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Threading questions

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Plastic
Joined
Feb 9, 2003
Location
Penna.
I searched the forums for this but came up empty handed.
I am pretty much self-taught on my machining abilities, and I know and am learning enough to do my basic repair oriented machining. The only thing I really have a question is threading.

using a JET GHB-1340W

its a pretty nice lathe coming from jet. swell for what I do.

I guess my main question is, how do you line your tool up with the next thread cut using a threading dial, which the lathe has. This makes no sense to me, could someone explain how you do this to me? Thanks

chris
 
I guess my main question is, how do you line your tool up with the next thread cut using a threading dial, which the lathe has. This makes no sense to me, could someone explain how you do this to me? Thanks
chris
Rather than try to explain it I have attached an explanation from Lathe Operation and Machinists Tables.....pg

ThreadingDial.jpg
 
I'm just starting to cut threads as well. Can someone tell me, how close to the marks on the threading dial do you need to be when engaging the half nut lever. I can't engage the lever at exactly where the mark is on the threading dial. It just won't let me as the threads on the lead screw and half nut are not yet fully alligned. If you look at fig 180 on the attachment you provided, that illustrates almost exactly when I am able to engage.
 
If you look at fig 180 on the attachment you provided, that illustrates almost exactly when I am able to engage.

The witness mark and the dial mark should be closer together than what is shown on fig 180.
As the dial mark comes around to the witness mark, engage the half-nut just as they are almost lined up. That should line them up almost perfectly.
Question: You do have your lathe in back gear when you are threading, don't you?.........pg
 
Sorry, I'm not sure I know what you mean in reference to having the lathe in back gear. I do have the lead screw engaged and when the thread dial approaches the stationary mark I try to engage the half nut lever but it doesn't engage fully until its a little past the mark. It happens every time so at least it's consistent . Maybe its not calibrated exactly and I should change the witness mark? :confused:
 
Sorry, I'm not sure I know what you mean in reference to having the lathe in back gear. I do have the lead screw engaged and when the thread dial approaches the stationary mark I try to engage the half nut lever but it doesn't engage fully until its a little past the mark. It happens every time so at least it's consistent . Maybe its not calibrated exactly and I should change the witness mark? :confused:
The back gear should be used to turn threads on the lathe. What this does is to slow down the lathe rpm and increase torque at the same time. The big thing is the slower rpm.
First, what brand lathe are you using?
Depending on the brand, putting it in back gear should be very simple.
Back gear for a 10 inch South Bend is done like this:
1) you raise the cover over the spindle.
2) pull out the plunger, located on the side of the bull gear.
3) and pull the back gear lever towards the front of the lathe to engage the back gear.
Your lathe now turns very slowly and you should be able to easily time the half nut engagement with the indicator marks lining up. Let me know if you have any questions.......pg
 
Last edited:
MrAR15,
I would look for a way to center the witness marks, in one of two possible methods:
Loosen a nut which holds the gear or dial to its shaft, line things up and retighten,
Or,
If the thread dial is mounted on a little stud on the side of the apron, you can find some way to loosen it and slide it a wee bit along the stud, or perhaps put a thin shim between its housing and the abutment/boss where the pin or bolt goes into the apron.
 
If it does not line up perfectly to the mark , it is not such a big deal .
I always just find a spot near a line , before, right on or after , it doesnt matter .
Stop the spindle with the split nut still engaged . Then take a felt tip pen and draw a line across the dial and outer ring . As long as you always engage it at the same mark , it will always pick up the thread . You will get proficient at leading in to the mark as it comes around . Not such a big deal to wait for it to come around to the same mark .
Considering how much time you may put into a part before you do a final thread , why would you be in such a hurry to mess it up now ?
I have had to cut threads on old lathes that did not even have a chaser dial, just an odd gear with a pitch to mesh with the leadscrew , and a shop built mount with a wire pointer welded to a bolt .
( see also the discussion on threading problems )
 








 
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