Hi,
I'm trying to understand how the various angles relate when setting up a lathe for UNC/UNF threading. I can buy the 1/tpi = the thread depth
which I've heard elsewhere- the math seems to work and suggests (assuming I did it properly) the thread is cut a few thousanths extra, presumably to make sure the outer edges of the threads are cut clean.
However I don't understand the purpose of the 30 degree compound setting- is it so the cut is primarily taken by only one edge of the tool? And second, why 30 degrees? I suspect its pretty simple geometry that I'm overlooking- but I'm a newbie too...
Thanks,
Gregm
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Most do 29 degrees which can be even more difficult to comprehend. In truth, the lathe cuts better when only one edge is doing the cutting as the bit won't work the slack back and forth while going down the cut. As such, you will get a better smoother cut for the same setup. Having the 29 degree angle also keeps the backside of the cut tight against the work with a light scraping of the surface. Note that the actual position on the metal of the thread as to where the start point is is pretty irrelevant so the cut can start at one point and gradually move around as the cut goes deeper without any problems.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!
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wrote:

Try using .750 /TPI for NC/NF threads if you are using the compound for infeed with a 29 degree offset. If your material is within specs for the nominal or called bolt sizes, that will usually give you a finished thread that will take an hardware store nut.

Actually, 29 degrees (measured from perpendicular to the center line) is better than 30 because that gives some clearance between the material and the cutting edge as you move away from the point of cut.
You are right about the rest of it as far as it lightening the load on the cutter and producing cleaner threads.
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Greg, I'm providing two links to another forum. If you'll take the time to read each of the threads you'll gain some very good insight to how and why you set up to chase threads. This will save you a lot of grief and agony, and is better spent time than others posting redundant information.
There's some wonderful information, including one very nice drawing that shows in clear detail what's wrong with setting your compound at the wrong angle. I highly recommend you read everything and save the information for future perusal.
http://www.chaski.com/ubb/showthreaded.php?Cat=&Board=gendiscussion&Number 066&page=&view=&sb=5&o At the bottom of each page you can click on another post related to the thread.
Enjoy!
Harold
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up
depth
extra,
clean.
you
agony, and

that
wrong
information
the
My .02 cents
You do not have to set the compound at 29.5 (or thereabouts) degrees but you do have to understand what you are doing when you do whatever it is that you do. IE I believe that it is DoN that does all his CNC threading without using the compound, using a straight in/out cross feed movement only.
The drawing on the first link, taken together with what the man said clearly indicate that the compound was set at 30 degrees off of the axis of the part and not 30 degrees off of the axis of the cross slide.
So in his words, he started the cut with the compound which formed a very wide, shallow cut with a 30 degree left face and a 60 degree right face. Then he finished with a straight in feed with the cross slide which gave him the proper 60 degree thread form shape but sadly the cutting he had already done with the compound had already sliced off a 60 degree angle segment from the top of what should have still been there.
Bill D
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Yeah, I've done a variation on that one already- I put the threading tool in a straight toolholder, pointing right down along the axis of the compound so a line drawn along the tool past its point was at 29.5 degrees relative to the work.
Gregm
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DoN may thread that way, and many guys with large CNC equipment do as well. I do not endorse the method, and indeed discourage it, especially for small machines.
The proper setting of the compound of an engine lathe is critical to outcome, and reasons why are well covered by the links I posted. Plunge cutting of threads leaves the door open for improper lead or pitch and should not be considered as an alternate to proper threading unless one has a rather substantial machine which is capable of resisting the generation of a drunken thread. I have personally witnessed such a thread that was generated on an industrial rated machine, a 15" Cincinnati. If a machine of that mass is capable, I would not recommend plunge threading for any of the home shop type machines under any circumstance unless improper threads would be acceptable. As one of the links testifies, you are almost assured of just such an end result. Learn to thread by proper techniques if you desire high quality work with a minimum risk of generating scrap.

