Angles and threading

Ah, Harold, on the *first* pass, it doesn't matter how you set the cutting depth before starting the cut. The cutter doesn't know, the work doesn't know, the carriage doesn't know. All they know is that a squarely set cutter is addressing the blank work at a certain depth of cut.

Only on the second and subsequent passes will advancing the depth of cut with the compound at an angle cause the cutter to bite more into the trailing flank of the previously cut portion of the thread (leading flank of the cutter), and thus load any lash out of the system.

It *is* the second and subsequent passes which correct any initial drunkenness of the thread. That's the whole reason for doing it this way.

Gary

????!

Gary

Yep, one of the most common errors one sees when novices set up to thread, is that they calculate the depth the compound needs to travel, they take one scratch cut, and then wind in the whole thing for the next cut.

For a sombody new to threading, it makes sense to take it slow and find out how the machine behaves.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

I did that!

And not all that many years ago. )o:

Bill D

Of course the first pass makes no difference, the tool has no reference point as yet, all it knows is there's metal to be removed. My statement alludes to the first pass only in mentioning that *should it have error*, it then would be difficult to eliminate when poor threading practice is employed. Succeeding passes will, indeed, follow the first one, they have no reason not to (with exception*, mentioned below). The carriage was free to do something it shouldn't have the first time, so there's nothing to prevent it from repeating once the pattern has been established. Regardless of the mindless statement from one individual that thinks he they can walk on water, drunken threads are a distinct possibility when

*improper* feeding methods are employed. That doesn't mean you'll get them automatically. *It's been my experience that when you encounter guys that don't have a clue about potential problems on machines, they are also negligent in machine care. They're the ones that likely have ways that are covered in black dirt, yielding so much resistance to movement that the friction loads the carriage such that they don't risk drunken threads. That's a poor way to guarantee good threads. Oil on the ways of my machines always looks like oil. My machines are wiped and oiled each time they are used, and often during the day as well. Moving members have "feel", a necessary feature for doing fine work. Black ways are a sign of a hack, not a machinist.

Correct. I'm not the guy that doesn't understand the concept, nor lacking in practical experience, and a lot of it. You should be addressing this to the one that walks on water, who appears to think he's exempt from the rules.

The first pass will also be correct under normal circumstances, which may account for a high degree of success even when poor practice is employed. The first cut dictates, so as long as it's good, the rest will likely be as well. I don't hope for that to be the case. I use proper threading techniques.

When plunge threading, the first pass can be ultra critical, especially if it's too deep. Because both faces of the threading tool are generating a chip, the loading at the tip is high, very high indeed. Early carbide failed almost routinely under those conditions. HSS is even prone to failure, thanks to tool tip welding because of the excessive pressure. Once a face has been established by proper feeding, the condition subsides considerably.

Harold

While my practice is to take slightly larger passes, it's hard for those that have never chased threads to understand what a threading tool goes through, especially on the first pass where there's considerable conflict with chip flow. Many of us can attest to tool tip breakage or abrasion from deep passes, particularly with the first pass or two. Jim has an excellent point, starting out light, learning to understand what each machine is capable of handling, and honing single point skills is the way to go. I recommend his procedure to the newbie.

Small machines may very well do better with tiny cuts. The only negative of taking light cuts is if the tool is not kept very sharp. All of us have had the negative experience where we feed a thou, take the pass, no chip, feed another thou, take the pass, no chip, feed a thou and it takes all of them, plus maybe a couple more, thanks to tool tip loading. Keeping the tool *sharp*, and *on center* is critical to good results.

Harold

While it may not be too kind to some, that's somewhat what I mean. I'm all for shortcuts that are effective, and all for new innovations, but plunge threading on an engine lathe when proper compound feeding is an option is nothing less than a *dodge*. I'm not a greenhorn, Bill.. I've chased threads by almost any possible means in an attempt to cut time, to increase production. Plunge threading saves very little time, but adds the possibility of error. I don't consider that a *legal* option. It's like choosing to drive faster than traffic is moving. Yeah, for the most part it works, but there are times when it is the cause of an accident. You can avoid that particular hazard by not doing it.

