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

[mmcmdl]

Nope. Unfortunately not. Why?

I was going to send a tool for you to use and try out . Worth it's weight in gold for threading tools . I know I still have it as it was made in my apprentice years . Some things I'll keep forever .

 

[WobblyHand]

I think my options are to either find or make some wedge shims, or try grinding a round piece of HSS to the right form. The wedge shims could rotate the square HSS bits (close) to the right position. Or I could just rotate a round bit a little in the holder to adjust for the lead angle. Sounds like the round HSS is my best option at the moment.

 

[WobblyHand]

I was going to send a tool for you to use and try out . Worth it's weight in gold for threading tools . I know I still have it as it was made in my apprentice years . Some things I'll keep forever .

Sigh, opportunity lost.

A SG is one of those things that I'm looking for. At the moment, there's no room at the inn - something I think you know all about. In my case, there really isn't room for one at my house. Too much other stuff in the way. Someday...

 

[mmcmdl]

A SG is one of those things that I'm looking for.

I'm not , but the neighbor has a Reid he's trying to pawn off on me .

 

[WobblyHand]

Been thinking about this a little more. Sketched something up. It's a holder of sorts to tilt the acme tool bit. Figured it could be made of aluminum to try out. Only issue is I don't have an accurate way to do the angle. Need a sine plate of some sort - which implies having either gauge pins or gauge blocks. Don't have that either. Hmm. The correct angle is 5.594 degrees. 5.6 is ok. Probably 5 degrees would be ok. I have a 5 degree tool, but not sure how I can use it at the moment! (If I cut it, I could use it, but I don't want to at the moment.). To mill this I'd tilt the piece 5.594 degrees clockwise, so the pocket was square to the cutter.

AXA holder. Ground a 3/8 acme 5 TPI tool. The holder tilts the tool 5.594 degrees towards the chuck. Shoot, a lot of effort to avoid spending $75 for the proper blade... (Aloris Acme 5 TPI)

 

[Jake M]

I made an attempt to single point an acme screw (nominally 3/4-5) and it didn't quite come out right. The tool was ground from a 3/8" HSS blank. It's 5 TPI, but the cut thread seems wider than it should be. The pitch is correct. I do know that I didn't make an attempt to compensate for the lead angle.

To sort out the lead angle problem, the first thing you need is the lead angle. Something like 5 degrees I think (longhand mathematics, verify that), which means that the leading edge of your threading tool, to RUB the work without (much) interference, would have to match that angle. Plus you want a working clearance. So that leading edge needs a clearance angle of MORE than 5 degrees.

What I don't understand is the relationship of the lead angle to the shim angle. (I think this is the amount that the tool is rotated to become more aligned with the lead angle.) My Aloris AXA-8 HSS threading tool has a means to make this adjustment, but I don't have acme blades. Not sure I'd want to buy an acme blade right now, as this is most likely a one or two of build. (5 TPI acme blade = $67 from Aloris, which isn't bad, if I was doing this a lot. But this is good for one pitch only.)

The shims, or otherwise rotating the cutting tool do just that, they rotate the tool. So if you did not have enough clearance, rotating the tool "towards" the cut will effectively add more clearance to the leading edge. And also reduce (or reverse) any rake angle on the top of the tool.

Can someone enlighten me on the subject? Or point me to a source that might show pictures and math? Don't want a "calculator" since I'd like to know what it means geometrically first. I haven't yet found a good explanation. Is it just set to the lead angle?

Yeah, that's the idea. I'm guessing it leans more towards insert tools, as they're harder to change such angles on, and/or require different cutters or whatever else. With HSS tools, it's literally just the clearance on the leading edge that gets in the way. Go to the grinder and knock that back (if it interferes), and you're done. Otherwise, or if that's impractical for whatever reason (including insert tooling that doesn't lend well to that), then you'd rotate the tool so that the leading face does not rub behind the cutting edge.

If you have a sample of known good to mediocre, but not "shot" acme rod, just by hand, stick the cutting tool into the thread. Straight, level, and eyeballed on center. If the cutter drops in well, you're good. If the cutter wants to "lean left" as you insert it, there's not enough clearance. That little science project in it's self might be the best explanation of what's going on that you could find...

Also, mmcmdl is onto something there. If you run the thread in with a "normal" threading tool, right to depth (with or without using the compound, whatever the lathe and/or the tool likes best), it won't make a full width thread. But it will "take out" the center of the cut you want. That makes life a lot better for the Acme cutter, which by the time 5 TPI comes around, is a pretty significant form tool. With the "middle" of the cut out of the picture, the two side cuts are collectively a lot easier. All the way down to the "point" of the "regular thread" cut. Just the separation of the left and right sides, the two simultaneous cuts are far lighter than when the whole chip was conjoined as one.

