Bridge Pin Offset

[Ron Nossaman]

>Hello Ron Overs - or anyone else that cares to chime in - in 
>your.....not-so-recent post below, you indicate that you use 10 degrees of 
>string offset (between front and back rows of bridge pins), and a 20 degree 
>pin inclination.
>
>I was laying in bed this morning trying to work through the process of 
>punching some holes in a bridge top (well, actually, the process of locating 
>the holes). Now I can do a pretty good job of copying a pattern off an old 
>bridge and transferring it to a new one - I can even straighten things out a 
>little bit. But one should be able to start from scratch and get a straight 
>line from the tuning pin, under the capo, down the speaking length, get the 
>targeted offset angles across the bridge and come out with a wire oriented 
>parrallel to the speaking length on its way to the hitch pin.
>
>It sounds simple enough, but when you actually get down to it, it appears 
>difficult to do with precision. Many take the course of marking the bridge 
>top with two dots along a straight line from the capo/agraffe to the hitch 
>pin. While that may be "good enough" in many cases, I would feel more 
>comfortable having complete control over this process.
>
>There are several factors that will conspire to make this process a 
>less-than-straightforward one: varying string diameter, bridge pin angle 
>(and resultant string position away from bridge pin hole in bridge top), 
>distance between forward and aft bridge pins, hitch pin position, and 
>softness of bridge cap (I suppose we are using maple/boxwood/whatever hard 
>enough to not worry about this?).
>
>Do you have a methodical approach for locating new bridge pin holes that 
>result in the desired 10 degrees of string offset? Thanks.
>
>Terry Farrell



A 10° angle deviates at a slope rate of 0.174:1 (sin(10)). Pin rows 18mm
apart will give you 3.132mm string offset. Subtract the pin and wire
diameter to get the pin offset. Make 1/4" clear Plexiglass patterns with
bridge pins for punches. Mark speaking length positions on the bridge,
string up your pattern on a unison, move it left and right (preserving the
speaking length position and keeping the front row parallel with the
agraffe), and see what you can get away with and still maintain spacing and
clearance. Settle for whatever looks like the best you can get. I assure
you it will look ten times better strung than is does while you're deciding
at what point to settle.

To see how much pin slant adds to offset at each pin, you can do: 
=((SIN(RADIANS(deg))* (Dia/2)+(Dia/2))* SIN(RADIANS(deg)))+
(COS(RADIANS(deg))* (Dia/2))

or for 20° pin slant, you can just take wire Dia*0.7, and get closer than
you can drill anyway. Fudge as you see fit for later compression and
settling. It won't take you long to decide that some hairs are just as well
left un-split.

As far as getting the front and rear segments parallel and maintaining your
10° offset, you're at the mercy of the hitch pin placement and diameters as
much as anything. Do what you can.

Ron N