[pianotech] crown and radius

[Gene Nelson]

 How are you "locating" the high
> points to accommodate the bridge?

**** First I locate approximately where the new ribs will be on the new 
board, then locate the bridge on the new board. Ribs are laid out and their 
locations are refined. Then the location of the inner rim is marked on the 
ribs and the ribs are then cut to length, width and rough height. Ribs are 
again laid out and the location of the bridge is marked on them. This bridge 
location mark is where the high points will ideally be. The  exceptions that 
I run into are: The first one or more ribs just above where the bass bridge 
ends and in the treble. If I were to locate the high points at the bridge in 
these places there would not be a smooth curve or line that the high points 
followed for the length of the board. There would be an unwelcome jog where 
the first rib past the upper bass rib high spot is. This would seem to make 
glue up a bit more difficult and possibly put unwanted strain on the board?? 
And in the treble the high spot would be too close to the bellyrail for my 
comfort so I compromise.

> If your crowns are uniform radius circle segments, the high points 
> couldn't be anywhere other than in the rib center.

****Hopefully I can describe this so it makes sense:
As I consider how the radius are cut onto the ribs - pulled and clamped into 
a caul mounted onto a sled on a table saw - the high point mark on the rib 
(bridge location) is aligned to a mark on the caul on sled and the finish 
height cut is made simultaneous with radius arc. The mark on the rib (high 
point) may not be in the center of the rib. Example: on a 12 inch long 
treble rib the high point may be 4 inches from the belly rail and 8 inches 
from the inner rim. It would seem to me that the radius circle should still 
be uniform??. It would also seem to me that if the high point were centered 
on this 12 inch rib, the bridge would be located so far away from it that 
there would be considerably less crown available for it. Make any sense?

Gene