Poor man's undersized plywood router bit

[chopnhack]

Let me preface by saying that I have neither a dado stack or an undersized plywood bit and was thinking of how to get around having to buy either one at the moment. I am trying to make a quick bookshelf and wanted to dado the shelves in for simplicity and rigidity.

Has anyone taken a thin slice of the desired plywood and used it as a shim on their table saw to register the far side of the dado, run it through the saw and then remove the "plywood spacer" and run it through the saw again leaving the dado'ed outlined?

Sounds like more work than its worth, but for the sake of thinking out loud.

[drumpriest]

I think that'd work out ok. I used to make my plywood dados in 2 passes with my router with a 1/2" bit. You can do this and really dial in the fit better than you'd get with an undersized plywood bit. Basically plywoods vary too much to rely on the undersized bit being even close to the actual plywood size.

Now I mostly use my TS with a dado, but sometimes rely on the router method still, like for really tall bookcases.

[Uncle Cracker]

That won't work. It will actually space the saw cuts right kerf to right kerf, where your slot needs to be spaced right kerf to left kerf to fit the plywood thickness. Your method will result in a slot that is the thickness of your plywood plus the width of one blade kerf.

You can use a spacer, but it has to be thinner than the plywood you are dadoing for by the width of one kerf, and then you still must make certain the workpiece (and the spacer) stay absolutely tight to the fence throughout the cut..

[JR]

Do you have a top-bearing straight bit for your router? If so, you could make a jig that would fit the plywood perfectly. Let's see, I had a pic around here somewhere...

Well, this isn't the perfect viewing angle.

The jig consists of two pieces of MDF with a variable gap between them. Slip a piece of scrap from you target plywood in the gap, bring the MDF pieces together and cinch them down. Drive up and down the gap with a top-bearing bit and you're good to go. The setup shown here has stops clamped to either end for a stopped dado.



Ah, here's a better view.



HTH,
JR

[LCHIEN]

Do you have a top-bearing straight bit for your router? If so, you could make a jig that would fit the plywood perfectly. Let's see, I had a pic around here somewhere...

Well, this isn't the perfect viewing angle.

The jig consists of two pieces of MDF with a variable gap between them. Slip a piece of scrap from you target plywood in the gap, bring the MDF pieces together and cinch them down. Drive up and down the gap with a top-bearing bit and you're good to go. The setup shown here has stops clamped to either end for a stopped dado.



Ah, here's a better view.



HTH,
JR

the only trouble with that technique is you have to have a top bearing bit with a cutting length that is NOT LONGER THAN THE DADO depth you want to cut. Might be hard to find.

[BrazosJake]

If the plywood is say, 1/2" or less and you have just a few dados to make in narrow stock, I'd just scribe them and do it on the TS with multiple kerf-width passes.

More than that, I'd make a router dado jig as JR suggested. It doesn't have to be as elaborate. I use a bushing collar 1/4" larger than the bit, then take 4 pieces of 1/4" hardboard or mdf. 2 pieces need straight, clean edges approximately 2" wide to index the collar against, and 2 pieces 4-5" wide for the base (to provide clamp clearance). Glue (and screw, if possible) the fence pieces on top of the base pieces, edges parallel but with the fence just a hair over 1/8" back from the edge of the base piece. Once the fences are secure on top, use the fences as a guide and run the router to cut the base pieces prescisely.

This is exactly like making a circular saw cutting guide, only with thinner stock, and you're making two.

To use, lay the plywood on edge across the piece the dado needs to go in, put each half of the guide up to the edge on either side, clamp securely, and you're ready to roll.

I functioned for several years without a table saw and made a lot of dadoes this way. You can get fancier and mount a square guide. Some people like to make T-guides, but that limits you to dadoes the size of the bit. Other dado jigs index the cut from the router base, but that limits you to one router, plus the bit may not be centered in the base. This dado jig works with any bit/bushing combo where the bushing OD is 1/4" bigger than bit diameter, and doesn't require a top-bearing bit.

[JR]

the only trouble with that technique is you have to have a top bearing bit with a cutting length that is NOT LONGER THAN THE DADO depth you want to cut. Might be hard to find.

True enough. In fact, the cut shown in these pics was too deep for one bit. I had to do it with a short bit, then follow up with a longer bit.

JR

[pelligrini]

Another way to use that type of jig without a bearing bit is to rabbit the top edges for use with a guide bushing and straight bit of your choice. The depth of the dado can be any height.

[Bill in Buena Park]

C&H,
I didn't catch how long or wide the workpiece is that you'd like to dado, but unless you have a nice big sled for your TS, moving a sizeable workpiece (unless it rides the fence) and keeping it square to the blade might be cumbersome, even for a SMT with the long miter fence.

The previously mentioned router dado jig would be my choice, or if you happen to have a RAS handy, making your scoring and waste-removal cuts on a well-tuned saw (arm dead-parallel to the table) will work.

[chopnhack]

Luckily its only 3 shelves and then rabbet the top. I think I will have to go the route that drumpriest described. I have done that in the past, but don't really like it because of the tedious setup. I would use a straight edge and have to clamp it front and back, reset to make the second pass, repeat for other dados. ugh...but it does work well. Food for thought on the next day off. Uncle C, your thoughts make sense, very good point! Thanks all.

[drumpriest]

DOH! Yeah, the 2 kerf thing came to mind, and in some 1/2 asleep way it made sense to me, but you are correct Uncle Cracker. Chopnhack, another thing that can speed up your process is by using 2 fences, one for each side of the cut. Once you have figured out how far apart they should be, rip some scrap ply to the width, and put a center mark on it. Then you can line it up, clamp it down, the clamp down the 2 fences, and finally remove the middle bit. One warning though, if you have a round router base, make sure you consistently reference one point on the fence, the base is likely not perfectly centered with the bit, and this variance can make a tight or loose dado.

[chopnhack]

Thanks for the tip drumpriest. It made me think about maybe making a jig, similar to the circ. saw guide so that the edge will be the line that is cut, that should help speeds things up abit.

[AlanWS]

Before I had a dado blade I used a technique similar to that. What you need is a shim the thickness of your kerf, and a bit of stock you want to fit. Cut one edge of the dado into your workpiece, and then move the fence the correct amount using this procedure:
1. Put the kerf shim against your fence and clamp a block to the saw spaced away from the fence by the shim.
2. Loosen the fence, moving it away from the clamped block, and swap the shim for a piece of stock you want to fit your dado.
3. Press your fence against the stock and lock it down. It should now be set to accurately cut the other edge of the dado.
4. Once the edges are cut, it's easy to shift the fence and make repeated passes to clear out the middle of the dado. I happened to have an old manual router plane that worked well to clean up the bottom of the dado, but that's not necessary.

In case you want to shift the fence the other direction, just switch which shim you use first. Or clamp to the other side of the fence, but I prefer to do this on the side away from the blade.