Need to guide a very wide mortise

[WestofLongBeach]

I am working with a 19x19" piece of 1/2" ply, and need to mortise ~18x18". Straight edges, square corners.

And, of course, resting the router on the work will only result in the mortise getting deeper and deeper, as the surrounding material is taken away.

What is the most reliable way to get a smooth surface?

Or, for that matter, squaring the corners (chisel?)?

Thanks.

Don.

[LarryG]

I think I understand the intended operation ... you have a piece of plywood that is 19" square, and you need to make a recess (of some depth) that will be 18" square, leaving a 1/2" wide margin all around. Is that correct?

If so, the usual way to do this is to fit the router with a homemade base plate large enough to keep it from falling into the recess. However in your case that would require a base plate some three feet square (!) in order to reach all the way to the corners of the recess. Unwieldy, to say the least.

Another way is to use "router rails," a method in which the router base is supported on a pair of rails and does not directly contact the workpiece. This technique is often used to flatten a large surface such as a work bench. There are various ways to set up a "router rail" arrangement; this YouTube video shows the basic principle in action and will jump-start your thinking.

When finished, square the corners with a chisel, as you say.

[JR]

Or just use thinner plywood and build up the outside 1/2" with hardwood cleats.

JR

[pelligrini]

I'd probably do it like JR suggests. Otherwise, I would make a template out of hardboard and tack it to the plywood and remove the material on my router table. You'd need a pretty big table to do it that way though. With my smallish router table I think I would only cut out half of the template (or maybe even just a quarter) so I could still have the face of the work on the table and not have to worry as much about falling off the edges.

[phi1l]

or if you have a decent sized router table. you can just clamp stop blocks around the periphery, & do it that way. I would look at just adding a 1/2" rim of ply around the edge though...

[Mr__Bill]

Another way is if you have a router table is to set the bit and put a fence arround the table. Then just start routing in the center and float the ply about the table until everything is cut out and you only have the edged left. The ply will always be supported by the edges so the cut should be of a uniform depth. Then clean up the corners with a chisel or leave round and round off the piece that sets in there. That's pretty much how I did it to recess a ceramic tile into a small table top.

You can also leave the ply face up and place the stops beyond the edge of the ply and start routing in the center and spiral your way to the edge. that way the router is always supported on at least half of the base.


Bill

[phi1l]

The "router rails" system works very well too, I have used it to make pizza peels a couple of times...

[radhak]

Don,
I'm interested to know what exactly are you gonna use this for? What is the project?

And if you are using ply - and just 1/2" thin ply at that - routing layers will only weaken material which is not very strong to begin with. As others have said, more intuitive would be to add cleats at the borders that give the same functionality.

[mpc]

Assuming the resulting plywood panel won't be too flimsy for whatever job you've got in mind for it, another way would be to use shims on the router itself:

* Fasten your panel to the workbench with double-sided tape or stubby bench dogs, whatever. It has to be secure and have nothing blocking the router.

* Fasten scrap boards (that are taller than the panel) around the panel to the bench as well, position these away from the panel so they act as stops/fences - i.e. when your router's base plate is touching the scrap boards, the bit is cutting to line of what you want removed.

* Fire up the router and cut along the edges of your final mortise, keeping the same spot of the router's base plate against the stop/fence boards. Use one reference point on the router base in case it isn't exactly circular or centered about the bit, standard router technique.

* Once you've made a moat around the edge, move the router half a bit-widge and start routing just one side of the panel. Pay attention to the feed direction of course - as always with a router.

* Move the router over half a bit-width and make another pass. The router is supported by the panel's edge and the yet-to-be-cut material.

* Repeat until one side of the router's base plate is about to "fall off" the panel edge into the mortise.

* Lift the router up and out, move over about an inch, and make a fresh start. Now the router is supported on both sides of the bit again - by yet-to-be-cut center material. You're going to leave a 1 inch wide strip un-cut for now, that'll support the router in place of the panel's edge.

* Move over half a bit-width, making another rectangular mortise, over and over, until the router base plate is about to fall into the mortise again.

Work your way across the panel that way. When done, you'll have rectangular mortises with 1 inch wide separations (and rounded, not square, corners).

Now the "trick":
* cut two scrap scrap sticks that you can tape to the router's base plate about 2.5 inches apart, separated by the router bit. Double-stick tape those to the base plate, or attach them with screws if your router base plate has screw holes available - most do. Make these "outriggers" the same depth as your mortise.

