Turning Precision Diameters, and lengths 03

Harold Hall


Overcoming the problem

From the above I have attempted to say that the lathe headstock, and the cutting tool, are both capable of machining to fine limits but that this is prevented by the inability to feed the tool reliably by the small increments necessary. How then can this be overcome.


The simple answer is to set the top slide to an angle of 6 degrees, precision is not required, the lathe's top slide calibration should suffice. In this case the ratio between the traverse of the top slide and the radial movement of the tool is very close to 10 : 1. Therefore, it can be taken, that feeding the top slide by 0.01mm will cause the tool to infeed by 0.001mm. This overcomes the problem of placing on very small increments using the cross slide.


However, if one is working to a face, then traversing the top slide will increase the length of the parallel portion made. In this case, therefore,  the parallel portion should initially be made slightly on the short side and finished to length once the required diameter has been achieved.


If wishing to work to even closer tolerances, typically if making a hole gauge, then an angle of 0.6 degrees gives a ratio of 100 : 1.  Unfortunately, using the lathe's top slide calibration will not be adequate for this, but can easily be set up providing a between centres test bar is available, see Sk. 1.


Other considerations.

The above assumes that the workpiece being machined is of a diameter and length that prevents it from being deflected, other than minutely,  by the cutting forces, if not, then turning precision diameters becomes much more demanding. All I can add regarding this is firstly to try to avoid the requirement.  If this is not possible, then a sharp tool is absolutely essential, and when nearing the required diameter very shallow depths of cut should be attempted. Also, make multiple passes at the same cross/top slide settings in an attempt to machining out the effect of the workpiece deflecting (spring cut).


A very slow feed rate should also be used which probably means that it will have to be hand fed using the saddle leadscrew as the power feed may be too fast. Of course, try the power feed first and check the result.


Even at larger diameters, making a second, or even a third, pass at the same cross/top slide settings is always worthwhile before measuring and adding more depth to the tool.


Workshop Processes