I prefer to leave thin jointed surfaces clamped up over night to allow the glue to cure fully before further work, and so I removed the parlour guitar soundboard from the thin panel gluing jig the morning after it had been glued up. Because I place panels face down in the gluing jig, there can be a slight step on the rear of the join if the panels are not of consistent thickness, so before surfacing the show face of the soundboard I removed this step from the inner face using a block plane.
When surfacing soundboards and backs, it is imperative to get the work as flat as possible, as well as achieving a clean surface free from tear out, and this involves constant checking of the work with a machinist’s straightedge. In the past I have used a No.5 jack plane set to a fine cut, followed by a cabinet scraper to remove any hint of plane tracks, but this time around I decided to give the No.8 jointer plane a go, and the extra width of the blade allowed me to achieve a flat surface quicker than I would have with the No.5, and left only a little work for the cabinet scraper. For difficult timbers (particularly the backs and sides of acoustic guitars, which use exotics like rosewood) I use a toothed blade, and I may have to look into getting one made for the No.8 for this purpose. Because the soundboard panels are bookmatched, it is important to change planing direction on each half of the work, so as to avoid tear out, and the cabinet scraper is invaluable for cleaning up the glue line as it seems to resist tearing out while working against the grain.
With the full panel surfaced, I marked out the shape of the soundboard using my plywood template, and then cut to shape (staying bang on the pencil line) with a fret saw.
The thickness of the soundboard is critical for the sound of the finished guitar, and with the outer face now surfaced I was able to start thicknessing the soundboard from the inner face. The purpose of thicknessing is to achieve the right balance between a thin soundboard which will be responsive and resonant, while remaining strong enough to withstand string tension without imploding. The advantage of handmade guitars over mass produced machine produced instruments is that each soundboard and back can be worked to the optimum thickness for that particular piece of wood, with the bracing further adjusted to reflect the properties of the soundboard and back.
Using my large calipers. I mark the thickness of the soundboard at 2″ intervals, and then work the high points down until the the soundboard is down to a nominal thickness, using block and bench planes. This is a very precise way of working, and with planes set for a fine cut I know that as soon as the pencil numbers have been removed from the soundboard I have reduced the thickness by 0.1mm of material. Once each number has been removed, I remeasure everything and write the new thickness of the soundboard at the same 2″ intervals. Once I am close to the nominal thickness, the numbers stop being as important, because I am testing the wood by flexing it and listening to the sound it makes when tapped with my thumb. The need to constantly remeasure means that thicknessing a soundboard will take a couple of hours, but the end result is worth it. For a yellow cedar soundboard I work to a nominal thickness of 2.7mm, and this soundboard ended up just a hair under that as the timber had good stiffness and tap tone, which allowed me to remove just a tiny bit more material.
With the soundboard thicknessed, I was then able to cut the rebate in the neck which accept the soundboard when the guitar is assembled.
This joint needs to be cut precisely, as the top surface of the neck and soundboard need to be completely coplanar. To achieve this, I set my marking gauge a hair less than the full depth of the rebate (2.5mm instead of the 2.65mm thickness of the soundboard) and once I reached my baseline I checked the fit using the soundboard, slowly sneaking up on the final fit with a very fine cut. To cut the rebate I bevelled the two outside edges with a chisel to prevent spelching, and then used a Lie-Nielsen to remove the waste.