Does it really matter if it is 29, 29.5, or 30

[ronm]

Good Grief!!! All these years I thought it was enough to just try a nut on it, if it goes on nice you're done...didn't know it could be a matter of life & death!!!
Just kiddin' Harold, all this is good info, & I do intend to get a set of thread wires someday...

[Mr Ron]

Armed with that thought, I choose 29°, and set to the mark.

Are you saying go by the 29° mark on the compound? I was thinking I had to use a protractor to set the compound relative to some other part of the lathe, the chuck or faceplate for example.

[Harold_V]

Armed with that thought, I choose 29°, and set to the mark.

Are you saying go by the 29° mark on the compound? I was thinking I had to use a protractor to set the compound relative to some other part of the lathe, the chuck or faceplate for example.

Yes, that's exactly what I'm saying. The reason is, the angle isn't critical, so long as you set such that it's less than half the included angle of the thread. 28° will work just fine, too, so it really doesn't matter. The primary reason for feeding with the compound is to give the tool cause to cut on one side only, so there's no conflict in chip flow. By choosing an angle that is just slightly less than the half angle of the thread, you ensure that the backside (trailing flank) isn't deformed by feeding at a rate which causes the tool to move away with each successive pass. Anything greater than the half angle would do just that, so it's risky to set the compound @ 29½°, assuming the marks are not true. The lesser angle provides for a wiping action with each pass, hopefully keeping the trailing flank clean and of proper geometry.

To put this in perspective, setting the angle plays no role in the final outcome of the thread form----UNLESS----the compound is set beyond the half angle. Anything less makes absolutely no difference in the thread form. That is determined by the tool, and if it is set on center, or not.

In practice, especially when machining materials that don't cut well, a few very shallow plunge cuts (less than .002" per pass) at the end of the threading process will generally improve finish on both faces, although a very sharp tool is required.

Harold

[coal miner]

Thread form , provided the tool bit is ground properly , will be determined by using the fishtail gauge carefully . Very carefully !

That is determined by the tool, and if it is set on center, or not.

29º or 29-1/2º does not matter as long as it is less that the 1/2 of the 60º , for US thread angle .

In practice, especially when machining materials that don't cut well, a few very shallow plunge cuts (less than .002" per pass) at the end of the threading process will generally improve finish on both faces, although a very sharp tool is required.

I follow this advice to the letter and usually just need some emery cloth to take the burrs off for a nice thread .

[shootnride]

In practice, especially when machining materials that don't cut well, a few very shallow plunge cuts (less than .002" per pass) at the end of the threading process will generally improve finish on both faces, although a very sharp tool is required.

Harold

Harold,
Could you explain the "plunge cut" process in a little more detail. Does this step require any changing of the compound angle ?
Thank you,
Ted

[JackF]

Ted,

Pretty sure Harold was talking about using the cross slide, will of course send the tool straight in, instead of the compound for your last couple of passes. Am also pretty sure Harold will correct me, wouldn't be the first time he has needed to do so, if I am wrong.

Jack.

[shootnride]

Ted,

Pretty sure Harold was talking about using the cross slide, will of course send the tool straight in, instead of the compound for your last couple of passes. Am also pretty sure Harold will correct me, wouldn't be the first time he has needed to do so, if I am wrong.

Jack.

Thanks Jack,
That makes sense. I kept re-reading the posts here and never did find an explanation of "plunge cut". Guess that shows just how much of a novice I am at this hobby.
Ted

[ctwo]

Don't forget about the "spring" cut...

[BadDog]

Sorry, I'm no authority, but I avoid "spring cuts" at (almost) all cost. If I'm shooting for a target, I shoot to hit it at the load expected. If I'm shy and take a "spring cut", I can never know if it will cut at all, or over shoot, it's a crap shoot depending on too many variables.

Also, make sure that you set < 30* off the cross slide, not the spindle axis. Some compounds are apparently marked with zero parallel to the spindle axis, which would require setting at > 90*.

The bit itself is often set to 60* included using a "fish gage" looking for light transmission through gaps till there is none. But that does not give you anything on point radius required based on thread pitch. For most HSM it's mostly a guess. Somewhat closer with good magnification, strong light, and pin gages. Better still using a projection comparator or even a simple pocket magnification comparator (Enco less than $50 IIRC). Then the fish gage is used again to set the tool perpendicular to the work.

