Riving knife - how wide? - Woodworking Talk - Woodworkers Forum
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post #1 of 11 Old 09-21-2020, 09:59 AM Thread Starter
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Riving knife - how wide?

Making a riving knife for my Delta hybrid TS and there is limited clearance from motor mount at lowest blade height. Probably can't make 1 1/2" wide RK.

How wide is your riving knife?

What would be minimum width for a riving knife to be effective if mounted 1/8" behind the blade?

Thanks

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post #2 of 11 Old 09-21-2020, 12:37 PM
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wider is better ......

Obviously, you can't make any wider than the insert will allow or it won't be able to move up and down. I've seen then rather slim and as wide as the insert will allow. My insert has the slot right out the back end to allow for the wider version:
https://www.google.com/search?q=rivi...h=722&dpr=1.25

Well, I stand corrected, at least partially. I have a splitter on my 22124 Craftsman Hybrid, not a riving knife, but the additional entry slot is what I said, and extends off the back end of the insert. I leave this saw without a splitter for operations that require a stopped kerf or rabbets. My splitters on my main saw are such a pain to realign that they stay on them all the time.

I used the hybrid without a splitter for these table leg braces:
https://www.woodworkingtalk.com/f2/l...allenge-33352/

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

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post #3 of 11 Old 09-21-2020, 04:35 PM
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the purpose of a splitter / riving knife is to prevent the kerf from collapsing on the blade and potentially resulting in kickback.


there is little benefit to having 'the thing' extend inches and inches past the blade. actually, there's downsides to that....


that dimension is merely a matter a mechanical strength - so adapt to the physical limits of your mechanism.
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post #4 of 11 Old 09-21-2020, 06:47 PM
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I think of the riving knife doing two different jobs:

1. As @TomCT2 puts it, "... prevent the kerf from collapsing on the blade ..." Sure, there may be some twisting action, but I like to think of this as the freshly cut wood "pinching" together, like tweezers.

The riving knife/splitter must be thinner than the kerf, or the cut wood would bump into it during normal cutting. Here is the rub with that (pun intended): If the riving knife is thinner than the kerf, then why doesn't the blade pinch the blade anyway? My response is: It does, but the riving knife limits how much "pinch" there is, so that you can retain control of the workpiece if it pinches. I like to think of the freshly cut wood as straight tweezers pinching together. The shorter the riving knife, then the greater the potential "tweezer angle" and the tighter the pinch around the blade and more wood in the rising blade path. A longer riving knife is better, because the "tweezer angle" will be less (straighter).
(... And yes, the end of the board might distort so much that it pinches the front of the riving knife. There is only so much that a riving knife can do.)

2. The other thing that a riving knife does is prevent you from turning more wood into the back of the blade, especially after the workpiece is cut apart. Imagine a push block on the right side of the blade, after the workpiece has cleared the front of the blade and is cut apart. Without a riving knife, you could easily rotate the workpiece counter-clockwise (anti-clockwise) on the table and give the back of the blade a good "bite" of wood to trigger a kickback. The riving knife help to prevent that, too. Again, the longer the riving knife, the angle is shallower, giving less wood for the back of the blade to bite. (Yeah, sure. Everybody pushes perfectly straight, keeping the wood directly against the fence at all times. Nobody sneezes while cutting.)

My belief is: The longer the riving knife, the better. Obviously you are limited by what will fit in the insert plate hole, and other mechanical limitations.

Last edited by Tool Agnostic; 09-21-2020 at 06:51 PM.
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post #5 of 11 Old 09-21-2020, 11:21 PM Thread Starter
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Quote:
Originally Posted by Tool Agnostic View Post
I think of the riving knife doing two different jobs:

1. As @TomCT2 puts it, "... prevent the kerf from collapsing on the blade ..." Sure, there may be some twisting action, but I like to think of this as the freshly cut wood "pinching" together, like tweezers.

The riving knife/splitter must be thinner than the kerf, or the cut wood would bump into it during normal cutting. Here is the rub with that (pun intended): If the riving knife is thinner than the kerf, then why doesn't the blade pinch the blade anyway? My response is: It does, but the riving knife limits how much "pinch" there is, so that you can retain control of the workpiece if it pinches.

