Riving Knife vs Blade Kerf Thickness

[JHZR2]

I have a DeWalt 7491 table saw. It has a nice stand, can be put away and out of sight, and it is powerful enough for my needs. It has a riving knife and a very easy means of installing and removing it. Id like to get a "standard" (?, 1/8") flat top grind Ridge Carbide blade (local manufacturer and Ive heard good things about them) blade.

The riving knife on my saw is sized for thin kerf blades. 0.94" or so, IIRC. I cannot seem to find a whole lot of info on riving knife application.

I can certainly understand that you wouldnt want the knife to be wider than the kerf of the blade. Then it would be its own source of binding. But what about thinner? Is there any intrinsic risk or problem with using a 3/32 riving knife for a 1/8" kerf blade? I cannot seem to find any wider ones from dewalt. Id imagine finding 1/8 stock and cutting a knife to the template would be very easy... But necessary????

Any thoughts or opinions?

Thanks!

 

[Astro14]

A properly sized riving knife is thinner than the teeth, but thicker than the body, of the blade to which it's paired. If the 0.94" is the widest riving knife that Dewalt makes for that saw, then it should be OK to use with a 1/8" blade but I would double check the body thickness with a micrometer.

Might I humbly suggest that you stick with a thinner kerf blade on that saw? It's a universal 15 amp motor, right? Not a ton of power. Dewalt likely sized it with a thin kerf (3/32") blade to compensate for its modest power.

Forrest makes superb blades. Extremely fine production tolerances yield a very smooth cut. I've got a couple and for a saw like yours, in general purpose use, the 3/32" Woodworker II 40 tooth (ATB) would be great at both crosscuts and ripping. IF you're looking at the flat top grind solely for ripping, then I would go with the 32 tooth ripping version of Forrest's Woodworker II, again in 3/32" kerf.

 

[Astro14]

Dewalt says this about sizing the riving knife:

All DEWALT blade body thickness and kerf widths are provided at www.dewalt.com.
If a different blade is used and the body thickness FIG. 28
and kerf width dimensions are not provided, use the
following procedure to determine the correct riving
knife thickness:
1. Measure the body thickness of the blade.
2. Make a shallow cut in scrap material and measure
the kerf width.
3. Select the riving knife (Fig. 28).
4. Slide the riving knife through the shallow cut
made in step 2 to confirm the correct riving knife
has been selected. The riving knife should not
bind or drag through the cut (Fig. 28).

IMPORTANT: If any dragging or binding of the
material is encountered as it reaches the riving knife,
turn unit off and disconnect machine from power
source. Repeat steps 1-4 to make the proper riving
knife selection before attempting another cut.

So, they provide an empirical way to test the knife, in the event that the dimensions of the blade aren't known.

But if you measure it, or the manufacturer provides a specification on the body thickness, and the body is thinner than the riving knife, you're OK.

 

[maxdustington]

Just run it as is or leave it off. I've ripped plenty on a table saw and have never had any kickback.

 

[JHZR2]

Originally Posted by Astro14
Dewalt says this about sizing the riving knife:

All DEWALT blade body thickness and kerf widths are provided at www.dewalt.com.
If a different blade is used and the body thickness FIG. 28
and kerf width dimensions are not provided, use the
following procedure to determine the correct riving
knife thickness:
1. Measure the body thickness of the blade.
2. Make a shallow cut in scrap material and measure
the kerf width.
3. Select the riving knife (Fig. 28).
4. Slide the riving knife through the shallow cut
made in step 2 to confirm the correct riving knife
has been selected. The riving knife should not
bind or drag through the cut (Fig. 28).

IMPORTANT: If any dragging or binding of the
material is encountered as it reaches the riving knife,
turn unit off and disconnect machine from power
source. Repeat steps 1-4 to make the proper riving
knife selection before attempting another cut.

So, they provide an empirical way to test the knife, in the event that the dimensions of the blade aren't known.

But if you measure it, or the manufacturer provides a specification on the body thickness, and the body is thinner than the riving knife, you're OK.


Thanks! Mind providing the link? I was trying to search for alternate part numbers for my saw and couldn't find any good results.

I agree a thinner blade is in the cards. Don't really care for ripping, more wanted the wider flat grind blade for certain joints.

