Acme thread

'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.
Reply to
Ed Huntress
Loading thread data ...
I've floated this question a couple of times before. I never got a solid answer, just possibilities.
formatting link
That's interesting, but, as you say, it isn't what the original instruction was about. We need someone who's even older than me to answer it. Most such people are dead. d8-)
Reply to
Ed Huntress
I may have one of that size. Ill check in the morning
Gunner
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. Gunner Asch
Reply to
Gunner Asch
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 the trash.
Thanks again,
Bill A
Reply to
BillMe
BillMe wrote in news:fj0c265vn1a5bugo6v9qssp36oo4h0btcf@ 4ax.com:
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.
Doug White
Reply to
Doug White
I've think I remember you asking before.
formatting link
>
There seems to be a problem with people winking out that have the old skills. I'm reading 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 big machinery.
Wes
Reply to
Wes
formatting link
>>
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, either.
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.
Reply to
Ed Huntress
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 with.
Bob Swinney
Reply to
Robert Swinney
'Sounds reasonable, but why do they plunge straight in with *external* Acme threads?
Reply to
Ed Huntress
John
Interesting. I suspect a lot of art was common knowledge, so common, no one wrote it down.
What was the method?
I took a look at K. H. Moltrecht's Vol. 1 Machine Shop Practice a hour or so ago before going out to wack the grass. He indicated using the compound at 14.5 to cut one flank on external threads and to use a follow rest, he didn't say anything about internal threads but I would suspect the same technique would work. I do the 30 degree thing for external and internal 60 degree threads.
Wes
Reply to
Wes
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.
Reply to
Ed Huntress
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.
Dan
Reply to
dcaster
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.
John
Reply to
john
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 small.
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. 1+.499+0=1.499
Feeding straight in, I get .252 units on each flank 1+.252+.252=1.504
Paul K. Dickman
Reply to
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 engagement between the gate and the actuating screw and after some wear the threads lose engagement and the valve cannot be opened. Not good! 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 some reason so I'll include it here:
formatting link
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.
Reply to
Phil Kangas
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 load.
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-)
Reply to
Ed Huntress
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 file to.
Paul K.Dickman
Reply to
Paul K. Dickman
Nice bit of work. I searched the drop box, I can't find your .txt file. I hope our benefactor keeps it in the box.
Obviously you have a taper attachent. Wish I did.
Wes
Reply to
Wes
Actually I don't have a taper attachment, but I do have an offset center device for the tailstock. Somewhere in the past I saw a pic of this thing so I made one. It is not the most secure thing in the tailstock so means must be made to sprag it in place to prevent movement. Shall I post a pic to the dropbox? Maybe I should as others may copy it. Tomorrow.......maybe.....phil
Reply to
Phil Kangas
Heck yes, post a picture.
Wes
Reply to
Wes

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.