Official Luthiers Forum!

Doing my first glossy in quite some time. I've taken a liking to hide glue for bridges, which means a perfect joint is needed. Ergo, bridge rabbet.

How's that jig go?

Here's ours that Dave Collins made around 8 years ago.

Sorry too I don't have photo editing software on this new Mac yet so the shots are sideways......

Hey, I think I have that attachment!

I don't have any pics for you Ed. I just use my Stew Mac binding router bit with the bearing for a .060" rabbet in a trim router with a custom base plate sanded to the appropriate top radius. I think the base plate took all of 5 minutes to make from a piece of 1/2" Baltic birch plywood.

Steve

Thanks...

Mine is just like Hesh's

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Same here Steve...

Here's mine (and I thought I was clever for figuring it out on my own!)

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"I've been had again"
Tim Benware
Creedmoor, NC

If you open them in Preview, you can go to tools > rotate. I love Preview for those quick things. The resize function gives really nice looking results, too.

How do you all make sure the rabbet is exactly the same thickness as the finish? It seems to me that unless it is perfect you either get a slight gap under the rabbeted edge on the bridge or less than ideal clamping around the perimeter of the bridge. What's the trick?

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Burton
http://www.legeytinstruments.com
Brookline, MA.

We still use a mask -- some day I may set up an operation like the factories and remove the finish using one of our CNC machines, for a hand operation I never could get my head around the idea of applying finish then meticulously scraping it off. I like the little rabbet machines, might get around to making one, in the mean time we continue to sand a tiny chamfer/bevel around the perimeter of the bridge -- it provides the necessary clearance and is invisible as well.

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Ken Cierp

http://www.kennethmichaelguitars.com/

You adjust the jig after measuring the finish thickness...

For example if the finish ledge averages a measurement of .003" we set the jig for .003" using sacrifice blocks of wood for the test cut(s).

The inset that we use is very, very little and I can barely see it to do it but we like around .005" inset which greatly expands the wood to wood gluing surface except or course if one removes the finish to the perimeter of the bridge which can be visible.

Many, many guitars if not most have lots of finish left under the bridge, an understatement. I measured one once and found 40% more gluing area available the way that we do it over the f*ctory mess... Of course the very opportunity to even be able to see what was under that bridge came because the thing lifted quickly in the life of the instrument because the maker left lots of finish under the bridge.

There is another issue too..... If you don't rabbit the ledge in the bridge and leave the bridge bottom flat AND don't clear all of the finish under the bridge's foot print the bridge bottom has to span that ledge. If the finish is thick, say .005" for a good distance inward under the bridge where there could be wood to wood contact the bridge is sitting in the air or on glue but not on wood..... Not good...

Details matter and this method is not new, lots of better makers rabbit the bridge bottom and so too does Collings Guitars maker of some really great stuff.

I was once asked on this forum some years back if the rabbit made getting a bridge off more difficult. No! It's likely that when we need to remove a bridge it's already lifted somewhere making removal no different than a non rabbited bridge.

One last thing. My original bridge design was lacking in that my wings feathered to nearly nothing. Rabbits and nearly nothing don't work well together.... trust me.....

Oh yeah..... two more details for those who love information as much as I do.

First the jig's table, the EIRW table that you see on our jig has a radius of around 25'. The slight radius lets the jig work with bridges with a radius which some makers do and I did and on existing guitars when the bridge lifts it's often taken on a bit of a radius.

Next when you clear the finish to match the inset cut that the jig does you also are making a pocket for the bridge to sit in without sliding around on wet glue when you apply the clamps. We use HHG and only HHG for bridges and this means that we like to have all clamps in place and snugged down in 10 seconds or less.... We do preheat the bridge too so we have more time available to us of course than 10 seconds. But for folks who have not worked out a way to slap a bridge on and have the clamps snug in say 15 seconds the pocket is a real help. We also make a "well" around the perimeter of the bridge once properly located in a dry clamping run. The well is masking tape and I use two layers to make the well even more profound.

With the tape well, the pocket from cleared finish and the matching rabbit around the bridge perimeter there is absolutely no drama when gluing a bridge quickly with HHG. A very good thing....

I lay a feeler gauge leaf in the recess. It's pretty easy to feel with you finger sliding across the seam between finish and gauge when you have the right one. test cuts are made in the edge of mdf witch cuts cleanly. The mdf is placed rabbit side down on a hard flat surface and the same gauge leaf slides under or doesn't.

A good friend of mine who builds classical guitars masks off the area that will be under the bridge. When finishing is complete he carefully removes the masking and measures the thickness with a caliper. He then subtracts the original thickness of the masking material to get the thickness of the finish.

