On 2011-12-21, wrote:
On Tue, 20 Dec 2011 08:43:30 -0500, "Jim Wilkins"
wrote:

[ ... ]

A thread indicator as shown here speeds up the job considerably.
http://www.micro-machine-shop.com/si..._threading.htm

[ ... ]

At 2 AM last night I was hoping nobody would spot the error and call
me on the depth of thread for 1/2-20. It is of course half of the
quoted figure, so only 12 passes are necessary.

You know better than to hope that nobody spots a mistake. :-)

I am pleased to report that with the adjustment of the change gears I
was able to cut two perfect 1/2-20 threads today! A nice Christmas
present.

Congratulations!

I re-did the cutter: It is amazing how many different instructions
there are to grind one, some from reputable sources which seem plainly
wrong (no side clearances). I added those and the cutting improved.

Yes -- the clearance makes a big difference. For production
work (lots of the same thread) it makes sense to calculate the helix
angle and adjust the side clearance angles to give sufficient clearance
for that thread in particular, which leaves the maximum meat in the tool
and increases its life.

The helix angle is calculated by taking the maximum diameter of
the thread, calculating the circumference, and the pitch (inverse of the
TPI for inch threads), which will give you the sine of the angle, and by
looking up or using a scientific calculator to take the arc-sine, you
find the angle. As an example, with your 1/2-20 thread, the diameter is
1/2" (0.500"), so the circumference is Pi times that, or 1.5708", and
the pitch (from 20 TPI) is 0.050". Divide the pitch by the
circumference, and you get 0.0318, and the arc-sine of that is 1.824
degrees. So -- you use that as a starting point, and add 5 degrees to
it on the cutting side, and subtract it that from 5 degrees on the other
side, giving relief angles of 6.824 degrees on the left and 3.176
degrees on the right (for a right-hand thread).

Note, you can ignore this for most thread cutting, except for
long production runs where you want to maximize tool life. (The carbide
threading inserts have available anvils which adjust the side clearance,
usually more necessary with multi-start threads or very coarse pitches.)

One of the joys of the 9x20 is the clutch at the lowest speed which
effectively limits the available torque. Changing the cutter geometry
stopped the clutch dis-engaging during the last couple of passes which
was a problem before. There are those who disabled their clutch by
epoxy. I have not addressed that because I tend to use higher speeds
most of the time but it may be something I shall have to look at.

It seems that there is benefit from that -- giving you a clue
that things are not as they should be.

I use RapidTap, seems to work fine.

Sounds good.

I do have a threading indicator but a cursory inspection would suggest
that it is not working. In any case I felt I wanted to take that
variable out of the equation for the moment. If I do more of this I
shall re-visit it.

What do you see which indicates that it is not working?

What you should see, if it is working properly a

1) When you crank the carriage from side to side without the
leadscrew turning, the dial should rotate.

2) When the carriage is stationary, but the leadscrew is turning
the dial should rotate.

3) When the half-nuts are engaged (for threading, or likely on
some lathes, for longitudinal feed as well), the dial is
stationary.

But -- to minimize wear on the pick-off gear and the leadscrew,
is is common to ship lathes with the pick-off gear disengaged. To
engage or disengate it, loosen the single screw attaching the dial to
the side of the apron, and to engage it. rotate it so the bottom moved
into contact with the leadscrew. To disengage it, rotate it so the
bottom moves out of contact with the leadscrew.

While there are some lathes made with the dial permanently
installed in the apron and no way to disengage it, I believe that your
lathe likely has it as I have described it above, and certainly the
image called "thread indicator" in the web site you pointed to is of
that type.

FWIW The thread indicator on my 12x24" Clausing had never been
*mounted* to the apron, and was in one of the pedestal drawers.
This is because the lathe was fitted with a bed turret, and
threading was being done with Geometric die heads, not single
point (really quick way to thread in production, but tedious to
set up properly for a single thread.

The choice of what number to close the half nuts on when using
the dial varies with the particular thread you are cutting. There
should be a table telling you what is what for your machine. But you
can get away with always closing them at "1" no matter what, though it
can slow you down quite a bit with fine threads, as with many of them
you can close on any marking, and with fine threads, the dial turns very
slowly. :-)

And for most lathes, your 20 TPI would allow you to close it on
any marking. For mine, the manual says:


================================================== ====================
When cutting even-numbered threads, engage the half-nut lever
at any one of the markings on the threading dial for each cut of
the thread.

When cutting odd-numbered threads, engage the half nut lever
for the first cut and all successive cuts at either the 1 or 2
positions of the dial.

When cutting half-numbered threads, engage the half-nut lever at
the same mark on the threading dial for each cut of the thread.
================================================== ====================

Note that the illustrated one has only the odd numbers showing with the
even ones between, while mine with the same eight index lines, numbers
only every other one, so 1 & 2 on mine would be 1 & 3 on this one, so
the instructions would be different. So check your manual -- and also
compare to your scratch test (make that one on '1', and see whether the
outhers track that or not. For 20 TPI, they should on yours.

So -- with the threading dial, you engage on your selected
number, wait until you get to the runout groove and disengage, crank
back the cross-slide, crank the carriage clear of the end of the
workpiece, crank the cross-slide back in, advance the depth of cut on
the compound, and wait for the right number to come up before engaging
the half nuts again. Note that the half-nuts won't engage at the
mid-points between marks, so you can start to engage about then and it
will fall in at the proper point. (And, if you start far enough clear
of the end of the workpiece, you can make sure that it is on the proper
number, and disengage if necessary.

And -- you don't have to stop the spindle and reverse it to do
this.

All in all I would like to thank everyone for their helpful advice. I
feel almost like a proper machinist now.

Being able to single-point cut threads does make you feel like
that, doesn't it?

Congratulations,
DoN.

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