* Then set the router+outriggers into one of the already-cut mortises and verify the bit is just touching the already-cut mortise. I'd shim the outriggers with a few strips of tape just to make sure it doesn't cut too deeply...

* Straddle the router+outriggers across one of the 1 inch strips remaining and turn on the router. Watch the feed direction again. You'll be able to remove most of the 1 inch strip; you just won't quite reach the sides of your panel - the outriggers will hit the edges of the mortise first. Ignore that for now. Sneak up on the cutting depth by removing tape to get a cut that matches your original cuts. When done, you'll have small almost-square islands along 2 sides of your panel to be removed.

* Once most of the 1 inch strips are gone, remove ONE outrigger.

* Now stick the router on the edge of the panel (the router base's side where you removed the outrigger) and let the other side be supported by the remaining outrigger. You should be able to chop off the last of those 1 inch strips. The router base will rest on the panel's edge and should "hit" your scrap stop/fence pieces too.

* Chisel the corners square.

I think that makes sense anyway... anybody see some flaw in my logic?

mpc

[LCHIEN]

seems much easier just to put a 1/2" solid wood lip around the outside of the 19x19 piece.
Mortised plywood will probably look like heck.

[WestofLongBeach]

Don,
I'm interested to know what exactly are you gonna use this for? What is the project?

And if you are using ply - and just 1/2" thin ply at that - routing layers will only weaken material which is not very strong to begin with. As others have said, more intuitive would be to add cleats at the borders that give the same functionality.

This may not fit everyone's aesthetic, but I like the way that plywood looks when rounded off on the edges and finished with oil/varnish. I like the lines.

So I'm making a 1/8" deep recess in the ply to receive gameboards for my daughter, which (the ply, not the daughter) I will then make as tidy looking as I can and mount it on a light TV swivel/stand. So I need to rout out a receptacle for the gameboard. And, as suggested, will chisel the corners, because I can't convince her to round off all the corners on all her boards.

And, in the application, strength (you're right about this most the time, though of course) just isn't an issue.

[WestofLongBeach]

Thanks, to all, for the intelligent replies. And even to lchien, who doesn't appreciate my idea of what looks good.

I tallied the answers, which were three basic ideas:

Router rails, favored by Larry & Phi1l
Float on remaining material, by Mr Bill & mpc
Use either cleats or blocks, if my table is bigger than it is, by JR, Pellegrini, Mr Bill & Phi1l

I wondered if anything like rails or floating would work, and I will obviously try one or the other, maybe including the extensive instructions by mpc, which were impressive.

I hope she likes it. And thanks again.

[radhak]

Done neatly, on good ply, I like the lines too. But after routing, the ply base itself is going to expose just another layer which will look similar to the top layer; maybe you could still add cleats around the original base, but cleats from ply instead?

I am just feeling routing out 18"x18" from ply is gonna be messy, and somewhat avoidable here.

Of course, if you do go down that path (or any path), some pictures please - before and after, with your experiences; should be interesting .

[LCHIEN]

I like the laminated look of the edge of plywood. but i don't think you're going to like the inside (altho it won't show when in use, it'll be visible when changing game boards) area.
Assuming you can "mortise" perfectly, you will expose one entire inner layer which is usually poorer quality wood than the outer veneer. If you can't make the bottom perfectly even depth due to router or to unevenness of the layers, then you will see some patchy areas where the underlayer or the glue layer between is exposed. I just don't think it will look good there.

If you like the lines on the edge, use a piece of 1/2 or 3/8" hardwood veneer plywood, then cut strips of the same material 1/2" wide and fasten them around the outer top, mitering the corners. Then You can router the top and bottom edges with bevel, roundover etc. of your choice exposing more of the laminations all the way up and down.
And the interior will show the exterior veneer of the plywood instead of the inner layers.

[LarryG]

For best results and the best look on the edges, I suggest using Baltic birch plywood, no matter how you do it.

Loring makes good points about what routing this big recess will expose in the inner plies. It won't be pretty, and you're almost certain to get a lot of splintering and burning from the glue.

To ease the workload and get the desired look on the edges, you could cut the 18" square opening all the way through a piece of 1/8" Baltic birch plywood, then laminate that to a piece of 3/8" Baltic birch. (If the ply structures of the two different thicknesses resulted in an unbalanced look on the edges that you would find objectionable, you could use three pieces: 1/8" top layer with the hole, 1/4" middle layer with no hole, 1/8" bottom layer with no hole.) Cut the pieces oversize and glue them together; then cut the piece down to the final 19" x 19' size.