My first mortice

[derekthedrill]

Absolute beginner here, so go gentle please.

Up to now I have always used brutish mechanical fixings of one sort or another. I feel that it is time to start learning some more pleasing techniques.

I am building a small platform, raised from the ground, with two posts at one end, the other attached to a ledger. I need to join the posts to a beam. This beam with carry joists.

I hope that makes sense.

I think that a mortice and tenon joint would be good for the joint between the posts and the beam. Strong and attractive.

All the guides I have come across, assume that the timber involved in the m&t joint is relatively small. Smaller than mine:

beam is 160x90mm
posts are 100x100mm

My understanding is that the thickness of the tenon should be around one third of the thickness of the beam. This makes for huge tenon. Chisels don't come that big do they? And what a force would be necessary to make that chisel cut! Even a double tenon would make for tenons of about
40mm wouldn't it?

What is the solution? Doubtless I am barking up the wrong tree here. Please, direct me to the right one. My throat is getting hoarse.


derek

 

[MikeW]

Are you certain the beam is 160 mm--60 mm larger than the posts?

In any case, usually when working with thick stock double tenons are used, like here...

Depending on the height of the tenons, they may well be further divided for expansion and or ease of making the mortises.

The ones in the picture are also haunched due to their proximity to the top of the legs. Yours I don't suspect would need to be.

Take care, Mike

 

[mickster]

Seeing as no-one else has replied yet...

The chisel doesn't have to be as large as the tenon, and you wouldn't cut the tenon to full depth all in one go. Using a smaller chisel, you would start at one side of the cut & move across in a series of shallow cuts to the other side...then remove the waste at that depth. Repeat until the full depth is achieved.

Kinda 'nibbling' the waste away.

Until someone tells me this is wrong, that is how I would do it. :wink:

MikeW got there first.

 

[derekthedrill]

Thanks for those replies. that makes more sense - unlike aspects of my question: when I said double tenon, I think I should have said twin tenon.

The beam is going to be 160mm in its vertical aspect, 90mm horizontal from back to front (is this called width?). Sorry, I am not sure which length width refers to. I think you can imagine it from those measurements though.

Thus a twin tenon would be better than a double?

Derek

 

[MikeW]

Hi Derek,

Double, twin...I use them (incorrectly) interchangeably. But you get the idea from the picture.

The picture I posted have rail thicknesses (the horizontal members with the tenons) of about 65 mm. The tenons are about 1/4" thick.

In your application if I were to make it the way I did on the credenza, I would use 3/8" (10 mm) to 1/2" (13 mm) tenons. 160 mm rails in width--I would use single width double tenons.

fwiw, when I make thicker tenons than 1/2" (13 mm), I use a brace and auger bit and simply pare the mortises square with a chisel.

Take care, Mike

 

[dirtydeeds]

when cutting long deep mortices by hand leave the last 6mm or so at each end until last

this way you can use this area as a lever for your chisel

 

[Evergreen]

Derek

If you haven't cut mortise and tenon joints before, I suggest that a project using big timbers is not the one to start on. Not knowing the purpose of your platform, you might well be better off with some sort of mechanical fixing. After all, most modern roofs are held up by woodwork that's fastened with only multi-nailed plates!

If you're still keen to go with M&Ts, practise on a good few bits of scrap first to avoid costly mistakes.

Regards.

 

[dirtydeeds]

my first foreman gave me a pair of garage doors to build

i was presented with a pile of PAR and T&G

"just for a bit of mortice and tennon practice"

and the second comment was

"if you waste any, you can plane up your own", said he pointing to a pile of sawn timber