Next size larger bridge pins

[Ron Overs]

Terry Farrell wrote;

>. . . .  what is a 0.1mm
>step drill set? Is this simply drill sizes that progress from bit to bit in
>0.1mm increments?

Yes

>And what guidelines do you use in selecting a drill bit
>diameter for a given pin diameter (I presume the drill bit selected would be
>some measure smaller than the bridge pin). Thanks!

Generally around 92 - 94 % of the new bridge pin diameter. So if we were 
to use a 2.5mm oversize pin to replace a 2.2 mm pin, we would probably 
use a 2.3 mm drill.

2.5 mm diameter = 0.0984"

2.5 X 92% = 2.3 mm

When redrilling the bridge pin holes, it is difficult to ensure that the 
axis of the drill is in line with the original hole. As an alignment 
guide, we might insert a 2.2 or 2.1 mm drill (whichever one fits 
comfortably in the original holes) in a nearby hole to the one we are 
drilling. This would allow us to hold the drill at the original bridge 
pin angle as we drill out each hole. People who lack experience doing 
this have been known to break off the odd drill bit in the bridge. It 
really makes your day if you're so lucky.

Don't forget to make up a depth stop for the drill (ours is turned out of 
brass, with a chamfer which matches the 20 degree angle of the bridge pin 
inclination - a small set screw locates it at the correct position on the 
drill). It is very important that the bridge pin holes are drilled to the 
correct depth. For those of you who are interested in bridge cap 
replacement, there is a link to a series of six detailed images showing 
the replacement of a bridge cap on a model D Steinway. The link can be 
found at the end section of the article "Bridge cap wood selection and 
conversion - for piano technicians" on our website.

After redrilling the holes when overhauling an instrument, it finishes a 
job nicely if the bridge top is cut clean with a flat block, and the 
notching recut prior to applying the new slip coating (which, we hope, is 
confined to the horizontal surface of the bridge only). The bridge 
notches should start just behind the centre line through the bridge pin 
group. When pianos are destrung, it is not uncommon to witness the 
strings contacting the bridge wood well into the speaking length, since 
the side draft - bridge pin angle combination tends to pull to strings 
down hard onto the bridge wood (this will be especially so if some 
over-enthusiastic technician has belted the strings into the bridge in an 
effort to eliminate falseness - which may even be due to some other 
cause). The bridge pin should act as the termination point, since the 
high hysteresis loss of wood makes it a highly unsatisfactory termination 
material.

Ron Overs

Overs Pianos, Sydney Australia
-------------------------------
Email:   sec@overspianos.com.au
Website: www.overspianos.com.au
-------------------------------