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28 February 2016

Finger Joints and other Gateway Techniques

It's such a lovely day today. The sun is shining and all is good with the world; A perfect time to hide myself away in my garage for a little while in the pursuit of joinery excellence.

Yep... I have more experimentation to share with you today, and once more my trusty table-saw is right at the heart of it. Step this way...

I plucked up the courage to delve inside my table-saw this morning and I removed the the riving knife. The riving knife is that attachment at the back of the blade (to the right in the picture) that moves up and down with the blade as you adjust the blade height. It's primary function is to prevent kick-back which is where the spinning saw blade will send loose bits of wood flying up and back at the operator. The top of the rivening knife is slightly thicker than the cut made by the blade which prevents cut wood passing over it. The second important purpose of the rivening knife is to act as a splitter and guide, keeping the cut straight and preventing the hole from closing in on itself.

I've wanted to remove the rivening knife for a long time but I've held off knowing full well that it is there to stop me killing myself. Today, I wanted to try a technique that just wouldn't be possible with the knife fitted. It turned out to be a really easy thing to do. I was able to unscrew a panel that runs next to the blade and there you can see it in the picture: the rivening knife is held in place by a single bolt.

Before I go any further I want to explain why I was brave enough to remove a safety feature from my table-saw. Have I lost my tiny mind? Maybe...

I made a sled jig that was to ride in the t-grooves in the top of my table-saw. Having made them with cut-back carriage bolts I was absolutely confident that the sled wasn't going to go anywhere except where I wanted it to go.

I've seen a number of different techniques for making guides for t-grooves and I reckon I must have chosen the hardest one to pull off. In the picture you can see one rail which is simply two M8 carriage bolts where I've filed down the sides of the carriage bolt head on either side. It fits perfectly... a little too perfectly... There isn't a lot of give once fitted. It doesn't take much for it to jam and stick.

Here's a better photo of how the rails work.

You can see that I fitted two bolts in either side of a board. I got them square and what do you know... it works! The only way to get the sled fitted and removed is to slide it through the ends of the t-grooves; The lip of the carriage bolt heads keep it safe.

But why on earth do I want so desperately to build a sled jig? Read on...

This was the start of my scheming. Check out this wonderful video that shows you how to make long-reach c-clamps. You can never have too many c-clamps... or at least that's how the saying goes. What struck me as I watched this is that this might well be the perfect technique for finishing my Digital Caliper project. Oh no... more to explain...

I'm not going to say too much about digital calipers other than to say that I'll need one when it comes to making my archtop guitar. It's a simple tool that will allow me to measure the thickness of the archtop wood as I carve and cut. I'll do a full post on this when I get there. For now, just pay attention to the picture above. This is a top-of-the-range SPCSM caliper that gives you a bit of an idea what over-engineering can do for you.

My caliper will be nowhere near as arty or clever as this.

Have you joined the dots yet? Yes... as I watched Matthias Wandel creating his c-clamps I began to wonder whether I could use the same technique to make the basic arm needed for a digital caliper. Of course I could! I set immediately schemed up an experiment to see whether I could do it.

And if this proves successful, you can bet that I'll have a go at making some clamps too! Ha ha.

Here is the magic that makes it possible to make c-clamps out of wood. Finger joints (aka Comb joints) are a way of making a strong corner joint, if only you can cut the bloody fingers to fit together. The more fingers, the greater the surface area holding the joint together... the stronger the joint will be.

If I can find a way of producing this joint, I reckon it will unlock all manner of projects. Back to the sled...

Here's is me doing a little further work on the sled jig. I've built some high-sided walls... well above the maximum height that I will raise the table-saw blade. This is important as I am going to run the blade right through the middle of the sled and I don't want the thing collapsing in on itself.

My test was going to be with some pine off-cuts that are 1 3/4 inch by 3/4 inch so I've sized everything in the jig to deal with this stock.

Here's the sled on the inside. You can see the slot that has been made by the blade. I need to find a way to position the blank such that each of the finger grooves can be equidistant. No problem. See the little metal line just to the right of the slot. This is actually an l-shaped bracket that is fitted to the wood block in the middle. If I've got it right, this is all I need to get the spaces right.

The process is to rest the blank against the the bracket... cut... move the cut blank to the right over the bracket... cut... repeat.

For my experiment I wasn't going to get to excited about clamping the blank... which as you will see shortly caught me out.

Here's my first effort. Look at the top cuts. I'm so wonky that I've got two to join! The bottom one on the other hand looks pretty good. I tossed the first and cut a third... 

Take 2

It won't win awards for beauty, but bloody hell if it doesn't fit together! I had to knock it a bit to push it all the way in, but even without glue this it strong. Will it be strong enough? I reckon so. I wonder if I can do it twice? Ha ha. That's for another day.

This would look pretty good in a rustic way if sanded. Maybe I should have made the fingers slightly longer to sand flat?

The general idea from here is that you glue it and for extra strength stick in a couple of dowels. I haven't glued for this experiment. All I did was to set it square and hold it there with a couple of screws.


Experiment successful!

Before I go... here's proof that I made it to the end of my spool clamp production run. What a long boring job that was! I think I might have ended up with 27 or 28 or 29 through an inability to count. I'll have to work with what I've got... NO MORE SPOOLS!

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