What's wrong with my threading technique?

[danlb]

Interesting question; was the left flank tearing or the right flank or both?

When feeding at 29 degrees the majority of the cutting should be on the left side, with the right just getting a very light cut. Only one side should be tearing.

Dan

[J Tiers]

The PD does not "set the fit" by itself. It has a tolerance range , just as the major diameter does.

It would be just as true as your last two sentences if one wrote

"The MD of a thread is the MD whether the PD is at max or min. It sets the fit."

A thread does not meet the specs for its class unless both the PD and the major diameter are within the tolerances for that class.

Please.... you are agreeing.... sort of. (Not about the PD, we disagree there)

point is that a particular numeric MD does not correspond to one and only one particular numeric PD. And the PD does not correspond to one and only one MD.

And the PD can be dead center at the "perfect" size, while on the same screw the MD can vary anywhere in its tolerance range. A tolerance is a tolerance.

I claim that the PD DOES "set the fit", but the MD does not. PD DEFINES the class of thread, because it sets the clearance between the A and B threads in any thread class. The major and minor diameters have to be in tolerance, but if you have to quote ONE spec for a thread, it will be the PD, because that is the spec that has the most direct influence on the thread fit.

I read your posts as suggesting that the nominal OD was located at the top of the theoretical sharp V thread, instead of at the flats. If I misunderstood, I apologize.

So your point here is that the OLD (sharp V) 5/16-18 was 5/16" nominal at the top of the V, where the NEW 5/16-18 (UNC) is NOW nominal 5/16-18 at the OD of the perfect 1/8 pitch flats? Just trying to get a handle on what you are saying........

According to that, then the PD of the NEW UN thread is slightly LARGER than the PD of the old sharp V. According to that idea, the PD would have to be bigger, since the MD is bigger by the difference of thread hieght, and the thread form is fixed as a triangle at 60 deg. So if the tip of the triangle was at 5/16" dia with a sharp V, then if it is moved up to put the truncated triangle at the same 5/16" diameter, then the PD will HAVE TO BE larger/

So, just to put you on record here, is that what you are saying?


P.S.: pay no attention the the numeric PDs I gave in a prior post about the two screws.... I think I read my scrawled numbers wrong off the note I had, because they do not make sense..

[754]

Re the title, problem was not technique, it was setup.
Threading for me us fun, with a bit of practice it's pretty easy..

[lakeside53]

As above.. it should not be a chore... and quite enjoyable. The key is to practice and eliminate variables. If you can create perfect threads of similar size/pitch on scrap (but known materials),but not on the gun barrel, then think material; hardness, type, speed, feed, tooling etc. adjust accordingly, but factor at a time..

Can you guys arguing about the minutia make a new thread? or maybe the OP should. The problem now is simply "quality of finish".

[J Tiers]

As above.. it should not be a chore... and quite enjoyable. The key is to practice and eliminate variables. If you can create perfect threads of similar size/pitch on scrap (but known materials),but not on the gun barrel, then think material; hardness, type, speed, feed, tooling etc. adjust accordingly, but factor at a time..

Can you guys arguing about the minutia make a new thread? or maybe the OP should. The problem now is simply "quality of finish".

OK Mr Moderator...........

But, since he is working with 4140, which does give a nice finish in general, it would seem that ir has to come down to speed, cutter and lube. Do we know all that for the bad finish case?

As I think has been mentioned a few times, carbide "tends to be" somewhat dull compared to HSS, and does not do as well when cutting at small DOC and slow SFM.

AND another point, the threading tool is often flat-topped, since that makes it easy to generate the correct shape of tool that will cut the correct threadform regardless of relief below the cutting edge. but a flat topped tool is basically a "snow plow", and will throw burrs, as well as having higher cutting forces, and more tendency to tear the metal.

A tool with rake away from the cutting edges should cut a cleaner thread than the flat fronted "plow" tool. It is more difficult to grind, though.

[lakeside53]

As I think has been mentioned a few times, carbide "tends to be" somewhat dull compared to HSS, and does not do as well when cutting at small DOC and slow SFM.

AND another point, the threading tool is often flat-topped, since that makes it easy to generate the correct shape of tool that will cut the correct threadform regardless of relief below the cutting edge. but a flat topped tool is basically a "snow plow", and will throw burrs, as well as having higher cutting forces, and more tendency to tear the metal.

A tool with rake away from the cutting edges should cut a cleaner thread than the flat fronted "plow" tool. It is more difficult to grind, though.

Not sure who "mentioned it", but quality carbide threading inserts are most certainly not dull (razor sharp is how I'd rate them) and work great at low rpm.

The most common type (ER lay down) are not flat topped - they are very positive rake with appropriate side rake and helix. For those that haven't used them, try some If you don't want to make the $$ investment and for some reason want to get the correct grind on a positive rake etc HSS threading tool, maybe copy the geometry of a lay-down insert.

[J Tiers]

Not sure who "mentioned it", but quality carbide threading inserts are most certainly not dull (razor sharp is how I'd rate them) and work great at low rpm.

