Lathe facing convex or concave

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Phil,

I presume that your question is capricious; but my answer is serious:

Find the fault [or combination of faults] that is causing the lathe to "face" convex, then rectify it accordingly.

MichaelG.

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P.S. You are obviously aware [although perhaps others are not] that the "target condition" is that the lathe should face flat ... The "acceptance" tolerances are simply a recognition of reality; and the permissible convexity is therefore specified as "Zero; plus nothing, minus a Gnat's Whisker".

Hi Neil (And others),
Ignore my comments in my post last night. When I woke up this morning I realised the picture I had in my mind when I was thinking about the problem was of the topslide. (Set parallel to the cross slide.) Non of that reasoning will apply to the cross slide as the component of the cutting forces along the axis of the lathe would not cause a twisting effect on the cross slide as it is always between the ends of the slide way. (Not past the end of the slide way as it would be with the top slide.)

Les.

Edited By Les Jones 1 on 22/03/2014 08:16:47

Edited By Les Jones 1 on 22/03/2014 08:17:52

Thank you Michael, you have got my point exactly! Everyone on this thread should read and inwardly digest the very wise PS quoted above. I think in this case OCD stands for obsessive compulsive DTI use. Stop testing and start turning, then if a problem arises, find it and fix it.

Phil.

Capricious? Me? Nooooooo.

It's not often that I write on here, but I do read this message board avidly. There is clearly a lot of knowledge here gained through real work experience and training, or through simply reading, asking and practising at home. This wealth of knowledge is shared freely and that is a credit to everybody.

Now the next bit is a bit critical, but please do not become offended. I have read through the flatness/concavity debate as it has developed, and it is not the first time it has been discussed, and I have to say that I have never seen so much written, about so little, by so many, so often. Concavity when facing is not a natural or designed in feature of a lathe. Indeed the machine is designed to produce a flat face. There may be tolerance limitaions caused by a number of factors including machine design, machine manufacturing quality and of coorse machine wear. Machine wear is inevitable particularly on the cross slide and the bed near to the spindle. How many of you actually know what flatness tolerance your machine is designed to produce I wonder? Remember, even if you do know it, that it is only truly applicable at the time the machine is new. Are there many old, worn machines among the gathered throng? There certainly are. I also agree with Phil Whitely in his re-defining of OCD.

Sorry to ramble on but, whilst all of this effort goes into debating the realities and perhaps the philosophical views on concavity, you are missing out on real machining time.

Remember this is not written to offend anybody. Rambling rant over.

Similarly you must be aware of the flatness tolerance called for in your component design, and as Michael has pointed out above surface finish is a key component of component/surface matching.

Assuming at first that the lathe mandrel axis is parallel to the bedways. and the lathe turns parallel. I think the best way to find out how accurate the cross slide axis is at right angles to the lathe mandrel axis is to do this. Place a parallel flat bar, approximately centrally, lightly clamped in the three or four jaw chuck. The length of the bar should be at least long enough to cover the full cross slide travel. Put a dial indicator on the cross slide or it can be mounted in the tool holder on the topslide. Bring the dial indicator into contact with one end of the horizontal bar and note the reading. Rotate the mandrel 180 degrees and again note the reading. Adjust the bar so that the two readings are equal. The outer face of the bar is now at right angles to the mandrel axis. With the bar set horizontally the dial indicator can now be traversed along the bar using the full travel of the cross slide to get a reading of the cross slide axis at right angles to the mandrel axis. (You can do this with the saddle locked and unlocked to note any difference). This will now tell you where the error, if any, lies. The only way this error can be corrected is to remove metal from the guide surface of either the saddle cross slide guide (front vee slide surface) or from the saddle bed guideway to get the cross slide guideway at right angles to the mandrel axis. It is probably easier to correct the error on the saddle bed guideway because it avoids having the mess with the parallel guideways of the crosslide. This may sound a dramatic fix but it is probably means only a few thou will be removed to get the desired result.

Alan

Edited By Alan Jackson on 22/03/2014 11:04:40

Edited By Alan Jackson on 22/03/2014 11:11:48

There is no argument about the requirement for a lathe to face perfectly flat. The real question is, given that perfection can never really be obtained, is it best to deviate slightly by turning concave or convex? My vote is concave.

Gary

My problem was showing up with a rule, not layout blue... I think some of it was a combination of a lack of front relief on the tool*, and loose saddle gibs. I spent ages setting everything up and once I take a few cuts, every single adjustment has gone slack. It's true what they say, not broken, don't fix

Neil

"Remarks....."Face up a large blank in the chuck. Test this with a straight edge and feelers(should be flat , to 0.001" concavity on 12" diameter)" ( I quoted this figure way back)"

I think you have the answer in the above statement ie, IT SHOULD BE FLAT, but can be concave up to.001 over 12" In other words, the maximum permissible tolerance is 1 thou over 12". unobtainable(except by chance) perfection is flat, maxumum permissable ERROR is 1 thou over 12" That is allowable error, NOT a requirement.

Many perfectionists end up wearing funny jackets that fasten at the back. I prefer to just get on making things and stay sane. If I chose to look for faults on my machines I am sure I would find some, but they do not stop me doing what I do. Maybe Myfords did have a machine that did what you say. I don't have one, neither have you, and Myford of Nottingham are no longer trading. I hope you don't lay awake at night worrying about this irrelevant non problem,

How many angels can dance on the head of a pin ? Would a surface created by 3D printing be convex or concave,

and if so, so what?