EDIT​

Here's a link to a quick visual. You can see what tilting the tool does, and what grinding a clearance does. The whole video is pretty basic, I linked to a timestamp that shows what I'm talking about.

 

[WobblyHand]

To sort out the lead angle problem, the first thing you need is the lead angle. Something like 5 degrees I think (longhand mathematics, verify that), which means that the leading edge of your threading tool, to RUB the work without (much) interference, would have to match that angle. Plus you want a working clearance. So that leading edge needs a clearance angle of MORE than 5 degrees.



The shims, or otherwise rotating the cutting tool do just that, they rotate the tool. So if you did not have enough clearance, rotating the tool "towards" the cut will effectively add more clearance to the leading edge. And also reduce (or reverse) any rake angle on the top of the tool.




Yeah, that's the idea. I'm guessing it leans more towards insert tools, as they're harder to change such angles on, and/or require different cutters or whatever else. With HSS tools, it's literally just the clearance on the leading edge that gets in the way. Go to the grinder and knock that back (if it interferes), and you're done. Otherwise, or if that's impractical for whatever reason (including insert tooling that doesn't lend well to that), then you'd rotate the tool so that the leading face does not rub behind the cutting edge.

If you have a sample of known good to mediocre, but not "shot" acme rod, just by hand, stick the cutting tool into the thread. Straight, level, and eyeballed on center. If the cutter drops in well, you're good. If the cutter wants to "lean left" as you insert it, there's not enough clearance. That little science project in it's self might be the best explanation of what's going on that you could find...

Also, mmcmdl is onto something there. If you run the thread in with a "normal" threading tool, right to depth (with or without using the compound, whatever the lathe and/or the tool likes best), it won't make a full width thread. But it will "take out" the center of the cut you want. That makes life a lot better for the Acme cutter, which by the time 5 TPI comes around, is a pretty significant form tool. With the "middle" of the cut out of the picture, the two side cuts are collectively a lot easier. All the way down to the "point" of the "regular thread" cut. Just the separation of the left and right sides, the two simultaneous cuts are far lighter than when the whole chip was conjoined as one.

​

My original tool was ground with a 15 degree relief on both sides. Tilting the tool by the required 5.594 degrees still gives plenty of relief on the trailing side, about 9.4 degrees.

I don't understand using a normal 60 degree tool in a 29 degree hole. It will be wider. Explain that please. I'm dumb, use a sketch or something, because I'm not visualizing it at all. The thread depth of a 5 TPI acme is 0.1". Oh, a light bulb is going off! Just did the math.

The width at the top of the acme is (1-0.3707)*P = 0.12586", (P=1/5) but the 60 degree tool cut a width of 2 * 0.1 * tan(30) =0.11547". Oh, very clever. I will do that next time. Nice trick.

At the moment, I'm thinking of making a holder to tilt the tool bit. Of course it will cost me $240 in tooling, (3" sine vise plus some spacer blocks) which I'm not keen on, although having a sine vise would be extremely useful. Far less useful would be just to buy the AC-5 acme blade for maybe $75. Unfortunately, the AC-5 blade is a one trick pony. It only cuts 5 TPI acme. I'd use my AXA-8 to adjust the lead angle. Leaning towards the sine vise due to the future utility.

 

[pontiac428]

Would it not be thread lead angle plus relief angle for the flank? Side rake only for the top, set the compound for 1/2 the included angle and go until you hit depth? Or am I under-thinking it?

 

[WobblyHand]

@Jake M thanks for adding in the video.

 

[Jake M]

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

My original tool was ground with a 15 degree relief on both sides. Tilting the tool by the required 5.594 degrees still gives plenty of relief on the trailing side, about 9.4 degrees.

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.

Oh, a light bulb is going off!

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.,

At the moment, I'm thinking of making a holder to tilt the tool bit. Of course it will cost me $240 in tooling, (3" sine vise plus some spacer blocks) which I'm not keen on, although having a sine vise would be extremely useful. Far less useful would be just to buy the AC-5 acme blade for maybe $75. Unfortunately, the AC-5 blade is a one trick pony. It only cuts 5 TPI acme. I'd use my AXA-8 to adjust the lead angle. Leaning towards the sine vise due to the future utility.

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.