Understand, Bill, that I don't tell folks to do it my way, but the way that is acceptable in industry. I am not the author of the process, I am simply a believer of information provided by those that went before me, many of which were far more wise than you or I can hope to be. Because I understand the ramifications of threading by other means, I strongly advocate *proper* procedures.

My personal habit was to work in the best way possible, to not take unnecessary chances where quality was concerned. Where I came from, you were judged by your ability to do it right. Nothing else mattered, not even high production. Ten good parts were far better than twenty good ones and three bad ones.

When I make statements about plunge threading, or twist drills not drilling straight holes, that is intended to send up a red flag for those that don't understand the possibility of error. There are those that can't seem to grasp the concept that it is a warning of potential failure (not impending failure), and choose to ridicule the advice. To the man, each of them will face the problems in due time, so I sit back smug, seeing them as fools. The best of us have been beaten by machines, regardless of the degree of skill we may have.

I hope you understand that my attitude is not one of a personal attack, Bill. It's not intended as such. Wise people read, file for future reference, and go on, better off for having been cautioned. That's what others did for me while I was coming up in the trade. I feel I have an obligation to pass it on.

According to a close friend, one that is a mold maker and very CNC literate, the feature of angular feeding is an integral part of software, but you pay in addition to the basic charge to have the feature turned on. I trust his judgment and opinion. I do not know it first hand. For many, the additional cost is reason to plunge thread. On CNC's, with ball screws (thanks, DoN) and higher spindle speeds, drunken threads are far less likely, and the higher spindle speeds tend towards fewer problems regards chip flow, so the success rate is likely much higher than a like operation on an engine lathe. As I said, I may endorse such procedures on the CNC, but would likely never do so on an engine lathe, if for no other reason, the poor results I've experienced in my years in the shop. I set my standard higher than that.

Harold

reference

statement

The Artillery cuts a thread.

Ha ha! I don't know why I do this. I guess I am a sucker for punishment: In a way I am lucky because I have about zero experience in turning and can only rely on observation and my own common sense. So, with no reputation to defend I am pretty much free to say what I think is right.

The reason I brought up threading without setting the compound to (30) is because I think it can be done and it is done with no adverse effect. I would expect that billions,,, well millions, of threads are cut in exactly that way every year. So I do not discount it as a valid method of cutting threads so people should be made aware of it, and it's shortcomings.

Next, In so far as this drunken thread bit is concerned. I do see where it could occur. Perhaps in cutting some very* (like a screw jack to lift a 707) large thread where you have the compound set at (30) in the wrong direction. I certainly know that it could not occur on a plunge cut because the cutting forces cancel one another out which means that the cutter and saddle have no means to resist being pulled along by the advancing thread in the half nuts and to suggest that some nonexistent force could somehow propel the cutter and saddle out ahead of the already advancing thread is to much of a stretch for even me.

If you have any doubt about the a single point cutter pushing itself and the saddle ahead of the rate of advance of the lead screw then just open the half nuts and watch and see if it finishes the thread under it's own steam.

My .02$ ; Anybody else, doctors, carpenters, whoever, have any thoughts on the subject?

Bill PS Harold I just read your last two posts and I do not suppose you think of what you say as a personal attack but this is a hobby group and most of the men are accomplished in their respective fields. You are respected for your knowledge in this, the field that so many of us want to learn, so your words are welcome but we are all free to say and think. You need not attack, browbeat or smother people with your machining credentials to drive them into submission. This group is supposed to be having fun. I am not interested in how well you oil your machine. The auto oiler in my 65 year old Monarch does that for me. You do not need to explain that no one else understands what you do.

I take your word for it. You are as good at your chosen trade as I was at mine, or the Doc or the carpenter. Get on with it and enjoy each other.

employed.

threading

especially if

generating a

failure,

considerably.

Hoyt, he's already mentioned that he's never done that, and that his comments were specifically steering *around* cnc machines and techniques. Honest.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Perform the following test: begin to cut a long thread on a machine, of fairly fine pitch, say 16 tpi or so. During the cut, grasp the carriage traverse handwheel and retard its motion - that is, push backwards against it motion.

Because the half nuts are loaded in that direction, obviously nothing will go amiss.

