Introduction: Turn a Fail Into a Feature

About: BongoDrummer is co-founder of Flowering Elbow. He loves to learn about, share, invent, and make things, particularly from waste materials. Check out his youtube channel:

So there I was at then end of the day, fitting the last ash wood window sill I had made for a friend's house, and oh no, it's not big enough!?

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

So you have seen in the photos what it looks like, the same method could be used to fit in any shape you like, but some are structurally better than others. The shape I used was based on the size of a scrap of teak I had left over from some other projects. Contrasting coloured wood is interesting, and makes it look like deliberate fun rather than a botched job.  

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 
Chisel (sharp)
Safety equipment (goggles, ear defenders, dust mask, dust sniper, etc)


Some wood
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

First find the wood you are planning to join on, check out the sizes, and then draw the shape you fancy. The shape I drew may seem random, and in many ways it is, but it has a few important features. The two sticking out bits near the top mechanically tie it to the end grain of the recipient piece, in this case the ash sill. And the side grain area (the useful glue-joint area) is increased by bulging out at the bottom right.   

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

Time to get the router out and set it up with a flush cut bit. I'm using a 1/4" shank with a 1/2"bearing and cutter dia. We want the guide bearing on top type. The router bit will replicate whatever the bearing rolls across, so before you start, sand away any unwanted ridges left by the saw blade. A bobbin sander is excellent for this, but we don't have one, so careful hand sanding is in order - a dremel type rotary tool can also come in handy here. You only need to saw the top of the edge where the guide bearing will run - that's the bit that the cutter will replicate. 

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

We need to firmly clamp the inset piece to the bottom of the wood we just cut a shape in. If the insert piece is small there is no room for clamps, because they catch on the router. In this case use three or more strategically placed screws to firmly secure it to the bottom. 

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

Unscrew the insert piece and mark up what needs to be removed - that's everything that was the other side of the the shallow grove you just routed. 

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

Now in an ideal world you use a disk sander and a bobbin sander to carefully sand away the remaining material until its a perfect fit. As it is I don't have either of those things so an upturned belt sander is clamped to the bench... The tight inside curves are impossible to get into with a sander, so use a sharp chisel to tidy up there.  

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

When it is fitting tightly in there (don't force it if it is too tight or bits will brake off), apply wood glue to the edge and tap or use clamp pressure to get it in place. Leave the bottom of the routed grove flush with the original wood (you can plane the raised section of the insert piece before or after it is secured in place - which is easier will depend on how big it is).

Wait for the glue to dry. 

Step 8: Finishing Tweaks

If it fit perfectly, that's awesome, skip to the next step and your'e done. If there are a few gaps, then hey, lets fill them to make it look better and make it stronger. I used two part epoxy mixed with some of the teak sawdust to make a runny putty which I syringe into any tiny gaps. Before I do that I create two channels with the hot melt glue gun either side of the joint line. It does a good job of containing the epoxy.

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

If you didn't already, plane off the plateau on the insert piece, and sand the whole area completely flush.   
Admire you new weird interesting unique feature.

Thanks for looking through. Would love to hear your comments, question and experiences.

Step 10: The Sills

So here they are all installed. They make some nice window hang out areas.

Thanks for reading, I hope some of you find the technique useful. If you interesting in more of what we are up to check out the Flowering Elbow website