When I’ve previously made control cavity covers, I routed the shape into the body and then, as best as I could, have cut the cover to shape. Getting an exact match is really tough and whilst my efforts have been adequate, they’re far from perfect. Look closely at this picture and you’ll see the ~0.75mm gap between body and cover.
It always struck me that there must be a way of using a single template and router to cut both the cavity (female) and cover (male). I finally stumbled across this page which shows a technique to do it. In essence it involves using an oversized template and different guide bushings (also known as collars).
I hit my first problem – all of the wide choice of router collars only seem to be available for Porter-Cable type routers, whereas I have a Bosch POF 1400 ACE, which has a different fitting in the router plate. I spent many fruitless hours searching for the parts I needed – mainly looking for what is called an “inlay kit” which combines two collars with a matching cutter
The way the geometry of this approach works (see picture below) is that the difference in collar diameter needs to be twice the diameter of the cutter. I currently have a 17mm collar, that came with my router. I finally managed to navigate to the correct page on Bosch’s UK website to see what collars were available that would fit. Put that part number in at Amazon, preceded by “bosch” and I found a supplier of the part I needed. You can find this at bosch.co.uk. Of course as soon as Bosch refresh their website this link may stop working, so I will reproduce the collar sizes and part numbers, should it be useful for you.
||2 609 200 138
||2 608 000 471
||2 609 200 139
||2 609 200 140
||2 609 200 141
||2 609 200 142
||2 609 200 312
So going back to that equation of the difference between collar sizes needing to be twice the diameter of the cutter, and given that I already have a 17mm collar, the only viable option for me was a 27mm collar with a 5mm cutter.
So how does this magical combo of two collars and one cutter allow you to make the male and female cut from the same template?
This diagram attempts to show how this works. The template needs to be 11mm larger than the final pieces. For cutting the cover we use the small collar, pictured at the top. Here you can see how the left edge of the cutter will trim the piece, to follow the shape of the template.
To cut the ledge for the cover to sit on we switch to the larger collar and here you can see it is the right hand edge of the cutter that replicates that same shape, albeit in female.
There’s more to this than initially meets the eye. Using the same principles you can make an exact sized cover for any cavity or make an exact sized cavity for any cover. For example, I have considered rebating a Telecaster’s control plate so that it sits flush with the surface of the body. I could mount the control plate onto MDF, use the small collar to create the 11mm oversized template, and then use the template and the large collar to cut the recess into the guitar body.
There is one limitation of this approach is that it is not possible to cut a corner smaller than the radius of the router bit. This means that the template corners must have a minimum radius of 22mm. It could cut a square corner on the male part, but it would mean that you’d need to use a chisel to square up the corners of the female.
And that’s the theory. When my 27mm collar arrives in the next couple of days I’ll report back with my experiences.