Riving Knife Problem

[Arry]

I recently bought a new Table Saw which came with a think kerf blade. I then replaced the blade with a FLAI U blade.

The riving knife is also meant for a thin kerf blade however the FLAI blade burns all the way down the wood when ripping.

The riving knife seems to be straight yet it still burns.

What I want to do is to say make a duplicate riving knife but only say, half the thickness of the current one, so say 1.7mm thick.

Questions: 1. Would it matter if the riving knife is that thin

2. Where could I go to get one made?

Cheers

[Arry]

no one :no:

[HazzaB]

Hey Arry,

any sheetmetal shop should be able to cut you a new Riving Knife, the flasher joints might even lazer cut or have a plasma cutter, I did one for my BIL using a Jigsaw with a metal cutting blade and some polishing with a flap disc, the biggest problem is getting it mounted in the centre of the cut, some spacers may need to be cut, it's not as big a job as you think.

HazzaB

[Avery]

I thought that the Flai blade was fairly thick (over 3mm ?). I don't see how a thin riving knife could cause burning.

I am really just a beginner with table saws so don't take my comments as anything but inquisitive.

I would be looking to make sure that the knife was in the centre of the cut and more importantly the blade was parallel to the mitre slot and the fence was parallel to the blade.
I really cannot see how a thin knife could cause burning.

I am very prepared to be educated here.

[fletty]

Arry,
Something is wrong! There's probably a reluctance to reply because of OHS issues but I don't think the burning has anything to do with the riving knife. Approach this logically and I would suggest the following;

measure the thickness of the blade and the riving knife (the knife should be equal to or a poofteenth thinner than the blade)
measure the distance from a blade tooth on the front of the blade to the fence and a tooth on the back of the blade to the fence to check if the blade and fence are parallel (this is where I think the problem is!)
measure the distance from the teeth on the blade to the fence and the inside face of the riving knife to the fence (they should be equal even if the riving knife is much thinner than the blade)
My guess is that the blade and fence are not parallel. If so the timber is being 'squeezed' between the blade and fence and the rubbing on the disk is burning the timber. The riving knife is only a passenger if this is the case.

fletty

[Mr Brush]

If the riving knife is too thin for the blade in use, then that's precisely what WILL cause burning.

A thinner riving knife allows the timber to close up behind the blade (which is what the riving knife is there to prevent), so there will be interference between the back of the blade and (probably) both sides of the cut.

For correct operation, measure the thickness of the body of your blade (i.e. where there are no teeth), then measure the thickness at the teeth (i.e. the kerf). Ideally your riving knife/splitter should be around the average of these two measurements. For example, one of my combo blades measures 2mm thick at the body of the blade and has a 3mm kerf, so the splitter is 2.5mm.

A 1.7mm riving knife sounds much too thin for a blade with 3mm kerf, so I'd say that's your problem.

[Avery]

OK Mr Brush

As I said I need to be educated here.

I don't understand why what you say is true and I would not dare to disagree, but (always a but)

On my Triton MK3, the blade has a 2.8mm kerf, the knife is under 2mm. I never get any burning unless the fence is not set properly.

On my H&F table saw , with a fine crosscut blade with 3mm kerf and a riving knive that measures 1.8mm, I have never observed any burning. Oh, and I do use this blade for ripping.

I'm afraid I just cannot see the mechanics of a thin riving knife causing burning of the stock.

[mic-d]

Arry your riving knife may not be the problem, wood doesn't always close up behind a cut. Try this, if you don't have measuring tools to check your fence: Get a piece of plywood and cut off a strip say 100mm wide using crosscut mode ie your sliding table, crosscut sled etc that you usually use. Does it burn? Then cut off another 100mm strip using the rip fence. Does it burn?
I'm guessing the crosscut will not burn but the rip cut will. If that's the case, you've eliminated the riving knife as the problem. As Fletty says, the fence is probably not parallel to the sawblade.

[thompy]

Avery, the riving can be smaller than the blade kerf, but where it is in relation to the centerline of the sawblade matters a ton.

Matched is best from memory, where the riving matches the kerf cut, centerline of kerf to centreline of the riving. if you swap in a different width blade/s the centres change. On your triton the riving is probably on the (away from fence) centreline of the blade, same for the H&F.

Offset to fence side, bad, will cause burning, as the wood can close back to the blade after the cut, better than no riving, but not the best, pushes the wood into the fence, like trying to force it into a funnel.

