Riving knife retrofit for unisaw #1: Riving Knife retrofit for Unisaw

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Blog entry by runswithscissors posted 02-23-2013 01:30 AM 28352 reads 27 times favorited 127 comments Add to Favorites Watch
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Like a lot of people, I wanted a true riving knife for my older Unisaw. I checked out the Bolt On Riving Knife (BORK) and realized it doesn’t maintain a uniform height relative to the blade, because it follows a bigger arc than the blade, being even farther out from the axis of the swing arm. Also, frankly, the means of attachment to the arbor doesn’t look very robust to me.

I find this analogy useful for picturing the functioning of a true riving knife. Compare it to your arm. Your shoulder represents the pivot point of the swing arm; your elbow represents the blade arbor; and your forearm represents the articulating arm, with the riving knife held in your fist. Now, hold your arm straight out and level with the floor. This represents the blade (and RK) at full height. Then, lower your arm slowly, always keeping your forearm level with the floor. Notice that to do this, you have to gradually bend your arm at the elbow. (If you are resistant to my suggestion to physically do this, at least imagine it.). If you can figure out how to do this on your saw, you can have a true riving knife.

I live close enough to the Bellingham Grizzly that it was convenient to go in and check out their saws. They have at least 3 different mechanisms for keeping the RK level with the blade. One avoids the swing arm altogether, and simply moves the arbor and RK straight up and down (I say “simply” because I couldn’t see the mechanism that does this; it may be very complicated). Another swing arm mount uses 2 toothed plates separated by a cog that rotates between them. Though I cranked this mechanism up and down several times, I couldn’t really see how it worked either. But it looked too complicated for me to attempt. The third used a swing arm, with “forearm” actuated by some sort of linkage, but again, I couldn’t see enough of it to understand exactly how it works. I suspect my method most closely resembles it in principle. It would be fun to take these machines apart to better understand their mechanisms.

I decided that the best way to accomplish what I wanted was to make a joint at the “elbow” part that could slip over the blade’s arbor flange. This would make it unnecessary to remove the arbor shaft. As the articulating arm (forearm) had to have a solid base, I had to figure out where and how this could be mounted. The arbor casting is somewhat triangular in shape, with the blade shaft forming one corner, swing arm pivot another, and worm driven gear for changing blade height the third. Within this triangle is a sort of web of cast iron (like a duck’s foot web, not a spider web). It would have been ideal if this web had been flat and coplanar with the rest of the arbor and table. But it was neither. So I tried bolting a plate (1/4” steel) to this by using spacers (small-diameter bits of tubing or pipe tack-welded on), ranging from about 3/8” to slightly more than 1/2” in length). After welding these spacers on, I could drill 5/16 holes for mounting bolts, and then, using a depth gauge, determine how much to grind off the spacers. I used 3 bolts, figuring I would need that many to have a solid mounting. The bolts tap into the cast iron web.

I then cut the hole in this plate so it could slide over the arbor flange. A 2 1/2” hole saw was about right for this, requiring a little enlarging on the oscillating spindle sander, only a few minutes worth. The “forearm” needed the same hole. To achieve concentric pivoting at the elbow, I cut three 1/4” wide arced slots about 1/4” out from the 2 1/2” hole, and concentric to it, and then bored and tapped 1/4” holes into the “forearm.” (The slots were in the mounting base). Wanting to have smooth studs (rather than threads) to run in the slots, I ran the threads down on the bolts to leave 1/4” of smooth shank, and enough thread to tighten while allowing free movement of the arm without wiggle. One bolt head had to be ground thinner to not clash with the arbor casting; one head had to be removed altogether for the same reason, leaving just a stud, but the remaining 2 were sufficient to hold everything in place. Those bolts (on final assembly), were put in with medium strength Locktite. When in the horizontal (full up) position, these concentric plates have a gap at the top about 1 1/4” wide. This is partly because it would be too high otherwise, and would interfere with the throat plate, and also because my Uni has flats on the shaft that accept a wrench for blade changing (a really nice feature that I’ve seen on very few saws). So I did not have to sacrifice this blade changing aid, for which I am grateful.

