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  #1  
Old 01-07-2008, 06:05 PM
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Cavalry Cavalry is offline
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Default Picking up an existing thread in lathe?

I have a job where I need to open up a threaded hole. The only way I can think of picking up the thread is turning over the lathe by hand on the existing threads. Chances are pretty slim that the dial is going to match my threads, so I was planning on putting a dot of paint or something to indicate where to start on the dial.

Am I on to the correct method or is there something easier?
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Old 01-07-2008, 06:15 PM
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You can only engage the dial (aka half nut) on the mark it will not engage in between.

It sounds like you are trying to do an internal thread. Not easy.

I have only done external threads.

What I do, and that is saying there is likely a better or easier way.

I set the lathe up for the pitch needed. Set the cutting tool just off the threads so you do not cut anything and then engage the apron, as normal for you lathe and pitch combination.

I set up the cross slide so it runs parallel to the z-axis (that is the long axis of the machine) and use the cross slide to make small adjustments until I am lined up with the existing thread. Then in small increments on each successive pass (cutting air) I feed in and adjust for being on center of the thread with the tool. It is time consuming. And a PITA.

If you are doing an internal thread see if you can get a left hand boring/threading bar and run the lathe in reverse. Chuck turning clockwise. So you have a better view of what you are doing.

And good luck!

Oh yeah and I will add all the adjustments need to be made while the lathe is cutting air so all the backlash is taken out of the threading gear train. Fun, eh!
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Last edited by Shade Tree Welder; 01-07-2008 at 06:22 PM.
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Old 01-07-2008, 06:48 PM
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That sounds like a tedious but sound plan. I am limited to turning in reverse as I have a screw on chuck and do not want to put a setscrew on the threads.
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Old 01-07-2008, 06:48 PM
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A right hand internal thread will be turned in forward unless you start at the rear and thread out.

Set your lathe as you would normally set it for threading.(compound at 29.5º) Engage the half nut and run the boring bar into the work just far enough to clear the first thread. Shut the motor off. Now you can adjust the tool to fit the thread by moving the compound and cross slide until the toolbit fits the thread groove,being careful not to move the carriage. This should get you real close. Make a skin cut for reference and go from there. The angled compound will let you adjust the bit to the center of the groove. Reset your crossslide dial to zero after you are satisfied with your centering or use any number on the dial as a starting reference.

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Old 01-07-2008, 06:54 PM
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Its tough to pick up an existing thread, but you already know that.

First, I'm assuming you've cut threads on a lathe before and know the proper setup techniques. This is one method I've used with some sucess and it would be easier to show you rather then tell you, but here goes.

I would set it all up like you are cutting a new thread only no cutting tool. Start the lathe, engage the half nut and then shut the lathe off without touching any thing.

Then put the cutting tool in what ever holder you are using and adjust so the tool is aligned with the existing thread. Back the cross slide away from the piece, disengage the half nut and crank the carriage back.

Start the lathe, engage the half nut and bring the cross slide back so the tool is close to the piece and see if you're close. It may take a little playing around with the compound slide to get it closer.

It still takes a little luck to get it right, but it can be done with some patience.
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Old 01-07-2008, 06:58 PM
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Quote:
Originally Posted by Cavalry View Post
I have a job where I need to open up a threaded hole. The only way I can think of picking up the thread is turning over the lathe by hand on the existing threads. Chances are pretty slim that the dial is going to match my threads, so I was planning on putting a dot of paint or something to indicate where to start on the dial.

Am I on to the correct method or is there something easier?
I've picked up threads on the inside without much trouble, and I don't
evenhave a thread dial on mylathes--- they are so old a thread dial hadn't even been invented.

The difficulty will depend on how well the TPI matches the threads on the lead screw. First thing is to contemplate the number of TPI on your
lead screw. For example my big ATW lathe has an 2 TPI lead screw.
You can accordingly close the half nuts anytime you want when turning threads of any number divisible by 2 which helps since thread dials hadn't been invented.

How I pick up threads generally, is to drive the boring bar into the work
under power until it is over a thread. Then I stop the lathe, and adjust the cross feed and the compound until I'm in the thread. Then I back out
of the thread with the cross feed, and back the boring bar out of the hole with the lathe in reverse with half nuts still closed.

Then I go ahead on it---slowly. I know the books say you can cut threads
as fast as you would for normal turning, but I ignore the books and get
in the back gears and turn real slow.

If you always stop the lathe by shutting off the motor/clutch release etc.
and never open the half nuts, and back out in reverse gear, figuring out what to do with the thread dial is irrelevant.
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Old 01-07-2008, 08:52 PM
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Thats why the compound has a little wheel on it...


And pray..
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Old 01-08-2008, 12:24 AM
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Quote:
Originally Posted by Cavalry View Post
That sounds like a tedious but sound plan. I am limited to turning in reverse as I have a screw on chuck and do not want to put a setscrew on the threads.
Cutting in reverse is certainly an issue with a screw spindle (what I have), but if you set the chuck fairly well, you should be able to start up the lathe and back out if needed in reverse without an issue. I just watch mine,
but then I've set my lathe to a very low gear when I do this.
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Old 01-08-2008, 11:18 PM
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I guess the only thing I would add maybe its already been said but after you set up your tool to the thread you can't use your compound to adjust your tool for the cuts if its set at 29.5 degrees you have to use the cross feed.
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  #10  
Old 01-09-2008, 04:42 AM
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Quote:
"I have a job where I need to open up a threaded hole."
I use the same method as in post #4, powerpooche.
Of course 29.5º on the compound would be in opposite quadrant for internal thread cutting.
SEE picture.

The compound is set at 29.5º and increments the depth of cut on each pass.
The use of the compound set at 29.5º allows you to cut on one side of the tool as opposed to a straight in plunge using the cross-slide that cuts on both sides of the tool.

The cross-slide is used to get you out of the V so you can move the carriage back out, then the cross-slide is set back to the same start setting.

The last two or three finish passes you can move the cross-slide .001 to clean up the V.
===

When threading its best to use the same number on the thread dial when engaging the half nut.

Theoretically you can use odd or even numbers depending on the pitch, but me and other people on occasions have cross threaded a part without a clear reason.
And when this has occurred is seems to be on the last few passes.
It doesn't make sense but I never had a problem using the same starting number.

Of course a double or quad thread will require different numbers.


Caution,
Once you start threading you can not disengage or switch gears at any time that includes the spindle gears.
If you do, you will lose the timing between the lead screw and spindle.
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