I can help. I installed my Phantom on an armboard I machined for a turntable I built.
Firstly, spindle to pivot = 217.5mm
Effective length = 235mm
Bob Graham gave me those numbers.
The mounting base I have is the Graham mount with 4 thru holes.
The 4 holes are located on a 2.200 inch bolt circle. I drilled and tapped 4 each 6-32 threaded holes and used stainless steel socket head cap screws.
Now I don't know how much rotation you will have with your armboard in order to orient the arm properly but the 4 holes on the Graham mount are basically 20 degrees of perpendicular.
The most important number is the 2.200 inch bolt circle, each hole 90 degrees apart on that bolt circle. Tell your machinist those numbers and you will have a nice fit.
I bored a 1.750 inch hole through for the arm base but you can get by with perhaps 1.500 inch, not positive on that.
Firstly, spindle to pivot = 217.5mm
Effective length = 235mm
Bob Graham gave me those numbers.
The mounting base I have is the Graham mount with 4 thru holes.
The 4 holes are located on a 2.200 inch bolt circle. I drilled and tapped 4 each 6-32 threaded holes and used stainless steel socket head cap screws.
Now I don't know how much rotation you will have with your armboard in order to orient the arm properly but the 4 holes on the Graham mount are basically 20 degrees of perpendicular.
The most important number is the 2.200 inch bolt circle, each hole 90 degrees apart on that bolt circle. Tell your machinist those numbers and you will have a nice fit.
I bored a 1.750 inch hole through for the arm base but you can get by with perhaps 1.500 inch, not positive on that.