The metric thread dial is intended for use with a metric lead screw, not transposing gears.

Conventional wisdom is that the normal threading dial is not useful with the transposing gears, no doubt why you are interested in the metric thread dial.

I have read a contrary opinion, that it will work if you reposition the carriage the same place for every pass (perhaps a carriage stop to the right of the carriage? might have to go on the rear v-way.) AND start on the same dial mark each time. It sounds reasonable to me.

I have been wanting to try this for some time, but haven't done it yet. Next best thing for me would be for you to try it.

I don't think I have the right words to explain.

Too long, don't read warning....

The thread dial indicates the phase of the cutter with respect to a thread being cut.

With the half nuts unlocked moving the carriage or rotating the spindle changes the phase.

With the inch lead screw cutting inch threads the spindle the ratio between the spindle and the lead screw is simple. More importantly, you can lock the half nuts anywhere the appropriate mark lines up and be in sync with the thread you are cutting. About every 1/8th inch or so. I think. Need to check. In any event -- places you can lock the half nuts are separated by a fraction of a lead screw turn. HTRL has some notes on which marks you can use for which threads per inch (multiples of 2, 4 or 8tpi make a difference). The marks where you can engage the half nuts are the ones where the spindle has rotated back around to where you can pick up the thread.

Add the metric transposing gears, or one of the approximations, and the ratios get really messy. So messy that there is no simple, usable, relationship between the phase of the spindle and the thread dial indication. Spindle rotations don't divide evenly into lead screw rotations.

There are three solutions to cutting metric gears that I have heard of:

1) Don't disengage the half nuts while cutting threads. When done with a pass run the lathe backwards to return the carriage to make another pass. This is the most commonly suggested method. Somebody is sure to pipe up that they prefer doing this with inch threads even if they have a thread dial. This definitely works.

2) I have heard (but not tried) that starting with the carriage at the same position (perhaps using a carriage stop to the right of the carriage, or the tail stock as a carriage stop) AND always using the same mark on the dial (perhaps the 0 mark) will work. This makes sense to me mathematically - I won't bore you with the details)

3) I have also heard (but not tried) of a method where you turn off the lathe at the end of a pass, note the thread dial position, disengage the half nuts, move the carriage right to a position where the thread dial is the same as where it was when the half nuts were released and engage there. Then turn the lathe back on. I think there may be more to it than that -- it seems at least semi plausible to me.

The safe thing is to go with #1.

I'm amazed if anybody has managed to follow all of this. The math of this fascinates me, but if I start blathering about Stern-Brocot trees, prime factors, smooth numbers, least common multiples and largest common factors I'll deserve to be banned from this list.

I don't think I have the right words to explain.

Too long, don't read warning....

The thread dial indicates the phase of the cutter with respect to a thread being cut.

With the half nuts unlocked moving the carriage or rotating the spindle changes the phase.

With the inch lead screw cutting inch threads the spindle the ratio between the spindle and the lead screw is simple. More importantly, you can lock the half nuts anywhere the appropriate mark lines up and be in sync with the thread you are cutting. About every 1/8th inch or so. I think. Need to check. In any event -- places you can lock the half nuts are separated by a fraction of a lead screw turn. HTRL has some notes on which marks you can use for which threads per inch (multiples of 2, 4 or 8tpi make a difference). The marks where you can engage the half nuts are the ones where the spindle has rotated back around to where you can pick up the thread.

Add the metric transposing gears, or one of the approximations, and the ratios get really messy. So messy that there is no simple, usable, relationship between the phase of the spindle and the thread dial indication. Spindle rotations don't divide evenly into lead screw rotations.

There are three solutions to cutting metric gears that I have heard of:

1) Don't disengage the half nuts while cutting threads. When done with a pass run the lathe backwards to return the carriage to make another pass. This is the most commonly suggested method. Somebody is sure to pipe up that they prefer doing this with inch threads even if they have a thread dial. This definitely works.

2) I have heard (but not tried) that starting with the carriage at the same position (perhaps using a carriage stop to the right of the carriage, or the tail stock as a carriage stop) AND always using the same mark on the dial (perhaps the 0 mark) will work. This makes sense to me mathematically - I won't bore you with the details)

3) I have also heard (but not tried) of a method where you turn off the lathe at the end of a pass, note the thread dial position, disengage the half nuts, move the carriage right to a position where the thread dial is the same as where it was when the half nuts were released and engage there. Then turn the lathe back on. I think there may be more to it than that -- it seems at least semi plausible to me.

The safe thing is to go with #1.