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Max Bishop
Brighton, Michigan

I just rout the ledge in small pieces of scrap binding and stop when it just slides over the edge of the finish.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Haven't built one of these yet, but I will soon. Here are a few related questions (more like, am I off base type questions):

1. It seems to me that, if I keep the vertical depth of cut the same for both operations (routing off the finish and routing the vertical depth of the rabbet), the rabbet will be the right vertical depth, no matter what. In other words, if the goal is to have the rabbet be exactly the same vertical depth as what is routed off, then I should use the same router with the same vertical depth of cut, and no adjustments at all. Am I wrong in thinking this?

2. It also seems to me that, as long as I rout off all of the finish layer, but don't rout off too much wood, I can rout off a tiny layer of wood along with the finish layer and be fine in terms of adhesion. The problem is in cutting too shallow, not in cutting too deep (within reason, of course). Am I wrong in thinking this, too?

3. Finally, it seems to me that the lateral depth of cut on the bridge rabbet is not super critical, as long as the lateral depth of cut made on the bridge rabbet is larger at all spots than the "underhang" of the finish. Obviously, too big is bad (not enough gluing surface is left), too small is bad (the finish edge shows), but within reason, there seems to be no serious need for sweating over the lateral depth of cut of the rabbet. Thoughts?

I am SO looking forward to this procedure, because I am not happy with the whole scribe and scrape thing.

Some really good thinking in your post, Don!

1) Yes - you are correct but we don't like routing the finish off for a number of reasons and prefer a very sharp chisel making the depth adjustment of the router moot at least for us. Part of the beauty of the rabbited bridge system is that you can greatly expand the wood-to-wood footprint for a more secure joint over time. We take the inset, more on your other question in a minute, so very close to the physical perimeter of the bridge that using a router, even a Lam trimmer obscures the view needed for the precision that we seek in hugging the line.

Sometimes being a Luthier means leaving the power tools on the shelf..... (did Hesh actually say that..... )

2) Removing a tiny bit of wood can be problematic anytime the continuous grain of the top is severed near the back of the bridge. It encourages the bridge to lift AND tear top wood with it. We frequently see this with small builder instruments where they ran an Xacto knife too deep in removing finish. The bridge lifts, tears the top wood too, and it's a harder fix to repair and in some instances can require the dr*aded.... ep*xy bridge reglue...... not good..... With WRC this can be a nightmare....

3) Right - you can expand the gluing area to what ever degree that you wish and it's not critical..... for now.... But with up to 40% more gluing surface available that we see on very famous maker, f*ctory instruments it's pretty clear that many bridge lifts might not had happened at all had the maker been concerned with exploiting as much wood-to-wood gluing surface as was available.

So you don't have to stress over it as I once did because Dave likes to take it so very close to the edge and I can't even see it..... These days .005 - .010" inch of inset is good enough for me.

Regarding the scribe and cut thing we cut and chisel and find no better way around it for now. We also read the top runout when using a chisel to avoid lifting fibers.

Great questions by the way!

Thanks, Hesh! I do have a follow up question about this part, though:



If I do rout the finish off, with no scribing at all, doesn't this protect against the type of grain severing that can cause a bridge lift? Certainly, scribing can cause this problem. You cut too deep, but leave the wood under the bridge, and the bridge is glued to spruce that is barely attached to the rest of the top, since the grain has been severed all the way around. But if I rout that wood off, without scribing, then the floor of the pocket I create will still be attached, via grain lines, to the rest of the top. The side walls of the pocket obviously are no longer attached, but the floor should be. Right?

I think that you are correct Don, I stand corrected and thank you kindly for that too!

Mine is a CNC....

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

I'm clueless about Mac's but in windows you can view the image and choose to rotate the image and that is saved to the image properties...no external software needed

nevermind, I see others have shown you the way

In my experience, Don's point is a big one. If you cut a fiber you better remove it. I often score around the very outside of the bridge. it makes me feel good gluing the whole dam thing down. One thing that really helps is to use the back of a #11 Exacto blade. It sounds crazy but the back acts like an impossibly thin scraper instead of a knife. It doesn't go skating off on it's own direction and you can feel when your through the finish but have not severed any wood yet. It's hard to know what you've sliced w/ the knife side.
you would think it would leave a visible gap after gluing but it just doesn't. It's high stakes and you have to be focused though.

One pet peeve of mine is factories that send out replacement bridges with their monster sized rabbits already in them. Pulling a bridge and cleaning up the old glue already costs some precious bridge height. If I want to put the replacement on with a smaller or no rabbit I am forced to thin the bridge further to get rid of a goofy sized one.

David Farmer wrote: "One thing that really helps is to use the back of a #11 Exacto blade. It sounds crazy but the back acts like an impossibly thin scraper instead of a knife. It doesn't go skating off on its own direction and you can feel when your through the finish but have not severed any wood yet."

Thanks so much, David! I've noticed this, but I wasn't sure if I was crazy (or at least, backwards). It's what I've been using to scribe around bridges, though, and it sure does seem to work.

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Tim Allen
"Never hurry, never rest."

I just am not setup to mill metal. But I would like one of those jigs. I realize there are questions about router type, etc. anybody want to work with me on this?