His drawing was posted in reply to one that had just done exactly that because he lacked the fundamental knowledge of proper thread cutting. Your description of the drawing is dead on, and well discussed in that thread. Again, learning to thread properly is the purpose of those links, something that is even more important to those that have small machines. Reinventing the wheel where threading is concerned is not a good idea. All of the problems of proper threading were known long before CNC machines were on the scene and had been addressed by the application of acceptable techniques. It is not a good idea to apply CNC techniques (plunge threading or strange compound settings) to engine lathe operation.
Harold
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Harold & Susan Vordos wrote:

Hi Harold (Been a while.)
If I am not mistaken DoN was describing threading with his emco compact 5 CNC lathe ( heavy duty??) and I use it simply as an illustration of just one of the possible ways to skin that particular cat. I think it wise to be aware of the options that exist and not just work by rote. Of course, you have to understand what you are doing when you do whatever it is that you do.
Cheers Bill

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Hi Bill--(Cal?) Yep, it's been a while. Hope you're doing fine.
I agree, it does no harm in knowing various dodges to achieving an end, but in this particular instance I'd be very wary of plunge threading. His machine would be small enough (far from heavy duty) to be easily influenced by various forces that may not be obvious to the casual observer. By not plunge threading, you eliminate the possibility that the cutting pressure can influence the carriage as it's being propelled by the screw.
As you likely know, Bill, I am not CNC literate. I spent my entire machining career on manual machines and have never even turned on a CNC. I have been told by more than one person that angular feeding ( as if by compound) is a feature that one pays for over and above right angle feeds. It is for that reason that many guys plunge cut threads, so the carbide industry has introduced chasers that are much more capable of withstanding the poor cutting conditions of such a cut. It is more a consideration of economy than machining practicality. Considering the size of the majority of the CNC turning centers, and the speed at which they operate, they are far more forgiving of feeding that way. I might have a different attitude in that situation.
I mentioned a personal experience of drunken threads being generated. The machine in question would bump the carriage slightly forward as the hand wheel handle came over the top of dead center, which ruined a large Acme thread intended for the missile industry by making the lead drunken. It was one of those embarrassing moments one "enjoys" on the machine when you are well known for your high quality, yet the simplest of operations turn into a scrap job. It is for that reason that I speak out against plunge cutting. The hazards are real, and learning to use a procedure that has the potential for making scrap is not to be considered as an option. In this case working by rote (without re-inventing the wheel) is a far better choice, especially when you realize that there is no benefit to not doing it right in the first place. There are no real benefits to plunge cutting threads aside from economics of software, but the negatives are numerous.
Harold
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Hi Harold
When you say "dodges" I get the feeling that you are implying slip shod or something lesser than the way you might prescribe. I guess beauty is in the eye of the beholder. In any case I do not know how CNC software handles threading. It could do a straight plunge, or it could mathematically cause the cutter to mimic the action of the compound, with one being no more difficult for the computer to achieve then the other.

influenced
By not

pressure
I do understand what you are saying and it is good that everyone should understand what is happening. In the same vain I do understand that when you thread the size stock that I do, on a machine as heavy as mine there is no possable chance of producing a drunken thread. If I disengage my half nut anywhere along the thread my carriage stops so fast you'd think it had disk brakes and if I had been foolish enough to leave my tip in the thread being cut, it would either, break, jam the cutter or cut a groove in the work. ( That's a pretty good indicator that the cutter engaged in the thread cannot pull the carriage anywhere.)

With your knowledge, I would bet that you could have a CNC lathe cutting parts in about two days if you had someone to run over the commands and the operation of the particular machine.

CNC. I

feeds.
Any CNC worth it's salt can apply X and Y moves that are fine enough to mimic the operation of a compound. As you yourself are aware you can mimic a compound set at 29.5 for threading on a manual lathe, by using a compound set at 90 by simply working out how much "in feed" and how much "compound feed" it would take. I believe the ratio to replicate the 30 degree compound would be one forward on the compound to every 1.732 on the infeed. (1 to 1.732) Actually rounding to 1 : 1.75 would produce a slightly steeper angle. A little closer to the magic 29.5

carbide
withstanding
of
majority
are
attitude
I think they do so well because the ball screws are so much more capable of setting and maintaining positional accuracy then the lead screws found in most of the old manuals.

The
hand
Acme
I'll tell you Harold,, my hand wheel handle in not heavy enough to influence the movement of my carriage I was going to say: " you must have a bigger knob then mine!" <BFG>
Cheers Harold Bill

you
turn
plunge
has
In
better
doing it

cutting
numerous.
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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.

the
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
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Crazy as a fart in a whirlwind, aren-cha? Years ago we used a device that went onto the lathe cross slide and cut threada automatically, by plunging straight in. Seemed to work fine,t me.
Lately, I cut threads on my Maxi, using CNC and feeding the cross slide, you guessed it straight in. Works fine for me.
Neither of these machines is heavy dty; the earlier one was a Cinnci with about 12" swing. My Maxi has 10" swing.
Can't imagine why anyone would say 'drunken treads' will result, unless setting cross slide screws up the lead screw pitch ... not likley.
The one thing that does happen is, cutting on both sides of the tool tip makes the cut a bit more apt to have rough finish. But so what? <-Paring off the last teensy bit with a spring pass cleans that up just fine.

