I did not expect to make this project into an instructable; this was made in hindsight. Therefore there are nowhere near enough photos for all the processes. It was interesting to try bird-mouth-joinery for the first time and this is not something you see often on this site - so I hope that despite the brevity and poor documentation it will still be an interesting read.
Step 1: Sourcing the Timber
The lid and base of this urn are made from Alder, which grows abundantly on our land. You can see in the picture above that due to a storm, we had a massive supply of the stuff. It was frankly heartbreaking to see what were healthy green trees end up stacked in a line like this. It did however provide a strong motivation to try and make something with the wood and not allow it to simply end up in the stove. So I took a few of the straighter and knot-free logs and seasoned them in a shed for about 12 months.
Step 2: First Cuts: Squaring the Sides
This was a lot more complex than I anticipated and in the end I had to use every trick in the book. The method I found worked was to start by using a chalk line and ruler to mark out roughly what a straight square run through the wood should look like. Then I used a few different sizes of hand plane to flatten out along the middle of one of those 'runs' of chalk line. This meant that I could then run that surface along my table saw. As the saw blade was nowhere near tall enough to cut all the way through the log - this would simply give a guide for the rip saw. Many calories later, some quick work with the draw knife would tidy up the cut ready for the repeat on the next side. These cuts became progressively easier and more accurate as the thickness of the log decreased - enabling the table saw blade to make deeper cuts.
The resulting square sections of Alder were then glued together and hand planed to make wide pieces. Approximate dimensions were about 10" x 10" x 3/4". The hand planing down to size was a long but very enjoyable job. Alder planes very well, it turns out.
However, because of the short seasoning time (12 months is really about half as long as they needed) some big splits sprang up almost overnight in the wood. I had to concede to buying the side pieces of the urn from an online hardwood supplier. This was very easy to do - and a 3 metre length of 1/2" American Ash cost about £50 including delivery.
Step 3: Bird Mouth Joinery
The idea to make an 8-sided urn came directly from watching David Henry's youtube videos on bird-mouth joinery. These videos are all excellent and watching them will explain the process far better than I can. However here is a very brief overview.
With a specialised router bit, called a 'bird's mouth' bit, a particular shape can be cut along the length of a plank of wood. The shape corresponds to the number of pieces that, if all cut exactly the same, will then lock together when assembled in a circular pattern. So for instance, the 8-sided bit used for this project will enable 8 identically milled pieces to form an octagon. A 6 sided bit would produce a hexagon, and so on. Regular bits come in 6, 8, 12 and 16 sided varieties, though I'm sure there are more. If you look in the images this should become clearer - you can see a very rough first attempt made out of scrap pine.
Finessing the height and depth of the bit is a critical and fiddly process, and depends largely on the exact thickness of the stock wood. I read some good instructions that helped get started - however a lot of tinkering followed with micro adjustments on scrap to get the bit in the exact spot. If done incorrectly the pieces will lock proud or shallow of each other.
Incidentally this was my first use of the Lumberjack RT1500 table router - which was excellent except for the fact that the height adjusting wheel vibrated with the table when turned on - resulting in the height slightly changing over the course of a few runs. So a good tip is to find a way to very securely lock the wheel if trying this with a similar table. Another tip is to make the cut in 3 or 4 stages as the bit is removing quite a heavy amount of wood and will cause splintering if rushed. Though tedious, this greatly improves the uniformity of the cuts.
Once all the American Ash was milled, I simply laid them flat on the workbench and taped the long edges together. I filled the cavities with regular woodglue and then carefully brought all pieces together with a final load of masking tape. By the next morning, the 8 individual planks had become one solid octagon.
Step 4: Finishing Touches
The lid and base were made from the Alder sections. This involved cutting them roughly to size before final dimensioning. I got them about 90% square and flat with the hand plane - this is about the peak of my ability. I am a very novice woodworker and don't own a thicknesser, so it was just a case of thinking it through and doing the best I could with hand tools.
The final pieces were then cut to match the octagon shape of the main body of the urn, using a fine toothed crosscut saw and the block plane. The base was simply glued straight on with regular woodglue and clamps overnight. I put a 5mm deep and 2cm wide groove around the perimeter of the lid, using a hammer and chisel, so that it had a bit of a seat when closed. I then added some very tiny cabinet latches and hinges. The second latch you can see in the picture is to help lock the lid down tight and account for the slight warp that occurred in the lid towards the end of the project.
I then sanded the whole thing down progressing through from 180 - 3000 grit paper, before cycling through 5 successive coats of tung oil with fine steel wool between them.
Job done! I was particularly pleased with the colour that came out in the Alder. Those who know the tree well might recognise that the tung oil nearly brought out that same colour you see when you cut the living wood - it goes a sort of deep orangey red. I am also excited to try out further bird mouth joinery projects as this demonstrates that even a relatively 'green' woodworker can produce a decent object with the technique.