Harold Hall


Photograph 1 shows that the dividing plate has a device mounted on its face consisting of two brass fingers. These can be rotated, one against the other, so that the space between them amounts to the number of holes to be traversed, 21 in this example. The fingers can then be locked together to maintain this setting.


The fingers can be rotated together on the dividing plate  but some inbuilt friction ensures that they do not move once set for each division. With one finger against the plunger the first division is made. Then, the input is rotated one full turn but continuing to the other arm, making it one full turn plus 21 holes. The arms are then rotated such that the other arm rests against the input plunger and the second division made. The process being repeated until all divisions have been completed.


Division plate errors minimised.

Sometimes, the workshop owner will need a division plate which he, or she, does not have but finding one to be more expensive than can be justified. In this case, making one will be a possible option, possible methods detailed on later pages. Fortunately, in this case, the worm/wormwheel configuration considerably reduces the effect of any division plate errors so making one in the workshop is more practical then some would consider.


Whilst the calculation is not one that the reader will be called to carry out, I feel that at least some viewers will find proof of the above interesting.


Consider a division plate with 18 holes, theoretically spaced at 20 degrees (360/18 = 20), but having an error of 1 degree on the 8th hole, that is spaced at 19 and 21 degrees between its adjacent holes. If now attempting 9 divisions (40 degree spacing) and using a dividing head with a 40:1 ratio, the arm on the dividing head will have to pass 80  [(40 x 18) / 9 = 80]  holes per division. This is 4 turns plus 8 holes for each division.


Workshop Processes

In this example, when moving to the position that finishes on the 8th hole, the angle rotated will be-


Four rotations, plus 7 accurately spaced divisions on the plate, plus one of only 19 degrees.


That is


{4 x 360} + {7 x 20} + {1 x 19} = 1599 degrees rotation


From there to the next division, the rotation will be-


 {4 x 360} + {7 x 20} + {1 x 21} = 1601 degrees rotation.


However, as the worm / wormwheel arrangement reduces the rotation by a factor of forty, the angles at the workpiece will be -


1599 / 40 = 39.975 degrees




1601 / 40 = 40.025 degrees


From this it can be seen that the second division will be accurately placed as it compensates for the error in the first, (39.975 + 40.025 = 80) but the + / - 1 degree error between the two holes on the division plate has been reduced to 0.025 degree, 1/40th that on the dividing plate. The value of 1/40th is no coincidence but any improvement will always be by the same factor as the  worm / wormwheel ratio.


This feature is a great help when wishing to improve on the accuracy of a home made dividing plate by using it to make a second, or from that even a third if you are after super precision, but I doubt the need.