BFD ... I have personally witnessed hundreds of parts threaded with plunge on cross slide, that were aerospace perfect.
Regards, Hoyt McKagen
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well.
small
No one said it doesn't work, what they (that would be ME) said is the variation opens the door to failure. Maybe you don't care about quality. I do.

Maybe when you understand threading the way I do it will make sense. Mean time, your comments have little value, exposing your lack of understanding of the real problems of threading when threads will be inspected by proper means. Whether threads are so inspected or not is no reason to use procedures that have the potential for failure. I do not endorse poor machining practice. I have no concern about practices that you see fit in your shop, assuming you have one.

The "so what" is the sacrifice of tool life, poor chip flow, and the potential for drunken threads, which appears to be a concept beyond your ability to comprehend.
<-Paring

And once established, assuming they have error in pitch due to poor machining practice, they will clean up "just fine" and be wrong. Again, when you have an understanding of that which I have spoken about, you might see it differently. On the other hand, perhaps you just plain don't give a damn. I was not afforded that luxury, I had to satisfy a myriad of inspection procedures and had no alternative but to learn to do it right, or lose my job. I get the distinct impression that you think you know a lot more about machining than your peers that went before you. Your comments prove to me you don't.

I can only guess that if you stand in a garage you also think you're a car? BFD, indeed. I made clear mention of the fact that threading by plunging on CNC machines has the potential to yield different results, often very acceptable. That does not mean that an engine lathe, especially a small one, will achieve like results. You do a disservice to those that are trying to learn to chase threads when you make light of proper procedure. You may bullshit them, but you aren't teaching me anything except that you are not knowledgeable. No more so than when you told me that you can drill a hole 12" deep and stay on location. You might, and you might not. That's the point. You'd do readers a service by telling the potential for failure instead of making it sound like it's a given. It isn't.
I don't take unnecessary chances when I machine. I earned a reputation as a highly skilled tool maker and machinist, a result of learning proper procedures and applying them. Plunge threading opens the door for error that would otherwise not be possible. Maybe some day you'll understand that the way I do. Maybe not. Should I have the opportunity to visit your shop and you're threading that way, I'll keep my mouth shut. I'll expect the same in return should you visit my shop (assuming the day comes when I have it back in operation, anyway. Damned house building project!).
Don't know about you, but I'm finished discussing this issue with you. You'll not hear any more from me on the subject. In the future I'll simply consider the source. Damned trolls.
Harold

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Harold & Susan Vordos wrote:

Arrogant bastid, aren'tcha? You remind me of a religious fundamentalist, who KNOWS he's right each and every time.

And nothing in theory or example can change your mind. Fact is, sucker, I do understand threads, but certainly not the way you do ... evidently my intuitions and practice go much deeper.

When the thread being cut drives the carriage instead of the other way round, you may have a point. I never saw that happen though and so far you're the only one who says it might. At least one other person here agrees with me. If you do have a problem like that, my suggestion is 1) use a tool with more bottom relief, or 2) find another job.

Ya see, only fools continue to argue against reality. I don't care if you believe me or not, but fact is, I cut plenty of threads by plunging and it always worked out fine. IOW, your contentions aren't supported by any realities in my world. So go find another dead horse to beat.

Pure arrogance. I do understand what you mean. You just aren't giving credit to what I've frequently observed.

My shit always passed too.

In fact it's quite the opposite.

As noted, we also used a Snap-Tap setup that plunged straight in. Worked fine!

It worked on a 12" Cinnci.

I didn't tell you that, but someone online here DID drill 1 mm holes about 300 diameters and managed to make the start from each end meet in the middle. But I suppose you think that's impossible too.

