Multiple start points

[ShawnR]

I wanted to do some threading today. I have done it in the past (a very small amount years ago ) but am pretty inexperienced at it. But it has been a frustrating day.

In a nutshell, it seems like I have very inconsistent starts. I have watched many videos on it and think I have the procedure down but it seems that when I start (using the threading dial), it never or seldom picks up the same starting point. I thought that it did not matter where exactly the tool bit was relative to the work piece at the start and as long as you hit the same number on the threading dial, it will pick up the same spot..?
I even tried positioning the bit consistently near the same start point but no difference.

Anyone have any suggestions? Probably a newbie error somewhere.

[digr]

I would think it must be the thread dial, check it to make sure it engaged. Try to stop it from turning with your thumb.

[digr]

Show us a pic of the way you have your compound set up

[ShawnR]

Thanks digr

I am wondering that too. If it has been off, is there only one way it can go on or should go on? It seems that it is just an indicator of a round gear. Of course, if it moves, not good. I read somewhere that if you are cutting an odd number of threads, then you should only use odd numbers on the dial but with an even number of threads per inch, then use any odd or even number...is this correct or is that one particular model of lathe??

[digr]

That is common but not always the case

[ShawnR]

I just disassembled the thread gear and not much to it of course. It all looks good. No indications as to orientation. I also tried always moving the carriage in the same direction last but no difference. It seems random, but of course, can't be. Would backlash in the feed screw do that? I can follow the cut line once or twice, then the offset cut occurs..

I also tried backing the carriage way back to that it moved about an inch or more before engaging with the work piece and no difference.

[ShawnR]

Here is the tool setup

[Scotts]

If there is a cover on the left I would open up and see if there is a key missing from the gear train on the lead screw or one of the other drive gears.

The lead screw should be timed to the chuck if you are using the same number on the thread dial and the threads are doing what you show in the pictures yo must have something slipping to cause this

Scott

What is make and model of lathe?


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[camdigger]

So, forgive me, but are you cutting the same system pitch as your lead screw? Even if you can get the right pitch through gearing, an imperial (inch) lead screw will not reliably synch up with a metric thread using the threading dial ( and vice versa). This is due to the odd gearing needed for the pitch.

The job can still be done, but the lathe will have to be stopped at the end of the thread and reversed to clear the work far enough that all backlash will be taken up before the Tool starts to cut.

FWIW, some very competent machinists do the reverse thing routinely to prevent these exact issues. The multiple start has to be offset using the cross slide and compound.

[milomilo]

Try keeping your hand on the lead screw engagement lever and see if there is any pauses while running. Are you sure the thread dial gear is fully engaged on the lead screw? I had a gear is the quick change box I had to replace a few years ago. Caused the same issues you have.