Ron's Bridge Notcher

[Ron Nossaman]

> I'm not sure what you're calling the hold down and what is being held 
> down.  The bridge?  Is the hold down that vaguely L-shaped wooden 'foot' 
> that is attached to what I'm calling the stop block?  And is that thing the 
> stop block?  

Yes, yes, and yes.


>It looks like it would stop the cutter table travel and 
> appears to be adjustable.

It is, though it isn't likely to be adjusted. That adjustment screw 
is mostly a stop to keep the stop block from being pushed too close 
to that big front height adjustment threaded rod. Too close, and the 
rod can get in the way of the bridge.


> If that thing is the hold down and does what I think it does, and the 
> cutter height is fixed relative to it, then the depth of cut is always the 
> same.  Correct?

Correct. It eliminates, or at least minimizes one of many potential 
ways to screw up. To make the cut shallower, which I'm considering, 
I would glue an appropriately thick piece of maple, grain parallel 
to the cutter travel, to the bottom of the foot. After notching it 
with the cutter, a second with a magic marker will make it contrast 
the bridge tops in use. If I blacked bridge tops, I'd leave the 
maple clean.


> So, you step on the foot switch and a pneumatic cylinder lifts the lower 
> table (with the bridge on it) up against the hold down?  What sort of 
> pressure are you using?

Right. Anywhere between 80 and 100 PSI seems to work well.


> With the pneumatic lift table why do you need this?  Is the travel of the 
> table limited?

I don't have room in the shop for a dedicated bench or rollaway 
stand for the thing, so I made it portable, and throw it onto a 
couple of sawhorses to use. Since the lift platform is pretty small, 
I have a movable platform that I can position and clamp to the base 
to support the bridge while I'm notching, and move it as necessary. 
I've got about 30mm of table lift, but I don't want the table to 
travel more than a couple of millimeters in use, so it won't have to 
lift the bridge too high off the auxiliary table, and because the 
less the bridge moves to clamp, the easier it is to keep it in 
position under the hold down as the table comes up.


> How did you decide on cutter diameter?  Is a typical notch about a 1 inch 
> radius?

Yes, it's a 2" diameter cutter. I chose that cutter because the 
limited chosen power supply (router) could drive it easier than a 
bigger diameter cutter, and I didn't think it would be as scary to 
run or as inherently dangerous as a larger diameter cutter. That 
leaves a bit steeper angle at the notch that I would ideally care 
for, but I'm counting on my petrified capping material to both take 
what pressure is there with less damage, and to not change dimension 
as much with humidity and produce the cyclic crushing. I expect a 
net gain in cap durability in spite of it. Look at the notch radii 
of the pianos you tune in the field. The old H&H grands, and most 
verticals appear to have notch radii smaller than that.


> I don't understand how the bridge is positioned in the machine.  How do you 
> insure that the edge of the cutter is aligned with the inside bridge pin 
> hole?  That the cut is square to the line of the three (or however many) 
> bridge pin holes (in other words, how do you insure that the bridge isn't 
> skewed)?  How do you position the bridge so that when the cutter table hits 
> the stop the cutter is at the center of the bridge pin holes?

The hold down was made to extend past where the stop block would 
allow the cutter to travel. The first test cut notched the hold down 
to the exact path and travel limit of the cutter. So the hold down 
is my positioning guide.


> Any possibility of a couple of pictures with a bridge in the machine?
> 
> Phil Ford

Eventually. I'm clearing a 10' space in a 8' shop for a 9' piano 
coming in tomorrow, and nothing is easily gotten to at the moment. 
It gets bridges, so I can get some in-progress shots of the setup 
and huge volume of chips the sucker produces.