Lead Screw and Half Nut Engagement Points
I think I've thought about this too much and have just confused myself.
For an 8 tpi lead screw, how many possible engagement points are there for the half nuts for each single revolution of the lead screw.
4, or 8? Or 3? Or just 1? (Or some other answer all together?)
Does the tpi of the lead screw make any difference in the number of engagement points?
All lead screws I've ever seen are single-start, so there is only one engagement spot per revolution. The pitch does not matter. That's leadscrew revolution, not lathe spindle rvolution. Perhaps you are thinking of engagement points on the thread dial? That depends on the thread to be cut and the specifics of your lathe.
Ok. That makes sense. And yes, the lead screw is single start.
I don't have a thread gauge (one on order), and no reversing switch on the motor (It's a really old lathe, but a really nice machine) - so leaving the half nuts engaged while threading is impractical.
I guess my question is really about - if the lead screw can only engage with the half nuts at one point, why do you need a thread gauge at all? It must have something to do with the ratio and relationship of the spindle speed to the lead screw as set by the selected lead screw drive gears. So in my case, to cut 8 threads per inch (which is the thread pitch of the lead screw), the spindle and drive screw would turn at the same rotational speed. For other thread pitches, I have to wait for the same multiple of turns, relative to the spindle, to recur before engaging the half nuts. (The common denominator of the spindle rotation to the lead screw rotation.)
Or, have I over thought this again?
Thread dial use
A simple answer is for you to get a copy of 'How to run a lathe". It describes the use of a thread dial.
For a lead screw of 8 thd per inch, there are 8 grip points per inch. When cutting a thread, you must use a grip point that EXACTLY matches the thread you are cutting. For any number of threads per inch to be cut that are an exact multiple of 8, you can use any grip point. For any other pitch, you must use the same grip point as the first cut. Thread dials usually have a gear that engages the lead screw and has a 2 inch circumference and 2 or more marks. This identifies the proper grip point for each new cut.
I hope this long explanation helps.
In all of my past experience cutting threads on a lathe with a 8-pitch lead screw, in my case on a 9" SBL, any multiple of 8 (8-16-32-64-128 etc.), you can engage the half nuts at any point on the thread dial. In fact, you don't need a thread dial for these pitches.
For any even pitches, you can engage the half nuts at any number or half number on the thread dial.
For odd pitches like 13, you can only engage the halfnuts at any full number on the thread dial.
Any half or quarter pitches like 4-1/4, 11-1/2, must be engage at the same number everytime. 11-1/2 pitch, you may be able to engage halfnuts at "1" and "3" or "2" or "4" on the thred dial.
This pretty much applies to 2 and 4 pitch lead screws too.
I've never cut threads on a lathe with a 6 pitch lead screw so don't know if this would apply.
Have a copy of 'How to run a Lathe', it tells the how, but not they why. It just took me some pencil pushing to work out all the ratio's and turn rates.
As with most things it just came down to doing the math. I can engage the lead screw anywhere IF I am willing to determine the rotational offset and compensate with the compound and cross feed, but that's too much math. I just ordered a thread dial which keeps me from having to align marks on the lead screw gears (which works, but is a pain).
Thanks to all. Sometimes it takes a post or two to jump start my brain. Kinda like thinking out loud - it helps!
anyplace you wish
If you have several lathes and don't want to bother with which mark to engage , then do what I do : engage the split nut anyplace you want , then use a sharpie to mark that spot on the dial . Even if it's 1/2 a mark before or after a scribed line , just engage it at the same place every time .
Sure , you may have to wait a few moments for the mark to come back up.
Unless you are in a production setting where every second counts , what's the big deal ?
What if you did 2 hrs machining and screwed up the thread as the last operation ?
How much time have you saved ?
I have a lathe that cuts inch , metric, modular threads . Hows that full/ half/quarter rule work for that ?