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Using screws to attach plywood.

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Forum topic by dpjeansonne posted 09-30-2020 09:29 PM 478 views 0 times favorited 12 replies Add to Favorites Watch
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dpjeansonne

75 posts in 4096 days


09-30-2020 09:29 PM

Topic tags/keywords: plywood attachment methods

I am wanting to build a frame like a torsion box, with a top using all plywood material. Is there any concerns with using screws to attach the top to the support frame with screws and glue.
I had heard it was not advisable to use screws into the end grain of plywood.
Is there cautions, tips, and/or advise to this work?

-- Cajun Don, Louisiana


12 replies so far

View Fred Hargis's profile

Fred Hargis

6502 posts in 3376 days


#1 posted 09-30-2020 09:39 PM

I built 2 torsion boxes, and both had 1/2” plywood webs (to save weight). One both I used screws and glue to fasten the bottom to the box (no screws in the top since I didn’t want to patch the holes). Each held wonderfully, the second one replaced the first one after it turned 15 years old and I sawed it up for disposal. The screws did their job…but I did use quite a few of them. You might have a few of them split the plywood, but use enough and with the glue there won’t be any problems. I should add, the tops and bottoms of my boxes was/is MDF, only the web was plywood.

-- Our village hasn't lost it's idiot, he was elected to congress.

View Rich's profile (online now)

Rich

5983 posts in 1472 days


#2 posted 09-30-2020 10:17 PM

Screwing into the end grain of any wood is weaker than going into the face. It’s the nature of the wood fibers.

Technically, plywood has alternating end grain on each edge, and it has alternating cross grain faces. If you are using the type with fewer layers and a fairly thick core piece, you can orient the boards such that you are screwing into the cross grain (assuming you mark and drive carefully). The layers on good birch plywood are so thin that a regular #8 screw will likely have some cross grain in its threads and hold well, especially with glue.

You could even skip the screws. Just use glue and maybe some pin nails if you need to keep everything in place while the glue sets. Since plywood has an odd number of layers, orienting the pieces so the face grain runs horizontally will maximize the number of cross grain faces you have for glue strength. I seriously doubt that it would have a large impact on the strength of the joint however, no matter how you orient it.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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SMP

2664 posts in 788 days


#3 posted 09-30-2020 11:03 PM

How thick is the plywood? Thats going to matter. And the actual thickness. I’ve made lots of things from 1/2” and 3/4” ply glued and screwed or glued and nailed with a finish/brad nailer. Years later when i dismantled some the glue tore the plywood to shreds.

View pottz's profile

pottz

11708 posts in 1867 days


#4 posted 09-30-2020 11:11 PM

just pre drill with a counter sink bit first and you should have no problem.

-- working with my hands is a joy,it gives me a sense of fulfillment,somthing so many seek and so few find.-SAM MALOOF.

View jkm312's profile

jkm312

41 posts in 286 days


#5 posted 09-30-2020 11:23 PM

In addition to the above

Use drywall screws, they have a wider thread and pitch.

Do some tests to see which drill bit size works the best for the screws you decide to use.

Set the clutch on your driver so you do not over tighten and pull the hole apart and loose the grip of the screws.

Size the plywood edge with glue, it will soak up a lot, then recoat and attach to your base.

View CWWoodworking's profile

CWWoodworking

1092 posts in 1062 days


#6 posted 10-01-2020 12:14 AM

I would use 5/8. Pocket hole to the bottom, #6 assembly screws on top. Depending on the quality of plywood, you may not need to pre drill. I use Hafele zip-r screws. But there certainly others.

View therealSteveN's profile

therealSteveN

6454 posts in 1457 days


#7 posted 10-01-2020 03:45 AM

I like to size the end grain, allow it to dry/tack, and then reglue holding the pieces together with 18 to 23 gauge nails. They will hold everything tight while the glue dries. A good glue bond will destroy the wood before the joint goes.

-- Think safe, be safe

View tomsteve's profile

tomsteve

1077 posts in 2102 days


#8 posted 10-01-2020 10:19 AM


Do some tests to see which drill bit size works the best for the screws you decide to use.
- jkm312

measure the diameter of screw at the bottom of the thread- the minor diameter. thats what size drill bit should be used.

View Robert's profile

Robert

4048 posts in 2363 days


#9 posted 10-01-2020 01:49 PM



just pre drill with a counter sink bit first and you should have no problem.

- pottz


This^^^

-- Everything is a prototype thats why its one of a kind!!

View Mellowpine's profile

Mellowpine

1 post in 23 days


#10 posted 10-02-2020 07:01 PM

It’s best down using a pockethole, don’t screw into the end grain of the plywood. Use a pocket hole and screw into the face of the adjoining piece.

-- Susan, Seattle, https://mellowpine.com/

View MrRon's profile

MrRon

5942 posts in 4126 days


#11 posted 10-09-2020 09:14 PM

It really doesn’t matter whether you use screws or not. It’s the glue that will make for a strong joint. A few brad nails to hold the pieces together until the glue sets is all that is needed. As others have noted on this thread, the joint would fail in the wood, not in the glue joint. Of course this assumes a good glue joint, properly applied with the correct glue.

View dpjeansonne's profile

dpjeansonne

75 posts in 4096 days


#12 posted 10-10-2020 12:12 AM

Thanks for all the responses. I have to agree that the glue is a major factor in the strength.

-- Cajun Don, Louisiana

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