Why don't you start RIGHT NOW?
Regards, Hoyt McKagen
To prevent virus propogation, please don't put this addy in your book Belfab CNC - http://belfab.freeyellow.com/belfab.html Best MC - http://batwings.freeyellow.com/best.html Camping/Caving - http://batwings.freeyellow.com/caving.html Two-Wheel-Tech List - http://batwings.freeyellow.com/2whfaq.html Tree huggers: try wiping your buns with a spotted owl
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[ ... ]

It is *far* from heavy duty. It has a total swing of 5", and I can (barely) lift it -- with its electronics, (which add considerably to the weight) -- from the bench by myself. I can't go very far with it -- just enough to swing around and place it on an alternate surface at about the same height. :-)
However -- (as mentioned elsewhere in this thread), it does not *have* a compound -- so the option of feeding at an angle does not exist -- unless it is emulated in the (rather minimal) software. Nor does it have a handwheel, so the weight of the crank handle going over center is a non-problem. What it *does* have is ball screws -- zero-backlash, very low friction, and mounted in a bearing which eliminates thrust backlash in either direction. So the control of position is quite
So -- I am *stuck* (with that lathe) with whatever way the makers designed into it for threading. I use it frequently -- for metric threads, and for threading up to a shoulder, as it does not depend on my reaction times. Though I do thread to near a shoulder, with a run-out groove pre-cut, on the manual lathe. The CNC *does* have a proper backing out of the tool at the end of each pass, so the thread ends with a groove getting shallower until it no longer exists on that machine -- no need for a run-out groove there.
Enjoy,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
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Well ... my CNC machine does not *have* a compound, so there is no real choice. I have no access to the internal algorithms used for the threading, so I don't know whether the lathe actually makes a diagonal move for each successive pass, though given the size of the memory, I suspect that it does not.
However -- note that the major problem which you have mentioned before -- a "drunken thread" -- comes from backlash in the leadscrew and its mounting.
My lathe, and any *real* CNC lathe, should have ballscrews, which are essentially backlash free, and the bearings at the end of the leadscrew should be paired ball or roller bearing assemblies tuned to eliminate end-play. This should eliminate the main contributor to the "drunken thread" phenomenon.
I can't stop the CNC machine in the middle of a cut to examine the chips, to see whether the cut is balanced or mostly from one side.
However -- with my 12x24" Clausing -- a manual machine, I *do* utilize the compound.

Agreed -- for any manual threading. The manual machine -- even if it starts life with little backlash, will almost certainly develop it as it wears.
Enjoy,         DoN.
--
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snip--------

Thanks for the description,DoN. That helps confirm my suspicion (I'm not CNC literate, as I've stated) and leaves only the problem of chip loading to be addressed. You can probably tell from the chip generated if the machine cuts both sides equally. I often plunge cut my last few passes to clean up threads, especially in 303 stainless. When you have a sharp tool and lubricate well, the chip comes off looking like a thread, thin 60 degree chip, equally generated from each face of the thread.
One of the problems associated with plunge threading is the tool tip pressure generated by the chips meeting at the center as they come off the machine. Carbide used to fail at the tip almost routinely under that condition, so plunge threading was almost a guaranteed failure. With the advent of superior carbide and higher speed threading due to CNC controls, I get the distinct impression that the operation is now far more successful. Turning at higher speeds, the chip has much better flowing characteristics.

It pays. You can get away using other methods, but occasionally it bites you on the butt. Why take chances, I say!

Plunge
has
of
for
almost
techniques
On the machine I ran that gave me grief, the wear of the machine, which wasn't really excessive, and consistent wiping and oiling, had made the carriage so easy to move that it could be done with a finger tip. Makes for nice handling and good feel, but it also allowed the hand wheel handle to bump the carriage as it came across top dead center due to the slight back lash in the handle assembly. It was not perfectly balanced, obviously. This happened to me at my place of employment, before I started my own shop. I was dumbfounded to find I had made scrap. Funny thing was I was watching the handle jump as it went across center, but couldn't imagine it was bumping the carriage. Talk about young and foolish! I was cutting a coarse Acme thread, fairly large diameter, perhaps only a two pitch. Don't really recall the part, just the failure.
Once you've made the first pass, there's no chance it will ever correct itself under those conditions. That's the chief reason to keep the compound set in the right direction, so the chip load itself pushes against the lead screw. When plunge cutting, the carriage may or may not be loaded, and is subject to the drunken condition.
Thanks again, DoN.
Harold
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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
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wrote:

against
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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.

trailing
cutter),
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.

drunkenness
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
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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.

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