Measuring string/bridge gap

[Phillip Ford]

Great, another data collector!

>In the interest in contributing raw data to the very
>interesting discussion of string/bridge seating, I was
>wondering about the best way for me (us?) to measure
>the gap between strings and the bridge.
>
>If there is an indentation in the bridge cap that is
>less deep than the diameter of the string, how would a
>feeler gage detect the gap?  Wouldn't it run into the
>side of the string and stop, yet there would be a
>0.0xx inch gap under the string?

The really thin feeler gages are quite flexible.  If the string really is 
above the indentation then it seems there should be an annular gap (a gap 
forming a ring around the string, so to speak.)  I think that a thin feeler 
gage should be able to go in there and bend around the string.  But this 
means that you probably need to stick it in tangent to the annular gap, if 
that makes sense, rather than parallel to the bridge cap.  (Please note 
that this is only my opinion.  I am not advancing this as a theory or 
stating it as an absolute fact).

I think the feeler gage check is only going to be effective if there is a 
fairly large length of the string that is above the cap (such as if the 
string is entirely off the cap and only touching at the pins) so that there 
is room to get the gage in.  For strings that are above the cap only 
locally at the pins then we may need to come up with a different means of 
measurement, such as the dial indicator you mention below.

>
>
>If I wanted to use a dial indicator, how would I mount
>it to get an accurate pre and post seating measurement
>at the pin?

I've started playing around with this a little myself.  My first 
inclination is to use a magnetic base and dial indicator stand and sit it 
on the plate.  Unfortunately the magnet isn't performing it's function on 
the cast iron.  I'm wondering if I could make a larger or heavier base that 
would be more stable just from its dimensions and weight.

>   What kind of tip would sit reliably on
>the string next to the pin?  Is there a commercially
>available tip for the job, or would I (we?) have to
>make one?

I think a flat tip is probably the best choice for this.  See:

http://www.longislandindicator.com/p133.html

http://www.dialindicator.com/products_index.html

>   Would a magnetic indicator base sit
>reliably on adjacent strings on one or the other side
>of the bridge?

I don't know.  I think it would be worth a try.


>Is there another way?

Another way might be to run some sort of beam, like an aluminum channel, 
across the top of the rim and clamp it to the rim.  Mount the dial 
indicator to this.


>I'll tinker around and figure this out (someday!), but
>was hoping one of my fellow listers had already hoed
>this row.
>
>Thanks,
>
>Greg Graham
>Happy to have aced the written exam at MARC
>Brodheadsville, PA

Congratulations.

Phil Ford