Harold Hall
Workshop Processes
Machining a chuck, faceplate, etc. to fit a lathe’s threaded mandrel. Harold Hall
The Requirements
There are just two main features to be machined when fitting a device to the lathe's threaded mandrel, the thread and a parallel bore, one needing to be very precise and the other with more scope for variation, added to these is the face that mates with the mandrel's flange.
Most important is the parallel bore that must be a very close sliding fit and determines the repeatability of the device returning to the same position each time it is fitted. Also vital, is that the end face is perfectly at right angles to the bore but this is easily achieved providing the face is machined during the same sequence as machining the bore. As the location of the device is achieved by the bore the thread needs to be just a little on the generous size so that it in no way attempts to influence the result, machining the thread is therefore less critical.
However, both present a problem as the item onto which it is to be fitted is not available for checking as it is holding the device for machining, either in a chuck or mounted onto a faceplate. This, unless a second lathe is available. Even so, in both situations, if you remove it for checking and replace it when machining the bore then if the chuck or faceplate mounting is not perfect it may not return to the same position resulting in an oval bore being produced. Remember, at the final stages ideally depths of cut in the order of 0.002mm (0.0001") will be being made so even the slightest error with it being returned will have this effect.
These comments are on the basis that one will be attempting to produce a bore in the order of +0.002mm to + 0.004mm, I have though seen tolerances of + 0.025mm and even 0.05mm quoted and whilst these values may just be acceptable if related to a faceplate they most certainly are not if for a three jaw or collet chuck. These values are hardly precision turning as better is easily achieved, but how do we achieve much better.
Gauges
The first stage therefore is to make gauges for both the bore and thread, Photograph
1, so as to eliminate the need to remove the assembly during machining. After use,
these can then be kept for future applications. Check carefully the diameters of
the lathe's mandrel and note these before starting to machine the gauges, neither
should be under size but the parallel bore is the most critical. For this I would
suggest a gauge size of -
1
Drawings
All pictures can be clicked on to provide a larger view