On Apr 26, 10:09*am, Winston wrote:
The proper answer is 'buy or borrow a table saw with a dado blade'.

However, I don't want to.

I want to use my plunge router to place a 3-1/2" square lap joint
about 3/4" deep on the ends of several pressure treated 2x4s.
I will don a respirator first, no worries.

I've designed an machined jig to allow me to do
this but I want to know how you pro's would approach this
challenge before I search my junk pile for parts.

My Google-fu was not up to the challenge. *Instead, it revealed
some table saw adapters.

My band saw refuses to cut anywhere near straight so that's out.

First prize answer would be an inexpensive jig I could pick up
at my local hardware store.

What would SuperWoodworker do?

Thanks!

--Winston

--

Harley was venal, arrogant, despicable and a psychologist.
He was the second most redundant man I ever talked to.

It seems to me that a fairly simple jig, based on the size of your
router base would make short work of this project.

I recently had to cut a number 3 1/2" x 1/2" deep recessed areas in
the middle of some boards and a simple frame made of 1 x 2 stock
clamped to the boards made quick work of them.

All you need is a square frame sized so that the router can only move
enough to cut a 3 1/2" square and a way to mount the jig on top of a 2
x 4.

How about this:

Size up and build the square frame and set it aside.

Take some scrap 2 X material and make a "U" shaped jig so that your 2
x 4's can slip into the "U", essentially giving you a "flat board"
just like I had. This will support your router as you rout the 2 x 4.

Mount the frame on top of this "U", positioned so the center of the
area you need to cut out is in the center of the frame.

Slip a 2 x 4 under the frame into the "U" and rout out your lap joint.
You can oversize the frame to allow the router to clean all of the
material from the end of the 2 x 4 and then just clean up the other
end of the joint with a chisel.

I don't know how this will come out, but I'll try a little ASCII
art...

The Frame:

|-------------------|
| |
| |
| |
| |
|-------------------|

The "U" (=) with 2 x 4 (x) inserted, obviouslY not to scale.

======================
=O==========O======O==
======================
xxxxxxxxxxxxxx =========
xxxxxxxxxxxxxx ======O===
xxxxxxxxxxxxxx==========
======================
=O==========O======O==
======================

Center the Frame over the "U", screw it where indicated (O) and rout
to your heart's content.