The most common type (ER lay down) are not flat topped - they are very positive rake with appropriate side rake and helix. For those that haven't used them, try some

I do not recall either. Probably lots of folks, it seems to be one of those "common knowledge" points that are not necessarily so.

However, my "stand up" carbide inserts are indeed NOT "razor sharp". They are more "dull". And the top is flat, so it "plows" instead of cutting. The lay-down "topping" inserts (full form) should be very good, but you need one for every pitch.

[lakeside53]

Not true.. Although the full-form lay down types are nice, the 16ERAG60 is rated for 8-48TPI. Several other sizes also; the A60 is is rated for 16-48 tpi. Small compromise though... the root is going to be that of the finest thread in the range.

[Jim Stewart]

I use carbide inserts for nearly everything on the lathe, but not for threading. For that I use an 1/2" HSS tool, ground (by me) to 60 degrees according to my fishtail, and honed with a diamond stone. Since my grinder (rest set to 7-10 degrees) hollow-grinds the flanks, the hone has only to touch up the top .050 or so on the finished tool.

I've had no problems at all threading gummy or hard steels (or aluminum or brass) - really crappy steel will need cleaning up, just as it does when turning.

I thread with compound infeed at 29.5 degrees.

-js

[J Tiers]

Not true.. Although the full-form lay down types are nice, the 16ERAG60 is rated for 8-48TPI. Several other sizes also; the A60 is is rated for 16-48 tpi. Small compromise though... the root is going to be that of the finest thread in the range.

You HAVE TO BE wrong on that.....

A "topping" or full-form insert has to have the size and form for a specific thread, or it cannot produce the correct form. They cut the entire thread profile, crest and root, so they would be specific to a particular single pitch (but not to any specific single diameter).

The stand up insert I have are as you say, with the finest root flat in their range. But they (and presumably the inserts you use) are not ful form, they cut only the inside of one groove, the topping type cut over the crest and into the adjacent groove (ok, yes it is all one groove, but you know what I mean).

[andywander]

.....So your point here is that the OLD (sharp V) 5/16-18 was 5/16" nominal at the top of the V, where the NEW 5/16-18 (UNC) is NOW nominal 5/16-18 at the OD of the perfect 1/8 pitch flats? Just trying to get a handle on what you are saying........

According to that, then the PD of the NEW UN thread is slightly LARGER than the PD of the old sharp V. According to that idea, the PD would have to be bigger, since the MD is bigger by the difference of thread hieght, and the thread form is fixed as a triangle at 60 deg. So if the tip of the triangle was at 5/16" dia with a sharp V, then if it is moved up to put the truncated triangle at the same 5/16" diameter, then the PD will HAVE TO BE larger/

So, just to put you on record here, is that what you are saying?


P.S.: pay no attention the the numeric PDs I gave in a prior post about the two screws.... I think I read my scrawled numbers wrong off the note I had, because they do not make sense..

Correct-except I not sure that there was ever a standard for a sharp V 5/16-18 thread. The Sellers thread, the precursor to the UN thread form, was around for quite a long time, and I believe it was one of the first thread standards.

[lakeside53]

You HAVE TO BE wrong on that.....

A "topping" or full-form insert has to have the size and form for a specific thread, or it cannot produce the correct form. They cut the entire thread profile, crest and root, so they would be specific to a particular single pitch (but not to any specific single diameter).

The stand up insert I have are as you say, with the finest root flat in their range. But they (and presumably the inserts you use) are not ful form, they cut only the inside of one groove, the topping type cut over the crest and into the adjacent groove (ok, yes it is all one groove, but you know what I mean).

Not wrong Look it up. AG60 are NOT full form and they certainly cut both sides of the groove. I rarely bother with anything other than "straight in" but I have the rigidity to do that.

[J Tiers]

Not wrong Look it up. AG60 are NOT full form and they certainly cut both sides of the groove. I rarely bother with anything other than "straight in" but I have the rigidity to do that.

OK, not full form. That makes sense. What I meant was the actual full form.

They cut not just "both sides", but also right round over the crest, for the "full form" inserts. So the total thread height is cut, and is of course specific to the pitch.

[danlb]

Not "both sides", but right round over the crest, for the "full form" inserts. So the total thread height is cut, and is of course specific to the pitch.

Jerry, you are arguing the wrong side of the argument. Lakesides is saying that the 16ERAG60 is partial profile, and the web sites I've found back him up.

You are right that a full profile insert is only for one pitch. No one is saying that it isn't. Properly used, a full profile insert can be used to not only cut the thread but to trim the crest to the specified diameter while also getting the thread depth and pitch diameter spot on.

Dan

[lakeside53]

To throw another money wrench into this I've used (with care) full-form inserts for other than the "correct pitch"; basically used a larger size and not the full depth - i.e using it like a partial profile insert. Doesn't work well with UNJ types (controlled root, very common around here due to Boeing), but if you don't have the "right" insert it sure is a good way to use up an excess of certain sizes, like a few boxes of Mxx X 1.5 and 1.0.