Now repeat the exercise on the next cut - but this time, slightly

*advance* the traverse handwheel - and of course you will see the lash in the half nuts (which is always present in a manual machine, to some degree) allow the cutter to ride forward and ruin the thread form that is present. It may even go a full 1/32 of an inch and cut off the threads entirely.

Now the thought process is that when the carriage is under way in the cut, the only thing that is really retarding it's travel is the friction of the ways. This can be fairly indeterminate depending on the kind of machine and its condition.

In most cases the friction of the carriage is plenty to load up the halfnut/leadscrew combination and prevent any lead errors, even if the actual cutting forces are ballanced.

And they are pretty well ballanced if the tool is cutting on both sides. In that case it really is the frictional forces on teh ways that keeps the halfnuts loaded up, and the lash out of the system.

In some machines however, the frictional force from the ways might be low enough so that a small imballance in the cutting force could allow the force from the right side of the tool to overcome the left side, and push the tool ahead of where the lead screw says it should be. Obviously the carriage is not going to zoom ahead of the lead screw by any huge amount, but one were to measure the force on the leadscrew bearing (that ultimately takes the force of teh cut) you would see it hovering around zero, with a fair amount of variation.

Using the 'angle feed' technique will cause the load on that thrust bearing to be a larger value, but more importantly it will be more constant. The accuracy in the lead screw will be better transmitted to the thread form being generated.

One can easily imagine that the conditions where a straight-in feed could cause problems would be:

1) machine with autolube on the carriage, 2) machine with low friction ways othewise, 3) very coarse thread pitches where the cutting forces are large in general.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Well, I don't suggest that. But taking little 0.001 skimming cuts is going to take all day. For example, cutting 1/2-13 at 0.001 per pass would take more than 45 passes.

I figured you must have slipped a decimal point.

Gary

I've slipped plenty of bits in my time, but that wasn't one of them. For a novice, first time out, it's the best way to go.

Sure if I were cutting 1/2-13 on my machine here at home, depending one *what* the material is, I know that the first three or so infeeds will be five thou or so on the compound. Then I would step down from there to about two thou, and the last few cuts would be a thousanth at a clip to get it to look nice.

But for a novice to factor in

How springy is the stock (diameter, extension)? What's it made of (delrin, or stainless?) What's the pitch?

That's a lot for a novice, better to just give them the 'short' answer to start with. I would however bow to Errol's approach if he would care to say how he tells 'em.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Like I said, arrogant bastid, aintcha? You seem to have no experience with preloaded *zero-lash* ball screws as used in CNC. There is no way the carriage can get out of sync with such a device.

Sheer presumption.

True!!

Ever run a good quality CNC?

(snip)

Intent here is clearly to swagger rather than to help the newbie who sought advice on using the machine he has. Telling him how you'd do it on a CNC machine is worse than no help; it's setting him up to fail. My respects and salutations to those posters who have offered good relevant guidance based on lots of experience (and success) with machines similar to the one in question and to those that most of us have and use.

Perform the following test: begin to cut a long thread on a machine, of fairly fine pitch, say 16 tpi or so. During the cut, grasp the carriage traverse handwheel and retard its motion - that is, push backwards against it motion.

Because the half nuts are loaded in that direction, obviously nothing will go amiss.

Now repeat the exercise on the next cut - but this time, slightly *advance* the traverse handwheel - and of course you will see the lash in the half nuts (which is always present in a manual machine, to some degree) allow the cutter to ride forward and ruin the thread form that is present. It may even go a full 1/32 of an inch and cut off the threads entirely.

Hello Jim

That is an excellent means to illustrate the slack that exists in the carriage / halfnut / lead drive system but such slack exists in any lathe I have ever used. I use the phenomenon routinely to break up the razor sharp hot stream of swarf coming off of roughing cuts. It exists independent of the compound or it's setting. (as you know)

Now the thought process is that when the carriage is under way in the cut, the only thing that is really retarding it's travel is the friction of the ways. This can be fairly indeterminate depending on the kind of machine and its condition.

Yes there is the friction load, but in all honesty you also have to add the inertia of the whole saddle system that requires a considerable expenditure of energy to cause it to accelerate ahead of the pushing vehicle. (somewhat like the second stage on a two stage rocket ,,, or, your hand on the hand wheel) On top of that I would expect that a threading cutter fed straight in to the work and merrily cutting along with it's so called, "balanced load", would require a considerable force cause it to start cutting more aggressively on the advancing face.

