Lathe turns convex

[KWILParticipant @kwil]

Ady1, maybe you cannot face flat on a hobby lathe but at least it should not be convex!

Edited By KWIL on 25/10/2012 09:40:31

[TerrydParticipant @terryd72465]

Ady,

As someone earlier pointed out, lathes should be set up to face fractionally concave in order that mating surfaces do not rock. sometimes 'flat' is not a good idea.  I understand that on Myfords the carriage is set to face concave by 0.001" in 12", so in the 3.5" Max Radius on the lathe  the concavity would be in the region of 0.0003".

Regards

Terry

Edited By Terryd on 25/10/2012 11:19:03

[TerrydParticipant @terryd72465]

I'm afraid I have the same problem John. It appears that the Chinese don't know about things like that so they make them accurate. Damn inscrutable lot!

Regards

Terry

Edited By Terryd on 25/10/2012 16:03:49

[Tony Pratt 1Participant @tonypratt1]

The idea behind a new lathe facing concave is to prolong it's useful life. As it wears it will face concave, then flat and then finally convex which is the least desirable condition.

Tony

[AndyfParticipant @andyf]

My Chinese one wasn't so hot. A thou or so dishing over 3" would have been OK, but 5 thou was too much. I had buy a decent size dovetail cutter, pluck up my courage, and realign the saddle dovetails to get the cross slide more perpendicular to the spindle. During the process, I found that very little on the saddle was square, co-planar or parallel to anything else,

Andy

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

(1) I'm afraid that the often quoted 'truism' that faced surfaces have to be concave to mate properly is model engineers twaddle . In any full size job that mattered great pains would be taken to ensure that faces mated flat on flat .

Doing otherwise would mean that in , for instance , a flange to flange joint in pipework carrying high pressure liquid :

(a) Very big stresses would be induced in the flanges when doing the bolts up .

(b) The zone effective in preventing leakage would be a narrow circle rather than a wide circular band .

Or in , again for instance , the backplate for a large chuck :

(a) The contact zone would again only be a narrow ring rather than a broad band . This narrow ring would crush almost randomly as the bolts get done up causing misalignment on first installation and new misalignment everytime chuck is taken off and put back .

(2) Quality machine tools have always had their theoretically perfect alignments tweaked in order to obtain better accuracy and wear characteristics .

As regards the concave setting for facing on lathes this is quite common and has several origins :

(a) It would be most definately undesirable for a lathe to face convex so the basic aim is to face flat . Since , however , in the real world everything has to have a tolerance this is set to ensure that a lathe always faces in the accuracy range of : ' flat to minimally concave ' . There is never any intention to deliberately turn concave .

(b) Deflection compensation .

(c) Life prolongation . Not as dramatic as stated above – just a general precaution to ensure that long term wear did not cause lathe settings to drift away from the (2)(a ) condition in a working lifetime .

There's a lot more to it .

Regards ,

Michael Williams .

[Tony Pratt 1Participant @tonypratt1]

The idea behind a new lathe facing concave is to prolong it's useful life. As it wears it will face concave, then flat and then finally convex which is the least desirable condition.

So buy an expensive lathe and it only cuts right for 1/3 of it's life? So if you buy a second hand one, make sure it is half way through it's lifespan and maybe it will cut correctly.

Something wrong with that way of thinking.

It's not a way of thinking it's a fact and a properly made lathe will face correctly until it cuts convex. I have only owned a Super 7 but worked on many lathes in industry and I must agree that Myfords are vastly over priced, under powered and short on specification. They are a joy to use and feel nice to the touch but if I ever change my lathe I will seriously look at the far eastern products.

Tony

[Cyril BonnettParticipant @cyrilbonnett24790]

Money no object

**LINK**

"The spirit of CY is to be outstanding, to be delicacy, and try best to satisfy customers."

[_Paul_Participant @_paul_]

You would have to be a bit nieve not to expect an old Myford like any old machine to have a little bit of wear, I know my 1949 ML7 has some.

Even if you consider them overpriced and or underpowered they really have stood the test of time and still have quite a following, myself included.

That said I do rather like the look of some of the modern far eastern lathes and they do seem on the whole very well featured when compared to the Myford, however I wonder as to their longevity as I read of problems with expensive electronic failures, plastic gear train difficulties, bearing failures and poor alignment on machines that in some cases are spanking brand new.

Back to the OP, the previously quoted "rear shear" fix works very well it transformed my old machine.

