Setting a milling mc. spindle vertical to its table. Often known as Tramming, and
the effects of errors on the finish achieved. Harold Hall
It was when commencing to use the lathe mounted milling head I had made that I finally
decided that it was about time to investigated an aspect of the milling operation
that had been on my mind for a number of years. This was the effect of the milling
machine's vertical spindle not being exactly perpendicular to the traverse of the
I think that many workshop owners tend to overlook that even quality machine tools
are made with permitted errors. For example, the precision three jaw chuck will not
be expected to hold the workpiece exactly concentric but with less error than thel
chucks that most of us use. It would be easy to assume that the lathe's cross slide
will be exactly at right angles to the bed but this is not so and an error within
limits has to be allowed, in this case, rather than having a plus and minus error
an error is permitted just one way, albeit very small. The permitted error is such
that a surface faced on the lathe can be very marginally concave, but not convex.
If such errors were not permitted then our machine tools would be astronomically
expensive. Where then is this leading us to?
Well, in the case of the milling machine spindle, an error from the perpendicular
is just one such a permitted error. It was when becoming involved in metalworking
I gained the impression from reading an item from a reputable author that the permitted
error was, rather like the lathe's cross slide, just one way and decided that I would
investigate the reasoning for this and its effect on the machining operation. After
16 years I have at last got round to it.
Now having referred to the appropriate British Standards regarding the accuracy of
milling machines I find that an error is permitted both ways, that is left or right.
It maybe that the information conveyed above was the authors own considered opinion,
a situation that I will elaborate on later, and to which I basically agree. With
regard to the error viewed from the side the error is only permitted one way this
also will be discussed in more detail later.
Know your milling machine
I will briefly introduce these pages by stating that they are not to say, this is
how it should or should not be done, but to introduce some facets of the milling
operation that are probably rarely considered. For some, this may even be new ground.
I would also add that they are technically more demanding than many related to using
the milling machine and may need reading more than once to fully comprehend their
meaning. However, I do consider that knowing the whys and wherefores regarding the
aspect of milling being discussed will appreciably increase the pleasures of using
a milling machine. I would therefore, encourage the viewer to reread the pages should
they not make full sense first time round. Also, read a third time if required.