Harold Hall


Turning Precision Diameters, and lengths.     Harold Hall

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.


Lathe setup

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.


Precision Diameters

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.


Workshop Processes