Precision, what is it?
Precision of course depends on the trade being practised, for a builder, maybe 5mm,
a carpenter, 2mm and a cabinet maker, 0.5mm. In the metalworking trades, say a sheet
metalworker, 0.2mm and for general turner, mostly 0,05mm. However, for a tool maker
0.02mm and better will often be the order of the day. It is in this range that these
pages seek to give guidance.
Providing the lathe's bearing are in adequate condition and the part being turned
is truly round then the requirements for producing a precision diameter are elsewhere.
First, there is no point in attempting a diameter of say plus and minus 0.01mm if
the lathe produces workpieces that are tapered, say by 0.1mm over 100mm. Of course,
the taper will be insignificant if turning a 3mm wide disk but all important if turning
a 100mm long spindle to run in some form of bearings. These pages though, assume
the lathe has been set to turn parallel as accurately as is possible for the machine,
this being covered elsewhere on the site.
What then are the requirements for producing an accurate diameter? They are, to
be able to accurately feed the cutting tool into the workpiece in very small increments,
and, for the tool being used to able to take cuts that are very fine.
The cutting tool
Taking first the easiest of these, the cutting tool. For this, it should be obvious
that a cut of 0.001mm deep cannot be taken if the cutting edge has a radius of 0.01mm,
or even larger, it will only rub. This limits the tool being used to one made from
high speed steel, rather than tungsten as the nature of this material is that it
cannot be made with such a fine edge. This prevents indexable tipped tools being
used for the task, especially as often they are, for strength of the edge, deliberately
made with a less than sharp edge.
The high speed steel tool will also need to be honed to a very fine edge and if a
number of parts are being made then such a fine edge will soon be lost and will need
to be re-honed. This can be delayed by using the honed tool just for the finishing
stages thereby extended its life.