Machining for the Brave!

So much for the theory that you can't/shouldn't gear a lathe to cut threads coarser than the leadscrew pitch. Only about 8 times the pitch there. Makes all the palaver about cutting 4TPI on a Myford seem a bit moot. But that is what comes from living where you can't just order anything you want on the net and have it delivered next day. Miracles performed daily, the impossible may require a short wait.

Drive loading is an issue when setting up small lathes to cut coarser than leadscrew pitches. Easy to ask for more than the machine is designed for if you get ambitious.

That one is a decently hefty machine and the cuts relatively light so strength almost certainly isn't an issue. Ensuring there is enough oil in the right places all the time might not be as easy as you'd ideally like tho'. Significantly larger than design speed variations between mutually lubricated components can have odd effects on oil distribution. Especially with plain bearings. Small lathes tend to be a bit on the crude side of simple when it comes to oiling.

Fundamental issue with going coarser than leadscrew pitc is multiplication of errors. Geared down as per standard the effects of screw errors are reduced.

Clive

Way back on a trip to Vietnam I saw a hand cranked machine with a coarse pitch for putting the rifling in gun barrels. Used what looked like a hand made Archimedean screw rather than the usual gears/leadscrew setup.

I have geared up a lathe leadscrew for a coarse pitch for an optical focuser - hand cranked - otherwise much too lively.

Physics accurately predicts the forces resulting when cutting threads coarser than the leadscrew, and the numbers provide strong guidance to anyone who understands them. They suggest unpleasantness like stalling, belt slippage, bent metal, smoking electronics and snapped shear pins.

The physics is a scientific Law, not a theory. The maths is rock solid, not a matter of opinion,

But I'm intrigued to know what other members think!

1. In my world engineering is firmly based on science and mathematics. Engineering projects are largely predictable, and failures are due to mistakes, or insufficient understanding. Engineering, science and mathematics all pursue a form of truth, and the way they work involves persistent rational improvement - refining the science, validating the maths, and investigating anomalies not line with theory. Failures result in reviews, not blame. Theories change until the evidence proves conclusive, at which point they become scientific law. The process is extremely powerful: offhand I can't think of a scientific law that's been found wrong!
2. Others reject these ideas. In their opinion, making things boils down to common sense, with little need to do sums, assess risk, plan, or bother with changing circumstances. Obviously anyone who learned on the job knows far more than accountants, lawyers, managers and degree trained engineers. No need for research because opinion doesn't require factual support. Experts are derided, statistics belittled, and evidence ignored in favour of untested beliefs and impressions. Failures are blamed on bureaucrats, foreigners, management, politicians, and anyone who dares suggest a fact-free approach might be mistaken. Woke views and improvement programmes are rejected out of hand, and criticism is met with emotional counterattacks, not logic. All lay opinions are of equal value, even if they're wrong, stupid, dangerous or merely inefficient. Best thing is there's no need to embrace change, because everything will be OK once the opposition is crushed.

Friends, which group are you in?

Dave

Depends what the question is, and who's askin'.

You know well enough that very many, if not most investigations, don't just break down that simply to the calculations of an engineer versus the opinion of the bloke down the pub.

Often the outcomes of conscientiously-set up tests are partially contradictory, and bottoming-out the reasons might entail a deal of work that quite outvalues the original question. Plus crucial differences between machinery and setups in the case in point might simply not have been recorded because their potential importance wasn't understood at the state of knowledge when the test was devised.

There are a lot more than 2 groups, and the lines are fuzzy.

I was taught that it is hypotheses that are unproven. When proof is established they become theories.

Russell

I am in Group-3 and here are my reasons:Trust

1-Group-1 is the logic one depending on the codes from Maths and Science and language involved in Engeneering. Now not everybody effected by engineering is an engineer, or will live long enough to study that field fully and having a life. Time and how long something perfectly engineered and manufactured also plays a role...Over Time....Therefore TRUST plays a big role here.

2-This group develop mistrust due to engineering failures or unavailability issues and they get away with "Engineering" not requiring perfection, they just want to get going any which way. To prevent this the Engineering & Manufacturing fields must be 100% on the ball like the maths and science theories/codes indicates, which in a real world would be very difficult.

3-I would belong to Group-3, where one hope for Engineering & Manufacturing processes to be as good as they can all the time, so I can have above reasonable level of TRUST in these systems, after all it will cost me money as life goes along.

Edited By Chris Mate on 14/01/2023 16:57:32

Dangerous ground... Much has been written by scientific philosophers about proof and the affirmation of theories. Popper pointed out that a theory could not be affirmed by weight of evidence, but it could be falsified by a single instance of a conflicting observation (provided, of course, that the observation was a good one).

