So I have fit many, many tone bars over the last week or so while doing some experiments with my new production mandolin design. It takes me a couple to three hours to get them cut, sanded, scraped, and shamed into joinable shape.
One of the problems is that the more angled the bars/bracing are, the more of the serpentine shape has to be truly transferred to the new bracing member. It can be really crazy geometry. A spindle sander does not really work except for very rough shaping.. you all know the drill.
I thought it would be nice to have a fixture which would automagically get the tone bars to match the geometry of the top.. at least within 5 or 10 thousandths of an inch or so. From there I can scrape it to final shape, no problem.
Here's what I came up with. It's just a mockup at this point, so don't be too hard on me.. the test is just in a piece of cheap plywood.
It's skeleton is a parallel torque reaction arm which I also use for binding. It keeps the cutter dead plumb but free to move.
It also counterbalances the router so it only "weighs" about 8 oz. Not enough to damage the spruce plate, but enough to keep the assembly grounded.
There is a Delrin probe which slides really smoothly but doesn't dent the spruce. The probe is coaxial with the cutter.
The workholding fixture above the unit holds tone bar stock.
Setup consists of aligning the tone bar's location with a line on the table, and clamping the top plate in its quick release rim fixture to the table with the blue hold-downs.
The torque reaction arm keeps the cutter from moving wildly. Because of the torque counter-action of the "elbow" on the unit, this router can be operated with one finger, it is very stable and does not "dance around" like a Dremel.
So far just proven the concept in plywood. It takes some thought to get a really smooth surface, but it is possible in probably 5-10 minutes per tone bar. Probably another 20 minutes left for final fitting, but I think it's a huge advantage. Works for X-bracing, too, just needs a different X-bracing holder above the unit.
It's basically just a 3D pin router. But I think it will work for me.