Saddle slot depth?

[Ken Reynolds]

I am currently undertaking my first real task in repair work, thinning the bridge on a Takemine. After carefully checking the neck set, relief, and seting the nut slots to the correct depth, I needed to lower the saddle, which after removing .060" worth of loosely fitting shims from under the saddle, there wasnt much saddle protruding from the bridge, maybe 1/8" max (Sorry, i dont have my notebook with me while I write this) so after looking at and measuring the bridge I decided it needed to be thinned by about 3/16th or so. After performing this operation, which went well, I wonder just how deep the saddle slot should be? It looks like there's about 1.5mm worth of bridge material left in the bottom of slot, with a minimum of 4mm depth on the treble side, up to about 7 mm in the center, tapering back down to 5mm on the bass side. I understand I dont want the slot to be too shallow, or risk cracking the bridge, but i'm not sure if I want to remove too much more from the bottom of the slot? Can I go right down to the top, like the old thru slot martin bridges??
I'll be making a bone saddle and nut once I have this bit figured out.
Any hints here??

[Ronnie Nichols]

If I understand your post properly, it sounds like you thinned the bridge down because the saddle didn't protrude far enough above the bridge. Unless the bridge was excessively thick, I would hesitate to thin it much. I've seen some old Martins that had thin bridges in an attempt to avoid the inevitable neck reset. A thinner bridge may even increase the volume a little, but it will also weaken the area around the bridge which needs all the help it can get to withstand the string pressure.The proper technique would be to check the neck set with a straight edge on top of the frets to see where it strikes the bridge. If the neck is set where the straight edge slides over the top of the bridge, everythng is pretty close. The action is created by the saddle. The string height (usually measured at the 12th fret) should be regulated by the saddle (or the set of the neck) and not by the bridge height. Frank has some good info on his site explaining this process. Removing material from the bottom of the saddle slot can be done but it must be done carefully and preferrably with a special jig to rout it without creating a tapered slot that is almost impossible to properly fit a saddle into. I'd, personally, not feel comfortable if the slot went through the hard bridge wood into the soft soundboard although it may not cause any problem. I have routed the slot deeper on some guitars with success with my home-made jig(inspired by Frank). I don't know if there is any hard-fast rule on how deep you can go without weakening the bridge. If you have already thinned the bridge, you just need to be real careful about keeping the saddle slot sides parallel. If you get this slot wider at the top and your thinner bridge requires a taller saddle, you may end up with a saddle that leans forward when it is strung up. I wouldn't panic because the worst you would probably have to do is replace the bridge. I hope your repair is successful.
Ronnie Nichols

[Magnus Hultin]

Ken,

I started on a reply earlier today but got tied up with other stuff. I now noticed that Ronnie have already covered more or less what I wanted to express in his reply but nevertheless, here are my thoughts...

I would say You don´t need to deepen the slot, in fact You shouldn´t. One function of the bridge is to transfer the vertical fraction of the strings vibrations (towards the guitar top), over the base of the entire bridge. By thinning out the material in the bridge below the saddle, the extreme being all the way until the bone stands directly in the top, the contact will not be the same.

The slot heights You measured seems fully OK to me. Don´t mess with it. Just make shure the new saddlebone fits tightly in the slot so there is no tendency of the bone tilting.

Regarding thinning the bridge, I have had several vintage guitars where this operation have been made in previous repairs, and quite honestly, I don´t really understand this method of fixing a high action. I can find a number of reasons not to do it, here are the two most obvious to me.

- Theoretically, the bridge, with its mass, is part of the parameters defining the sound of the instrument. If You remove part of the mass, most likely You will hear a difference. Maybe not much, but worth considering.

- If You thin the bridge out, risc is You decrease the string pressure on the saddlebone, and You may have to make "channels" in the front of the pinholes to get proper angle over the bone.

This method, to me, seems like a temporary fix to "buy time" before You need to do the neck reset. When You finally do the neck reset, to fix the instrument up correctly You really should change to a thicker bridge again to do get it right.

Magnus

[Christian Johnson]

I've only done this on cheap, imported plywood guitars where a neck reset is not economically justifiable. You can increase break angle to increase tone and volume, but it's so drastic I won't do it to a good guitar. Reset the neck to restore geometry.

[Doug Collins]

Magnus,

I wonder if you could clarify this for me? How does thinning a bridge decrease string pressure on th saddle? I can see that would be true if you deepened the slot, altering the pin to saddle string angle, but that wouldn't be so if you are taking material off the bottom, would it?

Ken, did you remove the bridge and take material off the bottom, or did it come off the top with the bridge still on?

Thanks,

Doug Collins

[Ken Reynolds]

Doug, No I did not remove the bridge, I used an orbital sander and (as delicately as possible) removed the excess material from the top of the bridge. I maintained the orignal contour. I retook the measurements, the slot is 3mm deep on the treble side, 5mm deep in the center and 4.5mm on the base side. the "wings" are 3mm at the contour and 2mm thick at the very ends. I was actually quite pleased to find a nice piece of rosewood underneath the plastic coating that covered the bridge originally.
I also made sure the neck was set flat, and using a straightedge over the center of the neck to over the center of the bridge, there is a 1.7mm (.035) gap at the leading edge of the bridge. attached is a picture of where I am at in this process. aft of the pins is stipe in the wood, not a contour.

[Doug Collins]

Ken,

That's a very interesting surface that the bridge is sitting on. I'd love to see a picture of the rest of the guitar. But I'm wondering now, if the neck angle is more or less correct, what is the purpose of lowering the top of the bridge? Are you ultimately trying to lower the action, or are you trying to correct the pin to saddle string angle?

Doug Collins

[Ken Reynolds]

Doug,
The surface you see here (the blue is what I asume you are talking about) is just painters tape, easy to pull off and easy on the finish, there's actually a nice spruce top under all of that blue stuff!
the reason for thining the bridge was two fold, mainly to increase the saddle height and regain some mechanical advantage after having removed the shims and getting the action near where I wanted it there was almost nothing above the bridge, also I was interested in seeing what would happen to the tone by removing some of the mass of the bridge as I have read that having less mass in the bridge would increase the responsiveness and volume (loudness) of the instrument.
I'm also thinking about ramping the sting slots on the unwound strings a bit to increase the break angle as the bridge pins are parrallel to the front of the bridge vs parrallel to the saddle.
If my saddle build goes as planned I should end up with a saddle height of about 3/16"

[Doug Collins]

Ken,

I get the picture now. Hopefully the bridge is still strong enough to hold your saddle without deepening the slot. (doesn't sound like there much room for play) But it's like Ronnie said, you can always replace the bridge if it cracks - but that sounds like it will be neck reset time as well.

Let us know how it works out.

Thanks,

Doug Collins

[Magnus Hultin]

Doug,

Not having seen Kens pictures, I assumed the pinholes to be in a slightly lower position than the highest surface of the bridge (at the center of the saddle-slot). This is the way most of my bridges are made.

On such a bridge You get the effect I was talking about, but it may not apply to most bridges.

Magnus

[Doug Collins]

I understand now. I thought that Ken had removed the bridge and thinned from the bottom.

Thanks,

Doug Collins