Help Me Fix My Thread Dial

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Hukshawn

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i was looking at my thread dial gear. it almost looks like someone made it. you can see scraping marks on the collar above the gear. why someone did this is anyones guess.
i started to figure out what i needed to make a new one.
i decided on 16t as i dont believe a 32t gear will fit within the dial housing.
the gear that is on there is 14t with a diameter of .600". i find this gear quite small and it jumps a bit when the keyway passes by.

so, if i take tubalcains math, i get a pitch of 26. (no idea if acme thread pitches are different... someone can correct me)
doing more math i determined i need a gear of 1.625" with a pitch of 26 with 16 teeth.

im sure i can find a website or an app that will help design a pie chart layout so i can just make this gear manually on the lathe, really. i was looking at a dividing head, and im still considering it, but, seems a bit much. but who knows when the next time ill actually need/want one and one will be available at a good price.
 
Hukshawn said:
......snip.........
so, if i take tubalcains math, i get a pitch of 26. (no idea if acme thread pitches are different... someone can correct me)
doing more math i determined i need a gear of 1.625" with a pitch of 26 with 16 teeth.
.....snip.......
Click to expand...
The "gear" on a thread dial is not technically a gear. (tubalcain does not tell you this because he either does not know or does not want to complicate things for the average person) So when you say a diametrical pitch of 26, this is not totally correct. Also with 16 teeth, the OD is not 1.625" That is way too big! The one on my South Bend lathe is 32 teeth and is only about 1.625" OD. A 16 tooth should be around .812" OD? I'm just guessing here.

Getting back the the thread dial "gear" Most are made to fit the profile of the lead screw. That is, the pressure angle is 14-1/2 degrees to match the flank angles of the lead screw thread provided it is Acme. (There are some odd one's out there, I know) Next, the helix angle of the thread has to be calculated so the teeth on the gear when cut will align with the thread on the lead screw. This is why a true gear will not necessarily work. The "gear" teeth will be tilted at a slight angle that will be equal to the helix angle of the thread. How do you do this in your own shop? It requires a vertical mill and a dividing head or a dividing head that can do 16 divisions. Or any other setup that can give you 16 divisions. The reason I say a vertical mill, is you can tilt the head to the correct helix angle to match the lead screw thread. Set you up a boring bar type of cutter in the spindle and use a tool bit sharpen to a Acme thread for 8 pitch since the lead screw is that.

I'm not going to go into detail on cutting the teeth right now. Have a lot of real work I have to get done today, too. It's also months end for me.
Oh, one last thing, make the gear from a piece of Delrin plastic. I have reasons for this.

Ken
 
Thats a good point on the gear slipping. Might be better using a piece of bronze instead. Mild steel works too. To me, steel against steel in this situation is looking for trouble. But now that I think about it, my SBL thread dial gear is steel. My Sheldon thread dial gear is bronze. Ken
 
The number of teeth on the thread dial gear should be a whole number multiple of the the 8 tpi lead screw pitch. So either 16, 24, or 32.

Before cutting a new gear, take a look at some of the other manufacturers who use 8 tpi lead screws. The lead screw pitch determines the gear pitch. If you can find one with the same diameter lead screw, the gear should work except for a possible difference in shaft size. It's a lot easier to rebore a shaft hole or make a bushing than to cut a gear.

I had seen elsewhere that KM shipped a lot of lathes to Colchester so that might be a place to start.
 
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4gsr said:
Thats a good point on the gear slipping. Might be better using a piece of bronze instead. Mild steel works too. To me, steel against steel in this situation is looking for trouble. But now that I think about it, my SBL thread dial gear is steel. My Sheldon thread dial gear is bronze. Ken
Click to expand...

The half nuts are bronze (or brass?) but the my lead screw is kept well oiled anyways. The steel thread dial gear is only engaged in the lead screw when it is needed for the job. Otherwise it is set off the lead screw and doesn't turn. What concerns me tho is that Shawn's lead screw has a keyway (I think).
 
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The old Atlas that I had, has a full length keyway on the leadscrew.

My current lathe does not have said keyway. My thread dail has 4 major divisions and uses a 32 t "gear".
Pierre
 
Tozguy said:
The half nuts are bronze (or brass?) but the my lead screw is kept well oiled anyways. The thread dial is only engaged in the lead screw when it is needed for the job. Otherwise it is set off the lead screw and doesn't turn. What concerns me tho is that Shawn's lead screw has a keyway (I think).
Click to expand...
The keyway shouldn't matter as long as the gear engagement is sufficient to bridge the gap.
 
pdentrem said:
The old Atlas that I had, has a full length keyway on the leadscrew.

My current lathe does not have said keyway. My thread dail has 4 major divisions and uses a 32 t "gear".
Pierre
Click to expand...

Pierre, does that mean you can close the half nuts on any line for any Imperial pitch?
 
pdentrem said:
The old Atlas that I had, has a full length keyway on the leadscrew.

My current lathe does not have said keyway. My thread dail has 4 major divisions and uses a 32 t "gear".
Pierre
Click to expand...
Pierre,

What is the TPI of your leadscrew?

Tom
 
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