Wood structural viability for clamps?

[rafezetter]

I need some advice - I've got a 3m length of 40mm x 40mm oak made up of those "offcuts" so loved by Oak furniture land, in other words sections glued together with those "zig zag" toothed joints.

I'd like to use it for making some wood hand screw clamps. Unfortunately to get the most out of it, each 375mm section of clamp will have at least 1 possibly even 2 of these joints.

Does anyone have any knowledge if bracing it either side with 6mm plywood (of which I have a lot, generously donated by Eric the viking) glued with epoxy would be enough to withstand the tension effects of clamping on that joint, and additionally which direction of those joints would have the most resistance. I'm inclined to think that having the teeth running vertically - so the tooth zigzag line is top / bottom would be stronger.

Or should I brace it all 4 sides. I know it sounds like extra work, but this length seems like a perfect donor among my stock - apart from the joint issue.

 

[NazNomad]

I'd wager that the timber will fail structurally before the finger-joint does.

 

[rafezetter]

I'd wager that the timber will fail structurally before the finger-joint does.

Really? It's that strong?

I know manufacturers say glues are stronger than the wood nowadays, but having seen Mathias's tests I'm always a bit sceptical.

 

[NickN]

Assuming they used a decent glue then I'd agree with Naz. Most wood glues are certainly stronger than standard soft woods, even on a simple laminated join, the wood breaks first, so I'd guess you wouldn't have any problem just making the clamps non-braced.

 

[MusicMan]

These are industrial joints produced with accurate machinery under controlled conditions, with well-chosen adhesive. Not home workshop joints. I agree with NazNomad.

 

[rafezetter]

Hokay! - Thanks for the replies, I'll leave them "as is" then.

 

[siggy_7]

With reference to Mathias Wandel's tests - he is showing the strength of joints. Glue it's only as strong as the surface it's given to act on. With loads of long grain area to act over and the tapered design of the fingers limiting weak spots in the parent material, finger joints are immensely strong.

Sent from my SM-N910F using Tapatalk

 

[skipdiver]

The reason they are toothed is to give a big surface area of glue, which makes them extremely strong. I agree with the others, that the wood will fail before the joint.

 

[ED65]

I know manufacturers say glues are stronger than the wood nowadays, but having seen Mathias's tests I'm always a bit sceptical.

His test is nothing to go on. Given he's an engineer he should be ashamed of himself for leaving it up there and causing people to have doubts when there's so much published data that it doesn't tally with.

All you have to do to prove to yourself that joints are stronger than the wood around them is glue some wood together and try to get it apart :lol: