Friday, August 26, 2011

Making a riving knife part 3

So now that the table saw's stripped down I can focus on the riving knife. I decided to focus on this first, and the dust collection second, since the riving knife will determine the room for the dust collection. Here is what I have to work with:

I guess I never really explained what a riving knife is. I'm not going to go into to much detail, but basically it's a table saw splitter that rides close to the blade. Wikipedia has a great description. If you google homemade riving knife what you mostly see are adjustable splitters, and while these are better than the factory splitters they are not riving knifes.

The problem with adding a riving knife is that most saws use a trunnion that rotates from a fixed point. If you just slap a splitter on the yolk, the top of the knife will not stay even with the top of the blade. It will just rotate down into the cabinet.

All of the pictures I found of riving knifes on newly manufactured saw change the trunnion design to lift the blade up and down in a vertical manor. Looking at all this additional complication makes me wonder how these tools will last in the long run. A pivoting arbor will wear and just not raise as high but for the most part will not go out of left to right alignment. But these systems have a lot more parts and allow a lot more room for errors. And besides, I don't have the capabilities of making an new trunnion.

I've been doing a lot of looking around on the Internet, the first homemade riving knife I found was on a saw just like mine! I studied this design inside and out and it works great. But there are a few drawbacks. I would need to make a new pulley for the arbor and motor. I would need to machine the arm, and I would need to buy preloaded linear bearings. The last part was the killer, the preloaded bearings are ~$250 each! Plus I'm not sure how well this will hold up under long term use.

The second design I found was right in line with what I was thinking. The yolk on the saw rotates about a fixed point. If I were to add a parallel arm of equal length a set distance from the from the pivot point as the yolk rotational center, and the arbor center, these 2 things would remain parallel. Here's a picture to illustrate my point:

If the arms are the same length and parallel then the part will remain level. So all I need to do is make a plate to hold the riving knife. In the picture above the top arm would be the Yolk in the saw and I would add the second arm.

The space is pretty tight I have about 1/2" between the blade and the yolk, and even less around the arbor because of the flange that holds the blade on the arbor. I will have to make some sort of ring to go around the arbor that mounts to the yolk. If you look in the first picture you'll see that there is about 1/4" of space to drill a hole and tap into to mount the ring. This might be a problem.

Fortunately this space appears to have been machined, and seems to be parallel with the blade. If not I will break out the file and level it off.

The parts that stick out on the yolk around the arbor hole, pivot hole, and gear rack are about 1/4" So this will provide some clearance for pivot arms etc.

The plan is to take a rubbing of the yolk, get measurements off of it, make a model in sketchup and play around until I get something that fits. Then make a mock up in wood, work out any kinks, and take the mock up and accurate drawings to a friend who is a machinist, make the parts out of metal and start sawing safely!

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