = .023"

The slant depth is the amount to advance the compound when it is set to 29.5 degrees.

 

[NickG]

Cheers Marv & Kvom

 

[NickG]

marv, just tried the program, something flashes up very quickly (milli seconds!) but the screen goes before I can read it! What am I doing wrong?!

 

[mklotz]

Read the very first line in the Introduction on my page.

 

[NickG]

thanks