I picked up an old driver plate for my SB 9A, and the threads are too tight - will not thread onto the spindle. (I checked the register diameters and they are OK.) I know the basics of thread cutting, but how does one "pick up" an existing thread? A pointer to a good reference would be fine - but I can't find this info in HTRAL, the Atlas book, or Moltrecht.
Set the cross slide for 29.5dg (for a 60dg thd), load a threading tool on the cross slide. Engage the half nuts (typ on the #1 mark) and let the
feed screw pull the saddle a bit, then shut down.
Adjust the cross slide and compound screws so that the cutting point is all the way in the existing thread. Push it in with the cross feed, and tweak the compound. When you get the tool as far in as possible, you can zero out your cross and compound dials.
Now, you're all set to back out of the cut (cross) move the saddle to the right, restore the cross and advance the compound and take a threading pass. Repeat until it fits [img]smile.gif[/img]
Everything Wally said and you will have to turn your compound to the 1 o'clock position at 29.5 degrees. Since this is an internal thread. You can see the whole set up in pics at my yahoo group...Bob
Because this is an internal thread it will be
tougher than an external one.
The compound should be set to the right
side of the carriage, at 29 degrees, point
to the *rear*. The best way to imagine this
is to point the compound straight back to
the rear, and then swing it towards the
right side, so it goes through 29 degrees
of motion. The numbers on the dial won't
read 29 though.
The advice so far has been good but because
it's a bit of a tricky job, I will say it once
First off mark up the inside of the existing
threads with a blue sharpie marker so you can
see exactly where you are cutting.
Get the threading setup right so it is cutting
the correct tpi.
Close the halfnuts and allow the leadscrew to
take up a strain on the carriage, so you get
all the slack out of the system. Of course
at this point you will have the threading tool
completely clear of the threads, inside the
bore of the part.
Stop the motor so the threading tool is
still inside the threaded portion of the bore.
Now it is simply a matter of juggling the
two dials at your disposal (crossfeed and
compound feed dials) so that the tool engages
the existing threads as closely as possible.
If you have the compound set at one particular
location, as you dial the crossfeed handle out,
the flanks of the tool will both touch the
flanks of the existing thread.
When you think you have it so, set both dials
When you are cutting, the lash should be loaded
out of the crossfees screw by unwinding it to
smaller numbers, and out of the compound screw
by winding it in to larger numbers.
Set up a trial cut with the crossfeed at zero,
and the compound maybe a few thou less than zero.
Take cuts, feeding the compound a thou at a time
until you start cutting metal. Then stop and
see what the bluing looks like in the bore.
By now you could imagine tweaking the position
by juggling the two dials to get the cut even.
If it's off by a thou or so that's probably OK
because you probably need to take of several
anyway, as long as both sides are cleaned up
when you are done it's fine. Don't make the
threads too tight, cut till the fit is snug
and then go several more thou.
One other thing you can do before you try this is make a small threaded plug the exact size of your spindle thread OD. The best thing to do is mic yours using three wires then make the plug the exact same size. It will come in handy on future projects anyway. Or you can run the carraige to the right with the half nuts still engaged, and carefully unscrew the chuck, turn everything around and see if it fits on your spindle. Its kinda tricky but it works. I just threaded a 40# part and didn't have a threaded plug for my spindle size. And had to turn the whole works around twice. What a job. My dummy spindle from a 9" won't fit my 10k its .006 too small. So if i thread a part to fit the dummy its too tight for my 10k...Bob