2. The other thing that a riving knife does is prevent you from turning more wood into the back of the blade, especially after the workpiece is cut apart. Imagine a push block on the right side of the blade, after the workpiece has cleared the front of the blade and is cut apart. Without a riving knife, you could easily rotate the workpiece counter-clockwise (anti-clockwise) on the table and give the back of the blade a good "bite" of wood to trigger a kickback. The riving knife help to prevent that, too. Again, the longer the riving knife, the angle is shallower, giving less wood for the back of the blade to bite. (Yeah, sure. Everybody pushes perfectly straight, keeping the wood directly against the fence at all times. Nobody sneezes while cutting.)

My belief is: The longer the riving knife, the better. Obviously you are limited by what will fit in the insert plate hole, and other mechanical limitations.
1. I plan to make the RK blade 0.115" thick, just 0.010" less than the kerf.
2. This one is why am very much focused on this project. I suffered my first kickback in 60 years because I was momentarily distracted, looked leftward and naturally my right hand followed my eyes just enough to put the piece into the back teeth.
3. So it appears that the consensus is that longer/wider is better (up to a point ), but anything is better than nothing.
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post #6 of 11 Old 09-22-2020, 01:07 AM
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wider is better ...

I have come to this conclusion over many years of using the older splitters that came with my Craftsman 10" table saws. Those saws had the blade guard and anti kickback pawls mounted to the splitter plate which was about 6" to 7" long from the rear of the blade to the rear of the splitter. This wide plate/plane keeps the work piece that's between the blade and fence from moving any direction except rearward from the feeding pressure. Without a splitter in place, any movement of the workpiece to the left, off the fence will cause it it to ride up the spinning rear teeth and get propelled right at the operator, a dreaded kickback.

It's other primary function is the keep the newly created saw kerf from pinching the rear of the blade and possible rising up and over it being forcibly and rapidly propelled towards the operator. When the kerf is pinched sufficiently the saw may stall and you will need both hands to control the workpiece. This means that you ON/OFF switch needs to be in a location you can bump it OFF with you hip or upper legs safely. I routinely turn my saw off in this manner even when there is no threat of kickback, it's just really handy and I never need to look under the saw for my OFF switch.


I have been advocating both of these concepts for about 15 years right here on this forum in my posts about table saws. This is not a new idea to me, I reinstalled the splitters after years of kickbacks and they were a great place to support an over the blade dust collection hose. The riving knife is a more modern device that moving in elevation right along with the blade. Although not as wide as a splitter, they will still perform those two basic functions of preventing "rotational" kickback and keeping the kerf open under "tension" wood. Plywood does not move in the way, closing on the kerf, but large panels ARE prone to move off the fence during a pass and will kick back. I speak from experience here!


The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #7 of 11 Old 09-22-2020, 11:37 AM
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My saw came with a riving knife that is 0.100 inches. It is intended to be used with a 0.125 kerf blade. Thin kerf blades would need a thinner riving knife. I would assume you should try to make it 0.025 inches less than the kerf.
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post #8 of 11 Old 09-22-2020, 01:33 PM
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I'm amused at the statements that plywood doesn't have the kickback issue.
the one and only time I've been wammied by kickback was a 1/2" piece of plywood.
so - plywood kicks back, believe it.
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post #9 of 11 Old 09-22-2020, 01:48 PM Thread Starter
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When I wrote, "probably can't make it 1 1/2" wide", I did not mean thickness but rather width of the RK.

How wide must it be to be useful was my question. I'm limited to 1 1/4", maybe could manage 1 3/8" wide.

How wide is your RK?

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post #10 of 11 Old 09-22-2020, 01:55 PM
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That's not what I said re kickback!

Quote:
Originally Posted by TomCT2 View Post
I'm amused at the statements that plywood doesn't have the kickback issue.
the one and only time I've been wammied by kickback was a 1/2" piece of plywood.
so - plywood kicks back, believe it.

Quoting from post number 6:
Plywood does not move in that way, closing on the kerf, but large panels ARE prone to move off the fence during a pass and will kick back. I speak from experience here!


The other function of the RK is to prevent blade pinching. Plywood will not pinch your blade BUT it will kickback as a result of moving off the fence just like any other material.


The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 09-22-2020 at 03:02 PM.
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post #11 of 11 Old 09-22-2020, 04:03 PM
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much more longer - but it also serves at the mounting for the guard/kickbackpaws....

don't forget to grind a 'knife edge' on the leading edge and taper the top point as well.
most all blades have/develop a bit of run-out - so the actual kerf will be a smidge wider than what is stated on the blade.


Riving knife - how wide?-dsc_4864s.jpg
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