All the above was relatively straightforward (but made harder because I hadn’t removed the table, and had to do everything reaching through the throat). Drilling holes for the mounting bolts was a real challenge, requiring a right angle drill and a drill bit I’d cut in half. I only had to attach and remove everything about 50 times to make small corrections and adjustments.

The tougher problem lay in figuring out how to actuate the forearm to make it rise and fall proportionally with the blade. I cut out poster board mockups of various schemes and pinned them to a piece of plywood, with pivots consisting of brass binder clasps. Everything I tried simply didn’t work. Spent a lot of sleepless nights puzzling over this. Finally, one idea I hit upon actually showed promise. I finally tried this scheme, using a mockup made from angle iron and a piece of 1/4” hardboard, and to my amazement, it did work. But there were two problems with it: it took up too much space where the riving knife was to go; and it required a precisely dimensioned, precisely curved slot. I hesitated.

About that time, I ran into a blog by TTalma showing an arrangement similar to mine, and he revealed the necessary placement and configuration of the actuating arm I had been struggling with. Once I had this conundrum resolved, I tried it and it worked. And I’m very pleased with the results.

At least two others have worked out schemes similar to mine, and they are worth checking out if you plan to try your own. In some ways, theirs are superior to mine (except in functionality) as they are skillfully machined and nicely finished (see TTalma”s blog “Home made riving knife,” and a SMC thread titled “Delta 34-350 12-14” riving knife retrofit”). Mine is crude, and plenty ugly, and shows signs of extensive trial and error—weldments, holes without meaning and even a slot with (now) no purpose. Several times, I had to cut something off and weld something else back on.

To follow my design, you would need a means to cut steel (I use a 4 1/2” angle grinder with 1/16” thick abrasive metal cutting disks); I also have an upright metal cutting bandsaw which I converted from a 14” wood cutting saw (Grizzly), with a jack shaft and stepped pulleys to reduce blade speed. I use a 1/4” bi-metal blade, which cuts slowly but surely, and can make fairly tight radiuses. I use this tool, on many projects. Others might use a plasma cutter or acetylene torch. And of course you’d need the usual drill press, wrenches, grinder, taps and dies, etc. Though I did quite a bit of welding, if you had a well thought out plan, very little welding would be needed. When you’re making it up as you go, as I did, expect a lot of false starts and blind alleys.

This took me from 10 to 15 days, and a lot of frustration. Was it worth it? For me, yes, because I like my old Uni, and I like the challenge in something that “can’t be done,” though admittedly there were moments when I wondered why I was torturing myself so. If you have a machine shop, you can make a much more elegant riving knife mechanism than mine, but I think mine works as well as any.

As for the knife itself, I found a one foot-square piece of 14 gauge plate, which seems to be a perfect fit for the Freud Diablo TK blade. If I decide to use a standard kerf blade, I’ll need to use 11 or 12 gauge. A good way to determine the right thickness is to cut a kerf in a piece of hardwood, and take it along with you to the steel yard. If it fits over the plate without binding, you’re good to go. The knife is held on with two 1/4” studs, spacers, washers, and nylock nuts (don’t want that thing loosening up). The slot in the knife is open at the bottom to make putting in and taking out easier. I decided instant removal and installation weren’t that important to me, at least for now. It doesn’t require removal to do blind cuts.

I’ve been having a heck of a time with photobucket, being a newbie at this sort of thing. I fear the photos may be cut off, and don’t know whether the text will be included or intact.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

127 comments so far

View BigAl98's profile


292 posts in 4374 days

#1 posted 02-23-2013 02:32 AM

Simply beautiful

-- Al,Midwest -To thine own self be true

View runswithscissors's profile


3134 posts in 3360 days

#2 posted 02-23-2013 06:56 AM

Thanks for your kind comment. If I can get squared away on photobucket, I’ll redo the photos and captions that were supposed to go with them.