Dennis

It will be Cavex ™

Edited By John Stevenson on 22/03/2014 22:44:47

Myford also used a kind of reference angle plate that clamped to the vertical shears creating a perpendicular surface across the bed, i.e, nominally parallel to the movement of the cross slide.

A clock was affixed to the cross slide to confirm the accuracy of movement and adjustments were made by scraping the fixed vertical shear of the saddle.

Cross slide travel was always set to be slightly concave.

Martin.

Right on Martin! The way it was explained to me by an old engineer who used to work scraping slides for DSG was that over time the cutting pressure, ie from the chuck end towards the tailstock end caused wear such that at "some point in the future" the lathe would start facing flat and later would start facing convex which is of no use at all, therefore all the best lathes were given an inbuilt but limited amount of concavity. That from the days when engineers were just that and not a bunch of folk with the knowledge to recognise a lathe and the nerve to call themselves engineers. CONCAVE ! FOR A REASON!  Not something up for debate.

Rick.

Edited By Rick Kirkland 1 on 23/03/2014 11:48:15

How many men do you reckon went thru this shop in the course of a lifetime ?

And the thousands like it ?

How many do you think knew about concave v convex ?

The answer is we will never know but the vast majority of them could blow all these armchair machinists socks off and as regards for blue sky thinking they knew sod all about this as well.

it was dark when they went to work and dark when they finished their shift.

Don't some of you have a car to polish ? obviously not allowed in the workshop.

I have never checked it on the small lathes I have since it gave no problems in the simple work I do, but is not the easiest way to check the facing of your lathe by placing a large diameter in the chuck, face it and than place a DTI on the crosslide. If you than move that a cross the full diameter of the faced surface any deviation from flat, concave or convex, will show up. Niko.

Hi Graham and all others, I have no wish to get personal or fall out with anyone, my remarks are not pointed at anyone except those I mention by name. What I try to express is facts and reality as opposed to opinion. I am firmly in the "so what" camp with stovepipe, and I can easily accept the logic of making a new lathe turn concave so that as it wears it goes towards flat rather than away from it. My point is that most lathe users do not have new lathes and would not notice any problem with a machine that faced slightly convex, flat, or slightly concave, until it was pointed out to them that it was wrong(?) and then they would believe that the machine would not produce good or even usable work untill this "fault" was corrected. This is simply not the case.

"Secondly I am no perfectionist, but I do like to do a job right though, and if I have the knowledge to do that job the correct way then for me that is the only way."

I completely agree with you on the above Statement, that is how I work as well, although I am not a model engineer, and use bigger machines to make parts for real world machines that I build and repair. My first lathe was a DS&G 13Z which I bought in about 1974, it was cheap, had been dropped on its front, and I repaired it as best I could. I doubt it was ever truly accurate in any plane, but I learned a lot from it and made a lot on it. My point is that it would be quite difficult for most users to make the switch to being machine restorers, and be able to correct the "problem" to any great degree. It depends whether you work on machines, or play with them(NOT play in the derrogatory sense), and I cannot see that a machine that turns 1 thou concave is capable of producing any better work than one that turns flat or even slightly convex. It depends entirely on the operator. Indeed if a perfectly flat surface is desired, it might be better to choose a different method, or even a different type of machine to achieve it. Many however will not have those machines, or access to them, and will have to make do with what they have. A perfectly accurate lathe is better than an innacurate lathe, an inaccurate lathe is better than no lathe at all. The problem is that even in the tool room, the PERFECTLY accurate lathe does not exist. Neither is it needed to produce good work. The accuracy comes from the operator knowing his machine, allowing for it's little innacuracies, and keeping it well lubricated and adjusted. When it comes to re-scraping I would not touch it at all unless it was to correct a problem that made the machine not capable of what was required from it.

Finally (and it is firmly NOT a criticism) If I was doing the above illustrated job on a lathe, I think I would fix the DTI to a test bar held between centres,and checked with a DTI to be parralell to the bed. I fully understand that the three jaw in the illustration is merely a fixed point in space, and any rotational innacuracies it posseses (like all 3 jaw chucks) will not come into play provided the chuck is not rotated. I believe what you are doing is accepting the risk that any induced innacuracy is acceptable in these circumstances. Please also accept that 1 thou over 12" is a far smaller tolerance than most engineers, model or otherwise, need to work to. Talking in tenths is vanity.

Phil

Might I suggest thast talkng in tenths is not vanity, it is a sign that you know what you are doing!

Here here!, although shouldn't that read" working to tenths is a sign that you know what you are doing"?

Rick

Working to tenths is a sign that you know what you are doing"

Talking in tenths means you are just talking...........

But who has the advantage of working in a climate controlled shop and the end use of the part is also kept in the shop ?

A steel bar 1" long grows 0.0001" [ one tenth] per 10 degrees C

So your 20" long choo choo frames are going to be out 2 thou and heaven preserve us if you ever use it and light the fire.

Hmm, all I can say at the moment is 'convex' is bad.enough to cause faced surfaces to rock on a flat surfaces, which means extra effort spent correcting for this from time to time.

I went out to try Graham's check, but instead I checked the headstock alignment. DTI on toolpost, very rigid. A bar of 19mm BMS in the chuck was 1 1/2" thou eccentric at 6" from the SC chuck (not bad, methinks). I turned the bar until it split the error. over about 8" the bar was about 1 thou towards the front, so that isn't the cause.

Neil