Yes I know... Don't give me that measure twice business, it still happens, and when it does what are you going to do?
Having made this particular sill from scratch (and I mean from scratch, having felled and chainsaw milled the tree myself (more on the DIY chainsaw mill in the future)), I didn't really want to give up on it.
Lets add a bit of length and make an interesting and unique feature of it. Adding length to a board of wood is difficult - even if you find a grain and colour matching piece of wood, end grain gluing doesn't work. In short trying to make an un-noticeable addition is very difficult and will probably look scrappy anyway (and it's a bit boring).
Better we thought to make a feature out of it, make it look like a playful and deliberate addition - something cool to remember the comedy of a too short plank.
I'll go over the method I used.
Step 1: General Idea, and Tools Required
The more complicated and large you make your shape the harder it will be to get it fitting tightly, without a gappy joint. I found this one quite a challenge.
Tools used in this method:
Router (with flush trim cutter bit - the bearing on top type)
Jigsaw, coping saw, and/or bandsaw
Safety equipment (goggles, ear defenders, dust mask, dust sniper, etc)
Your choice of Wood glue
2 part epoxy and some saved wood dust (useful if you expect to have some small gaps in you joints - you should expect this!)
Step 2: Draw Your Shape and Cut It Out
One note of warning: sharp bends and pointy bits do look super cool, but will be very hard to do using this method, as the router bit may not fit. They will increase the 'chisel time' significantly.
Anyway, draw on your shape, and when you're happy with it, cut it out with a jigsaw or coping saw (jigsaw is much faster if you have it, but can be less accurate if your not practised. Either method has a significant problem: the cut will rarely be completely perpendicular to the wood. When you cut tight corners and curves the jigsaw blade will tend to bow and deviate from a true up-down vertical path. As we are going to be both jointing to this cut edge and using it as a template to make our insert piece, this is significant.
Step 3: Tidy Up the Cut Edge
When happy run the router round the cut edge to smooth off the cut and ensure that our edges are exactly perpendicular. Depending how straight your saw cut is, you may have to flip the workpiece over and rout from the other side as well. This eliminates the undercuts - whereas the router pass from on top removes the slopes and makes the edge sheer.
Once you are happy that you have a clean and consistent edge to join to, it's time to use it as a template to make your insert piece.
Step 4: Rout a Small Edge on the Inset Piece
Once it is safe and secure rout a shallow grove in the insert piece that copies the cut edge. See picture comments for details...
Step 5: Follow the Line & Cut the Insert Piece
Ideally use a finely tuned bandsaw (if not a jigsaw or coping saw will do) to cut close to the grove. The more of a bandsaw jedi you are, the closer you can sneak up to the outside grove line (I aim to leave about 0.75mm of material between the saw kerf and the edge of the routed grove).
Step 6: Sand to Perfection
The little side-wall of the routed grove is a good indicator, when that is sanded off you know it's time to stop. Take your time and stop to check the fit often - it's easy to do too much.
Step 7: In Place
Wait for the glue to dry.
Step 8: Finishing Tweaks
Once set sufficiently all the excess glue (hot melt and epoxy) can be removed with the router, planer or whatever - this is best done when the epoxy is hard but not rock hard, like it goes after a few days - if it is like this it's pretty harsh on your cutting tools. Getting it after about 8 hours seems about right, but it varies with temperature a lot.
Step 9: Sand Flat
Admire you new weird interesting unique feature.
Thanks for looking through. Would love to hear your comments, question and experiences.