I started to rip a piece of 3/4” oak the other day, and it had hardly advanced beyond the blade before I ran into resistance. Sure enough, the wood had a lot of locked in stresses, and it had closed up on the riving knife. It was really reassuring to have that thing work exactly as it should.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View Nicky's profile


718 posts in 5427 days

#3 posted 02-23-2013 02:37 PM

I get it. The pics are good enough.

Good write-up. This is an elegant solution. I’ve got to try this.

I appreciate you taking the time to share. The blade guard/splitter was removed a few days after I purchased my saw because it was too cumbersome to use. I do use an aftermarket splitter but your design is far more robust.

Thank you for sharing.

-- Nicky

View Cosmicsniper's profile


2202 posts in 4493 days

#4 posted 02-23-2013 02:45 PM

Love it, my man!!! Thanks for the effort and thorough description of your thought processes in this project. I’ve strongly considered a DIY riving knife for quite a while now for my unisaw. This is a really nice solution!

-- jay,

View runswithscissors's profile


3134 posts in 3360 days

#5 posted 02-23-2013 11:13 PM

If anybody wants to try this, I’d be glad to answer any questions you might have. One part I seem to have overlooked is mounting of the actuating arm. You can see well enough how it bolts to a tab welded on to the forearm. The other end is bolted to a 1X1 angle iron, which in turn bolts on where the dust diverter (don’t know Delta’s name for it) attaches. There are two 5/16-18 holes there, and the arm pivots from about the middle of that. I made the 5/16” bolt holes oblong, in order to make any needed vertical adjustments by tapping up or down. The way I determined where that pivot point should be was to measure distance from the tab on the forearm to the front part of the trunnion. By measuring from two positions—fully raised and fully retracted—I could find the spot where the distances were equal.

I had to sacrifice part of the dust diverter where it came up against the base plate, but was able to save the lower part where it protects the blade elevation worm. Also kept the side opposite the blade intact. Had to shorten the spacers between the deflector and the trunnion by 1/8”, the thickness of the angle iron.

Also note that I lowered the very end (1”) of the forearm by about 3/8”, as otherwise the mounting studs clashed with the ZCI throat plate at 45 deg. when in fully raised position.

In case anyone is wondering, it is fully my intention that anybody is welcome to borrow any ideas from this that they might find useful.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View DocSavage45's profile


9071 posts in 4177 days

#6 posted 02-24-2013 07:12 PM

Well done,

My “but” is I would have to aquire some metal cutting tools? The Bork might be simpler for me and my ol grizzly bear. LOL!

Nice write up!

-- Cau Haus Designs, Thomas J. Tieffenbacher

View runswithscissors's profile


3134 posts in 3360 days

#7 posted 02-24-2013 09:01 PM

You can get a 4 1/2” angle grinder for less than $20 from HF. The cutting disks are cheap and expendable—maybe a couple bucks each (I just charge them at my local hardware store, and don’t pay much attention). As for the cuts I do with the bandsaw, some of them could be done with the angle grinder, but you also might be able to get a machine shop to cut them out for not much $. I used 1/4” plate for the base plate and the forearm, not for strength but rigidity; also because you can get a reasonable number of threads when you tap a hole. Taps and dies are cheap also, either as sets or individually (you’d only need 5/16-18 and 1/4-20). The actuating arm or bar is 16 gauge (a.k.a. 1/16”) because that is plenty rigid and strong for the application.

The materials for this are not at all expensive. I doubt if you’d go much over $50 for everything, probably less.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View runswithscissors's profile


3134 posts in 3360 days

#8 posted 02-25-2013 01:52 AM

One more clarification. In my original post I said I fitted the base plate before cutting the 2 1/2” hole that fits over the arbor flange. Of course that’s impossible. The hole has to be cut first, then you slip the plate over the flange before determining where the 3 bolts are going to go. (I actually did do it originally the wrong way, which proved to be the hard way; no need for anybody else to make the same blunder). And it’s easiest to start with mockups using moderately stiff cardboard, which are fitted and then used as a template.