Granted, I just ripped some 2x3s (though only 1/16th) with my OE blade and got this...

Last one was the end of an 8' piece and I couldn't safely and securely keep it hard up against the fence for the last 4-6". I don't have a jointer...

But those rips aren't why I want the flat grind.

 

[Astro14]

It's from the manual for your saw. After I first posted, I decided to go see what DeWalt said about the riving knife. My kid brother is considering the same saw. I had already looked it up.

I think you'll find that a good blade makes an enormous difference. Some carpentry/general blades have 0.010" or more in runout.

The good ones are at 0.001".


Dewalt service net

 

[JHZR2]

Aaah, ok yes I did see that. Thing is, it states, and the rivng knife does too, 0.094" min kerf and 0.67" max body thickness.

The issue with the manual is that it doesn't tell of other available knives. And any searches I've done have been worthless. So then the options are to use the existing one as a template and cut/grind my own, or use the existing one.

The blade I want to use is the ridge carbide 1/8 box joint blade. Reports seem to indicate the plate thickness as 0.087 or 0.01". I did find a new Freud fusion blade at a yard sale for $10. It's 1/8 kerf, 0.098" plate.

It's not clear to me how much the max plate thickness vs the knife matters. I get how a thicker roving kife than the kerf would be an impediment. I can see how thicker than the body might hold it up with too much friction. But a thinner knife would still hold the cut open, it would just be going backwards towards the limit of no knife. And be very capable all the same.

Have to wonder if it's written by the engineers or lawyers??

 

[Astro14]

The Riving Knife serves two purposes - keep the cut open and (more importantly) to prevent kickback by preventing the wood from moving away from the fence.

To perform that second function requires a delicate balance - the knife has to be thick enough that the wood can't twist and thin enough that it doesn't impede the movement. So, narrower than the kerf, by just a bit, but wider than the saw blade body.

If the riving knife is too narrow in relation to the blade, it's not going to impede cutting, the saw will still cut just fine, but there is a slightly higher risk of kickback, because it allows slightly more wood movement towards the blade.

I've got a splitter on my Delta Tablesaw, for the record, and that has to be removed for certain cuts, like dados. On that saw, I can reduce the kickback risk by sliding the fence itself toward the operator. The unifence (which I like very much, despite no longer being made) is guided by a single rail, and so, you can have the fence go well past the blade, or just past the blade, as you see fit.

If you're doing box joints, for example, on my saw, there would be no way to use the splitter. It would be an exposed blade. Riving knife is a safer alternative. But really, the riving knife is most effective when you're ripping, when long workpieces can twist as internal stresses are exposed by the cut.

I searched Dewalt.com and didn't see any other optional knives.

So, what to do? I would make your box joint cuts without a riving knife, or with this one. The riving knife isn't doing much in that kind of set-up. The workpiece is not up against a fence, it's up against your miter gauge, or sliding jig, however you set it up. With a short workpiece, your cut is over before the work contacts the riving knife anyway. Your safety comes from a solid set-up and control of the workpiece itself, not from the riving knife.

By the way, hadn't heard of Ridge blades. But their site shows some good quality blades, at a price commensurate with that quality. Forrest blades are made in Clifton, NJ, also local...

 

[JHZR2]

Thanks! Good insight into the knife application.

I'll have to look into Forrest as well!

I've never used an OE blade - always slapped a Freud on right away (onto all of the two other saws I own - miter and worm circular)...

I have to look into the roughness in my images above. Some of this is just practice. Doing silly things. I'm going to laminate 2x3s onto a 16x30 worktop for mounting my drill press. Not a critical job, but wanted to try and learn. But not happy with the result. Couldn't get the last 3" to stay tight to the fence and blade,
But don't know what's normal. Especially as noted regarding OE blades.

