Harold Hall


Similarly for the Y co-ordinates


        PCD              (P - 1) x 360

Y =  -------  -  R Sin-----------------

        2                      N


Some may say that holes below the upper line should be negative if normal geometric conventions are applied (that is if my limited knowledge of the subject is correct). My comments though relate to increasing values being read on the leadscrew dials. A drill placed over the zero line will move towards the lower holes when the hand wheel is rotated clockwise and the reading on the dial increases. However, some milling machines do not conform to this standard and the datum point will have to be other than in the top left as in SK2. It will not though alter the calculated values, only how they are applied.


Hole generation

A variation is often used in the home workshop to enable large holes to be cut in a workpiece by drilling touching holes on a pitch circle as illustrated in SK. 3. Whilst this can be done using relatively inaccurate methods it almost certainly will result in the need for more manual activity, filing, sawing, etc.


In this case the pitch circle diameter is not of importance, what is, is the diameter on the outer edge of the drilled holes. The closer this is to the ultimate hole size required will considerably minimise the finishing work necessary.


What is required is that with the chosen drill size, and on the calculated PCD, that the outer edge is close to the hole size being made. This may seem relatively straight forward but what will certainly be found is that the last hole does not meet with the first hole drilled. The drill size and PCD have to be chosen more carefully and involves some calculation.


Firstly, roughly estimate the number of holes to be drilled but making it divisible by four as this will make the holes identically positioned in each 90 degrees and reduce appreciably the amount of calculation required.


Having decided on the number of holes work out the angle (A) between each adjacent hole, that is -


A  =  -------   Where N is the number of holes.   



Workshop Processes