Oh, another tip: it’s really hard to see pencil marks or even Sharpie marks on steel, especially in bad light. So I get full-sheet (8 1/2”X 11”) Pres-a-Ply labels, cut them to the size or shape I need, apply them to the steel, and draw my cutlines on that. The only problem is the stuff is very sticky, and a PITA to get off. Next time I’m going to try some “repositionable” labels, which hopefully won’t be so tenacious.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View runswithscissors's profile


3134 posts in 3360 days

#9 posted 02-26-2013 12:49 AM

Well, I am figuring out how to do this. Imported directly from iPhoto, no help from photobucket.

Showing the spacers on lefrt; forearm position with blade raised. Slot in forearm was from an aborted trial configuration. Ignore

Forearm position with blade retracted.

Closeup on slots and studs. Extra holes were a blunder. Protrusion at bottom is where actuating arm attaches

Actuating arm. Curved shape is to let it go under the pivot elbow. Angle iron bolts on where dust deflector attaches (behind the dust deflector).

Holes in the arbor casting web. Threaded with 5/16-18 tap.

The assembly as it will be mounted (right tilt saw). Studs for RK on the left (1/4-20 thread)

Actuating arm comes out from behind vestige of dust deflector.

Another view. Gap at top of elbow pivot provides clearance for ZCI, and allows arbor wrench to be used for blade changing. One the Unisaw’s features I really like.

Another view

Blade and RK at intermediate height.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View DocSavage45's profile


9071 posts in 4177 days

#10 posted 02-26-2013 01:45 AM

Lookin good!

Just got my grizzly 1023sl cleaned up. Have to put in 220 and go slow as I had a 3hp 10’ craftsman before mama bear. I knew the parts of my craftsman and I will be figuring out the bear. Probably similr to the unisaw, but I don’t know until I get more involved in the mechanics and handling of the saw.

Until then I’ll keep watching?

-- Cau Haus Designs, Thomas J. Tieffenbacher

View toolie's profile


2213 posts in 3963 days

#11 posted 02-26-2013 03:35 PM

that’s a really excellent job of metal fabrication. is there any chance for a brief video of the RK in actions, being elevated and bevelled, so the action of the added parts can be viewed in motion?

-- there's a solution to every just have to be willing to find it.

View runswithscissors's profile


3134 posts in 3360 days

#12 posted 02-26-2013 08:01 PM

Actually, a lot of my parts appear to have been shaped with a hatchet (if there were a metal working hatchet). That’s because I was making it up as I went along. Tends to lead to randomly shaped parts.

I’ll see what I can do about a video. Thanks for the suggestion.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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3264 posts in 4010 days

#13 posted 02-26-2013 10:06 PM

Great job on this. Thanks for sharing.

View runswithscissors's profile


3134 posts in 3360 days

#14 posted 03-06-2013 10:46 PM

These are supposed to be videos. But I don’t know whether they will open as such. Another bump on the learning curve for runswithscissors.

In regard to cutting metal: I forgot to mention the jig/sabre/bayonet saw as a useful tool for this. In cutting the 1/4” wide slots, typical blades are too wide. Just grind the backs of these down so they will fit in the 1/4” hole. In 1/8” inch steel, they will cut reasonably quickly. In 1/4” it will be slow going, but doable. Oh, if your saw has an orbital mode, set it on 0. An orbiting blade will make it difficult to saw metal. Guess how I figured this out.


-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View DocSavage45's profile


9071 posts in 4177 days

#15 posted 03-07-2013 01:55 AM

Try again…LOL! Learning curve on everything.

-- Cau Haus Designs, Thomas J. Tieffenbacher

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