Sanding top kerfing, head and tail blocks
 

I have built a few instruments before at a luthiers workshop so this is not my first instrument, but this time i I wanted to build a guitar for a friend, so as I don't have a workshop I bought one of those Stewmac dreadnought kits. Looks good but checking the instructions (https://www.stewmac.com/freeinfo/i-5295/i-5295.pdf) I have a doubt.
At the part where the kerfing is sanded, I can see that the back radius is built into the kerfing, head and tail blocks. But when the guitar top is done, everything is sanded down flat and squared to the guitar sides. Is it ok to do it like this? The guitar top also has a radius, and I know some builders also build this radius on the kerfing and end blocks, so I wondered if sanding them flat could cause any problem when gluing down the top.

Thanks a lot in advance.

One of the biggest variables in the quality of a guitar is how well things fit together. The better the joinery, the greater the integrity of the whole, and the less stress in the structure. Wood is fairly resilient and can be forced, and joinery which is not perfect may hold adequately anyway, but if one is going to go to the trouble to make a guitar, why not do the best job possible?

Thanks a lot for your reply, Bruce. I have seen your guitars in forums and in FB, they look great, and yours is a valuable opinion. You might be right.
I wondered why they suggest this method, so investigating a bit I found this article ... https://artisanguitars.com/huss-and-...s-radiused-top. Looks like this is how Huss and Dalton build their tops on their Traditional line. So I guess this is probably the way Martin used to build theirs in the old days?

If the guitar top has a radius and the kerfing does not, it will touch mostly on the outside extreme. When you rout your binding channels you will eliminate the part most likely to have connected. Far better, IMO, to bias the kerfing the other way, if anything having too much angle so that it touches best on the inside extreme. Correct would be better yet, of course.

As some one whose primary job is to fix those loose tops and backs on guitars (primarily Martins :D), I would heed bruces advice and spend the extra time needed for fit and finish so this does not become an issue on your build.

Steve

Just a check, but are you only concerned with the radius (or not) being reflected on the width of the kerfing? (A 30' radius across a ⅜" kerfing-plus-side is only a couple of thousanths difference in height, even at the widest point, the lower bout.) If you expect to have the entire top edge sanded in one plane, and then intend to attach a top glued to arched braces, you'll have to clamp hard in the waist area. The could cause an issue. Here's a simple assessment: is the top edge of the sides a straight line, or does it have a bit of a "bulge" in the waist area? If it's a straight line, when you lay the arched top on it, before gluing, the top will rest on the edges of the upper and lower bouts and there will be a gap at the waist. Not so good. Time for some sanding.

Thanks everyone for your advice. To answer Yellowsty's question, the top is a straight line, it has no curve. But as far as I now that's how flat top guitars have been built traditionally.

Be it a top or back, if it has arched braces being utilised in its construction, then I always profile sand or utilise a plane to ensure the kerfing / heel / tail blocks match.

Typical profiles i see are 50ft, 40ft, 28ft, 20ft, 15ft.

Here is an example, i glued this up today

Supported

http://www.mirwa.com.au/images/MNBKR36.JPG

Marked out with just some pencil

http://www.mirwa.com.au/images/MNBKR38.JPG

20ft radius platform for sanding.

http://www.mirwa.com.au/images/MNBKR39.JPG

Profile sanded, no pencil marks remaining.

http://www.mirwa.com.au/images/MNBKR41.JPG

Thanks Mirwa. That looks great. Just out of curiosity when seeing your pics, do you always attach the neck before gluing the back? What's the advantage of this?

I am primarily a repairer not a builder, i build around 8 guitars a year, i repair around 1500-2000 guitars a year.

The guitar in the photo is an existing guitar that had been stolen from the owner and dumped in the bush for a month, they also smashed the back.

It fitted your question perfectly, hence why i used it and it was a set of photos I took today.

Removing the back and putting a new one on involves me making sure the neck is in the correct plane to the bridge.

Steve

Here is a photo of how I recieved it
http://www.mirwa.com.au/images/MNBKR1.JPG

I see, thanks. Wow, that's a big repair.

I did a tour of a guitar factory where they had a radius dish with sandpaper on it mounted to an electric potters wheel. They held the guitar body against the sanding surface as it spun. 30 seconds later, they were done.

Got the same setup, i also use mine to reshape banjo heels when doing a neck reset

Steve

That would make my life much easier! :)

Well allow me to be the outlier here...

I don't think it matters!

Having said that, I do actually put the proper radius on the linings but honestly when you look real hard you can't really see it anyway. It's more important for the back of the guitar which is arched a lot more. I simply use a plane and then finish it off with an arched sanding stick.

This issue has come up countless times since Cumpiano and Natelson's book came out in the 90's. In the book they demonstrate putting a radius on the top but then clamp the sides to it with flat linings. I imagine literally thousands of guitars if not more have been built using this method and none of them have fallen apart. I've built a few with this method and you always see squeeze out on the outer edge of the linings. It could be because when you clamp the sides down they sort of lean in. But if you do the math on it, it's a very very small distance that requires that angle.

Interesting in Irving Sloans book which was one of the very first of it's kind, he does talk about putting the proper angle on the linings.