Lathe Accuracy

As long as the work you produce on the Lathe is OK why worry about the accuracy? If it is clattering and banging then worry, but maintenance is always on going so you work and become aware of slack movement so fix it.

As to measuring the detailed accuracy of the lathe then accept what it does and do not worry about things. As long as it turns parallel in length I will accept it. Make to fit is fine as far as I am concerned unless its replacement parts then work to tolerances. Otherwise leave well alone!

Clive

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Very fair comment, Clive

For me, it's not really a matter of 'worry' ... just fascination.

For concentricity and roundness, I am aware of nothing that can improve upon working between dead centres [viz: the watchmaker's "turns" used with the bow, or the bodger's pole lathe]; but this arrangement is inconvenient, so we tend to rely upon the accuracy of the machine. ... I find it interesting to know how far the things can be trusted.

MichaelG.

Even that isn't a guaranteed way of ensuring roundness.

Myford (Nottingham) used to use a process to raise three small areas on some of their (Super7 only, I think) spindle centres to ensure they were perfectly seated whilst being finish ground.

Martin.

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Very interesting, Martin

It reminds me of a bearing arrangement that was used in some Clocks, and on some 'Unipivot' pick-up arms.

... A cluster of three [static] balls; with a conical point, bearing upon them.

Obviously, the ultimate accuracy of turning between dead centres will be prescribed by the quality of the fit between the male and female centres. ... Myford's arrangement sounds optimum.

Thanks for the info.

MichaelG.

Going back to the original post asking for the manufacturing tolerances of the Myford Connoisseur, on the Yahoo Myford lathe group website, in the files section there is

Myford-7-Series_Inspection-Sheet.pdf

I don't know whether the origin of the information was Myford itself or it is just someone's suggestion but the figures are a pretty good benchmark for a lathe of Myford size. When I checked mine, all but a couple were within tolerance. A little fettling brought them all within the figures quoted.

Andrew M

Just to throw in my three penneth - during my apprenticeship at Colchester, I spent many hours ( happy ? ) scraping in large Colchester lathes.

The specification for the alignments of the machine were very precise for a relatively low cost industrial machine.

From memory ( 30 years ago ! ), the headstock alignments were specified by clocking a nose bar over the top for horizontal and on the front for skew ( headstock should point up towards the tailstock to allow for the weight of the chuck etc. ). There were many other checks carried out on the spindle to ensure concentricity etc.

The saddle was scraped to turn concave by a few tenths of a thou' and the cross-slide movement checked for straightness on a 'T' bar in the spindle socket.

The tailstock was scraped to get the barrel horizontal and straight with a check made with a precision bar between centres to ensure the tailstock and headstock were level with each other.

After the machines were completed ( before painting ), a turning test was done, skimming the OD of the test piece. This piece was then checked for roundness using a Taylor Hobson Talyrond - maximum allowable out of round was one tenth of a thou'. A circular graph was produced ( rather like a truck tachometer disc ) and was included with the machine accuracy chart.

So, final comments, I started a thread early last year about the quality and accuracy of the Axminster Sieg C0 - the one I bought was unbelievably poor. Axminster were very good and I got my money back without any arguing. I phones Cowells and talked to Colin Childs - we discussed their accuracy policy ( just what the doctor ordered ) and, suffice to say, I am now the proud owner of a sparkling 90ME ( has 'some' swarf on it now ) - fantastic machine !

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Thanks for the link, Andrew

I will hang a copy on the wall, to remind me not to blame the tool.

MichaelG.

I am having trouble accessing the Myford 7 PDF

Message is " error in downloading the file "

Have tried on Firefox and IE, no joy.

Any clues as to why please?

Len. P.

Sorry Len, I can't help you with that but if you join the group here

**LINK**

you'll have access to that and much more.

Andrew M

I've just spotted, Andrew, that I can only access it through Yahoo groups as you say.

I don't do Yahoo groups, thanks anyway.

Len. P.

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How very infuriating !!

I clicked that link this morning, on my iPod Touch, and it opened the document.

... But, of course, I didn't save the file.

Now ... I get that same error message, on iPod Touch and on iPad.

I will try some other machines.

MichaelG.

From my experience , when you turn a piece in a chuck, part it off, place it into a Vee block and rotate , then you will see the error. Something turned between centres, especially dead centres, should be round on almost any set up. I have a modified Taig type lathe, and when I turn something in it, measures ok with a mic and looks ok with a dti. Then rechuck the part and get it round, just won't happen. I have a 0.001mm dti that use for checking in a Vee block. It is a lobed shape, not triangular and not very round.The Taig has since been corrected with better accuracy angular contact bearings, but even with those is still no better than 0.003mm, but is a lot better than the original 0.01 to 0.013mm it used to be. A better grade of bearing is available, but am not prepared to pay for them .

Neil L

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Bob,

It's good to see that Cowells met your well-informed requirements.

They seem to have developed what was originally quite a modest machine into a little gem.

I posted this link on another thread today; but I think it worth repeating here

... He has some good pages about his Cowells 90CW.

MichaelG.

Edited By Michael Gilligan on 02/02/2015 07:51:04

Or try this: Schlesinger

Neil

You might well find the lobed shape is caused by the 3 jaw chuck you're using, either by clamping pressure from the jaws or uneven wear upon the gripping surfaces, which allows the job to move fractionally. A collet should be used if you want to isolate (as much as possible) the error in the spindle bearings.

Martin.

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Sorry Neil, I don't understand the "Or try this" remark.

MichaelG.

Hi Blowlamp

I think Neil L is referring to the shape of the indicator tip not a turned part, some similar tips can be seen in the Verdict indicator lists, you could say it is 'tear drop' shape.

Emgee

When I make test pieces, I ruf down the od, then almost part it off, then I finish turn the diameter, and finally part off. That way the stresses in the bar are at the minimum. Some materials are not well suited to make test pieces out of due to the stress in the material stock itself. When looking for small errors it is not quite as easy as it may seem. Anyone who has worked in a metrology lab will know what I am talking about.

Neil L.

Does the fact its almost parted off not reduce rigidity of the part which could affect the finished shape?

Even if the work is held in a chuck or collet and then supported by a tailstock center there could be a bias due to misalignment of the tailstock ! Is it also said that a ground bar in the same situation as a turned bar is not round. In fact some of the measurements quoted are outside what I would call normal measuring. 0.01 mm is about the limit for our use and well within the tolerances offered by most work. I would not think 0.001 mm due to surface finish and the stated out of round condition is possible and in any case a lot of work carried out is only done to satisfy a personal desire to make something that works. Buy any piece of kit and it can be improved for convenience of use (DRO's perhaps) but to increase accuracy is an expensive task compounded by the cost factor. Linear ball shafts are beyond the need of the amatuer unless he is seeking what i stated before. Work to your best ability, ensure the tooling is new or sharp. Ensure the metal is right for the part being made and follow machining and work holding protocols. Even when a part is made, further work like honing and polishing change the surface and measurement which has to be taken into account, make to fit is all that is required for us but production work means you have tolerances to work to which is totally different.

Clive