Lathe cross threading

[pontiac428]

The spindle and the lead screw are LOCKED into proportion by gears. The lead screw and dial are LOCKED in a 1:1 relationship. All that matters is the engagement of the half nuts must occur at the same carriage position and the same dial position. You see, the other relationships are mechanically inseperable, locked in and fixed. The point is to select the ORIGIN, unitless and dimensionless, where all initial conditions (dial, carriage position) are initiated. The resulting movement is explicitly dependent on initial conditions. Replicate the initial conditions, replicate the movement.

You need to remember that every pattern is n+1. The initial condition is your +1, the iterations are n. You are looking at n using your example. Reorient your logic to establishing an origin, acknowledge the hard-locked relationship of the spindle and lead screw, and reconsider the exercise.

You aren't plunking a needle on a record, you are instead engaging mechanically liked systems. The initial condition, or the origin, is where everything lines up when you start. Repeat that, and the movement will repeat.

 

[Harry Knutz]

Are you SURE you can't disengage the half nuts? I mean, really really sure?

Because if you start at "1" on the dial and return the carriage to the same spot for every chase, you can disengage the nuts and start on 1 every time, regardless of system. You could be cutting threads per furlong or cubit for all I care, because you are establishing the origin (unitless) between the carriage, lead screw, and dial. The origin is the origin is the origin. Anything downstream has to agree with your measuring system, but the origin is fixed. You CAN unclamp, you CAN re-engage, as long as the carriage is in the same spot and the dial reads 1.

We learn rules to threading, but there is no substitute for understanding what you are trying to do.

This was just being discussed on another forum and someone posted a link to an article by a guy and the way I understand it is, You can disengage the half nut, But you have to stop the machine pretty quickly, Back out and then roll your carriage back to the number on your dial and re-engage it then start your thread again. This is because when setting the lathe up to cut metric, You are using the top change gear to engage your 127 gear and the bottom change gear is flipped around where it engages the 120 gear, So things can get wonky by the time the dial rolls around again. This is the way I understand what he was trying to explain. I have always left the half nut engaged.

Edit: Gear number counts were just an example I used.

Here is a link to the article.

Metric Threading

Metric Threading on the Lathe

www.conradhoffman.com

 

[woodchucker]

Are you SURE you can't disengage the half nuts? I mean, really really sure?

Because if you start at "1" on the dial and return the carriage to the same spot for every chase, you can disengage the nuts and start on 1 every time, regardless of system. You could be cutting threads per furlong or cubit for all I care, because you are establishing the origin (unitless) between the carriage, lead screw, and dial. The origin is the origin is the origin. Anything downstream has to agree with your measuring system, but the origin is fixed. You CAN unclamp, you CAN re-engage, as long as the carriage is in the same spot and the dial reads 1.

We learn rules to threading, but there is no substitute for understanding what you are trying to do.

nope, you can't.

What you can do is disengage, stop, reverse and reengage when your number comes back at you.. So you are really at the same spot.
But no, with an imperial lead screw, you cannot leave your number even for one rotation. you must be on the same tick and that needs to be the same spot on the lead screw, no rotation.. should I say it a third time.??

 

[Clunker1]

Kept the half nuts engaged and cut decent threads. Royal PIA though!

 

[Harry Knutz]

Kept the half nuts engaged and cut decent threads. Royal PIA though!

Cut a chicken groove (Relief Cut) and use your foot brake if you have one.

 

[WobblyHand]

So, what is really necessary? I don't know, I'm thinking there are some conditions that will allow it, but you have to unravel the math. Until we can figure that all out, use a relief groove, stop the spindle, back out the cutter, reverse the carriage, reposition the cutter and do it again, without disengaging the lead screw. It works.

I'm trying to do this with an electronic lead screw. Really think it's possible to do what @pontiac428 says, but it's a weird mind bender (for me) to work it all out. It should be possible to figure it out, given knowledge of your start position, spindle position, effective gear ratio, lead screw pitch, etc. If things engaged once, and you can go back to that same set of coordinates in space, with the same orientations, position and various angles, you should be able to reengage again.

 

[RJSakowski]

I ran across this video that shows another technique. I seldom do metric threads but I tried it once the way shown in the video and it worked for me.

Oxtoolco video:

I use this technique for metric threads. You still have to drive the lead screw back but it does eliminate the pucker factor when you app5roach the end of the thread.

 

[Dabbler]

Cutting metric threads with an imperial lead screw is just the same as cutting imperial threads with a metric lead screw. In these cases, you either never disengage the half nut, or correct the overrun as in the video from "oxtool". there is no other way I am aware of, and I've been listening for one and prepared to learn a third way for over 40 years.

 

[Harry Knutz]

So,What I was trying to convey when I mentioned setting up the lathe for metric threading is, When the top change gear is mated to the 127 gear, And the bottom change gear is mated to the 120 gear, In essence, your idler gear is turning 2 ratios instead of one as I understand it, Which is not the case when the top and bottom gears are mated to the same idler gear, That's why things get wonky by the time your number rolls back around.

 

[RJSakowski]

Using the half nuts and thread dial works when cutting inch threads on a lathe with an Imperial lead screw because inch threads are all some integral number of threads per inch so the pattern repeats every inch., You can engage the lead screw threads at any point exactly one or more inches from the original point and the threading tool will be in the same position relative to the spindle rotation. The thread dial ensures this because it is clocked to the lead screw so it makes one full rotation at some multiple of one inch of travel. The multiple depends upon the ratio of the number of teeth on the thread dial gear to the tpi of the lead screw. If that multiple is 2, the thread dial will have moved an inch for every half turn and engaging the half nuts at half turn marks will be the same as engaging on the original mark. For threads like 11.5 tpi, engaging that combination on the same mark works because there are exactly 23 threads in a full turn of the thread dial. Alternatively, even tpi threads can be engaged at every quarter turn of the thread dial since there are an integral number if threads in each half inch.

Metric threads repeat every 127 inches on an Imperial lead screw and create an impossible situation for use of a thread dial/ half nut strategy.