'Don't know. I think it's mentioned in the Atlas lathe manual, and maybe in
the South Bend _How to Run a Lathe_ book. But neither one, if I recall
correctly, says why.
I have a few really old, old lathe operating books, but they're in storage
right now as we juggle some family arrangements and I'm trying to make room.
I may have one of that size. Ill check in the morning
One could not be a successful Leftwinger without realizing that,
in contrast to the popular conception supported by newspapers
and mothers of Leftwingers, a goodly number of Leftwingers are
not only narrow-minded and dull, but also just stupid.
Thanks guys for all the insight. Think I will try a boring bar
approach. Its nothing too demanding of an application. The female
thread wore on a bench vice. I hate to throw a good condition vice in
BillMe wrote in news:fj0c265vn1a5bugo6v9qssp36oo4h0btcf@
Probably not an option here, but there is a thread (pardon the pun) on the
Clausing Yahoo group about this. One fellow made a tap out of a piece of
ACME threaded rod. Cut a taper on it, and then cut flutes into it.
Apparently it worked fine, but took a LOT of torque. Presumably
lengthening the taper would help with that.
There seems to be a problem with people winking out that have the old skills.
one of Tubal Cains books when I visit the reading room. ;)
I wonder if John from amdinc has experience cutting acme? He repairs a lot of
I can't think of who John is.
There are two old machining books handy, and I just looked in my 1940
edition of _American Machinist's Handbook_. Nothing there. I also looked in
Colvin and Stanley, _Turning and Boring Practice_ (1943). Nothing there,
There are a few interesting bits about single-point turning of threads,
however, one of which relates to what you said above about CNC turning.
Colvin mentions a method that sets the compound perpendicular to the cross
slide, and then using the compound to cut one flank, and then run back to
cut the other. There is no detail; it sounds like it requires some careful
thought. The finishing pass(es) are taken by plunging straight in. He says
this is "said to be a common method used in English shops."
Apparently plunging straight in was the more common method in production,
even for conventional 60-degree thread forms. It's also interesting that
Acme threads derived from a group of miscellaneous flat-bottom threads known
as "bastard threads." The 29-degree angle, says Colvin, probably was chosen
because Brown & Sharpe published a simple method for laying out cutting
tools with 29-degree angles.
Some time I'll dig into my other books to see what they say about cutting
Acmes. I have four or five others from that era.
Ed sez: " . . . . I've never seen an explanation of what you do that, Bob."
One reason is that going straight in avoids the difficulty of an angular feed in
the bore which can
be a clearance problem if you're threading from left to right Double chip
load, and chip
clelarance is reason to take it slow and easy. One of those options, hobby
types like to worry
Interesting. I suspect a lot of art was common knowledge, so common, no one
What was the method?
I took a look at K. H. Moltrecht's Vol. 1 Machine Shop Practice a hour or so ago
going out to wack the grass. He indicated using the compound at 14.5 to cut one
external threads and to use a follow rest, he didn't say anything about internal
but I would suspect the same technique would work. I do the 30 degree thing for
and internal 60 degree threads.
It requires an illustration. The edition on Google Books (1948) doesn't
allow sufficient searching, but it's on page 70 and some previous page, if
you want to get tedious about it.
My scanner sucks but I'll see if I can do something with it some time.
Hmm. That's the first recollection I have of someone recommending the
half-angle setover in print.
I think Colvin is wrong in this case. My understanding is that the 29
degree angle is the strongest angle. It is also the angle used for
gears (14.5 degree pressure angle. ). Brown and Sharpe came out
with involute gear cutters in 1858 so they are likely the source of
the angle. Brown and Sharpe probably published a method for laying
out tools with 29 degree angles as that would be what is needed to cut
a rack using a shaper to work with 14.5 PA gears.
This is just based on bits dredged up from memory and some guessing so
please correct me if I am wrong.
Plunging straight in or cutting on one flank is determined by the
stiffness of your machine. Every old time machinist I know with a good
lathe will plunge straight in. They cut in to the double depth and then
check it with a profile gauge, looking for any light coming under the
gauge and then try the thread gauge. It's easier because you just cut
to the double depth of the thread that is stamped on the back of the
Starrett fishtail gauge. With inserts and the tool holder set properly,
your thread angles will be correct.
CNC machines can cut alternate flank threading as well as single flank
or plunge cut. A good machine will cut either way with no problem. Some
of the special acme threads we do we plunge cut to the depth and then
shift the tool a couple of thousandths in the Z axis to widen out the
acme profile because of certain tolerances that are required on the
parts we do.
What it comes down to is if your machine can handle plunge cutting that
is the way to go, if you run into problems then go to flank cutting.
The fallacy in the argument is the assumption that feeding straight in would
double the chip load. This is true with vee threads because you are in fact
cutting twice as many sides.
I am too lazy to do the trig, but I drew the acme thread on a cad program
and found that the difference in chip load between the two methods is pretty
Feeding at 14.5 deg for each unit of depth of cut at the root of the thread,
I get .499 units on one flank and 0 units on the other.
Feeding straight in, I get .252 units on each flank
Paul K. Dickman
This looks like a good place to add my attempt at making
such a tap.
There is a problem with fire hydrant gate valves. What
happens is that
when the valve is closed there is only a very short thread
between the gate and the actuating screw and after some wear
threads lose engagement and the valve cannot be opened. Not
So I came up with a repair that involves a longer threaded
portion on the
gate. I've repaired several of them now with this tap. I
have posted a
pic to the dropbox, the text file has not appeared yet for
so I'll include it here:
This tap is a 1/2-4 double helix left hand thread used to
cut a thread to repair fire hydrant gate valves. It has
been used several times to cut threads in aluminum. Six
previous attempts were failures due to the wrong heat
treatment procedure. The last few threads are the only
ones that cut the final profile. The thread profile of
the actuating screw is definately a square thread. The
tap is made from W-1 drill rod and was done on a 9"
South Bend Model A workshop lathe. The flutes were cut
on a milling machine. Tap drill size is 25/64.
I don't do math on Sundays , but are you measuring "chip load" in terms
of metal volume being removed? That's the common definition of the term, and
I shouldn't have used it. What I meant was cutting force, rather than chip
I'm rusty on that but the primary factor determining cutting force in
turning, IIRC, is the depth of cut, with the feedrate being a relatively
minor issue. The depth of cut, or the effective equivalent in terms of
cutting force, in the case of a turning insert plunged straight in, is twice
the actual depth, multiplied by the secant of the flank angle. Add the
length of the flat at the tip of the cutter, if you want to get it all.
Right? I'll try it again on Monday. d8-)
I think it will still come out to less than you would think.
By either method, the big flat nose of the tool takes the deepest cut with
each pass and, unlike the flanks, it cuts the same width every time. The
result is that it swamps the numbers.
However, in the spirit of full disclosure, I have to admit that I do not
believe that I have ever cut an acme thread that I did not have to take a
Actually I don't have a taper attachment, but I do have an
offset center device for the tailstock. Somewhere in the
I saw a pic of this thing so I made one. It is not the most
thing in the tailstock so means must be made to sprag it in
to prevent movement. Shall I post a pic to the dropbox?
I should as others may copy it.