Prefinished Ply Jointery

[Greg McCallister]

Was wondering with all of you cabinet makers out there who use prefinished ply, what type of joinery do you use for the cases.
Being there is a finish on the plywood is there any particular glue you use if you use glue. Or is it biscuits, dado's etc...
Now this is not a mass produced project but my own kitchen and being the uppers will be 42" tall with 3/4 sides and shelves with 1/2 backs.
What do you find has the best strength (she loads them).

[Ed Peters]

have their bottoms and shelves housed in a 3/8" deep dado. This allows for a wood to wood glue joint as well as screws (McFeeleys Promax) through from the outside. The tops sit in a rabbet in the sides and the back and the back (3/4") is also dado'd to match the sides. Pretty much bomb proof.

Ed

[Peter Quinn]

At the shop where I work we use type I titebond, 3/8" dados and screws to build the carcasses, blind dados with solid wood edge banding for frameless, always decorative end panels so screws aren't exposed. One helpful detail is to make the dados so that a set amount of material is LEFT after the cut, the exact depth of the dado is irrelevant, but the depth of the remaining material is not. You want to mill your dados so that you leave 3/8" material after milling as plywood thickness may vary and the difference is to the inside where it will affect nothing, as opposed to the outside where it may cause problems over a long run as you subtract 1/32" from each end of every cabinet. Make sense? This way your horizontal partitions will be exactly the outside dimension of the cabinet minus 3/4" (3/8"X2) versus trying to cut partitions some fraction of inches under an even number to arrive at your final dimension, a small but helpful detail.

Another thing regularly done to deal with thickness variations is to run the dados a set width under 3/4", say 5/8" for instance, then to pare material off the blind side of the horizontal members to create a tongue to match this thickness tightly rather than shim a dado stack to match the plywood or hope an undersized plywood bit matches. With pre-finished plywood I have seen the thickness of 3/4" vary from as little as .710" to as much as .760" in the same lift of material which makes it hard make tight dados using a single set up. The afore mentioned technique cancels out any thickness variation when done using a tall fence on the TS with the panels run vertical, a slot cutter or rabbit bit on a router table, or an over arm router (which is admittedly not available in every shop). The critical detail is that the technique employed sets the distance from the pre-finished face to the bottom of the tongue precisely. The face opposite the cut must always be the reference face.

In instances where I must secure packing material to the prefinished face, such as for undermount drawer slides like Blum Tandems I find a few drops of PL glue bonds well to the finish. Otherwise for dados its just plain old tite bond, which bonds the cut surfaces well and cleans up easily should any squeeze out on the prefinished surface. In my own shop I usually spend a few extra bucks and buy prefinished two sides, particularly for sink cabs, boxes around dishwashers, or bathroom applications.

[Steve Jenkins]

"One helpful detail is to make the dados so that a set amount of material is LEFT after the cut, the exact depth of the dado is irrelevant, but the depth of the remaining material is not."

Peter what setup do you use to cut the dados. Do you use an overarm router?

[frank shic]

bag the dados. just butt joint and screw.

[Craig Coney]

Butt joints with dowels and staples.

[Chris Padilla]

With pre-finished plywood I have seen the thickness of 3/4" vary from as little as .710" to as much as .760" in the same lift of material which makes it hard make tight dados using a single set up. The afore mentioned technique cancels out any thickness variation when done using a tall fence on the TS with the panels run vertical, a slot cutter or rabbit bit on a router table, or an over arm router (which is admittedly not available in every shop). The critical detail is that the technique employed sets the distance from the pre-finished face to the bottom of the tongue precisely. The face opposite the cut must always be the reference face.

Good tips, Peter! I, too, have had the frustration of plywood varying on me. Incredibly frustrating to say the least!

[Brian Peters]

For face frame cabinets, pocket screwed and glued face frames with a 1/4" dado set in the stiles depending on how much reveal inside you want. (you don't want flush especially with prefin I usually go with 1/16-1/8") Then the plywood gets rabbeted 1/4" on the front 5/8-3/4" on the back for the backs to fit into with a little extra in case the drywall isn't flat. I really only glue whats hitting the solid face frame, the rest is stapled and screwed, applied to face frame. Makes for a top of the line long lasting strong cabinet.

[Ed Peters]

Butt Joint and Screws (no glue, no dado's)
Butt Loint and Dowels and Staples (no glue, no dado's)
Butt Joint (assumed), Staples and Screws, Dado's and rabbetts to attach face frame.

I couldn't be happy with any of these formats. A little extra effort will provide cases of incredible strength with no reason to be concerned with the load they support. In my world, the glue provides most of the strength of any connection. Screws and staples are merely disposable clamps. Screws or staples into end grain can hardly be considered a solid connection.

Ed

[Jay Brewer]

Butt Joint and Screws (no glue, no dado's)
Butt Loint and Dowels and Staples (no glue, no dado's)
Butt Joint (assumed), Staples and Screws, Dado's and rabbetts to attach face frame.

I couldn't be happy with any of these formats. A little extra effort will provide cases of incredible strength with no reason to be concerned with the load they support. In my world, the glue provides most of the strength of any connection. Screws and staples are merely disposable clamps. Screws or staples into end grain can hardly be considered a solid connection.

Ed

Not to say one way is better than another, but the backs and face frame has alot more to do with the strength of the box, than what type of construction you use to build the box.

Im not saying dadoes aren't stronger, just are they needed. I have ripped out quite a few 20 to 40 year old kitchens that were still in great shape, simply nailed together.

Heres how I build a typical upper you could do chin ups on if you like. Fast and strong.

3/4" pre finished ply for the box, butt joint and 2" quality screws ( not drywall screws). Sides have a 1/2" x 1/2" rabbit on the back cut on the shaper. The top and bottom are ripped 1/2" narrower than the sides. The backs ( 1/2" ) are glued and stapled to the sides and glued and screwed to the top and bottom. 3/4" hangers at the top and bottom.

Face frames are glued and pocket screwed to the fronts. Not a single dado.

[Greg McCallister]

Thanks everyone on your suggestions.

[John Hemenway]

There's a glue made for the pre-finished ply. Roo glue! Woodcraft has it.