Blind Routing

[Jude]

I'm not sure if that is the correct term. What I want to do is route inside the body from an existing opening using a fairly large diameter bit. Has/can this be done, or am I just asking for trouble?

Thanks

[Ripthorn]

Do you mean you want to deepen the cavity, or do you want to undercut the cavity? You can undercut. Use a rabbetting bit with the appropriate bearing for the undercut. Make sure that you aren't going to do something like leave too little wood for a screw to grab or anything like that. The thing about doing it this way is that the cavity has to be large enough for the rabbetting bit to get inside. A little more explanation of what you are trying to do would be most helpful.

[Jude]

Undercut, that's the word I was looking for. I want to add a switch without having to redo the access cover. I was worried that with wood surrounding the bit above and below that it would grab and send it out of control through the guitar and possibly me.

Thanks

[Ripthorn]

It certainly can grab if you are not careful, but most router bits have the cutters protruding from the main body some so that there is always some clearance. However, if you are going to try undercutting for a switch, I doubt you will find a bit that will allow you to undercut that much. You may have to go at it with some chisels/gouges or possibly a drill bit like a forstner or spade bit. Whatever you do, be very careful as you can wreck the entire body quite easily trying something like this. I did some undercutting both with a router bit and with a drill bit on a tele I built a couple years back. I wasn't taking off nearly enough for mounting a new switch, but it worked well enough.

[Prostheta]

What Ripthorn said. From the sounds of things, the cavity is so small that you would be working with tight corners or tight concave surfaces so the chances of a router bit grabbing are far higher. A high RPM bit running out of control in an enclosed space is akin to a small explosion or a bullet. I genuinely would never consider it on anything other than a router table and even then that has its own safety implications to consider.

If you post photos of the parts/cavity in question I am sure a safe solution will be formulated.

[curtisa]

The only time I have ever done blind routing was to add a wiring channel around the side of a battery box cavity, but that was using a 4mm x 4mm slotting bit with a bearing. It's not something I'd want to be doing too often. Anything bigger is going to raise all sorts of issues with practicality and safety that I personally wouldn't feel comfortable going ahead with:

A bigger undercut requires a bit with more "reach", so how do you get a wide bit down into the cavity to start with (bit may be wider than the opening you're trying to put it into)?

How do you stop it getting away from you if things go pear-shaped (can't just simply pull the router away)?

How do you safely finish the cut and the bit is still spinning inside the hollow you've just created (can't just simply raise the router)?