ACME agony

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?

Rob

I have only done a square thread a couple of times but I recall that I did it in stages

Stage 1 was with a pointy tool to create a normal deep thread, then used a square tool for the rest of the job.

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.

I would say thats a fair guess, the profile of the tool needs to be ground to match the form of the spiral/helix angle because on the deeper cuts you may not be able to clear the threads.

An acme thread is slightly more forgiving to cut than say a truly square thread, so i'm not sure if thats true or not.

Michael W

Edited By Michael Walters on 08/11/2016 05:04:10

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.

Or get your credit card out and buy an insert! Kennametal do one in the NA series for 4 tpi. I bet it ain't cheap, though.

John

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'.

Not diy,

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

John

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.

You've chosen the most difficult thread to cut, but a few suggestions. use a square tool the width of the bottom flat. Take the cut 0.020'' below the minor diameter. Then with a ACME form tool cut the flanks, thereby avoiding cutting the full form. Hope that helps.

"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

.

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.

Why do people answer a question that hasn't been asked. ?

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.

ndiy

But Robin is cutting an external thread. Not a feed nut or half nut.

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.

Don't know where the 5/8" Ø came from, Robin never mentioned that., or a leadscrew [surely not at a stub profile !!! ] Anyhow ,here is the thread data for,

5/8" stub Acme, at 4 TPI. Single start. [ makes me shake me head just looking at it !! ]

Lead 0.250" Effective Ø 0.5437" , Minor Ø 0.4550" profile height 0.0850" Helix angle 8.327°. Vardex states that is a non standard insert [ if you want to go down that route be prepared to part with a lot of cash ]

Best rough it out with a "v " then finish off with the tool ground to the correct helix angle. .best of luck....

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.

An easy help for this problem is to color up the tip of the cutting tool with a Sharpie-magic marker and see where the tool tip is rubbing.