Scientific laws are a a formal statement of how to think about things, rules of thumb to be followed, if you like, but they are not 'proven' - they are merely the best formalization yet discovered. Classical, Newtonian mechanics laws were overturned by relativity theory. But one can still employ the laws, provided one is aware of their limitations.

Science does not deal in 'Truths': it humbly acknowledges that the truth cannot be known. 'Truth' is the imagined speciality of areas of human thought like religion, politics, the legal system and so on. Scientific theories are always up for discussion and revision, provided that it is done properly, according to the scientific method. Disciplines which don't employ the scientific method are not Science, so political science, social science and 'ancient science' or the 'science' of aboriginal peoples are oxymoronic. Academics at Auckland University recently got into trouble for pointing this out. The Woke didn't like it. (Unfortunately, 'science' used to mean knowledge of all sorts; lots of people still get confused. The term 'Modern Science' is available to clarify.)

I'm not arguing, Dave, but reinforcing what you've said.

Edited By Kiwi Bloke on 15/01/2023 01:24:06

Hi Dave,

You raise some interesting points.

While I agree that cutting very coarse threads will impose large forces on the lathe which may damage it, I think if you tried to calculate just what those forces would be for any particular set of circumstances, and whether the lathe would be damaged, you would have your work cut out for you. You would need to start with the geometry of the tip, the rotational speed of the work, the amount of tool feed / pass, the properties of the material being worked, the geometry of the tool with respect to the toolholder, the geometry of the toolholder with respect to the lathe, the diameter of the lead screw and the way power is transmitted to the saddle, the properties of the materials of the components which achieve the power transfer, the co-efficients of friction between the belts and the pulleys and a host of other factors which I’m sure would need to be taken into account.

While this may theoretically be possible, I’m not sure if it could be done in practice, and I’m absolutely sure I couldn’t do it.

In short, if you are prepared to take the risk of damaging the lathe, I believe the easiest way to see if the lathe will be damaged is to conduct an experiment which is what the Russian gentleman on the video has done.

As far as scientific laws being proved wrong are concerned, the whole basis of science is that the validity of any scientific law is dependent upon a lack of evidence that it is false. It is not possible to prove a law is true, only that it is false. This was the case for Newton’s laws of motion which stood for 250 odd years until Einstein came along.

I used to be a research chemist and spent a long time doing experimental work, so I’m firmly in the suck it and see group.

Regards,

Alan C.

So where are the maths, the calculations, to support your unsubstantiated hypothetical assertions?

I'm in the group with the Russians in the video: getting stuff done.

Edited By Hopper on 15/01/2023 08:01:05

Edited By Hopper on 15/01/2023 08:02:08

Check out that trick near the beginning to mount the piece of bar off centre in the three jaw chuck: Whips out one jaw then winds the other two in by about the desired offset, then reinstals the third jaw. Sheer genius. I am going to remember that trick. See about minute 00:30 onwards.

Edited By Hopper on 15/01/2023 09:04:56

Edited By Hopper on 15/01/2023 09:05:38

Any basic physics book; they almost all start teaching physics with the most basic of all machines, the lever. The maths, first captured by Aristotle, is irrefutable and it leads directly to another fundamental engineering truth, which is Work = Force x Distance. This simple calculation underpins almost all machine design.

This video is an OK introduction. Many school textbooks available, and they've all being saying the same thing for millennia. The science of levers has been tested in every conceivable way, and so far no-one has found a flaw.

Lead-screws are a form of lever, as are all screw-threads, wheels, wedges, hinges, nutcrackers, pliers, cricket bats, jacks, gears and pulleys etc. Many machines, including lathes, are some combination of levers, making it possible to design them rationally. A lathe bed is designed to take a certain weight, as are the bearings. The lead-screw, gears and pulleys are all calculated to achieve particular spindle and slide movements, and the motor sized to do the planned work. Nothing breaks for 'n' years provided the machine stays within its predicted work-envelope - how hard and how long it's expected to work.

But, because materials are imperfect, machines are always designed with an additional safety factor. Customers rarely know what the safety factor is: 1.5x is common for lightly loaded non-critical items, and it can be has high as 30x for safety critical items.

It's the safety factor that allows machines to be ignorantly overloaded without breaking them instantly. But this kind of practical experience is misleading. The most likely effect of moderate overloading is an invisibly increased rate of wear, not catastrophic failure. Seems OK, but there's no such thing as a free lunch.

