What size wood screw?

[Mike King]

So, I need to screw my coopered seat to the undercarriage of a set of chairs I'm making. The seat is elevated above the undercarriage by 4 standoffs that will be through drilled to allow the screws to pass through, and move with seasonal wood movement. But there can only be 4 screws. So, do you think I can get by with #10 screws or must I step up to #12's? The seat is butternut.

[Paul Haus]

Think of the absolute worst case and use that as your minimum. What's the biggest person that might sit on them? What's the worst abuse (like kids jumping up and down and what age and size). I've seen 350 pound Aunt Kate (fictious name) flop into a chair at my brothers place. I've seen some roughly teenage kids climb the kitchen cabinets and jump from the counter onto the chair they used to get there.

Don't think it'll happen - good luck. I've seen kids even step through a cane chair seat because they didn't want to get a proper stepstool and used a fine antique chair for their purpose.

[Lee Schierer]

Any chance you can post a photo of the arrangement?

[Mike King]

Before carving the standoffs to fit. 4F6CB30C-1B3F-4E17-AC3E-3160B0E53ECE.jpg

[Andrew Seemann]

Are you screwing into the butternut seat or into the undercarriage? Butternut doesn't have a lot of screw holding power.

[Mike King]

butternut.

[Lee Schierer]

I think I would go with the #12 as they will have more contact surface with the butternut.

[Larry Frank]

On softer woods when I am worried about holding power, I drill the pilot hole and then put a few drops of thin CA glue in it.

I do the same for MDF and even plywood at times.

[Paul F Franklin]

Another way to beef up the holding power is to grind off the tapered points of the screws so you have full depth threads all the way to the tip. This is especially useful when you have relatively limited depth to screw into, like your chair seat.

[Jim Becker]

Alternative to "just screws"...threaded insert in the seat bottom with an appropriate length machine screw or bolt up from the bottom, countersunk in the frame so it disappears. The threaded inserts have significant holding power in the wood because of their width and typical deep threading. You'll need pilot holes to carefully locate the center spot and then attention to detail in sinking them in at the correct angle.

[Jacob Reverb]

Alternative to "just screws"...threaded insert in the seat bottom with an appropriate length machine screw or bolt up from the bottom, countersunk in the frame so it disappears.

I'm with Jim on this. Chairs and stairs take a mountain of abuse, and can hurt people when the fasteners fail. Tough to match the strength of through-bolts with screws threaded into wood for long...I might also try to make it such that I could tighten the bolts/machine screws when the wood wallows out and leads to looseness...

[Mike King]

Thanks for all of this. I've been contemplating using threaded inserts, but the seat bottom has a curve and will not be square to the standoff. Even worse, I can't figure out how to drill a larger diameter hole as the pilot for the threaded insert that is in the proper alignment, let alone how to start the insert given the significant offset on one side of the seat. So, I'm thinking the solution is #12 screws.

Mike

[Jim Becker]

Mike, you can make jigs to help with drilling for the inserts that deal with the angles and use a depth stop for the obvious reason. It's going to be a bit of work for sure. You'd have to do the same thing if you were going to use glued tenons into mortises rather than metal fasteners. Screws are easy and if you think you can get what you need from the #12 or even a #14 screw do a test on a mockup. Chairs are one of those things that you absolutely have to get "right" for both safety and longevity...a lot of stress is going to be on those connections.

[Mike King]

Sure jigs would be a possibility, yet each bottom, and undercarriage, is unique as they are coopered and shaped by hand.