If you can plunge the tool but not thread with it you probably have insufficient front clearance. 4 TPI requires a pretty wide tool nose. A small lathe may not be rigid enough to handle that width of tool without chatter. As Ady1 said, use a v tool to do roughing cut, then finish with the Acme tool.

Yes, it sounds like the tool has insufficient clearance on the leading side. You don't state the diameter of thread, but a 4tpi thread is going to have a steep helix angle. And the angle is steeper at the root of the tooth so the tip of the tool needs more clearance than further out.

You can work out the helix angle by taking the circumfrence of the thread, at the root, and the pitch (.250" in this case) and laying it out as a triangle. For instance a 1" thread will have a circumfrence of about 3.14". So a right-angle triangle with one side of 3.14 and one of .250 gives you a helix angle of about 4.5 degrees. So if you want to have say 5 degrees clearance, the tool must be ground at 9.5 degrees, or call it 10. But take the root of the tooth which has a diameter of .750 and your helix angle is more like 6 degrees, requiring a tool with more like 11 degrees clearance ground on it, and so forth. With a larger tool bit like used for 4 tpi you should be able to measure the angle with a protractor gauge etc.

Another thing that helps when cutting Acme therads is to set the top slide at slightly less than half the thread angle just like you do for a BSW or UNC thread. So for ACME you set the topslide to 13.5 degrees, that is 13.5 degrees from the cross slide axis, not the main lathe axis. This way when you feed the tool in, most of the cutting is done on that one leading edge, considerably lessening the cutting load. To get the right amount of infeed with the topslide set over at 13.5 degrees, you just use the standard thread depth, plus 2.8 per cent.

Or you can get the same effect by leaving the topslide in the normal position parallel to the lathe main axis, but for every 10 thou you move the tool bit inwards, use the top slide to move it 2.5 thou towards the leading edge of the tool. IE, top slide moves one quarter of what the cross slide moves. This shifts the tool at close enough to 13.5 degrees but has a couple of advantages over off setting the top slide. One is that thread depth is read directly off the cross slide dial, no need for adding the 2.8 percent etc (or multipy by 1.028 for duffers like me). The other, big advantage, is that at the final cut stage, or close to it, the top slide can be used to move the tool bit just a thou or so in one direction to take a small clean up cut along that cheeck of the thread, then moved just a thou or so in the other direction to take an equally tiny clean up cut along the other cheek of the thread.

Also, assuming that for brass you have no top rake or side rake on the tool bit. IE, top of tool bit is dead flat so there is no tendency for it to "self feed" either inwards or sideways. If you want to get really picky , top rake should be flat relative to the helix angle of the thread. One way to do this is to grind the toolbit out of round HSS then rotate it in its holder by the helix angle of the thread. I've been meaning to try this for years but always end up usiing my standard square HSS stock and grinding the extra angles on the one side.

This topic (in several different threads) has now been going on for over a month. Do the easy option. Partly cut the thread in the acetal and fully form it by 'heat 'n squeeze'.

What Ø is the thread ? The Helix angle will change with the Ø For instance if the major Ø is 25mm the helix angle should be 5.046° for 35mm major Ø it would change to 3.520°. It does indeed sound like it is rubbing on the front edge.

Vardex are threading insert specialists,and they would have a specific insert for you, but they are spendy.

"I'm trying to turn a 4 tpi external ACME stub thread in brass using a

home-ground HSS tool. It isn't working. Horrible chatter even with a steady

behind the cutter.

If I use my cutter like a parting or profiling tool, going in straight in, it works well,

so I'm thinking my problem is (given the steep helix angle) that I haven't got the

side relief right. Is that a reasonable conclusion would you say?"

First of all why would you make a leadscrew in brass ,at 5/8 inch od
with a .085 in thread depth ends up with a root diameter of 0.455 inch
with a 4 tpi thread ,hardly suprising that you are having problems.
Have a read of the recent article in MEW 241 to 243  Milling leadscrews these sort
of problems are history ,job set up ,full depth in a single pass , set it
running walk off and leave it, come back when its done.

John

Edited By John Pace on 08/11/2016 09:36:54

5/8" OD? Where did that come from? With a root diamter of .455", you would have a helix angle of 28 degrees or so. NO WONDER the side of the tool is rubbing. You need 33 degrees of clearance there. Definitely make tool from round HSS and turn it sideways by 28 degrees.

Lead screw? I thought the OP said it was a "stub thread"? I had visions of something an inch or two long.

Perhaps the OP could post a pic of what you are doing?

Edited By Hopper on 08/11/2016 11:44:43

Edited By Hopper on 08/11/2016 11:49:39

All very well if you're cutting the thread in acetal, but Robin asked for help for cutting brass.

You clearly missed my point. Do look back at the (several?) related posts from a month back. It is all to do with a leadscrew half nut for a wood turning lathe, I am sure. Existing nut is delrin.

Does it matter what he's doing or rather was doing ?

He's asked a question and as usual over half the people have answered a question he never asked.

Hopper suggests calculating the lead angle based on the circumference of the thread. Bear in mind that the lead angle changes according to how deep in the thread form the circumference is taken. The minimum value of lead angle would be measured at the outside diameter. The maximum value is obtained at the minor diameter. The most critical location on the tool for clearance angle is at the tip of the tool (it cuts to the minor diameter). I calculate the lead angle at the minor diameter and grind my tool so the side clearance exceeds the lead angle by several degrees.

Posted by Robin Graham 08/11/2016 01:20:16
I'm trying to turn a 4 tpi external ACME stub thread…

Presumably this is still in connection with this earlier thread as if he
had already solved this earlier query we would not be seeing this
thread "ACME agony".
Here is the first few lines of that post note the 5/8 x 4 tpi and also
the unwisley part.

Screwcutting Acme threads
Posted by Robin Graham 15/10/2016 23:27:44

Perhaps unwisely I've offered to help someone out by cutting an acme thread
in a half nut for a woodturning lathe restoration project. Options would be to make
a tap (the leadscrew thread is apparently 5/8 x 4tpi)…

Posted by John Stevenson 08/11/2016 12:16:08

Robin asked how to turn one as he doesn't have a milling machine and
therefore also doesn't have all the extra bits needed that are covered
in MEW 241 to 243.

Well we have all been there at some time with the lack of know-how
and equipment.

About 40 + years ago before i even had any machinery i was building
and flying model aircraft ,a particular project required a fan ,the place i
worked at that time used to make all sorts of things in connection with
the auto industry.
I went and asked someone in a department that dealt with such things
to try to (borrow) one ,the guy said he had nothing suitable and suggested
that i make one,i said i didn't know how ,he said go and learn .
This was a pivotal moment for me from which i have never looked back.

Perhaps the OP has just taken on a job that he had no chance of doing,
these sort of things are only learnt in time sometimes just by doing.
One of the problems with forums is they can provide a wealth of information
that is completely lost on those who are just starting.

Posted by Raymond Anderson 08/11/2016 14:41:36
Don't know where the 5/8" Ø came from, Robin never mentioned that.,
or a leadscrew [surely not at a stub profile !!! ]

Raymond
The 5/8 inch came from that earlier post .I also worked out the helix angle
and all of the other dimensions and got exactly the same figures as yours.

Hopper had obviously forgot to apply Pi x d in the calculation to get to
28 degree as tan .250/.455 = 28.78 degree.

Placed in album "Leadscrew" American Standard 29 deg Stub Thread
tables.

Hope this is of interest.

John

42.

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