Thread: Help with butt joints for 100x100 posts

T bridal joint at DuckDuckGo

Well done for hanging in there!

First off, you can do this with the tools you have. All you need is a crosscut saw (a cheapie from Bunnings would work), a cordless or power hand drill (or router), a few chisels, and a hammer.

You are going to make through mortice and tenons! Forget dowels - you are building for outdoor weather. Your enemy is rain and sun. Rain gets into the joints and will create rot, and the heat will cause expansion and contraction, and the movement will open the joints.

There is a rule for M&T joinery called the "one third rule". This means that the mortice and the tenon each measure 1/3 the width of the member. So, for 100mm across, you will have a 33mm mortice.

To make a through mortice, the secret is careful marking out - mark the lines around the post, and then mark out the mortice lines on each side. Work half-way from each side towards the middle. You can do this with a drill bit or a router. The aim is to remove as much waste as possible, and then use a wide chisel to clean up the sides. Use a block of wood to guide the drill and chisel vertically.

Mark the tenons from the mortice - that will aid if you go over a line with the mortice. Now here is the important part: where a stretcher enters a post from both sides, the tenon only needs to be half the height. In other words, the combined height of two tenons fills the mortice. This allows them to fit in line with each other. When glueing up (use Titebond 3), use bolts as well with these joints.

The outer tenons are wedged - this is what is done to add tightness and allow for re-adjustment when the timber dries.

Yell out for advice when you need. Keep in mind that this will take you a few months to build - take your time, plan out what you intend to do, and don't be hasty or get impatient. Check out my website for tips.

Regards from Perth

Derek

Following on from Derek https://youtube/LPBkO2chZxk

https://www.youtube.com/watch?v=LPBkO2chZxk&feature=youtu.be

Should be a YouTube link to Paul sellers Mortice and tenon

saw chisel pencil. No more no less. Also there is no shame in a 30mm mortice that turns into a 35mm mortice
cheers

I still consider myself a beginner and I agree that you can do a mortise and tenon. You just need very sharp chisel(s) and patience. And you need a little extra timber in case you screw up and need to redo a part. Also being outdoors, the joins are going to open up so IMHO there is no need to worry about making them a perfect fit. They will not be a perfect fit in 12 months of weather anyway.

Another idea would be to make them modular. Then bolt the modules together with galvanised bolts. That way you would not have a long span requiring a 3 way join. A modular design might also open up some alternative furniture arrangements as a bonus. You could then use lap joints and bolts so that the joinery is simpler. (Or if you have an impact driver, even screw the lap joints together with ceramic coated screws, because the lap will counter gravity and the screw the lateral forces.) Since it is outdoor and slightly rustic design, that hardware will not look out of place IMHO.

Thanks so much Derek, this is really helpful, including the photos on your website which make the technique really clear. I will indeed give it a go! Thanks also to the others who have contributed more tips.

So it looks like I’ll be going for mortises that have a horizontal width of 33mm, and a vertical height of (say) 80mm (to allow for say a 10mm shoulder on top & bottom).

Where two tenons use the same mortise entering at 180 degrees, they will be only 40mm in height (one above, one below).

And where two stretchers join at 90 degree join (in a corner), I guess I can do something similar, but it would only need to be a 40mm height mortise in each direction (one above, one below).

Hopefully I’ve got the idea. Off to Bunnings for a sharp saw & chisels.

do you have a battery drill and impact driver?

14G lag screws at an angle. slap 4 in each post (2 on opposite faces). sure them up with black brackets and screws if needed. you're going for a chunky rustic theme?
Otter 14g x 100mm Matt Black Bugle Batten Screws - 40 Pack - Bunnings Australia

850c0cdb-770e-4c04-a162-e9b70ad8eef5.jpeg


pilot hole them with something like this:
Sutton Tools 5.0mm Long Series Jobber Drill Bit - Bunnings Australia

Originally Posted by China

The Americans call them Dado's because. well, they are Americans, the trench is the female part of the joint the entire joint is called a Housing joint.

Heaven forbid that something could be referred to by a different name as it evolved over a couple hundred years in an entirely different part of the world eh! 😏

Derek has been giving you some really good advice.

Part of the aesthetic charm of your sofa design is from the exposed mechanicals of those joints. In my view, if you use bolts or screws then you will lose some of that charm.

Sofa - Pine.jpg

I often make maquettes (scale models) of new designs from cheap materials so that I can practice joints, assembly and just look at the aesthetics. A friend actually draws cartoon series on how he makes and assembles each piece; his work is comparable with Derek's. I think through how I make each piece.

Originally Posted by EB42

... And where two stretchers join at 90 degree join (in a corner), I guess I can do something similar, but it would only need to be a 40mm height mortise in each direction (one above, one below). ....

With 100x100 timber you can actually go with 50mm halved tenons - something like this:

Sofa Joints.jpg

... Where two tenons use the same mortise entering at 180 degrees, they will be only 40mm in height (one above, one below). ...

Halved tenons could be each 50mm high. Alternatively, they could be full height (100mm) but only half the length so that they meet in the middle of the mortice.

