Tuning pins with teeth

[Ron Nossaman]

I just tried a little experiment out in the shop. I took four new cut
thread tuning pins and a scrap of Baldwin's pinblock. I figured that if
anything would strip the thread burrs off when the pins were driven in,
it's that block. 

Drilled four "I" holes and drove in the pins to the same height, and about
as deep as they would be in a real piano. Two of the pins, I took counter
clockwise torque readings, followed by clockwise readings. On the other
two, I took clockwise readings, followed by counter clockwise. The idea was
to see if there was more initial resistant to movement in one direction,
than in the other. The torque peaked around 25 foot pounds either
direction, and settled immediately to around 20 after the initial movement.
There was no discernable difference in torque values between clockwise, and
counter clockwise movement of any of the four pins. 

Next, I turned two pins clockwise 180°, then back to their starting point.
The other two, I turned counter clockwise 180°, then back. Torque readings
were similar coming and going on each of the four pins.

At the bandsaw, I cut to within a millimeter of each pin from both
directions and broke the block apart to get a look at the pins without
further twisting them. About half of the threads ( those that protruded a
little further, I suppose)  were filled with sawdust.

current conclusions:
1: The "one way" nature of the thread burrs is, as suspected, nonsense.
Rebuilders have always known this to be the case, so why hasn't every piano
tech with this question tried this experiment for themselves?
2: The thread burrs are not taken off during driving, and survive enough to
provide some initial resistance to turning as they bite into the block and
fill with sawdust.
3: The thread burrs don't continue to chew up the block because they are
immediately filled with sawdust with the first turn, clogging the burr
"tooth", and the sawdust has nowhere to go to clear the burr for another bite. 
4: The sawdust between the pin and the block, trapped in the threads,
lowers the static friction, and raises the sliding friction, so the
transition is smoother when the pin is turned. It slides smoothly instead
of jumping.


In the interest of symmetry, I cut apart a block in which a pin had been
driven, but not turned. There was not nearly as much sawdust in the threads
as those which had been turned. So far so good. I then put the block halves
back together, clamping the halves in a vise, and cranking the pin back and
forth. When I took the block apart again, the pin hadn't picked up any
additional sawdust that I could tell, but there didn't seem to be that
initial resistance to turning at the first movement either. This one was a
tad on the inconclusive side, and just serves to supply some contradictory
evidence to the above experiment, lest anyone be too easily convinced. 

That ought to get you started. Go forth and ponder.


Ron N