Screw Length and Dado Depth

      Basics of dadoed cabinet joinery. March 20, 2006

Question
On dadoed 3/4" ply cabinet boxes, what length of screw and type does everyone use? What's best?

Forum Responses
(Cabinetmaking Forum)
From contributor L:
I dado 1/4", then use 1.5" Recex square drive screws through the dado into the cabinet floor. I generally put about 4 screws in a 24" cabinet.



From contributor J:
I dado everything I can, including face frames, 1/4" and pocket screw using 1" or 1 1/4" Kreg screws. I know they cost more, about $20 per thousand, but I use Kreg pocket screws for just about everything. 1" washer heads are great for attaching applied drawer fronts. Amazon sells the Kreg line now so I just order them online with free delivery.


From contributor A:
The rule of thumb for screws is 2/3 of length for minimum holding. 1/2" side (3/4"-1/4" dado) x 3 = 1 1/2". We use 8 x 1 1/2" for dados, 8 x 2" for butt joints. Anything beyond 1 1/2" of end grain penetration in plywood could split the sheet. At the end of the day, the screw is pulling the dado tight. A little glue will keep it from coming apart.


From contributor I:
I use 1 3/4" screws and the boxes are 3/4 ply butt joined. Haven't had a problem in 10 years.


From contributor O:
Pocket screws and butt joints work great for me. I much prefer to run the screw from the base into the side for better holding power. If I'm expecting a particularly heavy load, then I run a line of PU glue.


From contributor F:
I used 1.625" screws for many years. I recently switched to a 2" length especially for butt joints in 3/4" melamine. Am I the only one who drills a pilot hole and countersink before the screws are driven?


From contributor D:
I think so, contributor F. What kind of screws are you using? You can eliminate probably 85% of that countersinkin' and pilot holin' with the right screws. But if you're getting paid by the hour...


From contributor M:
You're not the only one. We use a Fuller tapered bit with countersink. Set .125 below face. Takes time but works for me. We also use type 17 screws.


From the original questioner:
Thanks for all the good info. Is 1/4" the standard dado depth?


From contributor C:
Butt joint, countersunk hole (Fuller bits), 2 1/4" screws. 2" have always felt a bit too short for me, 2 1/2" are too long. If I dado, it's 1/8" deep.


From contributor F:
1/4" is a good depth for a lot of dados. You have to weight what you gain in dado shoulder strength against what you lose in strength in the thickness of the dadoed panel or part. On dados that are primarily for alignment purposes, I often go as shallow as 1/8" deep. Also depends of course on the thickness of the part being dadoed or rabbeted.


From the original questioner:
I'm talking primarily dado depth to accept the floor, top of the cabinet, and 1/4" back. I was going to do just half or 3/8" in 3/4" material, but after hearing the forum, sounds like that is too much.


From contributor F:
Sounds like we're both talking about the same subject. I dado the bottom shelf (floor?) of a cabinet into the finished end at 1/4" deep. Like I said above, what's to be considered is the thickness left remaining in the part that receives the dado. Does it weaken the part in a critical area?

Also, the reason for the dados come into my decision. Is it for strength? Is it for accurate location of a part? Is it for removing a coating such as melamine so I can use yellow glue? Is it for cosmetics so that no visible fasteners will be needed?

I think most of us started out with the half the part thickness depth formula. As far as shallower dados go, that part won't be able to fall out or come out even if the dado is only 1/8" deep. The depth of a dado adds a little extra glue surface the deeper you go, but not that much. It is mostly a mechanical device that captures the part. If the part that goes into the dado is two sided melamine, there is nothing to be gained in glue surface area by using deeper dados.

By the way, 3/8" deep in 3/4" material is fine. I just like to use 1/4" deep and keep the part thicker. On most of my work, the dado is only responsible for carrying a small part of the overall load. The dadoed in part is usually supported by the face and the back of the cabinet also.



From contributor S:
We always pilot and use 2" auger point screws and Roo glue on melamine.


From contributor X:
1/4" deep dadoes are pretty common - the rule of thumb I was taught was no deeper than half the thickness of the material, so in a 3/4" piece, no deeper than 3/8".

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