Thread: Riving Knife rigidity question

If I interpret your description correctly, the knife you intend using is thicker than the saw kerf, so what ever you try to rip will bind on the knife
Correct me if my interpretation is wrong.

Are you able to get some Pics?

Cheers Matt.

Originally Posted by rwbuild

If I interpret your description correctly, the knife you intend using is thicker than the saw kerf, so what ever you try to rip will bind on the knife
Correct me if my interpretation is wrong.

Suspect he's saying the saw plate is 1.8mm so the kerf is likely to be 2.4mm. Pretty thin for a 12" blade.

Thanks for the replies, gentlemen.

The saw is an old Masport cabinet saw, which I got for free. The saw seems to be ok, but needs adjustment in pretty much every aspect, and I am nearly done with this.
Because I am new to using a table saw, and I want to enjoy all fingers for the rest of my life, I am scrutinizing the internet to learn as much as possible and also to make the saw as safe as possible.
I learned about the importance of a riving knife, but the supplied riving knife is short and only useful, when the blade is fully, or nearly fully, sticking out of the table. When the saw blade is only sticking out, let's say 3..4 cm, it is not there.

My DIY riving knife is much longer and has slots instead of holes for the screws, that way I can adjust it if needed.

IMG_20220113_094136.JPG


This picture shows my DIY riving knife in the setting when I am going to rip thin stock or plywood.

IMG_20220113_094223.JPG

This picture shows the blade and the DIY riving knife fully with the original riving knife in front of it.
If I want to make a cut that is not completely going through, I will have to loosen the screws of the riving knife a bit and slide it down until it is just beneath the top of the saw blade.

Regarding the thickness of the blade:
For working with delicate veneer ply or other thin stock material I plan to use the DX12100X, for everything else the DX1260X. The kerf is ~2.5mm for the DX1200X, and ~2.8mm for the DX1260X, which looks as if a riving knife thickness of 2mm is perfect. I just must never try to mount a blade where the blade itself is >= 2 mm thick...

Back to the original question,one can imagine, when this long riving knife is touched from the side, it will swing - the old, short one will obviously not, but that thing is not an option.

To my understanding the riving knife shall prevent binding of cut wood to the blade, and the DIY riving knife will do this, because the riving knife is thicker than the saw blade.
But it is necessary for the riving knife to be rock solid to prevent kickback? If yes, I have a problem, because the riving knife cannot be larger - if would not fit past the cast iron table (in fact, I already took of ~2mm material to make this one fit...).

Cheers
Ivo

The riving knife only has to be just higher than the highest tooth. You could remove 30% of the material from your device which would greatly reduce the flexibility

My homemade riving knives are just stubs about 3/4" high.

Measure saw plate thickness, measure kerf (blade) width, and riving knife should be the average of these two measurements (i.e. midway between the two). Definitely NOT wider than the sawblade kerf.

Originally Posted by Greg Q

The riving knife only has to be just higher than the highest tooth. You could remove 30% of the material from your device which would greatly reduce the flexibility

The only difference between the two pictures is that in the second one I cranked the saw blade all the way up. If I would shorten the riving knife so that it is just higher than the highest tooth in the second picture, it would not be visible in the first picture, which is my starting point of the whole exercise.

The slots in the DIY riving knife are there to enable me sliding the knife back a bit so that it is just a tad lower than the blade, when I want to not cut through a board (or use a cross cut sled, for example).

The removal of the riving knife is a pain, two M8 bolts and washers and a spacer plate, not something that you want to do for every second cut. So my idea was, to leave the riving knife as is when ripping...

Cheers
Ivo

Originally Posted by Mr Brush

My homemade riving knives are just stubs about 3/4" high.

Measure saw plate thickness, measure kerf (blade) width, and riving knife should be the average of these two measurements (i.e. midway between the two). Definitely NOT wider than the sawblade kerf.

Wait, what? 3/4" is 18..19mm, you mean, this is all it is protruding out of the table? Does it move up or down when you raise or lower the blade?

8F951662-6DFA-42ED-9A94-8D0CD02911AD.jpegThat knife is fine, it’s similar to the one on my Martin slider and it has worked perfectly for 45 years. Just make up a timber guard to go on top of it, that’s all the old man put on the saws he used to build, sold almost 100 and never an issue.

Originally Posted by Hanilein

The only difference between the two pictures is that in the second one I cranked the saw blade all the way up. If I would shorten the riving knife so that it is just higher than the highest tooth in the second picture, it would not be visible in the first picture, which is my starting point of the whole exercise.

The slots in the DIY riving knife are there to enable me sliding the knife back a bit so that it is just a tad lower than the blade, when I want to not cut through a board (or use a cross cut sled, for example).

The removal of the riving knife is a pain, two M8 bolts and washers and a spacer plate, not something that you want to do for every second cut. So my idea was, to leave the riving knife as is when ripping...

Cheers
Ivo

you dont need a riving knife at all if you’re not cutting all the way through the thickness of the board as the kerf will be held open and not close on the back side of the blade, which is what a riving knife is there to prevent.

Originally Posted by riverbuilder

you dont need a riving knife at all if you’re not cutting all the way through the thickness of the board as the kerf will be held open and not close on the back side of the blade, which is what a riving knife is there to prevent.

Yepp, I know. The problem with my saw is that it is a PITA to remove the knife completely, it is simply not designed for that. But to loosen the screws a bit and just slide it down or up is not a big problem, hence my construction.

So that timber guard is then stopping anything from falling into the blade, right?

I am astonished that the knife doesn’t raise/lower exactly with the blade, and don’t understand why they designed it that way. So in your two pictures the knife raised that much more than the blade without you adjusting it? In which case, good work. I think your springy knife is a function of its thickness. Tool steel is after all pretty much spring steel.

Thanks Greg - yes, that was astonishing for me too. The rotating point of the blade is the motor, which sits a bit towards the front end in respect to the blade. The blade is driven by a belt, and the mounting bar for the riving knife sits on the opposite side of the blade as the motor, which makes the knife move about twice the way the blade does.

It's an old design, I suppose...

The long slots in your knife worry me.

Not long ago there was post from a member who was using slotted riving knife and it struck the blade and the blade tore it out of the slots and fired it at the member and managed to embed it into his shoulder. I would remake the knife with holes and locate it at an average position and live with it.