Box Joints on Pre-finished Plywood

[Carl Schellenberg]

I am trying to make interior cabinet drawers out of 1/2 inch pre-finished maple plywood. I am using a box joint jig which works on either a table saw or a router table. The setup is fine but I get tear out and chipping on the back side of the piece when I use the table saw with the Freud Box joint blades. I get tear out and chipping on the front side when I use the router with a spiral up-cut bit. I have tried putting masking tape on the vulnerable side for both operations.

Any suggestions would be greatly appreciated as I have a workshop full of the plywood just waiting to be magically turned into drawers.

Thanks

Carl

[Jose Kilpatrick]

My only reccomendations would be to

Check the speed of your router.
Use a backer board.
Score a line along the cut with a straight edge.

[Carl Schellenberg]

Thanks Jose

Have tried using a backer and varying the speed of the router. As far as scoring is concerned, it is hard to do with a box joint.

Thanks anyway

Carl

[Chris Padilla]

Use sacrificial boards (MDF is good...cheap plywood is good...scrap is good) on the front and back of the board(s) you are cutting.

You may wish to hog out a box joint SMALLER than what you want in the end and then go back and cut the larger size but honestly, I think the sacrificial boards will solve your tear out issues.

[Brent Leonard]

My only reccomendations would be to

Check the speed of your router.
Use a backer board.
Score a line along the cut with a straight edge.

X2


Try sandwiching the plywood between other stock material when you cut your joints. Keep the pieces tightly together with 2" wide painters/masking tape.

[Phil Thien]

Any chance your spiral bit is dull? I recently was making box joints on 1/4" thick fir and was getting all sorts of chipout on the front of the boards. I was using a backer board and even getting a little on back.

New Whiteside upcut spiral bit did the trick. My old bit had really been around the block.

[Jamie Buxton]

Suggestion: give up on finger joints or dovetail joints in plywood. You'll never really cure the chip-out. Futhermore, doing joints like that in prefinished means that you can't sand down the tips of the fingers if they're a little proud. And on top of that, you still have to finish the tips of the fingers somehow.

A better approach in plywood, and particularly prefinished plywood, is the lock rabbet joint. You can make it all on a table saw, so it is faster than finger-type joints. It doesn't penetrate the outside of the drawer, so there's no chip-out. There's also no need to add finish to the drawer. http://www.sawmillcreek.org/showpost.php?p=129487&postcount=8

[Rick Fisher]

I have had the exact problem described and found that a sacrificial board makes all the difference.

[Charles McCracken]

Carl,

For the SBOX8 I recommend that you use a backer board that provides zero clearance and clamp the material to the jig so it is more stable and so the fibers are held tightly to the backer board.

[Thomas Knapp]

Suggestion: give up on finger joints or dovetail joints in plywood. You'll never really cure the chip-out. Futhermore, doing joints like that in prefinished means that you can't sand down the tips of the fingers if they're a little proud. And on top of that, you still have to finish the tips of the fingers somehow.

A better approach in plywood, and particularly prefinished plywood, is the lock rabbet joint. You can make it all on a table saw, so it is faster than finger-type joints. It doesn't penetrate the outside of the drawer, so there's no chip-out. There's also no need to add finish to the drawer. http://www.sawmillcreek.org/showpost.php?p=129487&postcount=8


I second that opinion! I have Baltic Birch plywood drawers that I made over 25 years ago with those joints and they haven't failed yet.