I'm amazed if anybody has managed to follow all of this. The math of this fascinates me, but if I start blathering about Stern-Brocot trees, prime factors, smooth numbers, least common multiples and largest common factors I'll deserve to be banned from this list.

On 1/3/21 10:56 AM, john kling via groups.io wrote:

Hm I don't feel particularly sharp today and have  not in the past thought about why the thread dial works - But would the imperial still coordinate the position of the carriage and the lead screw in metric operations on the imperial lathe. -if this is completely silly, ignore.

On Sunday, January 3, 2021, 10:08:19 AM EST, Steven H via groups.io <stevesmachining@...> wrote:

I have never heard of a metric thread dial. When cutting metric threads with an imperial lead screw lathe, one normally engages the half nuts and leave the half nuts engaged until you are done cutting the thread. Do not disengage the half nuts. Reverse the lathe to return the carriage to the starting point after backing out the cross slide. 

Steve Haskell

Troy, MI

On Jan 3, 2021, at 9:57 AM, Davis Johnson <davis@...> wrote:



The metric thread dial is intended for use with a metric lead screw, not transposing gears.

Conventional wisdom is that the normal threading dial is not useful with the transposing gears, no doubt why you are interested in the metric thread dial.

I have read a contrary opinion, that it will work if you reposition the carriage the same place for every pass (perhaps a carriage stop to the right of the carriage? might have to go on the rear v-way.) AND start on the same dial mark each time. It sounds reasonable to me.

I have been wanting to try this for some time, but haven't done it yet. Next best thing for me would be for you to try it.

On 1/3/21 8:58 AM, Al Costich wrote:

For some silly reason, I am in the hunt for a metric thread dial for a 9A.
Some time ago I acquired a 127/100 transposing gear set and some additional
gears.
Thinking forward, i would like to single point some metric bolts.
I'm not confident enough to make a thread dial myself.
So, should anyone have one that is superfluous to their needs,
contact me.

Thanks.

Alan

I’ve cut dozens of metric threads on both my Southbend and Atlas lathes. You don’t need metric change gears, metric lead screw or even a thread dial at all.  

Find a gear combination that gives you close to the metric lead you need (.5mm, .75mm, 1mm etc) you can calculate these, but the closest gear combinations are published or there are PC programs that will tell you.

Install the gear combination on your lathe to produce the desired lead (.5mm/revolution etc)

Turn your stock to the correct major diameter

Turn a short section to the minor thread diameter on the left end of the threaded section. You need this to let the lathe stop and not hog into the stock at the end of each cut.

Put the lathe in the slowest belt pulley combination and engage the back gear.  You are looking for the slowest chuck rotation & carriage movement.

Carriage should move toward the headstock when the the chuck  is rotating normally (top coming toward yo)

Adjust compound cross feed to be 29-1/2 degrees (assuming standard 60 degree thread)

Set up a single point thread cutting tool on center, perpendicular to the work piece, with point just touching workpiece. To do this set the MAIN CROSS SLIDE DIAL on Zero first then use the COMPOUND CROSS SLIDE to move the tool in until it just touches the workpiece.

Move carriage past the right end of the workpiece.

Using the COMPOUND CROSS SLIDE handle dial in a small cut (0.001 inch)

close the half nuts and NEVER TOUCH THIS HANDLE AGAIN until you are totally finished

Turn the ELECTRIC POWER SWITCH ON FORWARD to begin forward rotation with cutter moving toward the headstock. 

The cutter should make a light cut and you should see your thread stripe appear.

when the cut is nearing the end and is approaching your minor diameter relief cut, Turn the ELECTRIC POWER SWITCH OFF and let the lathe coast to a stop in the relief area.

Using the MAIN CROSS SLIDE DIAL move the tool away from the workpiece by turning the handle one complete turn from zero, one turn out and back to zero.  You have moved your tool away from the work one turn of the main cross slide

Turn the ELECTRIC POWER SWITCH TO REVERSE.  Allow the tool to move to the right until it is past the right end of the workpiece.  Approximately where you started.

Using the MAIN CROSS SLIDE DIAL move the tool back toward the workpiece by turning the handle one complete turn from zero, one turn IN and back to zero.  You now moved your cross slide back where we started.


Using the COMPOUND CROSS SLIDE dial in the next cut (approximately 0.005)

Make the second cut as before, stopping in the runout area. Move the tool out one turn main compound, reverse motor to beginning, move main compound in, dial in third cut etc. repeat this until your thread is complete. You can continue to cut until you hit your minor diameter.  You can also check fit with a nut or mating piece with part still in the lathe.

leaving the half nuts closed all the time ensured the timing of the gear train is correct. 

Dallas Shell