Thread: Biscuit joining options

Stinky,
I reckon "B" will better counteract the natural tendency of the two pieces to want to slide past each other when downward pressure is applied. However I reckon a haunched tenon? would be stronger. (I think that's what it would be called, terrible isn't it I'm a joiner by trade and I can't even remember the right names) Anyway when finished it would look like a plain mitre but it actually has a mortice and tenon concealed inside.
Question: Does the $300 cover labour and materials? I would've thought that you'd be costing your boss more than $300 a day. If not, come and visit so I can exploit you.

Mick

Just realised after posting that "A" will be a lot easier to cut than "B". With "B" you'll need to make a series of jigs to locate the slots relative to the outside point of the mitre, whereas with "A" it's just a matter of resetting your fence for every new line of biscuits.

Thanks Mick .....
I was thinking the same about the forces acting on the joint and that is why I though of going for option B. I did some trial runs in pine using option A as it was easy as it was a simple fence height adjustment but after thinking about it oprtion B would be better.

I think I follow your suggestion for the other joint but after applying four biscuits and plenty of glue I am assuming that the joint would be super strong already. I know that it is often better to use the more traditional joinery but I sometimes wonder how much difference in strength there is with the new glues and methods.

The boss is the one in the house after hours and not durnig business hours so labour costs really don't come into it I just take what hours I am given. As for the business hours boss he gets well over $300 a day for my labour, actually that does not even get past mid morning ... pitty I don't see that much ahh one day I will be my own boss

Stinky and Mick

A haunched tenon is the correct name, Mick. A different look, but stronger, I agree.

"A" vs "B" in terms of structural strength? I don't know. Stinky, this is your area of expertise. Does it make a difference? I also wonder about the different allignment of the grain in the biscuits, themselves. Is this going to make a whole lot of difference? Probably not.

The other factor is that the purpose of biscuits is to allow for some adjustment while glueing up. You have pros and cons here. Which direction is considered more vital to get exact - sideways or up-and-down?

For reference, two other methods of joining that come to mind, both very quick to make: (1) Butt joint and glue, then dowel by drilling externally and diagonally. (2) Butt joint and glue, then cut a spline with either a dado blade or router. In both (1) and (2) you have a choice of same- or contrasting timber.

Your call.

Regards from Perth

Derek

Hi Stinky

On a joint like that one I think that I would cheat a bit and use a bit of 1/2" rod 6 inches long and bent into a 3 * 3 right angle. Insert it down the middle of the timber and Araldite it in. This would ensure that the joint will outlast a medium sized earthquake and nobody but you will ever know why. In fact, using this method you may even be able to reduce the size of the legs and make them more elegant.

Thanks for the replies.

I was thinking that option B would be the better option as it is similar to putting in a retaing wall across a slope. It is preventing the tendency for the horizontal timber to slide down the vertical. Option A is like putting a retaining wall the same direction as the slope.

Derek .... I understand your thoughts about inserting the dowel. I was hoping for a clean finish of just the two 45 deg faces meeting. I may however look into joining with the biscuits and then inserting a few decorative dowels of 10mm dia solid aluminium. Could look good.

Bob - 70 x 70 is exactly the size I am after .... I know it seems big but in the overall scheme of the design it works well. I will post a few pics when completed (a few months after the garden to give you an idea. Do you think that the bent rod will be stronger than the biscuits. I have concerns about mixing timber and steel without expansion joints as the steel can expand and contract at different rates to the timber. I like the bisuits as it allows for movement when gluing as Derek mentioned and they will swell to lock the joint. If inserting the bar it would be essential to drill the holes spot on in the middle of the legs.

Sir S,

I am very dubious that biscuits will be strong enough to do the job. I think the best solution would be to use a couple of 3/8" x 1 1/4" x 2 1/2" floating tenons - easy enough to do with a morticing jig. The mortices for the tenons should be aligned as in A, but closer to the inside corner, and routed perpendicular to the mitre face.

By the way, if you were using biscuits, I disagree that option B would be the way to go. You would be gluing the biscit onto end grain.

Rocker

I'm not certain of the technical term, but I believe the joint to which Mick referred looks like this:

Sir S,

Thought I had better provide a diagram in case my description was not clear. Here it is. I think floating tenons will provide a lot more glue area than the solution suggested in Darren's diagram, and the mortices would be much easier to cut than than Darren's solid tenon.

Rocker

Hey, I don't claim any ownership of this joint, I've never even made one! I was a bit bored and decided to draw a picture of it, that's all

Either way, I don't reckon it will ever be the strongest joint around. The only long grain to long grain glue surfaces are at an angle to each other and most of the contacting surfaces have end grain in them.

Still, as Stinky says with modern glues that should be less of a problem. Although I wonder how people can know the durability of modern glues, given that they are modern. Will they still be solid in 50, 80, 100 years? More to the point, does anybody care?

Darren,

The long grain in the cheeks of a tenon in an ordinary M&T joint are at 90 degrees to the long grain in the face of the mortice, and that is reckoned to be a very strong joint. The fact that the long grain on my floating tenons would be at 45 degrees to the long grain on the faces of the mortices should, if anything, make them stronger still. Apart, from the strength of the glue, the geometry of the floating-tenon joint would make it mechanically stronger.

If, like me, you have one foot in the grave, you do begin to care whether your joints will last 100 years; they are your epitaph

Rocker

Sorry, I think my brain fell out my ear a little while ago. I'll try to find it and put it back in.

Originally posted by Sir Stinkalot
Thanks for the replies.



Bob - 70 x 70 is exactly the size I am after ... Do you think that the bent rod will be stronger than the biscuits. I have concerns about mixing timber and steel without expansion joints as the steel can expand and contract at different rates to the timber. I like the bisuits as it allows for movement when gluing as Derek mentioned and they will swell to lock the joint. If inserting the bar it would be essential to drill the holes spot on in the middle of the legs.

I think that the rods would be about 10 times stronger than biscuits, so if you need a joint that is 10 tomes stronger then this would be it. I cannot see that expansion would be a problem at all, especially if the holes were drilled in the middles of the legs. Also, I wasn't saying not to use the other joints, but to also use the rods with them.
This would also certainly satisfy Rockers requirement that the piece would last until well into the next century.

Bob,

I would guess that, unless you have a lot of metal-working expertise, bending half-inch steel rod to exactly 90 degrees would be pretty tricky.

However, your mention of metal reinforcement for the joint gave me the idea of using 3.2 mm x 25 mm brass bar stock as a spline, glued in with epoxy. It would be easy to cut a 12.5 mm-deep kerf in the faces of the mitres, using a tenoning jig on the table saw, and glue in a piece of brass bar; it might form an attractive decorative feature of the joint.

Rocker

Stinky,
Go for the metal - not for strength, but so that in 100 years time, you can look down and laugh as some poor B tries to put it through his table saw to 'recycle' the timber.

I don't claim any great expertise, but I think option A would certainly be strong enough, but keep in mind Mick's caveat about what modern glues will be like in 100 years.