 

[spasm3]

The ripped 2x4 photos look like it was done with a 24 tooth blade. If you want it better go to say a 40 tooth diablo. I rip oak with a 40 tooth for long pieces and short pieces a 60 tooth

 

[Astro14]

That rough looking cut is pretty normal because you're dealing with a couple of things. First, is lumber that doesn't have a single true side. Without a true side (the result of jointing), there's no good reference for the fence. Next is lumber with high moisture...yeah, I know, kiln-dried, but there are a lot of internal stresses in softwood 2x4s. Next is a blade that may, or may not, have a ton of run out. Finally, your saw is small. Far more skill is needed to keep a board tight against a short fence than is needed to keep it against a long fence. You're simply not going to get clean/nice rips on a saw that small with wood like that. Your results are what I would expect, and are fine for carpentry, which is what the lumber was intended for and what the saw is marketed to.

If you're serious about fit and finish, i.e. cabinet-making, or finer craftsmanship - you've got to start with being able to make a board square. That's all 6 sides in parallel, at right angles, true (flat, not wavy), and smooth.

Step one is to joint one face. One face has to be the reference. Jointer or jointer plane is needed, the former takes some skill to use, while the latter takes a lot of skill. From there, a run through the planer to thickness the board, and make a second parallel face. Next, it's back to the jointer to get one edge at 90 degrees to the two established faces. From there, rip the board to get an edge parallel to the other, that was cut at 90 degrees to the two parallel faces. Now, cut the ends accurately to length, and you've got all six sides done.

The problem is, you're working with wood that is, frankly, rough. The sides are roughly parallel. They're sort of straight. When you're ripping against a fence, and there are bumps and waves in the side that's against the fence, you're not going to get a smooth rip, no matter how careful you are, or how good the blade is.

As a compromise/alternative, a step towards a better board, rip the board on one side to about 1/2" larger than you desire. Take the freshly ripped edge, which will be smoother than the factory edge when dealing with softwood 2x4s, and place it against the fence. Move the fence in 1/4" and rip again. This second rip will be smoother than the first. Flip the board over again, move the fence to the final width and take the second ripped edge (the most smooth) and put it against the fence for a final rip. You'll have a much smoother product. Still not going to be as good as a jointed edge, but you'll be happier with the results. The shorter the board, the better this will work. Ripping 8' boards on your saw will result in rough results (and frustration) every time.

If you want to build a 30" x 16" work surface, I would start with 32" boards, rip as described above. Glue them up. Cut the ends when done. For cutting the ends, since we're talking about a 16" cut, your choices are to build a sled* (sliding table) for your table saw, or to get a guide for crosscuts. I use a clamp guide for lots of really awkward cuts, like taking 1" off the bottom of a door, which had to be done when I put in hardwood floors.

The tool guide that I have is a 50" (so, you can do full size plywood sheets) Clamp'N Tool Guide. An aluminum extrusion that provides an accurate face. There are many brands, and mine is 25+ years old, so I'm certain that there are a variety of them on the market now. With a good cast (not stamped sheet metal, too hard to keep it aligned against the guide) base on a quality circular saw, with a good blade, running against the guide, you can get very nice, clean, crosscuts on large workpieces, like tables, doors, plywood.


*A big flat plywood box with a true reference fence on the rear, a fence on the front to hold it together, and some tight fitting hardwood runners that slide in the miter gauge slot. Very useful when cross-cutting large objects like cabinet doors, sides of a chest, or panels. You'll lose the depth of cut from the box thickness, so, I would go with 1/2" baltic birch. It takes some effort to make, and you'll have to spend some time getting the runners spaced properly and the rear fence at 90 degrees. PM me if you want to know how to make one.

 

[JHZR2]

Thanks for all that info!

I only work on this little bits at a time. I get the issue with having a straight face. Today I decided that trying to cut down full lengths was for the birds, so I cut the 8' boards into thirds. You can see how much curve they had..

Then I used my level as a reference edge.

What I didn't do was use one of the straight edges to shave down the long sides to be flat and parallel. So there could still be twist.

I used my new Ridge Carbide blade, which is much smoother than the OE blade.

Since there was some twist, I built the puzzle to minimize it overall. I clamped to check.

With things ideally placed, at least as best as I could do, I made reference marks and then set up for 10x50 dominoes.

I didn't put them in all the spots, but most.

Having the dominoes in there allows me to have some additional friction and helps to set the board to board alignment so the top is nice and flat. Here it is dry fit with minimal clamping. I'll glue it up tomorrow sometime, hopefully, and use 4-5 clamps. Was going to use cauls, but the dominoes sort of do it for me.