Acoustic Guitar Construction

A quick question that’s been troubling me: in Cumpiano’s book he glues the neck block down on to the soundboard dead flat at 90 deg to the sides and yet his top is arched. In YouTube videos, by other well known luthiers, the head block and tail block are radiused along with the sides. Just wondered which is preferred and why?

I am not sure there is one preferred method. There are a lot of ways to get the geometry correct for a guitar. Different for steel string and classical. Even for steel string I have tried a few different methods used by more experienced luthiers. I know they can all be made to work as these luthiers all have completed guitars. All the methods take a bit of work to get right. One bit of advice I give to people starting is to not mix and match procedures of building a guitar as they do not mix and generally work as a system. For example some build methods put an angle into the neck and leave the heel block square to the work board, while other methods leave the neck angle square and angle the block. If you mix the two your neck angle will be way off.

I believe cumpiano on a steel string does glue the block on square but then he uses a fixture and shims to tilt the block, rims on the block a bit and the top to the correct angle to make sure the geometry of the neck to the top is correct. Many builders will sand the top rims flat and then sand the correct neck angle into the upper bout of the rims, yet still radius the top braces. His method assure the correct neck angle. His method is close to traditional classical guitar construction with changes made to accommodate the steel string geometry.

One issue with radiusing the entire top is that it is easy to get too much neck angle the fretboard is straight yet without flattening the radiused top you will be gluing it on a curve. It can be made to work by flattening the upper bout a bit and making sure the angle will be correct.

Having said not to mix methods I would not use his pinned neck joint method rather I would use bolts and inserts. I believe in updates Cumpiano moved away from it as well.

John - Many thanks for taking the time to reply, it’s very much appreciated. Your advice about not mixing build methods makes sense now, unfortunately I should have asked earlier. I’ve built 5 guitars using Cumpiano’s book, and they’ve all turned out well, but for some reason I decided to radius the rims and top and bottom blocks on this latest build. I’m now beginning to wonder if the neck angle will be too far out of whack. I guess I’m just going to have to carry on and hope I can get the correct neck set by shaving the bearing surfaces of the heel until I get the correct clearance at the bridge. Lesson learned!

If the top is not glued down clamp the top to the rims and run a straight edge along where the fretboard will go. At the saddle location it should be above the top about 2.5 mm. You can fix the rims to give you the right projection.

Unfortunately, I’ve already glued the top down. Just taken the measurement at the saddle location and it’s 8.5mm, is there anything I can do with it now or is it beyond repair?

Hmmm, Generally an over radiused body can be fixed with a wedge under the fretboard extension filling in the gap. It would be thick at the body join and thinning toward the sound hole. You can see what I mean by lowering the saddle edge of the straight edge on to something 2.5 mm thick at the saddle location; that is what you want your extended neck plane to be. Look at the gap between the straight edge and the heel side of the guitar. That is what you would need to fill.

I did wonder about a wedge under the fingerboard but thought I might not be able to live with the look of it. The only other solution I could think of was to try and separate the Titebond glue joint at the head and tail blocks and all around the kerfed linings, but I’m not sure if this is even possible. I’d be prepared to make new blocks and rims but would hate to destroy the soft spruce soundboard in the process.