Very thin rabbets?

[LCHIEN]

Is there a way to make very thin rabbet cuts... I'm talking 0.01" or maybe 1/128; 1/64th (0.016") is almost too big'

I.m wanting to fine tune a large hole.
One classic way of doing this is to use a precise Forstner and then use a precise rabbet (with bearing) for part of the depth and then use a flush trim bit to even up the hole.
Supposing you need a 2-1/2 and the closest is a 2-1/8 you have then you can follow with a 3/16 rabbet (half the diameter distance) to enlarge to 2-1/2

I need to sneak up on a dust fitting.by .02" steps so .01 depth rabbet would be nice. Multiple cuts and a flush trim after each can do that.

But how do I get a really thin rabbet with a router?

I have a rabbet set with a large number of bearing sizes but they are for larger fractional sizes.

[mpc]

Do you need multiple holes of ONE consistent size? Or individually fitted/sized holes? If one (or two) size(s) is sufficient, what about making a template (or two) using an adjustable circle cutter (fly cutter) bit in a drill press? Then use that template with a normal flush-cutting bit to make the real parts.

Another (expensive) option: CMT, and perhaps others, makes router bits with square carbide cutters that screw on... if one dulls, you simply loosen the screw and turn the cutter 90 degrees just like the carbide cutter heads available for jointers and thickness planers. CMT's web page shows a diagram of how to position the cutters when turning them: push them against a reference edge before tightening the screws. Thin tape on the reference edge might give you a way to dial in thickness changes. They show a "flush trim" bearing as one of the options for this kit; that bearing plus tape on the cutters could change it from "flush trim" to "very small rabbet" operation.

CMT page: Example carbide insert rabbet router bit

mpc

[leehljp]

My mind is not clear this morning, still foggy. I don't know how this would help in your case, but when I needed something in a minuscule amount, I would add the thickness of one layer of tape to shift the dimensions. In your case one layer of fine scotch tape might cause .01 shift, EXCEPT you want to take that much OFF. Tape on a dado bearing would do the opposite by that much.

I am curious how you will get this done, following it.

[capncarl]

Routers aren’t very high on my favorite Woodworking tool list. I go by the saying that if a hammer is your only tool everything looks like a nail.... or something like that.
I’d use a finger joint jig on the table saw. I’ve made box frames for a vegetable dryer using a 1/8” blade, where every finger was 1/8” I’m not understanding your application but if you only need one rabbit one each piece this may be the ticket.

[LCHIEN]

Basically I have a largish hole, maybe 1.5" say.
What I want to do it to enlarge the top section of the hole by .010" or so. Or a few thousands at a time.

[leehljp]

I know you will not want to invest in this, but here goes: buy a good saw tooth set tool and re-set every other tooth of your one of your hole saw by .005. That will give you the diameter you are looking for. However, how will you keep it perfectly centered?

[LCHIEN]

My mind is not clear this morning, still foggy. I don't know how this would help in your case, but when I needed something in a minuscule amount, I would add the thickness of one layer of tape to shift the dimensions. In your case one layer of fine scotch tape might cause .01 shift, EXCEPT you want to take that much OFF. Tape on a dado bearing would do the opposite by that much.

I am curious how you will get this done, following it.

I thought of the tape on the bearing trick, but as you say, its the wrong direction.

I don't want to go so far as modifying hole saws and stuff!

[mpc]

In addition to the CMT rabbeting bit that I linked to - where you can tweak the carbide cutters outwards a bit with tape - another option would be to get one of the cheap style trim bits. Instead of a bearing, they have a polished metal surface that is flush with the cutting edges. They tend to burnish whatever material they're riding against but, in your example, that would be the inside of the existing hole that you want to drill out anyway. Get one of those router bits, chuck it in a drill or drill press, and use sandpaper or a file to reduce the diameter of that metal surface slightly. Now you've got an "undersized" bearing. Such bits are normally intended to flush-trim laminate countertops but they'd get your mini-rabbet started. Then switch to a pattern bit (top bearing) to finish the rest of the hole. Example bit: Diablo trimming bit Just narrow down the diameter of the "bearing" area at the domed end of the bit. You could use calipers to measure the factory diameter before filing it; then measure as you file until you get the 0.05 inch change or whatever you need/want.

mpc

[leehljp]

MPC, you are on to something. Since he only need .01, if he gets a smooth 1/4 or 1/2 inch rod and extend it to the depth he wants, lightly burnish the hole all the way around. That itself should give a .01 depression. The key will be equal pressure all the way around.

Second thought. Wrap a piece of 400 grit sand paper on a 1/2" wood rod - glue it on and don't have any overlap. Put this in the router and extend it down to the depth wanted. Do NOT turn the router on. run this around the inside of the hole for one turn and measure the hole; maybe a second turn. You should be able to sand away .01 with one or two or three rotations inside the hole.

[d_meister]

Use the hole saw to cut a hole the next size up in 1/4" pattern stock. Shim the bearing as Hank suggests. Heat shrink tube may be just the thing. Use the oversize hole with the shimmed bearing to route a smaller hole in another piece of pattern stock. Repeat as necessary until the hole is within design specs. Remove shim and complete project using the pattern to enlarge the hole.

[leehljp]

IF you had a lathe, you could turn a bushing to fit tightly over a router pilot bearing that is smaller than the bit. just make it so the outside diameter of the bushing is .005 smaller diameter than the router blade.

BTW, that is not meant is a joke. In making pens, I often build up finish oversize and turn it down by as much as .001 or .002 for perfect fit to components of the wood part of pens.

[jussi]

If it's a one of, I may try sneaking up on it with a spindle sander.