In most cases the friction of the carriage is plenty to load up the halfnut/leadscrew combination and prevent any lead errors, even if the actual cutting forces are ballanced.

Yes

And they are pretty well ballanced if the tool is cutting on both sides. In that case it really is the frictional forces on teh ways that keeps the halfnuts loaded up, and the lash out of the system.

Agree, and the moment you try to unload the halfnuts you are faced with trying to accelerate (the speed of) the mass of the top carriage plus the increased load on the cutter when it is forced to deviate from the "balanced state" (In the center of the thread.)

In some machines however, the frictional force from the ways might be low enough so that a small imballance in the cutting force could allow the force from the right side of the tool to overcome the left side, and push the tool ahead of where the lead screw says it should be. Obviously the carriage is not going to zoom ahead of the lead screw by any huge amount, but one were to measure the force on the leadscrew bearing (that ultimately takes the force of teh cut) you would see it hovering around zero, with a fair amount of variation.

In theory I can see where minor variations in loading between the right and left face of the cutter would always be occurring but such variations would be indiscernible when viewed in the light of the effort needed to accelerate the carriage ahead.

The other facet of this whole business that no one seems to take into account, is the fact that the every cutter is always advancing toward the end of the cut. Even threading cutters. If your cut or thread is 6 inches long the cutter not only has to cut it's way around the work (however often) but it also has to cut it's way along the length of that distance (the six inches) It takes energy to force a cutter through that distance. Every bit as much as if it were a shaper taking the same amount of metal.

So, like it or not, the leading face of a plunge cut threading cutter has always got a more signifficant load then the trailing face.

In any case I feel confident that the plunge cut can and does work every bit as well as a (30) degree compound cut thread. One of the most advantageous features of plunge cutting a thread is that you can never set the compound in the wrong (30) degree direction and wind up with a drunken thread as have many experienced machinists. (O:

Using the 'angle feed' technique will cause the load on that thrust bearing to be a larger value, but more importantly it will be more constant. The accuracy in the lead screw will be better transmitted to the thread form being generated.

Even with my very limited experience I far and away prefer to use the compound set at a tad less then 30 for two reasons. The first is because if I use a sharp point cutter I know exactly how far I need to advance the compound to get to the proper thread depth. Secondly I prefer the cutter cutting mainly on the advancing face. It does produce a better chip flow and it seems far less brutal a way to cut.

In a nut shell though, I see no great problem with plunge cutting.

Cheers Bill

PS I very much enjoyed your post.

One can easily imagine that the conditions where a straight-in feed could cause problems would be:

1) machine with autolube on the carriage, 2) machine with low friction ways othewise, 3) very coarse thread pitches where the cutting forces are large in general.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

I think that harold's point (and I would tend to agree on this) is that under certain circumstances, the leading and trailing faces can have nearly the same loads on them, and this is the time (when the load on the leadscrew thrust bearing becomes nearly zero) when inaccurate threading can happen.

Because Harold is a 'suspenders, belt, and duct tape too' kind of guy, he objects on what basically amounts to moral grounds, to using a less then optimal configuration.

I myself use the 30 or whatever degree infeed because I know it makes the chip flow a good deal nicer, and this seems to make the thread cutting go a good deal smoother on my older machine. There have been some times when, for space constraints, I've used direct plunge.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Thanks, Don. Very well said.

There are times when I get the distinct idea that there are those that intentionally undermine the dispensing of good, sound practice, for reasons best known only to them. It does all of us good to know that there are those that feel they are benefiting by those of us that try to pass along hard learned lessons. It makes the time we dedicate to posting seem worthwhile, in spite of the trolls we encounter.

Those that went before us had worked out the problems and provided us with excellent solutions. It's the wise man that will accept their findings, as I did. Unless newer technology is employed as a machining source, there is little reason to re-invent the wheel. I will be forever grateful for the good and useful information that was handed to me as I learned the trade. It served me very well through my productive years. It is my hope to pass along to the readers the same useful information.

Harold

Not true. Relief angle of the tool alone will alter the load, thanks to the helix of the lead. The greater the lead, the greater the influence.