Regards

Paul

[Sam StonesParticipant @samstones42903]

Gentlemen,

I’m reminded of my own experiences with this issue of saddle wear on a Myford. The wear was present on a 1946 ML7 which I bought in about 1965, although I didn’t appreciate the problem until carrying out a deep boring exercise a considerable time later. Only when withdrawing the boring bar did the wear become obvious. The tool tip would dig in as the saddle rotated very slightly ACW.

Others have already mentioned the design weakness in terms of the `narrow guide principle’, and in effect how the shorter part of the saddle guide-way wears more rapidly.

Until I sold my Myford, I always intended to carry out J A Radford’s (NZ) saddle modification, which he did by bringing into effect the much longer (unused) shear face of the saddle. However, my plan to carry out this modification would have to wait until I owned a milling machine, because Mr Radford’s innovation included a small amount of milling.

I got very used to the slight sloppy movement, but the mod never happened.

My point here is that (I think) Mr Radford’s mod (from the ME index) appeared in the following ME magazine :-

Year – 1971

Volume – 137

Issue – 3418

Page – 541

Author – J.A.Radford of New Zealand

Title – Renovating a Myford Lathe (Saddle adjustment)

In my opinion, the changes are worth serious consideration.

Good luck.

Regards,

Sam

[Cornish JackParticipant @cornishjack]

Posted by MICHAEL WILLIAMS on 25/10/2012 19:41:54:

(1) I'm afraid that the often quoted 'truism' that faced surfaces have to be concave to mate properly is model engineers twaddle .

Interesting discussion but not overly helpful to those of us who lack an engineering background. MW's observation is, no doubt, entirely applicable (as he says) to FULL SIZE engineering. Why, then apply it to our sizes?

Both Tubal Cain (the REAL one), GHT, Dennis Chaddock et al have stated in their various writings that 'faced surfaces have to be concave to mate properly'. If I recall, they all used, at some time, the various Myford products and found them good enough to make some pretty impressive tools, models, etc. I think I might hesitate awhile before telling them that they had got it wrong!!

As regards Myfords going through TWO stages of wear before reaching convex turning, presumably the much vaunted Chinese product has only ONE stage to reach that condition? Could account for the extra value of the Myford?

Rgds

Bill

[TerrydParticipant @terryd72465]

Hi Bill,

Eric Whittle made his v8 engine on an old Myford including all of the milling on a vertical slide and I'll never be able to equal that.

Regards

Terry

[Martin KyteParticipant @martinkyte99762]

Look at it this way. It will not mate if it's convex. A certain amount of concavity is acceptable and as Terryd states three tenths of a thou in 3 and a half inches is well within a gnats wotsit. As the lathe wears it gets better before it gets worse. Longer life before you have to mess with anything.

In addition to all this when Myford grind the bed they purposly introduce a slight vertical hump in the middle by a central hold down bolt. Grind flat, let the bolt off and instant hump. With wear the hump gradually erodes through flat to slightly hollow. This doubles the acceptable life.

It's no different to toolsetting in production, set to cut undersize within tolerance and as the tool wears the turned diameter increases until the upper limit is reached then you have to regrind or these days change the tip.

[Ex contributorParticipant @mgnbuk]

In any full size job that mattered great pains would be taken to ensure that faces mated flat on flat .

That is what fitters are for !

[KWILParticipant @kwil]

A lot of this thread just boils down to whether or not the operator knows what he is doing or indeed what he is talking about. Properly set up and maintained a Myford can and has performed very well for many noted figures (and others) in the past. Many continue to do so now.

If you want to buy chinese then do so, they appear to be just as variable as some claim Myfords to be. When Myfords held sway, there was very little else to buy, now you do have a choice. Make it and be satisfied instead of carping about this that and the other.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Both Tubal Cain (the REAL one), GHT, Dennis Chaddock et al have stated in their various writings that 'faced surfaces have to be concave to mate properly'.

They were wrong .

(1) Face off a large diameter on any good quality modern lathe with modern tooling and test it for flatness – it will be flat within 0.0004 inch on 12 inches diameter .

(2) If the mating faces on the flanges of successive sections of a jet engine are not flat to the same sort of accuracy as (1) then they both don't seal and they can fret themselves to pieces .

(3) Flatness tolerance of mating surfaces is often shown on machining drawings . I have never seen one where anything other than truly flat with a minute deviation was acceptable .

No one will choose to believe this is true but people with the skill levels of people like Tubal Cain , GHT and prof. Chaddock are a halfpenny a dozen in the aerospace industry . Just for starters go into any jig and tool office and you will find a whole row of GHT's .

Regards ,

Michael Williams .

