I have gotten a bit lazy in my old age. Back in the past I would have calculated this by hand on paper. Later I wrote a BASIC program to do the calculations. These days I use one of two methods:?
If I am drilling holes in piece of metal that is not too big. I will just mount it on my rotary table, find the center of the bolt circle, move the table away from the center by the radius of the circle, decide my starting point and use the degree wheel to move the rotary table however many degrees it takes. 90 degrees for four holes, 72 degrees for five holes, 60 degrees for six holes, etc.. This would be the mechanical way of doing things and this works as long as I can fit the metal, wood or plastic on my milling machine.? CNC can also handle this with a servo driven table or by entering the center, how many holes and the first hole coordinates. I don't have a CNC mill so have to use some manual method, which is satisfying my desire to do things the more by skill than automation.?
If I am going to have to layout the bolt holes on something that cannot be done as above, I would just use a web bolt hole calculator, then layout the measurements. Find the center of the circle, draw a circle with a divider, find my starting hole position on the circle and set the calipers for the distance between holes. Then scribe lightly from hole position to hole position, making sure the points on the circle exactly match the scribe distance. Doing this in reverse around the circle is a good check. Once I am sure the points are all correct, I center punch each and drill.
Drilling a bolt circle in a business/production environment, I would probably choose to use CNC or a digital readout setup which can do hole position math.
