Thread: riving knife height.

G'day Zed,
I went through similar problems with my table saw, and replaced the riving knife with a drill bit lined up with the kerf into a zero clearance insert .
I found the curved shape of the knife was not consistent with the blade profile at different depths of cut, and for some reason stock climbed up the knife. (dunno why)
I first tried a drill bit the same diameter of the kerf width.........not good.......so drilled a new hole with the next size drill bit down. Worked great.
The only adjustments I make to the height of the "knife" is when I'm using the table saw to groove or rebate. This I do by screwing the bit further down, and then to revert...screw it up.
Dunno if this helps you....but it worked for me.

Originally Posted by Zed

Hi Guys,

Did a search but cant find a specific answer. so....

well... after to much pontificating and dicking about I finally bought a table saw. Got a MBS300 from Major woodworking in Padstow. Great mob. Great deal, great saw.

The saw was easy to set up and works very welll out of the box so to speak... HOWever (theres always one of them)...

I had a bit of cedar that was about 130mm wide that I needed to rip. the max height is about 105mm so i was gunna do it in 2 halfway passes. No problem I thiought - started the cut and noticed tha halfway thru the cut the timber started to rise up for some reason... I thought it was about to kick back so hit the emergency stop and stopped the cut. whenthe blade stopped I had a close look and noticed that the riving knife was ABOVE the height of the sawblade. I thought, ok its set too high... so I tried to lower the riving knife so it was below the height of the blade and about 10mm behind the circle of the saw. it was however at the bottom of its adjustment - Ie it could NOT be lowered any further... so I removed it and extended the groove the nuts sit in to make it adjust lower..... this I thought fixed the problem but I noticed that the back of the riving knife was fouling on the backstop so I hit it with teh grinder to remove a few mm so it wouild rise and fall clear of the backstop as I rasied and lowered the blade. all good I thought - got the verniers an measured tht it would a) not foul and b) be lower than the saw itself... I measured at the mioddle of the cut height range... proceded with cut and bobs yer uncle...

Zed, I have a TS10L tablesaw, a PM2000 clone, with a riving knife, but the guard is attached to this riving knife. So for captured cuts like you are talking about, I had to make my own riving knife, see here.

You will notice, that no matter what the the height of the blade, the riving knife sits just below it, I thought that all saws were like this.

Originally Posted by Zed

I did another check afterwards and noticed that the riving knife at the very bottom of the height adjustment was below the surface of the table and at the top of the cut height it was substantially higher than the sawblade itself. this seems to be as a result of the shape of the knife and the swing forwards backwards as the balde is adjusted up/down.

That doesn't make sense, so it is either by design, or somehow the riving knife is not installed properly.

Originally Posted by Zed

now my questions are :

1) has anyone else encounted this phenomenon ?

No, I haven't.

Originally Posted by Zed

2) does the riving knife in fact have to be lower and behind the blade ? ( I would think yes)

yes and yes IMHO

Originally Posted by Zed

3) do I need to adjsust the bastard every time I raise or lower the blade ? (I friggen hope not)

Seems silly if you have to do that

Originally Posted by Zed

4) can I narrow the top of the riving knife so the hump at the back of it is a clean cresent shape (like a dolphin's flipper rather than a ghurka's killing knife?).

I followed most of the shape of the original riving knife, but omiited the back part where the guard attaches.

Originally Posted by Zed

5) will the adjustment I speak about in Q4 above afect the performance ? or is the performance PURELY on the width of the knife rather than the shape of the wedge so to speak ?

Apprecaite answers to this concern - I dont want a kickback or compromise saftey.

ta

I would think mostly on the width (thickness) of the knife, with the proviso that it should extend back a reaonable distance to provide some material to stop kickback.

Just my 2c (well maybe 5c!)

Zed, I have a similar saw (12") with a Leda badge. At 2.5" the knife is higher than the blade so when I need to cut that deep I remove the knife. You are at greatest risk of kickback when the blade is low, so the risk is less when you have a large amount of blade exposed. Mind you, that much blade is intimidating by itself.

Another option is to make two passes - bottom first then sever by flipping and finishing the cut.