Offset to away from fence side, it will stop the burn,and "rive" like its supposed to, but it'll potentially "draw" the cut away from the fence.

I'm definatly no expert on the matter, but i was taught it was the centrelines that were important first up, then the fence to edges adjusted by packing. So to quote a bad analogy, it's not the size that matters, its where it is that does.

Neal.

[Avery]

Yep Thompy,

That sounds all OK with me.

The width of the knife is not so important as its position relative to the blade. If the riving knife is absolutely central to the blade , whatever its width, providing that the blade is thicker than the knife, then there should be no problem, certainly no burning of the stock..


I still do not see how the riving knife, no matter how badly adjusted, can cause burning of the stock. Only the spinning blade can burn the timber and that can only happen if the timber is pressured between the fence and the blade.

please correct me if i am wrong

[thompy]

yeap, the knife wont burn it but the effect it has as a rearside guide perhaps might, i think if the riving is outside the kerf either way it'll draw the cut into the front side of the blade one way or another, kind of like warping it and burning the timber, i'm not talking snap the blade type pressure (perhaps i am to illustrate) but as best efforts guide the wood straight in, the riving acts like a boat rudder from the back side, that has effects on the front side of the blade.

Kind of like using an old saw without a knife or guard in place, or the way a bandsaw blade flexes on the rear edge of the blade if the feedrate is too much. I'm not advocating people should go try to burn timber this way, but i rekon a setup thats properly adjusted shouldn't burn.

If the timber burns due to poor feedrate or wrong tpi or worn teeth or lateral movement of the feed then its a user fault. Just saying a badly adjusted knife can have similar effects.

Neal.

PS: i know what i mean but i'm not sure if i'm expressing this the correct way so others do. If you setup a knife to the blade centreline and keep it parrallel to the fence, you should be set.

[mic-d]

As long as the knife is within the bounds of the saw it will be OK. If it's out at the front it will catch on the timber, if it runs out at the back it may cause jamming and burning in both rip and crosscut. Since the problem is only with ripping, it is highly likely that the source of the trouble is the rip fence. As soon as it is even slightly closed up at the back relative to the front of the saw it will burn. When ripping, ideally the fence should only extend to the midpoint of the blade or a little before as an anti-kickback measure, but it also helps stop burning, best to check and adjust the fence as necessary for starters though.

[Arry]

Cheers for the responses guys, I probably need to explain myself better..

Basically at the moment the riving knife and the kerf of the blade are approx the same thickness...say 3mm.

I am wanting to reduce the riving knife thickness to say 1.7mm or thereabouts.

Basically no matter how hard I try as I pass the timber passed the blade it seems to either hit the riving knife on the left side or the right side.

This causes the timber to angle slightly into the fence, and the back of the timber is sqeezed into the blade, ie it makes pushing it through very difficult. thus causing burning ...well this is how it seems to me.

I want a thinner knife so that the timber does not touch it as such but is there to stop the it coming back into the blade, thus preventing kickback.

You can also see by my incra square that the blade is perfectly aligned ....

I have read your comments with interest, the thing I am trying to undertand is how is a thin knife a problem?

I have attached photos to show what I mean....I have exaggereated what I think is happening with the wood as it comes into contact with the riving knife and thus burning the wood all the way down.

[Avery]

It is very difficult to guess what is going on , but photo 2 seems to show that the blade is not parallel and the riving knife is quite a bit to the left of the blade. This may be just difficulties with taking photos of this sort of stuff but if the blade is angle that way then it could certainly cause the burning that you describe.

If the blade was angled in at the back, you would hear it recutting as the stock moved through. On flat pieces, say 3mm ply you would see it lifting off the table at the back of the blade

What happens if you put the fence on the other side of the blade?


How about removing the knife and making some experimental cuts? I know you shouldn't do it but just be aware of what is going on and there will be no problem. If it still burns without the knife you will know that the problem is with the fence and blade.

[Petethebutcher]

I'm with you Avery, isolate the problem by removing the suspect.Take the knife off and see what happens although if it is as thick as the kerf as Arry has indicated and not dead centre you need a new knife.
My old table saw does'nt have a riving knife and I have trouble burning thick ( say 40mm) hardwood when I'm too impatient to change my saw blade to a course tooth blade:doh:.
Cheers Peter

[Arry]

Cheers guys, good idea, I will try it without the riving knife.....I just won't stand in the firing line :-)