Are there flush trim router bits smaller than 1/2" diameter?

[JonW]

From a discussion we've been having on building some shelves, I think I'll want to cut some dados into legs and use 1/2" baltic birch plywood for the shelves. So I'll need a top bearing flush trim router bit to make the dados. I figure the bit should have a diameter less than the shelf thickness. I'm not finding anything with a cutting diameter less than 1/2". Googling around and all. Do such things exist? Thanks,

[LarryG]

First of all, nitpicking ... most people consider a "flush trim" bit to have the bearing on the bottom. One with the bearing on top is usually called a "pattern maker's" or "template" bit or (imagine this) "top bearing" bit. You will see both types lumped together in catalogs but these terms will likely result in less chance of confusion with a vendor.

That said ... flush-trim bits do come in sizes smaller than 1/2", but I've never seen a pattern maker's bit that is smaller than 1/2". They get larger, and the cutter lengths vary, but I don't think they get smaller in diameter since a 1/2" OD bearing fitted around a 1/4" shank is about as small as things can practically get.

[JonW]

Thanks!

OK, it's a "pattern maker's" bit. Didn't know that. (Heck, 6 months ago I didn't know what a router is!)

And for these shelves I want to build, it looks like that's what I'll need to cut the dados- a top bearing- if I use your simple jigs. So I'd better go measure my plywood to see if 1/2" is too wide or not. Or I could always get the 1/2" bit and make the 3/4" thick shelves, although that's not my first choice. But if the bits don't come narrower than 1/2"... I can't be the first person with such a need...? Well, maybe people just use table saws to make dados, I guess.

[jziegler]

Jon,

Another option, although harder to get the proper size, would be to clamp two boards to the piece the dado is in and use a straight bit. The router base can't move past the boards. So, for a 6" base, boards would be about 6 1/2" apart. I actually used some 1/4" plywood with a couple of boards screwed to it for something similar to this before. Now that I have a dado blade set, I haven't used it.

Using a pattern makers bit is a better solution overall. This just gets you around the size limitation.

-Jim

[JonW]

Jon,

Another option, although harder to get the proper size, would be to clamp two boards to the piece the dado is in and use a straight bit. The router base can't move past the boards. So, for a 6" base, boards would be about 6 1/2" apart. I actually used some 1/4" plywood with a couple of boards screwed to it for something similar to this before. Now that I have a dado blade set, I haven't used it.

Using a pattern makers bit is a better solution overall. This just gets you around the size limitation.

-Jim

Right. I thought about that. And my router has a collar guide inside of which the bit spins. I think there is about a 1/16" gap between the bit edge and the collar. So I could actually have the boards maybe 1/8" farther apart than the desired dado width. I tried playing with that the other night. One dado was a little too narrow and the other a little too loose. I'm new to all of this. So maybe the regular pattern maker's bit is what I need to do. I like the idea of actually using the real board as your thickness guide.

[Tarheel]

I would suggest investing in a set of CMT Plywood Groove Bits. They are slightly undersized for plywood that isn't actually 1/2". The set comes wth "1/4", 1/2" and 3/4" bit's. I don't have a dado set for my TS and find that these bits will cut a dado that makes for a nice tight fit! (I recommend getting the set with a 1/2" shank.) A lot smoother cut! Woodcraft, currently have them on sale for $52.40. (I paid $70.00 and I still think that they were worth every penny!)

Wayne

[RmeDad]

A little cheaper solution!

I would suggest investing in a set of CMT Plywood Groove Bits. They are slightly undersized for plywood that isn't actually 1/2". The set comes wth "1/4", 1/2" and 3/4" bit's. I don't have a dado set for my TS and find that these bits will cut a dado that makes for a nice tight fit! (I recommend getting the set with a 1/2" shank.) A lot smoother cut! Woodcraft, currently have them on sale for $52.40. (I paid $70.00 and I still think that they were worth every penny!)

Wayne

Just a quick note,

Holbren has a sale on the same three bits for $12.95!

[scorrpio]

Or see the jig I posted in that other thread - the one that allows use of no-bearing straight bits, either 1/4 or 1/2.

