On the milling plate I put an accurate counterbore around each tapped hole to a depth of about 0.15"; this is for a dowel bushing to align the part. Then I turn dowel bushings that fit this counterbore and also fit the hole in the part and use these in two of the holes to align the part. This means that the bolts threaded into the tapped holes can be used to clamp the part.
If it is necessary to face the top surface I use two clamping bolts and program to miss these in the facing. Then have a stop in the program to allow bolts to be put in the holes that have been face around and remove the other bolts to finish the facing. When this is done i use the dowel bushings in all the holes.
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