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Screws for 18mm ply dado joints

Posted: Sun Dec 04, 2016 10:55 am
by PeteW1959
I am making a built-in bookcase/tv/hifi unit from 18mm ply. I intend using dado joints for the shelves/uprights and want to know the best size/type of screws to use.

I have read that 35 x 40mm drywall screws are good, but what size clearance/pilot holes do I need, or is there anything better?

Re: Screws for 18mm ply dado joints

Posted: Sun Dec 04, 2016 9:02 pm
by promhandicam
Welcome to the forum. Screwing into the end of a ply board isn't recommended as the screw thread tends to separate the ply resulting in a weak joint, especially as the majority of ply is very poor quality or for the good stuff ridiculously expensive. A thinner screw, like a drywall screw would probably be best, although 40m might be a bit short. I would look at perhaps using pocket hole screws to hold the carcase together, relying on the glue and the joints to give the strength. If you haven't bought the ply yet, I would maybe look at alternative materials - moisture resistant mdf being the favourite if the unit is going to be painted when completed.

HTH

Re: Screws for 18mm ply dado joints

Posted: Tue Dec 06, 2016 8:59 am
by PeteW1959
Thanks for the advice. MDF isn't an option as I want a woodgrain finish, and I couldn't find suitable veneered MDF locally. The ply I have bought is the best quality the timber merchant had, and was £36 a sheet.

I'm not sure that pocket screws would work as I want them hidden. If I were to use pocket screws though, would that still be with dado joints, or would butt joints be better?

Re: Screws for 18mm ply dado joints

Posted: Wed Dec 07, 2016 8:22 am
by woodsmith
With dado joints you don't have to use screws at all as the glue and the housing should give you all the strength you would normally need. The best way of holding the joint whilst the glue cures is to fire brad nails into the internal corner of the joint, a couple from each side will hold it in place and make a more or less invisible fixing.

Re: Screws for 18mm ply dado joints

Posted: Wed Dec 07, 2016 10:10 am
by mw40784
You need long and coarse thread screws.

Re: Screws for 18mm ply dado joints

Posted: Wed Dec 07, 2016 2:26 pm
by mark270981
To be fair Steve if it the housing snugly fits the board thickness this will alleviate any splitting with the correct pilot hole.