A crane hook rated to 100kg (5x safety factor) will apparently lift a 500kg load without bother, but that doesn't make it wise or safe. Doing it a few times proves nothing, because the overstressed hook is only starting to fail, a process that might be quick or slow depending on a multitude of unknowns. Assuming he's got it right, the designer knows that the hook will last almost indefinitely within specification. The practical man has no idea: he can only suck it and see, and probably only has limited facilities. Outside the design parameters, everything is a gamble. Engineers are allowed to take calculated risks, but they should never rely on luck.

Hobby lathes are unlikely to be designed with the same safety factor as industrial machines.

I often take risks with tools to get the job done, but it helps to have a feel for how bad the abuse is before starting. One way of finding out that lathes can be damaged by cutting threads below lead-screw pitch is to jump in optimistically at the deep end and damn the torpedos! A better way, not always possible, is to understand the extent to which the work will overstress the lathe and approach the job with suitable caution. Popping the electronics on a mini-lathe is an expensive way of confirming the machine was asked to do too much, especially if intelligently slowing down the metal removal rate would have avoided the extra expense and bother.

Anyone who doesn't believe in the science of levers is invited to do a 6 hour motorway journey at 70mph in second-gear only. Perfectly possible to do hill-starts in second-gear, and most cars will accelerate well up to 70mph and stay there. Although driving that way works in practice, theory predicts it's not a good idea...

Dave

Chris puts his finger on a major issue! Trust is essential. But what makes one approach more trustworthy than others?

A logical contradiction I notice in Group 2, is many reject the approach preferred by Group 1 because Group 1 do not have a perfect track record. Actually, Group 1 have never claimed to be infallible. They strive for perfection rather than achieving it, expect mistakes, and have a rational approach to avoiding them next time. The Group 1 approach progressively improves, rather than guarantee results. Believing that any imperfection in Group 1 means the whole system should be disqualified is wrong. Particularly wrong to believe that when the Group 2 approach is a hodge-podge of unvalidated good and bad ideas.

Professional football adopts the Group 1 approach. Players act as a team, organised to create goal opportunities, and cover the defence. Players have roles, and although encouraged to exploit opportunities, results depend as much on intelligent cooperation as individual thrusting. Goals are enabled by the team feeding the ball foward to a player positioned to score. Small boys play soccer using Group 2 methods: they all chase the ball to the best of their ability, each convinced they're playing the game, but actually a disorganised mob. Great fun, but poor results! Getting boys from Group 2 to Group 1 starts by imposing discipline. This is necessary because individual talent takes time to see any advantage in helping others. Only later does understanding emerge, and there are many players who never get it. Which is why managers, coaches and captains are needed.

I like the idea of Group 3 though because neither Group 1 or Group 2 provide a universal solution to life's challenges. Probably most of us blur between Group 1 and Group 2 approaches, perhaps seeking the best of both. And maybe Group 3 provides the shakers and movers. Group 2 can and do have good ideas, but it's hard to trust faith based notions with weak evidence. Group 1 is more about establishing ways and means than changing the world; they enable rather than initiate.

Dave

It is obvious that Dave is an engineer, certainly not physicist.

Andrew.

True, but please explain why!

Dave

So where does a lack of trust feature?

I would suggest that anyone who invokes 'it's just common sense' should be ignored for eternity because they are deliberately exposing their ignorance.

On the other hand, in trying to decide how best to design my steam-lorry's steering-gear box, a 2-start RH thread of 1/8" pitch but 1/4" lead would seem best but would I need try to classify myself in some philosophical group..?

(Some of my pals probably think I should be "classified" but that's by-the-by.)

The answer is no. I am not trying to cut an Archimedean screw but perform a task best approached by seeking advice, which in my case means using a lathe that can stand it (the Harrison L5 better than the Myford ML7 ) and seeing what the Operator's Manual says about it.

Yes, it is possible to cut threads coarser than the lead-screws' 4TPI but while the instructions give the change-wheels necessary, they also warn it should be approached very carefully: i.e. appreciate it's unkind to the machine and use it only very gently.

'

Whilst it is interesting to consider such things as fact, opinion, experience, claim, etc., it is possible both to over-think things and to take them at face value. For example, has Relativity replaced Newtonian Physics? No, not at everyday, practical scales. It might in Cosmic and Quantum phenomena but it's too easy to quote the claim as absolute, without qualifying it. That's little better than glibly quoting the E=mc^2 formula without having any idea what it's really about. (Which I haven't, by the way, so I don't go round quoting it!)

I think it's that lack of constructive questioning so to comprehend, that leads to the problems of unwitting ignorance and mistrust; whilst ignorance itself leads to lack of trust.