Official Luthiers Forum!

Core drill size for the bridge pin hole matters. A string ball is is about 0.155" - 0.160" in diameter, but needs a minimum hole size of about 0.180" to pass into the body. A 3/16" bit cuts a hole that is about 0.008" oversized for that task, although we have seen closer to 0.200" used on some factory instruments. For thick bridges (>> 0.350"), thicker tops (>0.120") and thicker bridge plates (>> 0.100"), the thickness stack-up may result in the reamed 5 degree taper not fully extending to the underside of the plate, which can leave a gap between the plate edge and the pin. One remedy seen is a deeper chamfer, but too deep a chamfer risks issues with string changes and tempts the owner to employ extraction methods that can damage both pin and bridge. If a gap is the result of appropriate reaming and moderate chamfer to house the turned ring under the head of the pin, anything under about 0.025" seems of little consequence given the half-diameter of the string ball is about 0.080", With the same pin diameter under the ring, a three degree reamer will extend the hole taper to or just short of the bottom of the plate, eliminating all or part of that gap, but again, that seems inconsequential given the nature of the string ball.

In summary, the potential gap at the underside of the plate arising from overall thickness of bridge-top-bridge plate is not a problem with unslotted pins, but may become one with deeply slotted pins (in an unslotted bridge).

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For the times they are a changin'

- Bob Dylan

I use 5° taper...

I spent the better part of a week trying to make a bridge pin taper gauge where reference pins (carefully measured and identified) were placed into holes in a block of stable hardwood. The infinite, miniscule surface and dimensional variations from pin to pin, and day to day (same reference pins- same index holes) made it unpractical (thinking impossible) to get any degree of accurate indication or repeatability of taper or "fit". It was very surprising.....

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Measure Twice,

Karl Borum