It can. There is no guarantee it will. That's the point. In my younger days I used to steer my car with my knees (you remember those days, don't you?). It worked, too, but presented certain risks that might not be there had I been using my two arms and hands (busy doing other things!).

Exactly. And often due to circumstances beyond your control. For one, a moving carriage that is well lubricated is easily propelled and is influenced by everything, including the hand wheel of the carriage, especially if it isn't well balanced. That's the source of my one embarrassing moment, as I've stated elsewhere.

Just a moment while I adjust my suspenders and tighten my belt!

Thanks, Jim. That's precisely the point. That I got bit on the butt from having a negative experience helped to formulate my attitude. Why is it some guys never have time to "do it right", but always have enough time to do it over?

Guilty as charged. For me to say I've never plunge threaded, or never will, is, of course, absurd. There is a time and place for any type of operation, even those that may break some of the rules. I make it a policy to ALWAYS plunge the last few passes of my threads, which yields the best looking thread you can hope to chase. I also usually sharpen my tool before doing so.

I, like you, have also plunge threaded when it was appropriate, especially in soft materials like brass or aluminum, where tip failure is not likely. In such a circumstance, to prevent a drunken thread, I usually load my carriage by some means, a snugged lock, or a little pressure on the hand wheel..

The idea that is hopefully conveyed to the reader in this thread is that drunken threads are a possibility if you plunge thread, as is tip failure. The newbie should read and understand the possibility and make decisions based on their application. Armed with the information contained within these posts, they would have enough knowledge to address problems, should they exist. To suggest to them that plunge threading is the equal of proper technique is doing them a disservice.

Harold

And Harold's anecodotal evidence involved a machine with an unbalanced handwheel on the carriage. Note that it is geared to the rack gear to advance the carriage under hand-feed or power-feed conditions. Note also that it almost certainly has some backlash in that gear train. The bigger the lathe, the more likely that it will have multiple gears in the train, to give lots of mechanical advantage to the operator -- thus also producing more backlash in the geartrain.

What happened on his machine (an old one which was really his employers, not his personal machine) was that at a certain point in the thread, the crank handle on the handwheel moved over to-dead-center, and took up the backlash to thump against the gears in the opposite direction. This thump was probably enough, with the assistance of the geartrain, to bump the carriage ahead of position (taking up some of the slack in the (well-worn) half-nuts, and possibly in the end support bearing of the leadscrew, as well.

Result: The thread, at that one point in the passage would jump ahead -- every pass -- because there was no force resisting the inertial bump from the handwheel advancing to take up the slack in the geartrain.

It is probable that the length of the thread was insufficient to have this occur more than once per threading pass -- or if it did occur more than once, it would be not too often, and probably at different angular positions of the chuck and workpiece, so you would not even see these "bumps" lined up. It would take careful inspection to see what happened.

Solution: -- several possible:

1) Cut with an angular infeed, so the force applied to the halfnuts by the carriage would counterbalance the "bump" from the handwheel. 2) Drag a thumb along the rim of the handwheel when you see the crank approaching TDC. Once it is well past TDC, carefully move it forward, taking out the backlash, without letting it get up some speed before running out of free travel. 3) Connect a weight to a rope, running from the carriage over a pulley at the headstock end, so there is a constant extra load on the carriage to counterbalance the "bump". 4) Change the lubricant to something more viscous, which would make the carriage resist the "bump" somewhat better. 5) Take the time to add counterweights to the handwheel, so it is properly balanced.

Of these -- (1) above worked for Harold, and did not require taking time to modify the machine (as in (3), (4), and (5) above, or to have to spend a lot of time looking for that particular condition to control it, as in (2) above.

I can agree with this, too.

Agreed. And of course, a 30-degree (or near) infeed is only right for standard 60-degree threads. For Whitworth threads, it would have to be 27-1/2 degrees or slightly less, and for Acme threads (which is what he was cutting in the above situation), 14-1/2 degrees or slightly less. There are probably other thread angles which were common at times. On old concertina endbox screws, I have measured a 90-degree thread angle (with very rounded crests and roots), used for too coarse a thread on too small a diameter screw. :-)

Enjoy, DoN.