A few months ago I picked up an old Rockwell 10″ contractor saw that needed some work which I got up and running pretty quickly. Up until this point I has only used cheap portable table saws (my current saw was a $250 Ryobi), so moving to the 1.5 HP, belt-driven, cast iron contractor saw was a big upgrade; but even though it was a more stable and accurate tool (and thus less dangerous to use), I found myself nervous about using it, fearing a catastrophic kickback despite never having having this problem with any prior saw (perhaps my fear was due to all of the horror stories and warnings I read while researching how to restore the Rockwell).
I’ll make another post that goes into a little more detail about the saw and the repairs I made, but for now I’ll focus on my most recent modification: adding a riving knife.
A bit of background: A riving knife is essentially a fin behind the blade that rises and lowers with the depth of the cut. It is approximately the same width as the blade and helps to keep the material in line with the blade after it is cut. It also helps to prevent the freshly cut material from pinching the back of the blade, which can result in dangerous kickback and serious injury. Most American saws have featured a “splitter” type system, which places a fixed fin a few inches behind the blade. Riving knives have been standard on European saw for some time, and are now standard in high-end table saws (American or otherwise). Splitters are better than nothing, but since the original splitter was missing from my saw, I decided to see if I could add a riving knife instead.
I started by tracing my saw blade onto a piece of 1/8″ aluminum plate and drawing a knife shape that would closely follow the blade and then cut it out with a jigsaw.
My saw is set up with a thin kirf blade, that measures about .0925″, so I knew I would have a bit of metal to remove before the blade would be the correct thickness.
I worked on it for a long time with my grinder, sander, and hand file, but was still too thick, and was having trouble maintaining consistent thickness along the blade
At this point I did a little more research on the appropriate width of a riving knife and found out that the goal is not to necessarily match the kirf width, but rather to land somewhere in between with kirf width and the blade plate width, since the purpose of the knife it to keep the wood from grabbing the plate. My blade plate width is .0745″ so anything in between .0745″ and .0925″ would be fine.
I went to the local metal supply and picked up a new piece of aluminum that id 0.09″ thick, that I will be making the new knife out of.
Since, at the moment, my wife and daughter were taking a nap and I couldn’t use any loud tools (i.e. the jigsaw) to cut out and shape the new blade, I used the existing blade to finish the construction of the mounting bracket for the knife within the saw itself.
I had already cut the basic shape of the mount out out 1/4″ aluminum and refined the shape by clamping it in place with a pair of vise-grips and raising and lowering the arbor assembly to see if the mount interacted with and of the other saw components. My final shape is shown below:
This right end is designed to mount to a flat surface on the arbor assembly that is parallel to the blade. I drilled (using my cordless (read: “quiet”) drill) and tapped the flat surface in the arbor assembly to match the three holes in the right end. The two holes on the left end are placed to thread in screws to mount to blade itself.
A test fit showed everything to work properly:
I then removed the bracket and attached it to the knife using a couple of nuts as spacers to bring the knife directly behind the blade:
I was then able to install the full assembly and test the movement:
I am happy with the movement overall, of course I will need to finish the second knife before I can test it, but since I won’t need to adjust the thickness of the new knife, I should be able to finish is pretty quickly.
I may also revisit the knife length. You can see in the video that the blade comes up before the knife, and if only parallel to to the top of the blade when the blade is at full height. This means that the knife will not be present in shallow cuts. This is not a problem on newer saws because they are designed with a fairly complex system of linear slides that keep the riving knife in the same position relative to the blade.
Since my saw raises the blade by swinging it along a radius, and my riving knife if effectively an extension of the arbor assembly, it is moving on a larger radius, so each degree of movement is exaggerated in the riving knife relative to the blade.
I can resolve this my making the riving knife longer, but with the consequence that the riving knife will be over the top of the blade when the blade is at full depth. I haven’t decided which problem is worse, so I’ll have to update you with the results of further experiments.
Video of the full process below: