Seating strings

[Phillip Ford]

>>>However the pin can rise out of the bridge, due to hydraulics. As 
>>>moisture is absorbed by the bridge, force is applied to the bottom of 
>>>the pin, moving it upward.
>>
>>This would seem to argue against bottoming the pin in the hole.  Some 
>>rebuilders (maybe some builders too, I don't know) drill the holes deep 
>>and don't drive the pins to the bottom of the hole.  Do you think this 
>>would prevent this?


Ron N writes:
>I drill deeper just to not have to file the tops of the pins, but yes. 
>That's another good reason. It's an even better reason not to drive pins 
>to seat strings, because the pins will be pushed right back up where they 
>were in the next dry cycle. Even with the hole deeper than the pin, the 
>bridge will still push the strings up and down the pin with humidity cycles.

I thought you were saying in a previous post that the pin was being pushed 
up.  I think you mentioned measuring an 0.011 inch change in height above 
the cap over a humidity cycle.  But here you're saying that the string will 
be pushed up and down the pin by the bridge cap.  Are both happening?

>  The point of zero relative movement between the pin and the bridge tends 
> to be somewhere toward the bottom of the cap.

How did you determine that?

>  Perhaps the glue line has something to do with it.

Maybe so.  Perhaps the drill bit doesn't enlarge the hole in that region as 
much because of the presence of the bond.  I suppose the ideal situation 
would be if the point of zero relative movement between the pin and cap 
were at the cap surface.  I wonder if there's some way you could bond the 
pin to the cap right at this point.

One idea:  I'm not sure how much interference you normally look for between 
hole and pin.  The supply house drills and pins tend to give about 3 - 5 
thousands interference.  For the sake of discussion let's say we're going 
to use 5 thousands of interference.  Suppose you had a bridge pin that was 
not intended to bottom in the hole.  This bridge pin was also turned down 
on its lower end to be 5 thousands smaller diameter than its upper 
end.  The upper end is sized to give 5 thousands of interference to the 
hole.  The pin goes into the hole smaller diameter first.  It would be a 
nominal fit going into the hole until it reached the larger diameter 
portion of the pin, at which point it would be driven a millimeter or two 
further.

>I have more tests planned, with epoxied in pins, and some with my latest 
>capping material.

What is your latest capping material?

This brings up another topic.  I know that Steinway uses a lot of maple in 
their pianos, so it probably has some magical acoustical properties, and 
was specifically chosen to complete the magic circle of sound.  But since 
you're experimenting, perhaps some other materials might be 
interesting.  As far as American woods go, maple is not the hardest 
according to my various wood manuals.  There are several which are harder:

Hickory
Some Oaks (specifically Live Oak)
Locust
Persimmon (of which golf club heads used to be made, back in the old days 
when 'woods' were
      made out of wood)
Osage Orange (which I think may be one of the hardest American woods - I 
would think some of this
      might be found in your section of the country)

A couple of potential problems here.  There are many hickories and oaks, 
some of which are harder than maple and many of which are not.  So, getting 
the right one from your wood merchant might not be easy.

The woods that tend to be very hard also seem not to be very dimensionally 
stable for some reason.  That's a disadvantage versus maple.  But, if you 
put a thin top lamination on a more stable base laminate perhaps it would 
hold together and provide a hard top surface.

Also, there are tropical hardwoods which are probably harder than American 
woods.  Lignum vitae comes to mind.  Ron Overs may know about some 
interesting woods from down under.  Perhaps Tasmanian Devilwood or 
Queensland Kangeroowood or some such would be a lot harder than what we can 
get here and perhaps might be more stable.

Also, I wonder if anyone has tried putting a thin layer of metal on the top 
of the bridge.  Say a 15 or 20 thousandths sheet of steel (or other metal) 
bonded to the top of the bridge.  I would think it would keep the strings 
from indenting the cap and also the pins from enlarging their holes.  Don't 
know how it would affect the magic circle of sound.  Notching might be a 
little tricky.  Think your notching machine might be up to it?  In a 
factory environment I wouldn't think it would be too hard to deal with, but 
it might be more problematic for rebuilders.

>  I want to see what it takes to eliminate this curse in rebuilding or 
> manufacture. Yea, I know Del, bridge agraffes.

I think agraffes or something like them are ultimately where we want to 
go.  But until such time, it seems like time well spent to improve on the 
bridge pin design if possible.  As the numbers we've been throwing around 
show, at least for static loads, the bridge pin angles, side to side offset 
of the string, choice of pin material and pin surface finish all affect the 
string to pin friction.  I don't know how much thought or investigation has 
been put into optimizing this.

Also, there seems to be some thought that the string is trying to describe 
a straight line between some point back behind the notch and the front 
string termination.  If that's the case it seems likely that the further 
toward the pin line the edge of the notch is, the more likely it is to be 
crushed.  We know that moving the notch back from the pin line doesn't 
cause false beats.  This might reduce the crushing of the notch edge.  It 
may, however, slightly reduce support for the pin because more wood is 
taken away, so over time might lead to loose pins and flagpoling.  I'm 
curious as to which notching method remains most beat free over time.

There's also the suggestion that I made to Ric, perhaps curve the top of 
the bridge to describe the path that the string naturally wants to 
take.  This might result in less string grooving at the notch 
edges.  Perhaps this is impractical or provides little return on investment.

Phil Ford




>>Would tapping the strings down without tapping the pins down not 
>>accomplish the same thing?
>>Phil Ford
>
>It would, with fewer complications.