Official Luthiers Forum!

Nothing is flat. The dish and the guitar plate are curved in all directions like a section of a ball. I could go on about this, but really nothing else needs to be said.

I understand what you’re, saying Brad. I’ve thought about that in the past, too. I just don’t think the difference is significant enough to actually matter. I think we’re talking a thousandth or so from the centerline to edge of the brace, and that should be a gap the glue will swell the wood enough to close. I’ve tried to play with this with arch calculators online, but don’t understand the equation terminology enough to be able to use them. [SMILING FACE WITH OPEN MOUTH AND TIGHTLY-CLOSED EYES]

I have the brace making jig from Luthier Suppliers. It’s easy enough to run through a saw. Then I perfect it with the Brace Sander jig from LMI.


Sent from my iPhone using Tapatalk

Yep, I think this is the answer.

Brad


Sent from my iPhone using Tapatalk Pro

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

I think this is the right answer. Imagine a brace 3 inches wide. Would you want that to be flat across the width on the bottom?

If you have setup blocks, take a 1/4” or 3/8” or 1/2” setup block and touch its edge against the concave face of an arching template. That will give you an idea as to how small of a gap theoretically exists. Then think about the fact that the material is not steel or acrylic, but spruce. The gap across the brace is small enough not to worry about. I do greater harm to the glue joint by trying to arch the brace across its width, and risk messing up what I really care about, which is the lengthwise arching. Or I can mess up the edge of the brace. I arch lengthwise using templates and don’t worry about arching across the width of the brace.

For the top, I just made a 3/4" X 24" long radiused sanding block. Planed it close and finished on the sanding block. I didn't give any concern to the miniscule amount of a gap that might exist down the center of the brace. I only use a dish for the back.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Thanks, Don. I agree with you, it’s nothing to concern about. I just could not visualize how it could be exactly the same. It cannot. Does it matter? No. Is it the same? Also no.


Sent from my iPhone using Tapatalk Pro

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

Brad,

1. For where in the dish you sand, and assuming you are using a true radius dish, it just doesn't matter where you sand since the dish is a uniform radius all over its surface.

2. When considering the effect of flat vs radius on the width of the brace you might want to check out a circular arc calculator like the one I've used here.

- Leaving the brace flat across the width just doesn't matter for practical application since the fit error is much less than 0.001".

- For example, consider a 1" wide brace (the widest back brace most of us use) on an 18' radius back. When I make them the brace is radiused lengthwise but is flat across the 1" dimension. The center of the brace will be less than one thousandth of an inch (0.00058") off of the radiused back. Obviously much less (0.00004") for a 1/4" wide brace.

Note that some of units are feet and some inches

_________________
Steve Smith
"Music is what feelings sound like"

I was hoping someone could do the math. Thanks.

I love that, thank you, Steve!

Also, here for fun, Brian Cox on Joe Rogan’s podcast talking about how the universe appears flat because we can only see a small portion.

https://youtu.be/ne3HV9tIITw?si=rs3Or3LWlBNxDDEh


Sent from my iPhone using Tapatalk Pro

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

OK, in respect of the radius being the same everywhere on the dish:
This is true relative to the origin of the radius (the center of the circle formed by the radius).
Relative to the bottom of the brace it is true, the radius will be the same regardless of position on the radius dish.
But relative to horizontal top of the brace, the POSITION of the radius on the brace will change.

Draw a cross section of the radius dish on a piece of paper, then draw a square cross section representing the brace; the position of the radius, RELATIVE TO THE HORIZON- HORIZONTAL, will be in a different place.


Sand a brace on one side of the dish, keeping top horizontal. Now place the brace on the opposite side of the dish and the top will no longer be horizontal. It will of course be straight but tilt away from the horizontal.

The guitar top/ back is a radius as well, a rather compound and complex radius. The same concept applies when placing a brace sanded on a dish. Ever had a brace that seems to "tilt"? I sand my braces in the same position in the dish as it will be located on the plate, relative to the position on the dish where the brace was sanded.

_________________
Measure Twice,

Karl Borum

I apologize for the pic size, I don't have an elegant way to manage it....

_________________
Measure Twice,

Karl Borum

Karl—

I recognize that some builders, in fact, do care about what you have described, but I struggle with understanding why they care. So what if the back braces don’t have sides that are all parallel to each other? I care about each brace being perpendicular-ish to the part of the plate to which it is glued, not whether the braces appear to conform to a perpendicularity constant. The box is closed; nobody should be able to see this. I see no functionality difference. Are some buyers so uptight that they check this with their little mirrors, along with all the other fussiness some of them have about the inside?

Well I'm with you fellers.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Like I said… can of worms.

However this does have me thinking and it’s why I asked the question to start with.