[KWILParticipant @kwil]

Who is building a jet engine on a hobby machine?

[Tony Pratt 1Participant @tonypratt1]

Michael, the 3 gentlemen you quote are of course no longer with us to defend themselves so we will have to accept your word on what they did or did not say. The truth of the matter is that obviously flatness is the preferred option, then concave and the worst case scenario is convex.

What I will say is that Tubal Cain (the REAL one), GHT, Dennis Chaddock were I believe highly qualified engineers and also extremely skilled and practical machinists etc. In my experience having this combination of skills is not all that common.

Tony

[TerrydParticipant @terryd72465]

Posted by MICHAEL WILLIAMS on 26/10/2012 15:48:29:

Both Tubal Cain (the REAL one), GHT, Dennis Chaddock et al have stated in their various writings that 'faced surfaces have to be concave to mate properly'.

………………………….. Just for starters go into any jig and tool office and you will find a whole row of GHT's .

Regards ,

Michael Williams .

 

Professor Chaddock CBE was teaching those and myself at Loughborough, and was highly respected. I knew him slightly as I was in residence at Quorn Hall. He lived in Quorn, hence the name of his tool grinder.

"Attending a lecture by Frank Whittle (later, Sir Frank Whittle—developer of the first British jet engines), Chaddock became highly interested in the challenges involved is high speed turbines. Working with flash steam as the power source, he developed bearings and turbines gearing systems…………his home made turbines and bearings eventually achieved speeds of 120,000 rpm. ……………….with speeds in excess of 400,000 rpm."

I think that he could quite manage in the average Jig and tool office, at least in any I have worked in.

Best regards

Terry

Edited By Terryd on 26/10/2012 16:21:44

[KWILParticipant @kwil]

Reference is ME4026 page 396+ "Cross Slide at right angles to spindle (to face flat or concave only)

Schlesingeer Limit lathes to 16" 0.001"/ft

31/2"model engineers lathe 0.002"/ft allowance, actual 0.0015"/ft

65mm instrument lathe 0.0008"/ft allowance, actual 0. 0002"/ft

These are lathe inspection limits

Lorch precision lathes actual 0.005mm/300mm, 0.02mm/300mm allowance

[Sub MandrelParticipant @submandrel]

Kwil is spot on, the Schlesinger limits have historically been applied to lathes and are still appropriate for any hobby lathe.

The only material fault on my Clarke mini-lathe is facing convex and it's a right pain sometimes. this was a known fault on some of the earlier batches (the 3-jaw that came with mine is dated 1998).

I have considered milling the slideways true, there's only 2-3 thou to come off. I won't be able to get it perfect, but the Schlesinger limits show me which way to let the error be.

Neil

[Ady1Participant @ady1]

Who is building a jet engine on a hobby machine?

——

There's a bundle of pulse jet stuff out there

It's a wee bitty noisy and they run white hot pretty quickly, not good when you live cheek by jowl with hundreds of neighbours

Intruiging stuff though

[Ian S CParticipant @iansc]

There are model gas turbine engines being built too, they don't run quite as hot as pulse jets, but they make a good amount of noise, some of the little turboprop engines would go well in a boat. Ian S C

[Ex contributorParticipant @mgnbuk]

If the mating faces on the flanges of successive sections of a jet engine are not flat to the same sort of accuracy as (1) then they both don't seal and they can fret themselves to pieces .

All well and good, but the mating surfaces of the flanges of various sections of jet engines are only a couple of inches wide on sections that are 4 feet or more in diameter – the sections tend to have a big hole through the middle !

And the sections are almost always turned on vertical borers with upwards of 54" diameter tables – add "borer" to the title of a machine tool & the accuracy requirements become much tighter. The rebuilt machines that I used to commission & install for my previous employer sold for £180-200,000 depending on spec & options – new machines are £400,000 plus, so it would be not unreasonable to expect them to be a bit more accurate than a small hobby lathe.

[TerrydParticipant @terryd72465]

Posted by MICHAEL WILLIAMS on 26/10/2012 15:48:29:

Both Tubal Cain (the REAL one), GHT, Dennis Chaddock et al have stated in their various writings that 'faced surfaces have to be concave to mate properly'.

They were wrong .

(1) Face off a large diameter on any good quality modern lathe with modern tooling and test it for flatness – it will be flat within 0.0004 inch on 12 inches diameter .

……………………..s .

Regards ,

Michael Williams .

And the same work on the Myford would be within 0.0005" flat – if it could turn a 12" diameter job. Not too bad for a hobby lathe then.

Best Regards

Terry

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