[JonW]

These plywood thickness bits are interesting. But they don't have a bearing. So how do you folks typically get the straight dado, when set up with a jig?

[JonW]

Or see the jig I posted in that other thread - the one that allows use of no-bearing straight bits, either 1/4 or 1/2.

OK, I just read that. Very interesting. I've got 1/4" and 1/2" straight bits at home. Maybe I'll play around with that. Hmmm...

[LarryG]

A caveat on undersized-plywood bits.

Disclaimer: I don't own any undersized-plywood bits, in part because I have a dado set for my table saw that can be shimmed to provide whatever width I need.

But the main reason I've never bought a set is because the plywood I can get locally is all over the map, thickness-wise. An undersized bit will indeed cut an undersized groove, but it can still cut only one width, and that width has to match the thickness of the plywood you're using.

If you have a consistent source of plywood AND its thickness matches your bits, cool. If not, you're basically right back where you started: trying to cut a groove with a bit that is almost-but-not-quite the correct size.

[JonW]

Larry-

I think you're totally right. I'm new to this, but I don't know that 3/4" baltic birch is the same thickness as 3/4" AC plywood as 3/4" MDF as... So I think using some sort of jig is the way to go.

I'm a little surprised there aren't pattern maker bits thinner than 1/2". Maybe that's when it's time to use the table saw.

Maybe I'll try scorrpio's jig. Just fool around with it and see if I can get it to work. I like the idea, in theory.

-Jon

[gjat]

There is an even easier and more accurate way to cut dados with a router.
You need a straight board, a 1" piece of your shelf, a router, and a straight bit thinner then your shelf thickness, but not less than 1/2.
Step 1- Clamp your straight board accross the upright so that the top edge of the router bit will cut the top of the dado. This will be where the top of the shelf is.
Step 2- Clamp your piece of shelf so that it moves the router bit down the exact width of the shelf. As long as your bit is at least 1/2 the width of your shelf thickness, you will have a dado exactly as wide as the shelf thickness.
-TWEAK- I always sand down the shelf thickness spacer a teeny bit to make sure I get a tight fit.

[LCHIEN]

Right. I thought about that. And my router has a collar guide inside of which the bit spins. I think there is about a 1/16" gap between the bit edge and the collar. So I could actually have the boards maybe 1/8" farther apart than the desired dado width. I tried playing with that the other night. One dado was a little too narrow and the other a little too loose. I'm new to all of this. So maybe the regular pattern maker's bit is what I need to do. I like the idea of actually using the real board as your thickness guide.

Actually this (using a template guide ) will work very well AND solve the problem of pattern bit too long AND too wide (not available less than 1/2").
The trick is spacing the second side of the jig to account for the difference between the collar (bushing) and the bit diameters.

You can use a straight bit and a template guide bushing. the bushing will always be oversize since it has to have a wall and not touch the bit.
For example I would use a 3/8" straight bit, spiral cutting if you desire ,
with a 5/8" OD guide bushing. This will allow plenty (somewhere between 1/16 and 1/8") clearance for the bit in the bushing and you have to make the guide groove 2/8" or 1/4" oversize.

This is accomplished easily with the prior explained jigs and stuff by inserting the shelf and a 1/4" bar or sheet of material alongside the shelf.

A setup bar will work OK but they generally run about 2" long, not the width of a shelf. You can buy whole 1/4" bars at the hardware store for around a dollar for a 12" piece, also you can get 1/4" square wood stock where the wood dowels are. The metal bars are extremebly accurate.
Another alternative would be a sheet of 1/4" acrylic plastic (very accurate). I would not use 1/4" plywood because that is not really 1/4" - other laminates may or may not be accurate. Even a 1/4" dowel rod would work.

Just make sure you offset your jig by 1/8th inch it takes to center the router in the bushing. E.g. the dado will be 1/8" from each jig edge.

[just4funsies]

I have seen a 1/4" upcut bit used as a pattern follower, by just letting the 1/4" shank ride on the pattern surface. I guess it depends on the material the pattern is made of, because surely there would be some abrasion or heat buildup, I would think.