Harold Hall

First Page

Workshop Processes

Making the curve along its length presents another problem, not only must the arm be the correct distance from the table's centre but both holes must also be the same distance from it, within normal workshop tolerances. If this is not achieved the arm will be thinner at one end compared to the other.

Absolute precision not being required it may have been acceptable to use a rule from the central locating spigot juggling the position of the arm until both ends were reasonably equal. However, this would be a tedious operation. I therefor chose a more engineered approach and made a clamp that would both hold and position the arm. This has a hole to locate on the centre spigot and two pins to enter the holes in the arm. Having machined one side it was easy to turn the arm over and repeat the exercise on the other side. If you had a batch to make this would certainly be the way to proceed. The arm, ready to be machined, together with the central locating pin and the clamp plate, can be seen in Photograph 8. Note the pins in the clamp plate for locating the ends of the arms.

Photograph 9 shows the process being carried out and, as can be seen, almost at the edge of the table. The design actually called for a shallower curve but this would have theoretically required a larger rotary table. I say theoretically as being a light weight task an extension could have been made to enable the arm to be machined beyond the diameter of the table. It can be seen in the photograph that thin card was placed below the arm allowing its edge to be fully machined without fear of the cutter contacting the table. This approach was also adopted for the part being machined in photographs 6 and 7.

It can also be seen in photograph 9 that an additional plate has been fixed to the table and against the end of the clamp plate. This ensures that when the clamp is removed for machining the second side that it will return in exactly the same place in terms of its angle on the table. As a result the same rotary table readings can be used for the second side. The finished arm, apart from a little manual tidying up being required, is seen in Photograph 10.

A much heavier duty task than those above is to machine a large hexagon, as was seen in photograph 4. Photograph 11 shows the part being positioned for machining but this though would only be satisfactory if the table had previously been lined up with the machines spindle as in the case of requirement 2 mentioned earlier. The easy way to do this with the commercial table is to also add a centre into the tables taper and visually align this with the one in the machine's spindle, as Photograph 12 illustrates. Whilst this is not precise it should be accurate enough for most applications. Where a greater degree of precision is required a dial test indicator mounted from the spindle and rotated around the centre , or table’s edge, will provide greater precision.