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-----Original Message-----
From: 7x12minilathe@...
[mailto:7x12minilathe@...] On Behalf Of andrew franks
Sent: Wednesday, March 14, 2007 5:38 PM
To: 7x12minilathe@...
Subject: RE: [7x12minilathe] THREAD CHASING


John's right. When a leadscrew is used to cut a pitch of its
own "nationality", the right place to engage the half-nuts
turns up frequently. Imagine holding two rulers, ruler 1
graduated in 16ths of an inch (representing an imperial
leadscrew) and ruler 2 graduated in (say) 28ths (representing
a 28 tpi thread you want to cut). The graduations will
coincide exactly very frequently - every quarter inch
(because 4/16 = 7/28). So,the right place to engage your
leadscrew, as indicated by your thread dial indicator, occurs
very frequently.
Now, imagine you want to cut a 1mm thread, and that you
have a 127 tooth gear to do an exact conversion from your
imperial leadscrew. This time, ruler 2 is metric, and you
will only get an EXACT coincidence of the graduations every
127 inches (10' 7"). The "right place" is only going to turn
up infrequently on your TDI, and you will need a microscope
to distinguish it from all the near misses which will turn up
while you wait for it. Of course, you haven't got a 127T
gear, so you have to use some fancy geartrain involving 57T,
42T, or whatever to get something which is almost, but not
quite, 1mm (25.4 tpi). This might make matters worse.
I worked out that, though I could use my imperial leadscrew
to cut a 1.25mm pitch thread accurate to 0.02%, the "right
place" on a TDI would occur only once in about 30,000
revolutions of the spindle. The gearwheels would only be in
precisely the same juxtaposition every 30,000 revs. At an
unlikely 1,000 rpm, this would occur every half hour, and
would be beyond my eyesight to recognise from all the near
misses which intervened.
So, if you want to cut a reasonably accurate thread which
is "foreign" to your leadscrew, the only option is to leave
the half-nuts engaged, and reverse the lathe to get back to
the beginning of the cut.

Does this make sense? It does to me, but I've only just got
back from the pub!
Andy

John Dammeyer <johnd@...> wrote:
Hi Ian,

You are making an assumption that when the lead screw is at a specific
point when the spindle is at an equally repeatable location. That's
essentially how the thread chasing dial works. If you were cutting a
thread that required engaging the half nut at any line it
means that the
lead screw and spindle are linked in multiples of your lead
screw pitch.

But if you are cutting a thread where the #3 position and only the #3
position is the right place it's possible that the spindle is
90 degree
offset for the #2 position, 180 for the #1 position and 270 offset for
the #4 position.

Now follow that same line of thinking for metric and you may find that
the nut would have to be engaged at the #1 position the first
time. The
1.27 position the next time and so on.

John Dammeyer

Automation Artisans Inc.

Ph. 1 250 544 4950

-----Original Message-----
From: 7x12minilathe@...
[mailto:7x12minilathe@...] On Behalf Of steam4ian
Sent: Wednesday, March 14, 2007 3:16 PM
To: 7x12minilathe@...
Subject: [7x12minilathe] THREAD CHASING


G'day all.
On another topic, John (born4something) and I have been discussing
thread chasing dials. If you have a metric lead screw you need a
range of pinions on the dial to suit different threads. This is
tedious. to cut an imperial thread (with metric lead screw)

you have

to keep the half nut closed the entire time and reverse the

lathe out

between cuts, also tediuos.
There are two common methods to keep the tool "in the groove" for
each pass, namely, using the thread chasing dial or

reversing between

cuts.
I think there is a third method, does anybody know of it?

In the wee small hours of the night, my designing time, I

dreamt up a

method. It involves using a saddle stop at the RH end of the cut
(assuming R to L cutting). At the end of each pass you

dissengage the

half nuts, withdraw the tool and crank the saddle back hard against
the stop. You set you tool and then gently engage the half nuts by
feeling them into position. This last process would be made

easier by

a circular clip on the lead screw to set indicate when the screw is
at the same point of rotation corresponding to the first

engagement.

The gib for the half nuts would need to be well adjusted.

Comments and advice is sought, over to you.

One good turn deserves another. (Luke6:31 Machinists Bible)
Regards,
Ian




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