Grinding Cast Iron

[Bruce Thompson]

Finally ready to start making a riving knife for my old saw. Forrest 10" blade with 1/8" kerf. RK blade will be 0.109" thick and sickle-shaped, with leading edge 3/16" behind saw blade. Clearance from the saw blade to this motor mount casting limits width/depth of the RK blade to 1 1/8". I would feel better with more steel behind my saw blade to prevent pinching, so I'm thinking of grinding a 1/4" wide x 1/4" deep notch in the casting right where the blue tape is. That would enable clearance for a 1 3/8" RK; 22% bigger.

IMG_3520.jpg

The highest point where the blue tape is, makes about a 120º angle, with the motor mount, barely visible to the right. Minimum thickness of the casting seems to be about 1/4" or a bit more. The back of the high point of the casting is curved in what feels like maybe a 1 1/4" arc (can't see it). About equidistant from the center (blue tape) and the motor bolts (one is barely visible), on the back side, are a pair of 1/4" thick webs, about 2 1/2" deep at the center, spanning edge to edge of the inside angle of the casting.

My intuitive sense is that such a notch would not materially affect the strength of the casting.


What do you guys think?

[Bill Dufour]

I do not see any tape of any color. 1+inch seems plenty big to me. A steel adjustable square ruler is 3/4 to one inch wide. As I understand the geometry there is no real twisting force on the RK. It just keeps the wood parallel to the blade so it can not grab and get twisted by the motor. I would do it as is and make a bigger RK and the groove if needed later.
Bil lD

[Bradley Gray]

I agree with Bill - 1 1/8" is plenty wide

I use a plastic splitter that mounts in the insert and have also seen a steel pin used -not a lot of twisting force.

[Chuck Saunders]

Cast iron is not a notch sensitive material and the reinforcing ribs are outside of your intended area. Strength wise you should be fine. A deeper thicker RK will give a closing cut more to grab while both sizes prevent the wood clamping onto the blade.
Chuck

[Jim Becker]

I do not see any tape of any color.

There's a piece of blue painter's tape right on the "square" part of the casting in the photo...

[Matt Day]

I say go for it.

And Bill, you’re having some computer issues!

[Brian Holcombe]

Don’t cut any ribs on cast iron, sure path to warpage.

[Bill Dufour]

I say go for it.

And Bill, you’re having some computer issues!

The picture is too large. I had to click on it and it shrank to fit the window. That exposed the tape. The picture I was seeing was only about 50% of the image.
Now that I see the tape there is no way I would cut that rib. It may warp the casting and that may not stay stable after it warps. Meaning you will have to tweak the mount after every temperature change. It should probably also be checked after every cut to see if forces have tweaked the casting. cast iron can move a lot more then most people think. Not nearly as flexable as glass but they both fail the same way. fast and completely not bent.
Bil lD

[Erik Loza]

No opinion on whether or not you should do it but if you do, I would make sure to tape off any bearing or articulated joint inside the cabinet. If metal filing get into the arbor bearings, you will have an very un-fun project. Good luck, whatever you decide to do.

Erik

[Bruce Thompson]

Cast iron is not a notch sensitive material and the reinforcing ribs are outside of your intended area. Strength wise you should be fine. A deeper thicker RK will give a closing cut more to grab while both sizes prevent the wood clamping onto the blade.
Chuck

By giving a closing cut more to grab, are you suggesting that deeper RK would bind more with pinching wood? My 0.109" thick RK (cut from 12ga sheet) will be 0.016" less than the kerf; so 0.008" less on each side.

[Bruce Thompson]

Thanks for the tip.

[Bruce Thompson]

"…-not a lot of twisting force."

Thanks, but I was concerned more with burning if two sides of the cut pinch together. Since the RK will be 0.016" thinner than the kerf, a deeper blade would be more effective.

[Bruce Thompson]

Stupid me, I forgot that I could look in from the cabinet access door for a side view.

The part of the casting where I would grind is 5/8" thick (blue tape is just visible in the background), so 1/4" grind would leave 3/8" undisturbed. The vertical part where the motor mounts is 3/8" and the two re-inforcing webs are also 3/8" thick (the back web curves out like that to anchor a spring on the motor mount pivot).

IMG_3524.jpg

[Chuck Saunders]

By giving a closing cut more to grab, are you suggesting that deeper RK would bind more with pinching wood? My 0.109" thick RK (cut from 12ga sheet) will be 0.016" less than the kerf; so 0.008" less on each side.

Well, yes. But since the RK is not spinning like the blade there is no risk of increasing kickback, that's the good stuff. usually as the wood closes on the knife it continues to slide out the back. Sometimes it grabs hard and it ain't moving, but it's not grabbing the blade

[glenn bradley]

Ribs cast into cast iron are strengthening areas. Can you notch the RK instead?