[HELP] How to measure bolt circle diameter

[Cross Slide]

I'm machining a backing plate so I can mount my 5C Collet Chuck.
I'm at the point of needing to drill and tap the holes for the studs that will hold the backing late to the lathe. My lathe has 3 studs equally spaced. 
How do I determine what the Diameter is of a 3 hole bolt circle?
Trevor

[randyc]

(04-11-2018, 08:47 PM)Cross Slide Wrote: I'm machining a backing plate so I can mount my 5C Collet Chuck.
I'm at the point of needing to drill and tap the holes for the studs that will hold the backing late to the lathe. My lathe has 3 studs equally spaced. 
How do I determine what the Diameter is of a 3 hole bolt circle?
Trevor

For a three-hole, equally-spaced bolt circle, the bolt circle diameter is equal to 1.155 times the spacing between two holes.  If my memory is correct, there are tables in "Machinery's Handbook" that cover bolt circles for different numbers of holes.

[TomG]

There's a chart in the reference section of my website as well, but 1.155 is the constant you need (1/.866).

Tom's Techniques

[Cross Slide]

Thanks guys. 
That worked perfect. Once I had my diameter figured out, the DRO made drilling the holes a easy process!!

Trevor