Screws not holding in 2x6 douglas fir

[Michele]

Hi all,

I've got a mystery here. Trying to make a bar cart out of 2x6 douglas fir, 42" long. 3 shelves with 4 boards each. We have done this exact same design before in the fall and had no issues. I'm using 2 1/2" coarse thread screws. Each of the 3 joints / shelf has 5 screws, 2 going one way, 3 the other.

The issue we're having this time is every single shelf has "flex" to it. I do NOT get it. I've verified the settings on the drill bit via the tool AND a measurement of 4 1/8" from stop to shoulder of the bit. We're screwed in as far as they can go. Yet the joints still flex.

Here's a picture of the last one we made and we're just trying to replicate this. Any guesses? This is really getting on my nerves. Happened on the first shelf and we thought ok let's try th next one, same thing......

[Tim Grace]

Whenever I edge join 2X lumber, I cut the rounded edges off, either using the table saw or jointer.  

I also will edge glue to add the strength and stiffness.

The fact that you've built in the past and had no problems, tells me it may just be the lumber itself.  

The cart looks really cool!

[Michele]

What in the lumber would cause this do you think? I can get some plates done up to see under each shelf to stiffen it up but only after I give a good yank on it and make sure it's not going to pull apart. Since I need to beat this with hammers and chains to "age" it, I want to make sure it's solid first.

Here's something else I built with 1x4 cedar. No issues there either so I'm really scratching by head.

[Tim Grace]

My thought on the wood is that the screws may have hit "soft" spots that are further from the heart of the tree. As I think about this, it doesn't make sense.  Especially since you are drilling & driving into both boards.

The only other thing I can think of is the chuck setting on the drill you are using to drive the screws.  I use an impact driver to drive the screws and forget that the chuck setting is crucial to how tightly the screws are driven to draw the pieces together.

[Doug]

It is just like mentioned. The pieces of wood have to be square on the edges where the wood is joined together. The rounded edges have to be cut off.  When I use the construction lumber from the big box store, I trim off the rounded edges.  If you buy the more expensive poplar, pine, or oak boards in the section separate from the construction lumber, those boards will come with flat edges.  

[Michele]

And especially since it's happening on every stinkin' board and we've done this same build before with no issues. I have the torque setting at like 11 or 13 on the drill tho I did think maybe I should try the impact driver (Christmas present and haven't used it yet so why not) tho I don't think I should have to. PITA.

Tim Grace said:

My thought on the wood is that the screws may have hit "soft" spots that are further from the heart of the tree. As I think about this, it doesn't make sense.  Especially since you are drilling & driving into both boards.

The only other thing I can think of is the chuck setting on the drill you are using to drive the screws.  I use an impact driver to drive the screws and forget that the chuck setting is crucial to how tightly the screws are driven to draw the pieces together.

[Michele]

But I've done this same build in the past with no issues and nothing cut or sanded ahead of time. If I hadn't done this before with zero flex, I'd go with your theory but.....

Doug said:

It is just like mentioned. The pieces of wood have to be square on the edges where the wood is joined together. The rounded edges have to be cut off.  When I use the construction lumber from the big box store, I trim off the rounded edges.  If you buy the more expensive poplar, pine, or oak boards in the section separate from the construction lumber, those boards will come with flat edges.  

[Doug]

You should use a impact driver. That is what the cabinet installers use to screw kitchen cabinets into the studs in the kitchen wall.  After using a drill, one can find that one can take a screwdriver and still drive the screw futher in.   Also, the brand of the tools makes a difference.  Contractor grade Dewalt, Makita, and Milwaukee tools work better than other cheaper brands.

On the rounded boards you are fastening together, some of the holding strength is going into the curved area which causes the weakness in the hold.  You can find two pieces of flat edged boards and join them with a kreg and it will hold solid.   At a store like Home Depot, you can take the common pine boards or the more expensive select pine (without imperfections), they have flat edges and will solidly join. 

Michele said:

And especially since it's happening on every stinkin' board and we've done this same build before with no issues. I have the torque setting at like 11 or 13 on the drill tho I did think maybe I should try the impact driver (Christmas present and haven't used it yet so why not) tho I don't think I should have to. PITA.

Tim Grace said:

My thought on the wood is that the screws may have hit "soft" spots that are further from the heart of the tree. As I think about this, it doesn't make sense.  Especially since you are drilling & driving into both boards.

The only other thing I can think of is the chuck setting on the drill you are using to drive the screws.  I use an impact driver to drive the screws and forget that the chuck setting is crucial to how tightly the screws are driven to draw the pieces together.

[Ken Kortch]

Be careful with an impact driver. Mine can drive a screw clean through even thick boards. You don't want to overdrive the screw through the pocket hole.

[Michele]

I was wondering about too much torque actually since on visual, the screw heads are plenty far enough into the pocket. All Dewalt power tools around here 

I'm going to take some short pieces (about 12" - what's left after chopping the 12' board down to the 42" pieces I needed) and see how it does with just 2 screws after I rip off the rounded edges.

*scratching my head*

[Glen S]

I've been following your thread Michele. Not sure I have anything to add further other than speculation on my behalf. So here goes.

Just throwing something out there;

Were these boards joined together right away after bringing them home? Was there a sudden rise in humidity for more than a day or 2 or 3?  Then the boards joined together followed by a drying out spell for the boards to shrink? Have you tried to re-tighten the screws?

Most likely cause;

Were the edges exactly square to the faces?

Were these boards cupped across the face? If so then the edges cannot be square to the face.

BTW-- I love that rolling cart!! If we had the space I would make one for my Wife's kitchen!!

edited for the correct words!! LOL

[Michele]

Well apparently the faces were the issue. We took about an eighth off on the table saw and so far two boards are together tight and strong. Guess it was just dumb luck last time. Good excuse to get the old jointer going that someone gave me because ripping an eighth off with a table saw is somewhat dicey.

[Tim Grace]

Glad to hear you resolved it!