I've now managed to drill the cylinder and cover but seemed to make hard work of it and I'm sure the way I did it wasn't the easiest but here's how I tackled it.
First I made a close fitting threaded plug to hold the cylinder cover down and stop it rotating while drilling. I guess this is overkill but I had a piece of scrap round stock to hand that only needed a couple of minutes work so I thought it worth doing.
I don't have a fancy DRO with PCD functions so it's time to dust off the rotary table and get it centred under the spindle. The co-axial indicator makes it a 2 minute job
With the table centred and it locked in position I centre the cylinder using the same method, although not difficult I found this fiddly and time consuming.
Time to fasten down the cover and give it all a quick check again using the co-ax indicator. I also set the cylinder port face inline with the x axis of the mill.
I'm using 7ba studs so I need 6 2mm holes drilling and the cover opening out to 2.5mm clearance. All that's needed now is to move the mill X axis to drill 4.5mm from the outer edge and rotate 60
o between holes.
After doing all 6 then tapping the cylinder I have this.
I'm pleased with the even spacing and they are at the right radius but they are slightly rotated anti clockwise from what I wanted so are slightly off parallel to the port face
I guess it's not a lot out but still a bit annoying
Peter.