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Do biscuits add strength? Solved!

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Oakay

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There is a lot more to the strength of a joint than just what is used, ie biscuit, dowel or M&T. You have the actual type of wood or material, the application which will determine the type of forces the joint needs to withstand as a chair will be very different to a kitchen unit, so all methods will have their place somewhere and in my opinion biscuits are for alignment, the glue does the rest. When you fit a biscuit you are introducing a weak spot as you remove material from the workpiece, the question is does the biscuit compensate.
Sometimes it compensates, sometimes stronger (especially with opposing grain direction of plys), sometimes weaker. Situations vary as do the clamping application and adhesive used and the open-times, and whether or not there will be exposure to outdoor weather (not recommended for beechwood biscuits).
 

Thingybob

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I have a set of miniature saw blades with arbor and some .4 mm ply that i make miniature biscuits from the blades are available on ebay i use for gluing up mini table tops etc 1/12 scale
 

Inspector

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I made an oak screen door for the front door that I walked through multiple times a day. It slammed behind me every time for over two decades. I took it down to bring with me when I moved and may put it back up but storm doors are more practical here. It was made with biscuits and none of the joints opened the tiniest bit. His butt joints wouldn't have lasted for a week. His tests are set up to advance his theories, not to find out what the forces acting on joints really are.

Pete
 

Oakay

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I made an oak screen door for the front door that I walked through multiple times a day. It slammed behind me every time for over two decades. I took it down to bring with me when I moved and may put it back up but storm doors are more practical here. It was made with biscuits and none of the joints opened the tiniest bit. His butt joints wouldn't have lasted for a week. His tests are set up to advance his theories, not to find out what the forces acting on joints really are.

Pete
There is something to be said for a short 'stub tenon' which a biscuit joint has similarities to because less shrinkage plays upon it, keeping the shoulder tightly shut. I too made a simple half-glazed softwood outer porch door for myself (North facing and under cover so well sheltered) with double biscuits many years ago and suffered no problems. Doubling the biscuits helps when the wood is over 30mm thick.
 

TRITON

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I think biscuits are a bit on the brittle side. Ideal for lining up, especially joints like miters to stop them moving while being clamped.
 

JimB

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This guy again….

next up, do carrots really make you see in the dark? That other thing we all know but really, they don’t.

At least he’s stopped measuring force in kg
You mean my mother was wrong. ;) Next you'll be telling my that eating bread crusts doesn't make your hair curl!
 

Oakay

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I think biscuits are a bit on the brittle side. Ideal for lining up, especially joints like miters to stop them moving while being clamped.
How brittle? For what application? Are you over-engineering? Compared to what alternative joint? It takes quite a lot of force to snap a biscuit, but I agree, you shouldn't use them without considering the suitability; just think people sometimes under-estimate them. Compared to a small dowel, they can actually spread the load and in some cases remove less material from the main structure, which can also cause weakness, (which can also be a reason not to use them).
 

Garden Shed Projects

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This guy again….

next up, do carrots really make you see in the dark? That other thing we all know but really, they don’t.

At least he’s stopped measuring force in kg
Carrots do help you see in the dark as they contain vitamin A which helps you produce rhodopsin. Also you never see a rabbit in glasses 🤓
 

TRITON

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How brittle? For what application? Are you over-engineering? Compared to what alternative joint? It takes quite a lot of force to snap a biscuit, but I agree, you shouldn't use them without considering the suitability; just think people sometimes under-estimate them. Compared to a small dowel, they can actually spread the load and in some cases remove less material from the main structure, which can also cause weakness, (which can also be a reason not to use them).
Not a great deal. Had a frame topple off the bench before and it broke on all the joints. That would be end grain to side grain reinforced with biscuits. As it would be the side of a cabinet, fully glued up with a panel in veneered mdf it would be strong enough for application, but just the 4 joints it clearly wasnt.

do carrots really make you see in the dark
As in Vitamin C, or better put as Vitamin 'see'. It's just a joke most people dont seem to get.
 
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