One of the best ways to open a can of worms at a woodworking store is to ring up and ask the question:
“I’ve made an acoustic guitar body. What’s the best way to cut the rebates for the bindings?”
The main reason this is a tricky job is the multiple grain directions that need to be dealt with. The grain on the guitar’s sides runs around the body, while the grain on the top and back runs straight from head block to tail block.
The rebate will cut down through the top or back, through the side and into the linings. So, each cut will be dealing with multiple changing grain directions. It will also slice through glue lines. The timber is likely to be figured, or otherwise difficult to cut cleanly – but this the kind of critical cut where any tearout or chipping is absolutely not allowed!*
The rebate also needs to be clean and square. Any material left in the corner of the rebate, and any wonky angle, will result in major problems later on when gluing the binding.
So, how to give yourself the best chance of success in such a challenging operation?
As with most operations in woodworking, you can either tackle the rebate by hand, or with a machine.
By Hand
You’ll need to cut the sides of the rebate with a special tool called a gramil, which is much the same as a mortice gauge, except it registers off a thin pin rather than a broad, flat surface. Make sure the gramil’s blade is sharp. You’ll be slicing across the grain on the softwood top so you want to avoid crushing the fibres. You’ll also be cutting along the grain in the hardwood side; a sharp blade has the smallest chance of following the grain off the line of the cut. Make multiple, light passes with the gramil. Cut as deep as possible without ruining the crispness of the rebate edge. Then, cut away the waste with a sharp chisel.
Simple! But time-consuming. You’re aiming for a square, level rebate around the whole guitar, top and back. Take your time. Sharpen your chisel often. We recommend listening to the cricket.
By Machine
The machine in question being most commonly a small router aka laminate trimmer. The trimmer will register off the side of the guitar and rest on the top or back.
Rebate bits with bearings are a good option because the bearing is concentric with the bit, which means that it doesn’t matter which way the trimmer is oriented, the cut is always the same width (i.e. the difference between the radius of the cutter and the radius of the bearing).
Using a bit without a bearing means that you will need to use a guide that is part of the router base assembly. This works, but because the guide is less likely to be concentric with the cutter, you’ll need to maintain the router at a constant angle to the side. The great advantage of this approach, though, is that you can use any bit you like. A spiral downcut is a huge improvement over the standard straight cutter. It slices downwards through the delicate endgrain at the butt end of the guitar top, giving a much cleaner cut.
Because of the radius in the guitar’s top and back, the standard router base will not deliver a consistent rebate depth. You’ll need a base with a raised ‘donut’ around the bit. The donut means that the router registers right at the edge of the top or back, and the trimmer doesn’t tip from vertical – but it also means you’ll need to keep the trimmer parallel to the side either manually or with a jig of some kind.
Finally, two tips that will help you get clean cuts:
1. Put a coat of shellac over the area to be rebated. This will stick the fibres together and help stop them getting schmooshed sideways – especially on the top end grain.
2. Precut the rebates with a gramil before using a trimmer. It can be tricky to make sure the trimmer is set to exactly the same dimension as the gramil cut, but it is very effective at preventing tearout in figured and/or chippy side timbers.
A clean binding rebate is a thing of beauty. Make sure you sit back and admire your handiwork!
* if you do get a little chipping, there are luckily some techniques for fixing it – to be covered in another blog entry!