A follow up question from my request for trig. help. For a person with a
fully manual bridgeport and a rotary table, what's the method most likely to
be successful and efficient to drill hole circles, up to say 6 holes. I use

I use to use a rotary table until I bought a DRO. Its much easier to use
XY coordinates than to setup the rotary table. The DRO also can be used
to make sure the bolt circle is related to other features on the project.

Also I read in Moores book "Precision hole location" that XY is more accurate.
There are more error sources with the rotary table. However if your machine
has worn leadscrews, I suspect the RT would probaby be better.

I have tried using the RT to drill the cylinder head and the DRO to drill
the cylinder using XY and they always match up. Of course there is some
clearance in the head bolts.

chuck