Harold Hall

Workshop Processes


Making the stub mandrel

Making the stub mandrel would appear to be a simple exercise, and so it is. However, a tip on how to achieve the precise diameter will be of benefit to many beginners. To attain an accuracy of within say 0.01mm., requires precise control over the tool cross feed. Making adjustments to the cross slide of 0.005mm. with confidence is impossible, but the following method will achieve fine adjustment to the diameter with ease. Set the top slide to an angle of about 0.5 degrees such that the diameter will increase as it is moved toward the headstock.


First turn the mandrel, using the saddle, to the total length required (parallel plus tapered portion) and about 0.2mm. larger  than the eventual diameter required. Now control the depth of cut using the top slide. Do this by winding the slide away from the headstock, 0.1mm. movement of the topslide will approximate to 0.001mm. taken off the radius, 0.002mm. off the diameter. Using this method the parallel portion  will quite easily arrive at the required diameter. Turn only the parallel portion to the length and diameter required and, on the final cut, start winding the top slide forward when arriving at the start of the tapered portion. By this method the taper will blend into the parallel portion.


It is essential that, when taking such fine cuts, the cutting tool is honed to a very fine edge. If it is attempted with a blunt tool, essentially a radiused edge, the tool will rub as the radius will prevent the tool from getting below the material surface. Do take note that a tipped tool will not have a sufficiently fine edge and an high speed steel tool is essential for this.


Shape of workpiece

Thus far we have considered round workpieces and these are by far the most likely and easiest to work with. Parts made out of square or hexagonal material, or any other regular or irregular shape, are not though totally out of the question. The main problem will be the intermittent cut making it more probable

that the workpiece will slip on the mandrel. With care as to the depth of cut taken, any of the methods should suffice. If however the need for many light cuts makes the time taken excessive, particularly if several components are to be made, then either the end fixing, or split, type would be preferable.


More complex uses

These pages do not exhaust the subject as other more complex applications are possible. Typically, Sk.5 shows a two diameter taper stub mandrel. One diameter to hold a gearwheel, used for indexing, and the smaller diameter to hold a disk to be calibrated. This setup can be seen in Photograph 8.