Originally Posted by GraemeCook

Part of the aesthetic charm of your sofa design is from the exposed mechanicals of those joints. In my view, if you use bolts or screws then you will lose some of that charm.

Agreed, I came to the same view and have committed myself to doing through tenons as Derek suggested. I’ve actually just made my first (practice) joint. Phew it’s pretty hard work taking out that much timber! It actually fits nice and tightly, it just looks very ordinary. I’m finding that this cypress timber seems to crack and splinter quite easily when using some force to cut such a large mortise; in particular the top of the post (just above the mortise) started blowing out, so I will need to leave more timber exposed at the top and start the mortises a bit lower. I probably also need to do most of the grunt work with a smaller chisel rather than a 32mm wide one that spans the whole mortise, as that seems to be gentler on surrounding timber. I’ll keep trying.

Originally Posted by GraemeCook

With 100x100 timber you can actually go with 50mm halved tenons - something like this

Thanks so much for the diagrams, that make it very clear.

To make your life simpler drill the mortise out first with a large drill e.g. an auger then you just have the square it up with a chisel.

EB42

Thank you for the new description of your proposed work. For the moment let us return to the dowel. To my mind there are two alternatives there. You could use multiple dowels of the largest size commercially available, say four, but then you do have an alignment issue and it is getting a little fussy. Why not go to a single large dowel? Buy suitable size round timber (a broom stick would do) around 30mm diameter and cut it to the required length. Drill your holes from both sides and allow them to meet in the middle. Go from both sides to minimise any error away from 90°. There a number of simple jigsthat can be used to aid this process if you decide to give this a try and we can recommend subsequently.

The big issue with dowels and glue is, as you have identified, the ejection od surplus glue. The fluted dowel achieves this. before they were available woodorkers would just plane a small flat on one face of the dowel. A large dowel has enough room for at least three flats. Alternatively you could create a number of flutes using a hand saw. The size of the dowel makes this task very easy.

I believe you are using Cypress Pine and although it is an oily timber, I have found that the better PVA glues (I use Titebond 3) work well enough. However, you could use an epoxy glue and with it's gap filling properties would also work very well.

If you decide to go down the more "adventurous" route with M&T, I am sure there are others here to guide you.

Regards
Paul

Edit: Whoops! I missed that there was a page 2 and people have already stepped up to the challenge. So my suggestion is just another possibility. By the way, I am a big fan of chunky outdoor furniture so I hope it goes well for you. Be sure to post the results here.

Originally Posted by EB42

... I’ve actually just made my first (practice) joint. Phew it’s pretty hard work taking out that much timber! It actually fits nice and tightly, it just looks very ordinary. I’m finding that this cypress timber seems to crack and splinter quite easily when using some force to cut such a large mortise; in particular the top of the post (just above the mortise) started blowing out ....

Nice post, EB, you are asking the right questions and heading in the right direction. We have all been there, sometimes repeatedly ...

That erosion of the shoulder of your joint may be caused by a number of factors including:

• Removing too much timber in one bight,
• Chisel not sharp enough,
• Levering chisel against shoulder rather than always pivoting it on its point away from shoulder,
• Cypress is too dry.

Here is a fairly good demonstration of technique by Paul Sellars:
woodwork videos paul sellers cutting a mortise - Google Search

I prefer to cut mortises with a 3/8 or half inch chisel to within about 1 mm of the layout lines, From then on, it is a smoothing operation and the wider the chisel, the better. Chisels must be sharp, the sharper then better. And as you gain experience, your concept of sharpness will change. You can speed up mortise cutting a little by drilling out bulk material with a smooth cutting drill bit such as a forstner or brad point.

Originally Posted by Bushmiller

For the moment let us return to the dowel.

Thanks a lot for the detail on dowels. Planing flat surfaces on the large dowels is a great idea that wasn’t aware of. As you say, I’ve already started down the M&T route, for better or worse (although now feeling slightly silly that perhaps dowels could have worked, but no matter). Anyway I’ll save your helpful suggestion for a future project where perhaps there is less weight to support and so less need for ultimate mechanical strength from the joint.

Originally Posted by GraemeCook

I prefer to cut mortises with a 3/8 or half inch chisel to within about 1 mm of the layout lines, From then on, it is a smoothing operation and the wider the chisel, the better. Chisels must be sharp, the sharper then better. And as you gain experience, your concept of sharpness will change. You can speed up mortise cutting a little by drilling out bulk material with a smooth cutting drill bit such as a forstner or brad point.

Thanks for the advice Graeme. I have been using a 32mm Forstner bit to drill out most of the material before squaring it up, and I went down to a 16mm chisel utilised more gently with gradual removal, but even then I found that the edge of the post kept chipping off. I realised that this happened when I tried to chisel from both sides of the 100mm material to meet in the middle. Since I reduced the mortise depth to only 65mm (i.e. from one side only, blind not through) I haven’t had a problem. I know this is not quite the optimal way, and hopefully Derek doesn’t shame me for this compromise! Perhaps I can compensate for this by putting small dowels (or bolts) through the side of the M&T joints later.