I need to cut an acme thread in a steel block. Roughly 5/8" X 6. I
forget what I measured! Is that done with a tap or a boring bar?
Friends have not heard of an acme tap. Thought someone might know.
Thanks,
Bill A
There are such taps. I have one, 1/2" x 10 Acme.
Boring a female Acme thread is something I've never tried. I still scratch
my head over single-point turning of *external* Acme threads. Some of the
old books tell you to cut both flanks at once, but I've never figured out
why. I made a couple of Acme threaded shafts in 1/2" x 10, and cutting on
one flank, just scraping the other, seemed to work Ok.
On Fri, 25 Jun 2010 10:44:27 -0400, BillMe wrote the
following:
Sure it's ACME and 'Murrican?
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shows a 5/8-8 ACME tap.
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shows standard
ACME tap sizes at 5/8-8 and 3/4-6.
UNC is 5/8-11, UNF is 5/8-18.
--
Pain makes man think. Thought makes man wise. Wisdom makes life endurable.
-- John Patrick
ACME taps are readily available but are expensive. They also are often
in sets for roughing and finishing, though some are multi stage with a
roughing section followed by a finishing stage and are even more
expensive. A quick look on the McMaster site shows a number of ACME
taps, including the tandem ones, ranging from $60 to $300.
Typically unless you need to make a bunch of parts, it's cheaper to buy
a ready made ACME nut assembly and install it in your part. McMaster
lists a lot of precision ACME nuts, but the typical bronze thread in
ACME nut that you thread into your part and secure with a setscrew,
locktite, etc. runs $20-$30. These also make the nut readily replaceable
for wear.
That worry kinda goes away if you grind an accurate Acme profile and feed
straight in, slowly.
Bob Swinney
There are such taps. I have one, 1/2" x 10 Acme.
Boring a female Acme thread is something I've never tried. I still scratch
my head over single-point turning of *external* Acme threads. Some of the
old books tell you to cut both flanks at once, but I've never figured out
why. I made a couple of Acme threaded shafts in 1/2" x 10, and cutting on
one flank, just scraping the other, seemed to work Ok.
I've never seen an explanation of what you do that, Bob. Why plunge straight
in, nearly doubling the chip load, rather than feeding in on an angle and
cutting on one flank, as with conventional threads?
There must be some simple reason that I've just never come across.
(top posting fixed)
Doesn't that mean then that you need a different threading bit for each
diameter and pitch of Acme thread you'll ever do? Or do you just need a
different bit for each pitch, with enough clearance for the smaller
diameters?
============
Given the cost of the taps [Acme usually has two, roughing
and finishing] you may want to consider a threaded insert.
This makes the thread easly renewable. Also if a
replacement part, left hand acme threads are frequently used
for adjustment. Be sure which one you have. There is also
a metric trapizoidal thread with a 30 degree thread angle
rather than the 29 degree acme.
mother lode of information on Acme threaded products
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for some examples see
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?tag=155vvvvv-20
also see
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If you machine your own threads you will need a thread gage
to sharpen the lathe tool to the correct profile.
That's the question I'm asking. And that's what I did. But that's not what
Bob is saying -- and what Bob is saying is the same thing said by every
source I've ever seen.
Plunge straight in, they say. Why?, I ask. It nearly doubles the chip load,
and I can't see a good reason for it. There probably is one, but I don't get
it.
I'll wait to have it explained. Now if we were using cnc's we could get
creative in how
we cut the profile. I seem to remember reading in the Sandvik book a technique
for
alternating between both flanks to reduce cutting forces.
Google is my bud.
D. Alternating flank infeed
This method alternately feeds the insert along both thread flanks, and therefore
it uses
both flanks of the insert to form the thread. The method delivers longer tool
life because
both sides of the insert nose are used. However, the method also can result in
chip flow
problems that can affect surface finish and tool life. This method is usually
only used
for very large pitches and for such thread forms as Acme and Trapeze.
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This wouldn't work very well using engine lathes with screws with backlash which
is about
all of them, mine for sure.
Wes
Acme taps do exist, and the better ones are longer with a
roughing and a finishing section separated by very little.
Looking through MSC's on-line catalog, 5/8" typically has 8 TPI,
and 7/8" has 6 TPI.
The 5/8-8 goes for $81.59 for the combination tap.
The 7/8-6 goes for $221.25 for the combination tap.
Be sure to order the correct one. Leadscrews turned by
handwheels often have left-hand threads so clockwise advances the nut.
As for single-pointing it with a lathe and a boring bar -- I
think that it is unusually coarse for that, though I guess that it could
be done by someone sufficiently skilled.
Good Luck,
DoN.
I've single-point turned an Acme thread in a bronze nut (for a
friend's log splitter). I think that it was something like a 1-1/4"
major diameter or so. There was enough room for a reasonable boring bar
to hold the tool, and I used a sine plate and some other tricks to grind
the tool itself. But if he really is working with a 5/8" major
diameter, and 6 TPI, that does not leave much room for a rigid enough
boring bar.
Enjoy,
DoN.
[ ... ]
I fed in at an angle of 14 degrees (a little less than the
half-way point). But I ground a bit specifically for the thread I was
cutting, including side clearance angles calculated for 5 degree
clearance with the pitch and diameter I was working with for maximum
strength. A more general tip (more clearance to adopt to various
diameters and thus different thread angles) would have to be weaker.
And yes -- you make (or buy) a separate tool for each pitch you
cut. My 12x24" Clausing can handle insert tools for above 10 TPI, but
not up to the big bronze nut I was making -- so I had to make my own
tool bit from HSS there.
Enjoy,
DoN.
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