IS THAT ACCURATE? FLAT?
BY GRAHAM HOWE

IT IS SO EASY to become obsessed with accuracy and I am forever reading remarks on various model engineering forums about how accurate this machine is, or how well it performs, which I normally translate as the writer saying how clever they are or how special their lathe or milling machine is!  I have met many very clever and able engineers over the years and some of the best seem to do things the wrong way according to my learned colleagues on the forums. 

For example, never mount the lathe on a wooden bench as the wood will move.  I completely agree with that sentiment, yet a close friend has his ML7 on a wooden bench and makes parts of the very highest accuracy such as small steam injectors, and when he makes these he usually makes a batch to give them out to club members, each stamped and apparently has no making failures!  My ML7 is mounted on a relatively sturdy steel bench but I have yet to make an injector that compares well which must mean that I am less skilled (accepted) but also that the lathe stand has no influence simply because this guy has the skills to take this into account. 


Sin committed

Then comes other ongoing forum arguments about the type of lubricating oil to be used for the spindle and this usually ends up having to buy 5 gallons of the stuff of very high quality of course.  This is difficult to argue as it makes absolute sense to use the recommended specified oil but in my case I used basic engine oil!  I must have committed the worst sin to any lathe that can be imagined, according to the wise men out there, yet my lathe is 45 years old and only recently have I re-scraped and re-fitted the spindle, and to be honest, it was more a case of me wanting the challenge of doing this than need. 

So why is my lathe spindle not eating its way into the very expensive white metal bearing shell and why does it rotate within, as delivered tolerance, even after 45 years of hard work?  The answer is simple and does just apply to the ML7 lathe which uses a drip oil feed to the bearings and then loses the oil through seepage which while beneficial to the bearings, makes the machine somewhat dirty!  The thing here is, that the critical requirement is for the oil drip feeders to work and feed oil at regular times and supply a known quantity of oil to the spindle journals. 

What happens in so many cases is the oil fails to drip regularly or in extreme cases, not at all, and in a very short time the bearing is worn to the point of scrap, but does manage to keep the lathe nice and clean!  I have seen many nice clean lathes, especially ML7’s and know that while all looks good and shiny it is often operationally a disaster and totally inaccurate. Don’t get me wrong, using the correct oil is good sound advice but using a lesser specified oil is better than poor, or none at all. 


Accuracy banter

Next is the ever ongoing forum banter about accuracy.  ‘I only work to very high standards of accuracy measured in tenths and demand a machine capable of performing at this level’ is so often written, which again often translates as I am  very clever and have a special machine better than most and of course very accurate.  It is undoubtedly true that a well manufactured and well fitted lathe or milling machine will perform better and more accurately than one which is of lesser quality.  This argument is especially the case in production environments where time is money and machines that produce output made to exacting tolerances require accuracy in the base machine  thus negating the need to rework or scrap parts that fail on later testing. 

The problem is that many of the forum model engineer’s are good talkers and few are good makers and I have met many who fit into both camps.  If I visit a workshop that is pristine and tools are clean and like new then I wonder if this is a ‘show house’ or a real working environment.  Believe  me there are many who keep the workshop just like a ‘show house’ but when you ask what they have actually made there is little to show, and often you see comments like ‘I need a new part for my lathe, where can I get it’ when they have the machines all around them able to make these parts.   


Not ‘accuracy’

Accuracy is also so often used wrongly with regard to parts that need to fit together to perform various tasks. The accuracy of those parts vary according to the overall need and design requirements but so often ‘new’ engineers make or try to make parts to an accuracy level that is of no benefit.  Take a simple example, if the slides on a lathe or milling machine were ground very flat to very high flatness standards then on mating to the other equally flat surfaces the parts would ‘stick’ together and perform badly if at all!  Agreed, these parts have been made very accurately but in this example too accurately. 

I met one guy who set himself this high accuracy mentality and after fitting parts together he used to introduce abrasive dust, to as he put it, ‘wear the parts in’.  So, the message here is to make parts as accurate as they actually need to be for the task intended. Incidentally, this is why there are established standards for different fits exist which need to be understood when making or designing tools because they matter a great deal not only in the manufacturing process but also in the performance of the finished product. 

I sometimes doubt the experience of ‘engineers’ when I read about people producing highly accurate work to tenths dimensions using the milling machine and even more so if that milling machine is designed for the typical home  workshop. I have never thought of the milling process as a highly accurate one and certainly not in the tenths area yet it seems some people claim this is what they always achieve.  I guess it is possible but as I see it to achieve this sort of accuracy it needs another process following on from a milling or turning and done on a grinding machine which has the capabilities to produce even more accurate fits and dimensions.  The milling machine is very good for location accuracy such as used in an array of holes or producing square sides but often fails when it comes to surface finish accuracy.


Basic skill

Finally,  another related accuracy story about my new import lathe and resolving a problem, which I thought was inherent in the design of a typical import machine.  I mentioned before, knowing that the fitting of parts from the Chinese made machines often leaves a lot to be desired I re-scraped the cross and compound slides. Prior to doing this remedial work the sliding surfaces showed the so-called scraping marks but in practice these were of no value whatsoever!  It takes time and some basic skill to blue a surface and scrape the metal to produce a flat surface which when mated to another flat surface will glide across with a ‘silky’ feel yet maintain a very accurate location and this is the ‘real’ purpose of the scraping indentions. 

The aim is to have a flat surface to provide oil retention between the mating surfaces at random places such that the oil thickness is of almost microscopic thickness yet still effective in the task of providing essential lubrication.  This is what I call flat for sliding parts but certainly not flat for other, different purposes which, by design may require more accuracy. 

This became very evident to me with the new import lathe, when I machined the end of a 2” diameter mild steel bar the finish, while good, showed a rippling effect as though an image had been printed on the machined surface.  The ripples were of microscopic dimension but evident in the light reflecting and so, once again, jumped to the conclusion this was due to the roller bearings in the headstock!  Because this problem was of academic interest I thought little about trying to resolve it as that would probably entail replacing and using a higher grade of bearing which would probably cost more than the lathe! 


Some time later...

It was some time later I removed the QCTP and was admiring the superb ground and blacked finish to the base of the main body that fitted on the compound.  Yes it was another Chinese import but I have to say of very high standard regarding the hardness and ground finish and yes the bottom surface was very flat!  Looking casually at the compound surface where the QCTP was fixed it showed some obvious machining marks which were either from milling or I rather suspect, rough grinding, but did imply that the surface had been machined for flatness. 

Another wrong assumption! 

Checking the actual flatness revealed it was way out to such an extent that my small reference surface plate rocked about showing probably 1 or 2 thou. variance across the surface!  After a fair bit of scraping and bluing the surface was made flat not to very precise flatness but at least now the QCTP base would touch for at least 60% across all corners and edges.  Job done and next time I surface cut a 2” bar from the same stock using the  same cutting tool the surface finish was both flat and had no rippling effect even when examined with a light and eye loupe.


Solution to non-problem

That was the solution to a problem that never existed in the roller bearings but also a lesson that if a surface is designed to be flat then check to make sure it is as flat as required and never trust machine or scraping marks which often are done to imply quality but sadly do not live up real useful purpose.  It is so often assumed by seeing those beautiful artistic curled scraping marks that this demonstrates very high quality and precision but in reality few, even expensive quality machines, are scraped so artistically as the skills have been lost as machines are now capable to do these tasks better and cheaper.


ALSO BY GRAHAM:

CHOOSING AND BUYING A NEW LATHE

CURING STRANGE NOISES ON A NEW LATHE