This all started innocently enough while I was in the process of building a flow bench to be utilized in the flow testing of Small Hole EDM tubes, which will be the subject of an article for the Fall issue of EDM Today. Part of this project consists of a heavy cast iron base to which I mount a stainless steel rod flange from a marine application that will support a one inch diameter stainless steel upright. The stainless steel rod flange, purchased on Amazon, came with no information regarding the bolt circle for the holes. Thus, I first had to determine the bolt circle diameter before calculating the coordinates for drilling the mating tapped holes in the cast iron base. Standard procedure for determining hole-to-hole centerline distance is to use a caliper to get the hole edge-to-edge distance and then add or subtract the hole diameter, depending on whether you use the hole insides or out-sides. I chose the outsides, since the caliper I.D. jaws are much sharper than the O.D. jaws. I also checked the center-to-center distances for all the holes to be certain they were equally spaced. Thus, the three center-to-center distances formed an equal lateral triangle. My sketch with the measurements is shown in Fig #1.
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