From: null@void.com   
      
   I have a long long way to go along this path!   
      
   I got "accurate enough" arrangement with dividers.   
   Was it you mentioned trammel points, which would have been better.   
      
   Methods best seen online...   
      
   Finding circular plate centre - scribed chords with rule-and-scriber   
   then divider from each end of chord method.   
      
   All the rest I did with dividers.  Sole method.   
      
   Worked well.   
      
      
   Then from that centre identified, with dividers mark the PCD   
   (pitch-circle-diameter), centre-pop one starting position on it and use   
   dividers again on PCD radius to mark off the six positions   
   (you'll be knowing the special "law of the universe" property of a   
   compass/dividers to create a hexagon).   
   From the disk centre I also marked-out the as-best-measured diameter of   
   the tube, and fiddled the position of the circular plate on the tube end   
   to give the best averaged fit before marking-out the six "things" (block   
   with a clearance hole for the thread) circumferential positions.   
      
   There is one "best fit", and its 180deg rotation is almost as good.   
   Other 4 of the 6 positions - could force it, but don't.   
   Thing is, got the job done, and the tube isn't machine-precision-round   
   anyway.   
      
   Experience I had - do use the specific tapping-size drill.  The tap   
   "picks-up" and wants to run true to the drilled holes.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)