What sort of steel for a riving knife?

[Alpha-Dave]

Thank you for looking.

I have bought a flat-top blade for my table saw so that I can make box joints, but the riving knife is designed to support an overhead guard an therefore extends above the blade, and I need to make a shorter one. I picked up some 2mm mild steel plate from my local merchant and have cut it to the size and shape I need with an angle grinder and files.

Now when I compare the original and the new one, I can feel that the mild steel one has a lot more flexibility (they are both 2 mm), if I put my thumbs together in the middle as a fulcrum and pull the edges the mild steel moves a mm or 2 while the original barely shifts.

I therefore assume that the original was made from harder steel.

So 2x questions:
1) will using the mild steel make a difference? (I suspect the answer is ‘maybe’: it won’t make a difference in normal use, but if there was a catch then who knows)
2) what other grade of metal would be more suitable? Does it need to be hardened? I suspect that it should not be so hard as to be brittle.

Thanks in advance!

 

[Chrispy]

Not made one myself but if i do I'll cut up an old saw plate.

 

[LancsRick]

The riving knife doesn't need to take any horizontal loading so personally I wouldn't worry about it. The important attribute is the thickness relative to the blade kerf. If you find the riving knife flexes in use then I'd check your fence as it will be out of parallel with the blade.

 

[flh801978]

If all you are using the new riving blade for is box joints then really it won’t be doing anything so your mild steel one is fine.
If you want better material search for ground flat stock or gauge plate.

IAN

 

[Steve Maskery]

You don't need a RK at all for box joints. A RK prevents the wood from pinching on the back of the blade when it is separated into two. That doesn't happen when just cutting a notch. So yo can ditch the RK altogether.
The bigger issue here is how are you going to guard the blade? I sincerely hope that you are not intending to use it unguarded. You will be leaning over the blade more with a BJ jig then when doing more conventional ripping. My BJ jig is (well, rather was, I don't have it any more, I'll need to rebuild it next time I want BJs) used on the router table rather than the TS and has the guard built in so that I can't touch the cutter. You can see it here.
On the broader note of what kind of steel from which to make a RK, mine is made from gauge plate. It's not that it is harder, but it is flat and stays flat. Mild steel is all too easy to distort, especially when cutting it, and if it is bent it is just as dangerous as not having one at all, as the workpiece will catch and the temptation is then to force it.

 

[AES]

I don't even own a TS (or anything similar) but I THINK (don't know) that in theory anyway, the purpose of the RK is "simply" to keep the 2 halves of the cut apart as the blade passes through the wood. If so, then IN THEORY, any old bit of steel (or even ali) would do. BUT, in practice, just as Steve has said, I can see the RK receiving enough side pressures that a piece of MS only 2 mm thick would easily get bent - and ali even more so!!!!

So personally I'd 2nd Steve's idea of a piece of gauge plate. It's not hard to cut (with a decent hack saw blade, or a thin cut off disc), and you should be able to find it in any model engineering supplier. Have a look at the sticky covering suppliers at the top of this section for a range of possibilities.

 

[SammyQ]

The riving knife doesn't need to take any horizontal loading so personally I wouldn't worry about it

Lansrick M8, NO! It CAN have lateral thrust ("horizontal") put upon it by wood that is twisting as forces are released by cutting. Stiff as a Puritan's code of ethics is what you want in a riving knife! See the suggestions above and for the sake of the late J.C., please avoid monkey metal or biscuit tins!

I cut mine from an old Wadkin 12" saw blade well past its best and impossible to sharpen any more. Good gear.

Sam, now breathing slowly again....

 

[deema]

First you need to decide what sort of blades you will be using. There are a few types, but mainly used are those with a fairly standard Kerf of 3mm or thin Kerf that can be 2~2.5mm. Typically most people will use blades with a 3mm kerf. The kerf is the width of the slot that the blade cuts.

The teeth of the blade are mounted on a disc, the disc is thinner than the kerf normally by c0.5mm. So a 3mm kerf typically has a disc of 2.5mm. The sizes are normally engraved on the blade. You need to make any riving knife with a thickness slightly more than the disc thickness and less than the kerf. Typically a riving knife is 2.7mm thick. (3mm kerf / 2,5mm disc) Steel plate has a tolerance of +/-10% on nominal thickness. I’ve very rarely come across plate rolled thick, it’s usually nominal and down. When rolling steel they start the roll and then pinch in the rollers to get to the size.....which means they roll on the thin size unless you get a bit at the start of the roll. I usually use cold rolled steel 3mm nominal which comes out at c2.7~2.9mm typically for riving knives.

An old blade may seem a good idea as a source of steel, however these are normally tempered and on the better ones have a tensioning ring to help keep the blade flat. These are usually hopeless for making into riving knives as they distort as soon as they are cut. Equally they are only the thickness of the disc which is too thin.

 

[SammyQ]

Mine was a very old one Deema, pre-carbide. Didn't distort on being cut. Guess I got lucky.
Sam

 

[deema]

I’d forgotten about the old ones pre carbide, they are often brilliant for riving knives as they are usually the right thickness and good quality steel without a tempered ring.

 

[sammy.se]

I'm curious as to why the riving knife should be thinner than the kerf, shouldn't it be just as wide the kerf to avoid the work piece pinching on the teeth?

Sent from my Nexus 5X using Tapatalk

 

[Rorschach]

I'm curious as to why the riving knife should be thinner than the kerf, shouldn't it be just as wide the kerf to avoid the work piece pinching on the teeth?

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Cutting your tolerances a bit fine there, if the riving knife isn't perfectly positioned it will interfere.

From what I have been told (and measured) it should be less than the kerf but wider than the plate.

 

[Bodgers]

I'm curious as to why the riving knife should be thinner than the kerf, shouldn't it be just as wide the kerf to avoid the work piece pinching on the teeth?

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The reason is that, if the riving knife is the same or greater than the kerf width, it will be difficult or impossible to push the work piece through the knife.

If the knife is narrower than the plate/body of the blade, it won't do the job of separating the wood (if the wood starts pinching) to make space of the spinning blade body; the blade will get pinched, and the riving knife will be useless.

 

[memzey]

I made one a couple of years ago out of stainless. It’s definitely more resistant to warping and pressure than mild steel. A lot harder to work though!