Harold Hall


Holes on a PCD (pitch circle diameter)

     The mathematical method. Harold Hall

The need to place holes, equally spaced, on a pitch circle diameter can occur in many differing circumstances but most often only involving a small number of holes, typically 3 or 4 when drilling a back plate to take a chuck. In model engineering, five or six holes to fix the cylinder cover on a stationary steam engine cylinder is another example, Photograph 1.


The obvious first choice for positioning the holes will be either a dividing head or rotary table. Whilst discussing these is not the purpose of this item I will very briefly comment on their pros and cons.


The dividing head is the more accurate of the two methods in many cases but it is limited to only those numbers of holes that it can achieve. Typically, 96 divisions, common in clock making, is not achievable using the very common Semi Universal Dividing Head. On the other hand, the rotary table can in theory produce any number of holes but accuracy is variable. For example, if requiring to place 40 holes on a PCD then the angle between each one will be 9 degrees and should therefore achieve an accuracy sufficient for practically all  requirements. If though the number of holes required is 42, then the angle between each hole is 8.5714 degrees, obviously not practical using a rotary table.  


Therefore, even if both a dividing head and a rotary table are available, it is possible that the required number of holes will not be available. The method to adopt to overcome this shortcoming being the subject here.


Why then would one want to produce a large number of holes on a PCD? Most likely, whilst there are many other possibilities, it will be to produce a dividing plate for use on the dividing head for a number that cannot be achieved with the plates available. Also, those involved in clock making often use very simple dividing techniques, and in this case a plate with 96 holes, as required above, would enable these to be used.


The purpose of this item it though to describe how to work out the coordinates for each hole so that they can be set using the X and Y dials of a milling machine.  The advantage of this method is that any number can be achieved and to a fair degree of accuracy. To use the method the position of the holes need to be calculated. Describing the method is the aim of these pages.


Workshop Processes

Holes on a Pitch Circle Diameter (PCD), Steam engine Cylinder Block