For all but the finer threads, feeding with the compound at less than the half angle is certainly the way to go. The lighter the lathe and tool (or flexy material, or hard/ductile, etc), the more important it is to feed at an angle. With heavier machines, heavier/better tooling, granular (cast iron mainly, some bronze) or softer (aluminum) material you can get away with straight in-feed with cross slide. I've seen it done on smaller lathes with compound removed for added rigidity too. I've also seen "up-side-down" back side threading (and parting) tools always fed straight in, but for a variety of reasons have little use for that myself.

As an example, using my rather large 4.5k 7.5 hp lathe and a Kennametal Top Notch threading tool (I can't recommend highly enough, I love mine!), I have no hard and fast rule, but for typical structural or annealed steel of sufficiently rigid cross section and a pitch of greater than around 16 tpi, and where fit/finish/tolerance are not of too much concern, I often do feed straight in with the cross slide and get good (enough) results with no drama. But if I want to shoot the moon for whatever reason, I do feed in the compound at ~29* and make the last few 2-3 thou in-feeds on the cross slide for "cleanup". The times I don't are, for instance, the "torch light" base adapter piece I made not too long ago. Hollow to pass a wire through, something like 10 mm in diameter with (IIRC) a 1 mm thread pitch. No thread wires, no worries, a suitable size piece of tubing with a glob of epoxy would have been fine. Heck, it could have been cut with an old TPG insert and worked fine too. I probably was not at the machine for more than 2 minutes start to finish. And that is where I just dial the in feed and go. NOT for making what most would consider "machinist work".

Just remember, feeding straight in causes all the forces working against you to increase dramatically rapidly. And attempting to address one of the introduced problems generally works toward increasing the problems in other areas. IMO, for most, best to just follow the rule of thumb to use the half angle in feed.

[Harold_V]

In practice, especially when machining materials that don't cut well, a few very shallow plunge cuts (less than .002" per pass) at the end of the threading process will generally improve finish on both faces, although a very sharp tool is required.

Harold

Harold,
Could you explain the "plunge cut" process in a little more detail. Does this step require any changing of the compound angle ?
Thank you,
Ted

Sure. Happy to try to help.

No change of the compound.

As Jack suggested, you feed with the cross slide, simply by going beyond the established stop point, which is usually 0. That's another of the benefits of using wires---as you approach the required pitch diameter, you stop feeding with the compound and begin taking the last few thou with the cross slide----by plunge cutting.

I normally do NOT endorse plunge cutting threads, but if you follow my recommendations, the reason becomes clear instantly, as you witness the improvement in the thread. Doesn't work so well if the tool is dull, so I commonly remove the threading tool for a grind before taking a thread to size. I've even done that in a production environment, when quality is on the line.

Yeah, it's slow---but the practice in picking up a thread is very good, especially for those who lack experience--and the improved finish makes it worth the effort. Besides, as you become more skilled, it takes very little time.

Harold

[Harold_V]

Don't forget about the "spring" cut...

In reality, the plunge cut *IS* the spring cut. One develops a routine---and stops short of the desired pitch diameter, often for a sharp tool, so there is no real spring cut--and the cuts that are taken are with a very sharp tool---the material cuts just as it should. The one exception may be when cutting mild steel, where tearing is an ongoing issue.

In so far as taking the last few passes by plunge cutting, it all depends on what you're willing to settle for in the way of threads. I had a reputation to uphold--I spent the time to ensure the end result was good. Some folks don't care---and often it just doesn't matter.

No right or wrong in this issue---just a heads-up on how to improve the quality of threads you generate.

Harold

[shootnride]

Harold & Russ,
Thank you both with the assistance about "plunge cutting" and threading in general. I have very limited experience with threading, and even though the threading I have done seemed to work o.k., I think it was just pure luck because I certainly wasn't doing things correctly.
I am currently working on a little shop project and have a need to cut both internal and external 1 1/4-12 threads. I plan to do some practice so I don't destroy the parts that I've already invested several hours in. I think all of this "tutoring" will be a great help.
Thanks again,
Ted