Official Luthiers Forum!

Is there a preferred blade style/ type and method to do this? The double bevel #11 Exacto can slice into the bridge, and the lacquer gets tiny little chips that are noticeable after the bridge is glued on (tiny, but noticeable). I have double beveled disposable scalpel blades that don't look much different, but I have not tried them. The main issue I want to address is the little chips on the edge of the line. I've only scribed two tops, this is the third.

_________________
Measure Twice,

Karl Borum

Karl, are you set on clearing the full footprint? What many folks do is remove a reduced outline of the footprint, maybe 1/32 or so reduced, and then cut a rabbet around the perimeter of the bridge so that it sits on top of the finish around the edges?

Some pics of the job and the jig I use are below. I first learned this here from Colin North I think was the first one to show the jig.

I use the 3M 06528 tape to mask the bridge footprint and fretboard extension footprint. I have also used frisket film which is a little thinner than the 3M tape.

I use an xacto to cut the tape out which gives a good edge but since the footprint is reduced it doesn’t have to be perfect. One tip about the xacto that could help, the first pass flip the blade around and scribe with the backside of the point. Do this lightly. The blade has less of a chance to wander when used reversed this way and subsequent passes will follow the first pass pathway. Dave Farmer shared this with me and it works really well.













Brad


Sent from my iPhone using Tapatalk Pro

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

+1 on the backside of the blade, and IIRC I got the jig idea from Hesh/Dave Collins.
I'm still using the Frisket film, 1 thou, matt, extra tack.to mask off, I prefer it as it is covered by the finish so doesn't leave a ridge but is still visible.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I use #16 Exacto blades to scribe around the bridge and they work for me. I haven't had the tiny chips in the finish you mentioned. What finish are you using?

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Used Behlen Stringed Instrument Lacquer over 2 coats Behlen vinyl sealer, Behlen lacquer reducer, 3 to1 lacquer (75%) to thinner, 5% Behlen retarder, level sand every third coat. Finish thickness before final flat sanding is around .005" thk (measuring the lacquer on the soundhole mask).

This is the last one with Nitro, I'm switching to Target EM6000. This lacquer has not been friendly to me and is prone to cracking. This is the third guitar (of three) that I removed all the lacquer from the top and re-applied due to cracks. The first one I built developed cracks a year later. This build, the top lacquer was so thin I buffed through it (StewMac Buffer, Med followed by Fine). T be fair, this one cracked after it fell on hard concrete- cracked the top too. I've been 1 year 11 months getting to this point (this included the cold weather pause since I relacquered the top). Sorry for the blathering reply.

_________________
Measure Twice,

Karl Borum

I position the bridge with two 1.4mm brads through the saddle slot which gives excellent stability. Then tape off and use a #11 Xacto to cut through the tape and proceed exactly as bcombs showed leaving a small rim of finish and routing a ledge in the bridge.

The one pitfall I had early on was not getting the corners of the wings fully cleaned up. Be sure the ledge is not hanging up on a piece of finish.

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

This is a good tip! I cut the tape using a proxy bridge that is reduced by 1/32” and then I cut the rabbet at closer to 1/16” so that it’s just inside the edges of the “pocket”. This still leaves a gluing surface that is 1.125” tall so plenty of surface for small body boxes with light strings.


Sent from my iPhone using Tapatalk Pro

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

What Brad said and I use the actual bridge but move it inward over the footprint area maybe around 1/32nd" also as Brad said and use the actual bridge with a new, #11 blade carefully scribing making sure not to cut the grain. If you do cut the wood it's no biggie just try not to.

As for the rabbit sure it's good idea but folks here are not tooled up like we are to do that so you can forget this step entirely.

We do not clear finish to the bridge perimeter because you may be able to glue it on this time without it showing but after it lifts and/or has to be glued on next time it will show and look like crap.

Now I know without the rabbit/ledge there is a gap where the finish ends that has to be spanned with no wood-to-wood contact. But since you clear almost to the perimeter it's still way better than a factory bridge glue. We've measured up to 40% of available area not used by some factories for wood-to-wood contact.

We noticed it after their bridge lifted not surprisingly.

I find I prefer the results I get with a very sharp violin knife compared to the disposable scalpel blades I’d used in the past. In repairland we are, often, dealing with cheaper instruments with horribly thick finishes and the rigid violin knife cuts deep through the slab of poly with authority where the whippy scalpel blade may deviate if appropriate care is not taken. I just find the violin knife easier to control and of course I’m in charge of bevel angle too since I’m the guy sharpening it, can run single or double bevel, and I have knives set up in both configurations. Of course there is a trade off in time spent sharpening/lower cost vs quickly grabbing a fresh disposable blade but having another consumable to keep in stock.

If you are cutting lacquer and getting a chippy edge I’d suggest that either knife is not sharp enough, blade geometry is wrong for the task or perhaps alter your technique. It should be possible to knife a clean edge through a few thou of nitro, especially fresh nitro on a new build and Behlens/Mohawk stringed instrument lacquer is pretty soft as a fresh coating compared to some other nitros. Apologies if I’m stating the obvious but you would take the cut gradually in several passes with an exacto, the first pass an extremely light scoring, the next pass a little heavier to deepen it etc. Chipping suggests to me maybe pushing with a little too much force?

Thanks Josh I have to try a violin knife sounds like a better idea.

Brad, what attachment are you using to cut the rabbet with your Dremel? I don't see clearly enough in the pics to recognize the tool… thanks. I don't build, just do repairs, and tools can be somewhat different.

I do lots of bridge reglues and I find this discussion very useful as people are sharing their different approaches to the task.


Pierre
Guitares Torvisse

_________________
Pierre Castonguay

Is this what you are asking about?
https://www.stewmac.com/luthier-tools-and-supplies/tools-by-job/tools-for-binding/precision-router-base/precision-router-edge-guide/
The one I used myself was a Laminate Trimmer Guide Assembly which came with one I bought - a Makita 700 series or Bosch Colt IIRC.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Yep, that’s it.

You flip this part upside down:


And put a radius dome platform (the MDF) and a riser (the 1/2” birch ply) between the guide and the dremel base:





Does that help?

Brad


Sent from my iPhone using Tapatalk Pro

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

I love this thread and for those learning to glue on a bridge this thread is priceless.

Ah, I get it. It reminded me of something I'd seen somewhere. Thanks!


Pierre
Guitares Torvisse

_________________
Pierre Castonguay

Sure does! Thanks for taking the time to take those pics. I had a feeling of déjà-vu in the first ones, now you've made it very obvious. Now I'll have to get one of those, even if I never get to work on a binding channel. Great use of the thing, thanks!


Pierre
Guitares Torvisse

_________________
Pierre Castonguay