How to determine anvil or shim angle for single pointing an acme thread with a lead angle of 5.59 degrees

[Ischgl99]

Instead of a sine bar, you can make a custom angle plate to hold your fixture to machine that angle. Joe Pie shows how to do it in the video below.

The attached Carmex document is meant for inserts, but might be useful to you for determining the lead angle/tool relationship.

 

[WobblyHand]

Sorry I snuck in there on you, it was an afterthought. Glad you caught it.



If that was the case, there is no need to tilt the tool. At 15 degrees on the leading edge, it will not rub. At 15 degrees on the trailing edge... There was no need for relief, except that the grinder is probably set up that way by default. No harm in it (that's how I do it), but it is not necessary. And all in all, this does not seem to be an issue. There should be no need to rotate that tool.



Indeed... A 60 degree V thread tool is proportionally wider at the top, but an acme thread is proportionately shallower than a V thread, so the V thread never hits full depth...

While I still can't do 5 tpi, without serious workarounds, that light bulb took me and my little South Bend from "this lathe is not rugged enough to cut Acme threads" to "this can do acme threads "within reason". (Alas 5 tpi Acme is still not in the 9A's vocabulary, but I'll take 8 max and call it a win.,



Don't let me talk you out of new stuff you'll use... But I really don't believe that you "NEED" to do this. If your threads are coming out wider than the tool, AND you have clearance enough for the lead angle ground into the tool... Well, if it were me, I'd want to diagnose the problem before I spent money. And clocking the tool (money) is, in the end, achieving the SAME effect as providing clearance on the leading edge. I don't pretend to have an answer yet, but I think something else is going on.

It's against the rules to talk someone out of new (good) stuff to buy.

Think you are right about something else going on. I'll have to spend some quality time with the tool and the short screw I cut and see if I can figure out what happened. I'll put them both on the desk (maybe under magnification) and see what might happen.

 

[WobblyHand]

As a wag, the bottom of the thread is about 1/8" wide! The crest of the thread is correspondingly narrower, about 0.04. The pitch is 0.200. Both the valley and the crest ought to be about the same and around 0.0741". Hmm.

There could be an ELS problem, but at least for 3 full threads, the pitch is dead on. I hit the mark every time, and I used #1 each time. I know I didn't miss it, because I start with enough room to catch it in case of a mistime.

I suppose there could be serious deflection going on since a) I went straight in (did not do modified flank cut at 14.5 degrees) and b) I didn't pre-cut using a 60 degree tool. The lathe was working hard. Guess I will try fixing both a) and b) before doing anything further. I definitely have enough relief. I have 15 degrees relief at the nose and on both sides.

 

[mmcmdl]

You still working on that broom ?

 

[Jake M]

As a wag, the bottom of the thread is about 1/8" wide! The crest of the thread is correspondingly narrower, about 0.04. The pitch is 0.200. Both the valley and the crest ought to be about the same and around 0.0741". Hmm.
View attachment 508997View attachment 508996
There could be an ELS problem, but at least for 3 full threads, the pitch is dead on. I hit the mark every time, and I used #1 each time. I know I didn't miss it, because I start with enough room to catch it in case of a mistime.

I suppose there could be serious deflection going on since a) I went straight in (did not do modified flank cut at 14.5 degrees) and b) I didn't pre-cut using a 60 degree tool. The lathe was working hard. Guess I will try fixing both a) and b) before doing anything further. I definitely have enough relief. I have 15 degrees relief at the nose and on both sides.

Still no answer, but A, it "looks" like some chatter on the left side... Not sure that's relavent, because I can do a better job of making chatter marks than that, but it doesn't misshape the cut by that much...

The bottom appears pretty flat. The bottom is wider than the tool. That would suggest to me that "somebody" lost some time, "somewhere"

I havn't scienced this, but I'm pretty sure that if your dial was out "that much", you'd see it. I don't think "missing the mark" by the 1/32 increments that I "presume" match your threading gear would account for that. I think it'd be a big miss.

ELS? I don't recall knowing you had that. Cool.... If that ELS lost position somehow, so close to the end of the job that it got the bottom flat, it looks like the last cut would have had to have taken a sixteenth of an inch off of one side or the other. I'd suspect you'd notice that, if you were expecting a normal infeed and the same cut you'd been getting....

Very curious...

 

[WobblyHand]

You still working on that broom ?

Not really. I found a couple spare handles. But I'm a stubborn guy and want to learn how to do Acme threads, just because sometime, I'll need to make one for real.

 

[WobblyHand]

Still no answer, but A, it "looks" like some chatter on the left side... Not sure that's relavent, because I can do a better job of making chatter marks than that, but it doesn't misshape the cut by that much...

The bottom appears pretty flat. The bottom is wider than the tool. That would suggest to me that "somebody" lost some time, "somewhere"

I havn't scienced this, but I'm pretty sure that if your dial was out "that much", you'd see it. I don't think "missing the mark" by the 1/32 increments that I "presume" match your threading gear would account for that. I think it'd be a big miss.

ELS? I don't recall knowing you had that. Cool.... If that ELS lost position somehow, so close to the end of the job that it got the bottom flat, it looks like the last cut would have had to have taken a sixteenth of an inch off of one side or the other. I'd suspect you'd notice that, if you were expecting a normal infeed and the same cut you'd been getting....

Very curious...

There is chatter to be sure. It's only a 10" lathe, but more importantly, a 4Nm stepper motor. The stepper is closed loop and it did not lose steps. If it loses a step, it stalls! I can assure you that a stalled stepper inside a thread is not good.

I'd agree that it looks like it's out of time. But I've never made a bad V thread with my ELS before, especially with 8 TPI and greater threads. The threads are better looking than me.

I was threading at 100 RPM, so I can't see that I lost spindle counts, the system has been tested for rates over 5000RPM and doesn't lose count. (Using an encoder simulator, not the lathe - my chuck isn't rated that fast.).

I'll check for half nut backlash. Next try will be pre-cutting with a V tool first (30 deg modified flank) and then acme with the 14.5 deg modified flank. Probably overloading the platform, it's not a big lathe by any means.

 

[RJSakowski]

Is your tool height centered on the spindle axis? If the tool was too high, it would cut into the left flank.

Regarding the thread dial, my thread dial markings on the 602 were not concentric with the shaft which could vary the timing by +/- a quarter number. I made a new thread dial which solve the problem. Note that since you are using the same number each time,it is not as likely to be your problem. https://www.hobby-machinist.com/threads/threading-on-a-grizzly-g0602.34230/

 

[Ischgl99]

Is it possible your work piece rotated in the chuck? If it did during the heavier cuts, then I think it might look like that. Maybe try another one and use a Sharpie to make a mark on it relative to the chuck jaws and see if it is moving.

 

[WobblyHand]

Is your tool height centered on the spindle axis? If the tool was too high, it would cut into the left flank.

Regarding the thread dial, my thread dial markings on the 602 were not concentric with the shaft which could vary the timing by +/- a quarter number. I made a new thread dial which solve the problem. Note that since you are using the same number each time,it is not as likely to be your problem. https://www.hobby-machinist.com/threads/threading-on-a-grizzly-g0602.34230/

It should be, I'll check again!