John Hartley. Luthier and furniture restorer. Building mandolins and classical guitars

[John hartley]

Hi everybody, I use the router for edging rebates on guitars and mandolins but I find great difficulty in routing a mandolin back, where the curved back is at an angle of around 100-110 degrees to the sides. I have tried, with some success to add a timber angled wedge to the router base, but as this angle changes with the shape of the back, it is not very accurate. Any ideas? John H.

 

[Yojevol]

How is the curved back achieved? Would it be possible to do the routing before the back forming?
This might mean the routed shoulder ends up at an angle but could you live with that?
Brian

 

[Ollie78]

I suggest you could make up a jig or fixture that surrounds the part with a wide track at the correct angle to support the router.
Using a tiny trim router or possibly a dremel style tool to reduce the required size.
I used to have a Makita router which had an offset attachment for getting into corners which might help.

Of course this will only work if many parts are the same and it can be repeated. Otherwise it will take too much time to make the jig for a one off.
It could be done on a cnc router but would involve a lot of probing or an accurate 3d model, also some form of fixtures.

Ollie

 

[Ttrees]

Hello John
I tried doing a search on google for a binding and purfling jig with the key word...
" official luthiers forum " with no good results.
I suggest you join there and look in the archives, as there is many clever designs which aren't showing up on a regular search.
10 years ago the place was really booming, and all the pros were fr,equently posting
it seems quieter now, but does seem to be picking up again.

That's likely the best resource you will find.
Edit: There are one or two other Scots there too, still posting away.
All the best
Tom

 

[baldkev]

Or, is it a router bit with bearing? if so, you set your bit up in a router table and move the piece, not the router
I just engaged brain... sorry, i get you. You need it to stay at 90° to the sides, but as the back is curved, you need to keep it flat. Going via the sides wont work as the router base wont fit the curves.

So the only thing i can think is to fit a flat base the same shape as the body outline and measure off a set height, say 20mm to get over the curve, and glue in packing pieces to the flat board so the packers sit on the back at the same height up all the way round. That at least gives you a flat base to work from.
Hopefully that made sense?

 

[profchris]

I understand John's problem as I build ukes and guitars.

Because the back is domed, either:

1. the binding channel is cut at an angle (if the router base is held flat to the back), making the channel too deep and with a slanted base, or

2. if the router is held vertical, the binding channel is too shallow where there is doming because the router base is sitting too high.

I don't have a solution where the router does it all, though I have seen complex jigs with swinging arms which claim to achieve a channel of uniform depth with a flat base.

My plan is to make a simple gramil - a scrap of wood with a rebate of maybe 6mm, and a blade stuck through it at my binding depth. I'll run this around the body of the instrument before binding, and as it will only just overlap the back or front of the instrument the dome shouldn't affect it. This will give me a scribed line of uniform depth.

Then I'll rout holding the router vertically. In places it won't cut down to the line, and so there I'll finish the channel by hand using a chisel.

A far from perfect solution, but it should work!

 

[BillK]

Sliding router carriage? aka 'router binding jig' - I made this one using CNC rails & sliding bearings, people use draw slides too. Or they are out there to buy.
There's a curve-sided doughnut the router bit goes through, which rides on the body edge, without hitting carves.

This below's flat back/top below obviously but for arched or carved bodies, make a base with edge holders which keep the body edges at 90 degrees to the flat. Something like 2nd pic (borrowed from TLC in Holland).

If the top is perfectly flat then you don't need that fixture to rout the back.

If you have scrolls and such it won't get everywhere of course, but teardrop shape etc should be fine.