Never stand behind the timber inline with it, if it does kickback then you might have troubles. But if you are off to one side kickback is harmless.

In Europe the riving knife supplied by manufacturers must higher than the blade teeth at all times.
Otherwise it will not get a CE label.
Big trouble for any industrial user if he allows staff to use a saw without this certificate.

Hobbyists in Europe alter the machined slot of the riving knive so that this can drop below teeth height for tenon, rebate and double resawing cuts, without the blade guard fitted as well.

Perhaps Australia are going down that path.

Jon, do you know the reason why the knife must be higher than the blade?

Originally Posted by Wongo

Jon, do you know the reason why the knife must be higher than the blade?

I thought it was to stop the timber lifting over the teeth and creating a kickback.

next reason if the knife is over the top of the blade it deters the operative from making two rip cuts.

~The next step likely to come from EU regulations is likely to be that the mitre slot guides will not be allowed. In fact some of the top end T,S. now do not have these guides in Europe.

Originally Posted by jow104

I thought it was to stop the timber lifting over the teeth and creating a kickback.

next reason if the knife is over the top of the blade it defers the operative from making two rip cuts.

~The next step likely to come from EU regulations is likely to be that the mitre slot guides will not be allowed. In fact some of the top end T,S. now do not have these guides in Europe.

Jon, it would be easier for the nanny-state regualtors to regulate that a circular saw blade could not be fitted to these machines! Quickest way I know of to make them safe!

My saw has a riving knife that sits higher than the blade. It comes up and over the blade. This, I understand, protects against kick back by keeping the wood off the back of the blade - all the way up to the top of its arc. It sits a fixed distance from the blade - about 5mm - and travels with the blade. The gap between blade and knife doesn't change at all.

For grooving/dados, I have to remove the riving knife. This is ok, because - as long as your fence is absolutely parallel to the blade, the kerf cannot close up (because it is just a groove/dado) and it can't twist (its against the fence).

On my saw, having removed the riving knife, I've also lost the pawls and the guard.

For all other cuts, you leave the riving knife in place. Still, I understand that in the US it is possible to buy a shorter riving knife for this saw that sits below the top of the blade. I've seen a reference to it in a US forum.

The other option I've got is to remove the riving knife and use a short splitter. I have an MJ splitter that pops into a couple of accurately drilled holes in the back of the zero clearance insert. But I haven't used this yet. I haven't found the need.

Originally Posted by jow104

I thought it was to stop the timber lifting over the teeth and creating a kickback.

My saw has a riving knife that sits higher than the blade. It comes up and over the blade. This, I understand, protects against kick back by keeping the wood off the back of the blade

I still don't get it. Why does it need to be hgiher than the blade. Should't it work when it is level with the blade?

It depends on how the height adjustment mechanism works, but usually they pivot from a point forward of the blade, so as you wind the blade up, the carriage assembly swings in an arc. Because the riving knife is fixed to the carriage assembly, it also swings in an arc, so when the blade is at full height, it's at the top of this arc and over the blade, facing forwards. If you adjust the knife so that it is below the blade at this height, the knife would be well below the top of the blade at or near the bottom of the arc.

My saw must be different from these then, the riving knife goes into two slots behind the blade and travels up and down with the blade, always sitting at the same height relative to the top of the saw. The supplied riving knife as ant-kickback pals and the guard mounted on it, the guard has a pivot so always sits down on the tablesaw top.

The riving knife I made the blade, I now have 2, one for the standard 10" blade and one for the 8" dado blades.

quote Silent C
If you adjust the knife so that it is below the blade at this height, the knife would be well below the top of the blade at or near the bottom of the arc.

end quote


And then when you wind the blade down lower the blade guard fouls the tabletop and and also you have restricted the maximum height you can cut with your saw.
Mine would only cut a depth of 50mm instead of its 85mm maximum

Originally Posted by Big Shed

My saw must be different from these then, the riving knife goes into two slots behind the blade and travels up and down with the blade, always sitting at the same height relative to the top of the saw.

Same here. I set my riving knife about 5mm lower then blade. It doesn't matter what the blade height is. The knife is 5mm lower. Anyway.