Screws for plywood?

[photobug]

 My experience is that with this approach, the plywood (11 layer pine plywood from HD) doesn't hold the screws too well. Would it work better if I pre-drilled the dividers?

Mats

Here is your two biggest problems.  Poor materials and poor technique.

While the price is right the plywood from Home Depot is junk, plagued with voids and separation in layers.  I have used it a lot but often have to reglue and clamp the loose ply layers before assembly.  You can use HD ply, but it IMO it is more hassle and work than is worth the savings.  

Consider investing in better plywood available at a "Hardwood Store".  I just drove to the big city yesterday to visit my Hardwood supplier.  I normally use Baltic Birch which is as stable as you can get and in 5x5' sheets, is extremely reasonable.  The imported Birch there is about the same price as HD ply but of much better quality.  You owe it to yourself to check out better plywood to see what a joy it is to work with.

Irrelevant of the quality of plywood you use, you need to predrill. Get a tapered drill with a counter sink, get one for #6 and one for #8 screws, although I often use the #6 for a number 8 screw in plywood without any issues, for hardwood you want the correct size.  You can line up and drill all the screw holes in the back of the Dados before hand, make sure you counter sink so the screws will sit below the surface of the plywood.  If you use good plywood and #6 screws I find you don't really need to predrill into the "endgrain".  The predrilling the Dado side creates a big enough hole that the screw goes through easily and allows the screw to bite into the other piece and pull the gap tight.  If you don't predrill and the two pieces are not pulled tight you run in the likelyhood that the screw creates threads in both boards preventing the two from being pulled tight.

A technique I like to use it after predrilling the Dadoed boards I apply glue and assemble the boards, sink one screw to hold the boards in place then drill into the other predrilled holes to ensure a clean hole in the "endgrain" boards.  Titebond extended time glue makes this process less stressful.

[Mike77]

I often use screws as clamps to hold plywood cabinets together until the glue dries, especially in non-visible and hard to clamp places. My go-to type screw has been coarse thread pocket hole screws because of the flat head but they don't work too well holding plywood end grain. Maybe they're not beefy enough? Is there a better choice?

Mats

Confirmat screws

[museumguy]

Definitely pre-drill. It's funny, I'm "old school". Was taught to always pre-drill. Then, the new self drilling screws came out and my coworkers said to stop pre-drilling. It's amazing how much material is ruined and snapped screws because no one wants to take the few seconds to pre-drill. I had to replace some damaged wall cabinets that a coworker installed and he only drilled a clearance hole through one face frame, no pre-drill into the other cabinet. Every place a screw was driven, there was a split about 2" long in the face frame. Some people just can't be taught. On another project, small pieces of plywood were attached in the middle of a shelf lengthwise, as a shoe stop. Clearance holes were drilled from the bottom of the shelf and the stops were screwed in place. Several pieces are "blown out" because the person assembling the shelves did not pre-drill. I always get the evil eye when I pre-drill everything but I never have had an issue.

[woodmats]

While the price is right the plywood from Home Depot is junk, plagued with voids and separation in layers.  I have used it a lot but often have to reglue and clamp the loose ply layers before assembly.  You can use HD ply, but it IMO it is more hassle and work than is worth the savings.  

Consider investing in better plywood available at a "Hardwood Store".  I just drove to the big city yesterday to visit my Hardwood supplier.  I normally use Baltic Birch which is as stable as you can get and in 5x5' sheets, is extremely reasonable.  The imported Birch there is about the same price as HD ply but of much better quality.  You owe it to yourself to check out better plywood to see what a joy it is to work with.

Irrelevant of the quality of plywood you use, you need to predrill. Get a tapered drill with a counter sink, get one for #6 and one for #8 screws, although I often use the #6 for a number 8 screw in plywood without any issues, for hardwood you want the correct size.  You can line up and drill all the screw holes in the back of the Dados before hand, make sure you counter sink so the screws will sit below the surface of the plywood.  If you use good plywood and #6 screws I find you don't really need to predrill into the "endgrain".  The predrilling the Dado side creates a big enough hole that the screw goes through easily and allows the screw to bite into the other piece and pull the gap tight.  If you don't predrill and the two pieces are not pulled tight you run in the likelyhood that the screw creates threads in both boards preventing the two from being pulled tight.

You are right that most plywood from HD is poor quality. For closet shelving and basement cabinets, I have used their 11 layer Chinese pine plywood now for many years and it has very few voids and does not delaminate. The challenge is that the outer layers splinter easily so you have to pay attention when you cut it. It also bows if you store it for any period of time. In my case, making a 70" long bank of drawers, 5' x 5' BB would not work and the Denver price for 3/4" BB is $65/sheet so it is not cheap here.

Based on the recommendations here from everybody, I did a comparison test (the curse of being an anal engineer) pre-drilling the plywood end grain vs using the self-drilling capability of the pocket screws and the spax screws and found indeed that it improves the holding power of both screws so thank you all for that advise!

Mats

[Axehandle]

I use a Spax drilling screw for nearly all the times when I use screws on projects. I just built some large drawers and used them as clamps on the best 3/4 ply I could find at the BORG. In this instance I am just going to leave the screws in permanently

[photobug]

You are right that most plywood from HD is poor quality. For closet shelving and basement cabinets, I have used their 11 layer Chinese pine plywood now for many years and it has very few voids and does not delaminate. The challenge is that the outer layers splinter easily so you have to pay attention when you cut it. It also bows if you store it for any period of time. In my case, making a 70" long bank of drawers, 5' x 5' BB would not work and the Denver price for 3/4" BB is $65/sheet so it is not cheap here.

Based on the recommendations here from everybody, I did a comparison test (the curse of being an anal engineer) pre-drilling the plywood end grain vs using the self-drilling capability of the pocket screws and the spax screws and found indeed that it improves the holding power of both screws so thank you all for that advise!

Mats

Are the 5x5 sheets of BB $65 or 4x8s?
Keep in mind pocket screws in their traditional use are always predrilled.  As long as the first board is predrilled it allows the two boards to be pulled tight.  You could avoid pre-drilling if you added clamps and cauls, but at that point you could skip the screws.

[Richard D.]

If you are screwing into the edge of plywood without drilling a pilot hole, you are just splitting the ply to get the screw in. Drill a 1/8" pilot hole and use drywall screws, I have literally done this a thousand times at work for various temporary solutions. 

For cabinets, you would be better off figuring out your clamping procedure and do a practice clamp up before you get glue down and you are in a hurry.

[Cooler]

I treat screws into the edges of plywood as a substitute for a clamp while the glue dries.  Strength-wise the glue is doing 90% of the work.

Here is some data on screw strength in plywood.  Not too encouraging.  Less than 200 pound pull out strength:

file:///C:/Users/Howard/Downloads/e830.pdf

[Hank Knight]

3/4 to 3/4 ply.....predrill with a tapered bit with countersink head, and I use 2" #8 screws.

Don't use a tapered drill bit with SPAX screws. The shank of a SPAX screw is a constant diameter, not tapered like classic wood screws. Size the drill bit to match the shank diameter, the threads will cut their own path. This method produces a very strong grip with very little splitting of the workpiece, and it holds well in "end grain" plywood.

[Stwood_]

Don't use a tapered drill bit with SPAX screws. The shank of a SPAX screw is a constant diameter, not tapered like classic wood screws. Size the drill bit to match the shank diameter, the threads will cut their own path. This method produces a very strong grip with very little splitting of the workpiece, and it holds well in "end grain" plywood.

I don't have a Spax screw on the property.