The castings provided have a rough hole in the centre. In the past, I have made eccentrics for steam engines using solid castings. I machined them to approximate size, drilled holes for the bolts to hold the halves together and then cut the body in halves across the centre using a thin slitting saw. The halves were bolted together and the centre of the work established along the sawcut. The centre was clearly identified and initial drilling and work with the boring bar soon established the hole for the eccentric - a comparatively easy set of operations.
Not so with the current casting. How is it possible to locate a centre (photo 1)? I came up with a solution that I have not seen in print before so thought it worth recording. The bolt holes were drilled, and the strap cut across the centre using a fine slitting saw. The location for the cut was found from the middle of the two flats on each side.
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