Bridge-slot floor
 

I've worked on 150 +/- guitars, most of them being 2nd/3rd hand, wood bridges, and I use a Stewmac bridge-slot file to check for and correct any uneveness. The most common, if not only, deviation is a scoop in the middle portion. Just how flat do router bits cut the bridge-slot floor?

I think they rout the slots very accurately. I've measured with my calipers and they are dead flat. Hat could happen is when you glue a flat bridge to a slightly arched top the bridge might also slightly arch causing a hump in the middle, the opposite of what you notice. But top distortion in general could cause enough bridge slot distortion to make USP's have volume issues.

How do you like those StewMac files? I've been tempted to try those but am a bit weary of it.

For me, the difference/improvement is critical, but it wasn't until the last time I used the file (I actually have two of the thicker size, as their width varies enough so that one will fit when the other doesn't) that I followed a step by step process, using the same strings that the guitar came with.

1) Performed on a brand new Rainsong dread, it's sound was thin: flattened the bridge-slot floor's center scoop, leaving the underside of the saddle untouched (it was arched slightly in the center). The sound filled out, but still it needed improvement.
2) flattened the saddle's underside, rounded its edges, and the sound filled out even more so. From there it was only a matter of saddle material to get the guitar to its respectable self.

Interesting. I have postulated that, except for under-saddle transducers, the fit of the bottom of the saddle is irrelevant to acoustic sound. I've never done any testing to support or refute that postulate. Your results seem to suggest it matters.

I assumed that Bax was talking about the electronics.

The thing I wonder about the stewmac file is when you think about it you will be spending much more time filing the middle then the outside edges. I'm sure there is some technique involved, just thinking out loud.

I assumed that he was talking about acoutic sound without electronics. It is a given that with a piezo-type under-saddle transducer, the bottom of saddle and slot have to match in order to have even response. Often, for transducers, to restore balance, it is easier to put a single piece of paper under the saddle in the area that is less responsive.

When flattening the bottom of a saddle slot, I use a narrow chisel, held vertically, in an action like a scraper.

Correct, no electronics.

Pretty flat, however most people when they use a router move it to fast, even a cnc machine when you cut a slot, will have tool flex, so this leaves the centre of a slot ever so slightly proud.

When i use my cnc to cut a flat section, doing a second pass with no adjustments to the height will ever so slightly trim more wood

Steve

thats exactly what I do too, use a chisel as a scraper.

I'm not sure I would notice any acoustic tone differences between a saddle fitted into a slot with an uneven floor versus one that is slightly off. But yeah for sure the differences is obvious when using a piezo.

Same goes for installing the piezo's themselves. Some say the USP is bad for the tone of an acoustic guitar. Seems like that would be the case but I could never tell.