I have experienced an issue with the X brace where it appeared that it was fighting against itself to sit down properly. Like I mentioned in the original post, I normally sand each leg of the X separately. I used to do this wherever I felt like it in the dish, for the exact same reasons that many of the posts are explaining. The time I had the issue I switched to sanding in the dish in (or near) the location of the brace, and the issue went away.

Now, it very well could be that the sole reason I even hit that issue is because I sanded then individually and opposing the way that Karl described.

You would not see this if you sanded them side by side or better yet joined at the lap.

Interesting, and thanks for sharing your thoughts, Karl.


Sent from my iPhone using Tapatalk Pro

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

Yeah, perpendicular position matters, but you can sand a tilt any place on the dish if not careful. I also had an X-brace that would not lie flat so I fit it together and sanded away. Once I sanded the high side down everything fit well.

Steve went above and beyond by showing the math that demonstrates "across the brace" arching is not needed. Here are some photos that demonstrate the touchy-feely experiment I described above.

For perspective, here is a zoomed-out photo of a CNC-cut 15' radius template, balanced on top of a 1/2"-1"-2" setup block, straddling the 1/2" edge:



Here is a closer shot, showing the spot where the 15' radius arched template and the 1/2" setup block touch:



And here is an even closer shot, showing the same spot where the 15' radius arched template and the 1/2" setup block touch:



Does anybody see any daylight? I don't. I even put on my Mr. Magoo magnifying glasses, and I could not see any daylight between the template and the setup block. And this is with the tightest radius I use, matched up with the widest brace I use.

Brad, as for your prior issues with the X brace fitting, I'm going to speculate that the combination of a very tight lap joint cut before arching the braces, and the independent later sanding/arching of each of the two legs, is what caused the issue you describe. Maybe you could arch the braces first, then cut the lap joint? Also, are you using the bottoms of the braces as the reference point for the lap joint, or the tops of the braces? I just see a lot of opportunities for things to go wonky with the X braces fitting together (for all of us, not just you), but none of them point to a problem with the braces being flat instead of radiused "across the brace."

I think you’re right, Don.

Moral of the story for me is to just touch up the X while the lap is joined. This is how I’ve done the last half dozen guitars with no issue. I likely attributed the symptom to the wrong cause.


Sent from my iPhone using Tapatalk Pro

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

I think there is a particular issue with the X brace that does not affect other braces, and it is the fact that we connect the two legs of the brace together with the lap joint and make a somewhat rigid X structure, which can wind up with some 3 dimensional weirdness when expected to match up with a sphere. All the other braces just butt up against each other, so they easily conform to the sphere (if arched right on the bottom). You can make the X and get the arching right afterwards (as Brad is describing), or you can get the arching right for each leg, then cut a “bespoke” lap joint (I go this route).

I always radiused each leg, put wax paper down in the dish and glued them with weights in the dish.

Then I sanded the entire assembly once the joint glue dried in the dish taking care to view the dish as a clock 1, 2, 3, 4 etc and turning the assembly as I sanded so the errors in the dish, and there are errors in your dishes cancelled each other out.

This final sanding in the dish removes very little material and is really dialing it all in and nothing more and ensuring that the entire assembly sits with full surface contact in the dish.

Also I used abrasive sheets the same size as my dishes so I could use the full dish and cancel out spot errors in the dish.

Some food for thought on a related topic. We've seen a cloth patch as Gibson used on the X intersection and we see others cap it and some with substantial caps.

Because of the full height intersection of the X-legs in this area of the top it's stiff as can be and that, in my way of thinking may not be the goal. As I've said before we are not building ottomans we are building structures that are supposed to be capable of moving and more specifically pumping.

With this said the cloth patch we see far more frequently in vintage instruments may have the intended purpose of being flexible and may help produce a more efficient instrument if volume is the desired measure.

I never went to cloth but had I continued to build I might have. My last caps were only around .010" thick and tapered to nothing about 3/4" from the intersection but even still they made that junction super rigid and I always wanted to try to loosen that up and see what resulted.

Maybe no glue or reinforcement at all for that joint would be best. It aint going nowhere.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Brad

The first thing are we using spherical dishes? If so then the curve is by definition the same everywhere in the dish so it can't matter where you do your sanding. If it is not spherical then all bets are off and your method is required.

In terms of the curve across the width then you do actually want this for the best fit. Imagine a brace say 1 inch wide. If left flat across the width then you certainly won't get a good fit. The same principle will be true of normal size braces, even if the curve required is pretty tiny.
I certainly try for the best fit possible. Cheers Dave

Sorry for repeating didn't read all the posts Dave

Unless you are like Karl and want your braces vertical and parallel.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

It’s about tone!

….aaaaaand scene.


Sent from my iPhone using Tapatalk Pro

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