Preventing Tablesaw Kickback: It’s Not “An Acceptable Risk.”Comments (0)
sucker-punched by your tablesaw? You may know what I’m talking about: You’re
happily sawing along when–BAM!–in a flash, the board you were ripping flies
back and slams you in the gut. Worse yet, maybe you were carelessly feeding
with your bare hand instead of using a pushstick, and the escaping board
suddenly directs your fingers into the blade. Ouch! Or perhaps the wooden
projectile just crashed through a shop wall. We’ve all heard the stories.
Whatever the case, don’t blame your saw for being temperamental. If it’s lashing out at you, it’s only because you’re not using it properly. Kickback is not a risk you have to learn to accept when using a tablesaw, since you can easily prevent this kind of misbehavior. By understanding the mechanics, you can counter the forces involved and maneuver your stock with assured safety.
Here, we’ll take a close look at why kickback happens and how you can prevent it by adjusting your tablesaw properly and outfitting it with an appropriate commercial or shop-made anti-kickback accessory. If you use a tablesaw, this is stuff you absolutely need to know.
When working at the tablesaw, there are two primary ways that a piece of wood can be thrown back at you. One is when a ripped strip–typically a narrow one–ejects rearward like an arrow from between the blade and the fence. This usually only happens if your pushstick slips off the stock. To avoid injury in this case, never stand directly behind a board being ripped, which is standard practice anyway.
The more common–and more dangerous–form of kickback is when the entire workpiece is thrown upward and backward at a fierce speed toward you. This has probably happened to anyone who has worked at a tablesaw for very long without a splitter. Unfortunately, few woodworkers understand what causes kickback because it happens so fast. But here’s what is going on:
As shown in Figure 1, applying pressure to opposing corners of a board causes it to rotate. During typical ripping, your pushstick applies forward pressure to the rear right-hand corner next to the fence, while the blade entering the work near the diagonally opposing corner applies rearward pressure. This causes the leading end of the board to wander over against the rising teeth at the rear of the blade. The teeth then carry the board up onto the spinning blade, launching it backwards. The other kickback scenario is when internal stresses in the wood cause the kerf to close onto the rising rear teeth, which also throws the board. The worst-case scenario is when the board suddenly disappears and your hand continues its forward motion into the blade. Uh oh.
Rip fence alignment
The first step in
preventing kickback is aligning your rip fence parallel to your blade. The most
serious misalignment is when the fence is “toed-in” toward the blade–meaning
that the distance from fence to blade is less at the rear of the blade than at
the front. This creates a dangerous funnel effect that causes the edge of a
workpiece to press against the rising rear teeth.
To align the fence, mark one tooth, rotate it to the fore, and measure its distance from the fence (Photo A). Next, rotate the tooth aft, and perform the same measurement. (Using the same tooth as a reference removes any blade warp from the equation.) Adjust the fence until the measurements match. If you’re unsure of perfect parallelism, angling the fence outward 1⁄64" or so at the rear of the blade is a safe bet.
Riving knives and splitters
When it comes to preventing kickback, the best tool for the job is either a riving knife or a splitter. Each of these plates sits aft of the blade and, when properly aligned, as shown in Photo B, keeps the workpiece against the fence to deny it access to the rising rear saw teeth.
The main difference between a riving knife and a splitter (or spreader) is that a riving knife is mounted in such a way that it rises, falls, and tilts with the blade (Photo C). Because its top end can be set just below the blade’s top-dead-center, it doesn’t need to be removed to make grooves and other non-through cuts. Also, the knife’s arced shape closely mirrors the blade’s curve to minimize the gap between the two, preventing any potential board wander before the board’s edge reaches the knife.
All tablesaws sold in the U.S. these days are legally required to include a riving knife, and it’s a good thing, as it’s the best approach for defeating kickback. Unfortunately, tablesaws cannot be retrofitted with riving knives, so a splitter is the next best alternative if you have a legacy saw.
A splitter does not rise and fall with the blade, and sits at a fixed distance behind it. Therefore, when the blade is lowered to cut thinner stock, the gap between the blade and splitter increases, along with the potential for the teeth to grab the board before the kerf reaches the splitter. A splitter must be removed for making grooves and other non-through cuts, but a number of aftermarket splitters (Photo D) are designed for easy, convenient removal and reattachment, so you’ll actually use them, as opposed to the troublesome splitter/guard assemblies on most legacy saws. (See sidebar at left.) You may have to do a bit of research to locate the right one for your particular saw, but you’ll find many examples online by typing “aftermarket tablesaw splitters” into your search engine.
An easy way to incorporate a tablesaw splitter is to install one in a zero-clearance insert (ZCI). These throat plates are available commercially, or you can make them yourself. (See Issue #53.) The main purpose of a ZCI is to create a zero-clearance blade slot to reduce workpiece tear-out, but the throat plate provides a perfect platform for a shop-made wooden or aluminum splitter (Photo E). Alternatively, commercially manufactured splitters are available for mounting in a ZCI (Photo F). These splitters obviously can’t be used for bevel cuts, but they work fine for square cuts.
Although not as efficient or effective as a riving knife or splitter, a featherboard is probably the next best option for preventing kickback. To avoid pressing the sawn workpiece section against the blade, a featherboard must be set up in front of the blade instead of behind it. This will keep a workpiece from wandering into the blade until the featherboard loses contact. However, at that point, all but the shortest of boards will have mostly passed the blade, greatly minimizing the danger of kickback.
You can buy or make featherboards that either mount in your table slots (Photo G), clamp to your tablesaw wing (Photo H), or attach magnetically to a steel saw top. Probably the biggest drawback to using a featherboard is that it typically must be reset for every new cut.
Photo A - Check
fence-to-blade parallelism by measuring from the same marked tooth–rotated
first to the front of the saw and then to the rear.
Photo B - Like any properly aligned splitter or riving knife, this aftermarket Beisemeyer splitter is set in line with the sides of the saw teeth that face the fence.
Photo C - This curved riving knife rises, falls, and tilts with the blade and, when positioned as shown, doesn’t need to be removed for non-through cuts.
Photo D - The Beisemeyer Snap-in Spreader shown here is just one example of commercial aftermarket splitters that are designed for quick and easy attachment and removal.
Photo E - You can glue a shop-made wooden splitter into the blade kerf of a zero-clearance insert, or attach a piece of aluminum angle into an aft slot with screws.
Photo F - ZCI-mounted splitters available from MICROJIG include a guide that registers in the throat plate slot for easy, accurate drilling of the installation holes.Photo G - When setting up a featherboard, press its flexible fingers firmly against the edge of the stock in front of the blade, and secure it in place. A slot-mounted featherboard (left) is easier to set, but you can also clamp a long unit to your tablesaw wing.
Kickback In Action
One of the reasons kickback is so dangerous is that it happens in a split second–far too quickly to react to it. That speed is also the reason it is so misunderstood; kickback simply happens too fast to see. To remedy that, I pulled these stills from a video for a close look.
To prevent personal injury or shop damage, I used a 2-oz. piece of lightweight rigid foam insulation for this demonstration. Note that I’m standing to the right of the fence, which keeps me out of kickback’s path. The rip fence is perfectly aligned to the saw blade, and I have removed the splitter (which would prevent kickback).
First, I turn on the saw and begin feeding the panel, pulling it against the fence as the cut begins (Photo 1). The downward spinning teeth at the front of the blade help drive the panel down against the table. Everything progresses nicely, even as the teeth at the rear of the blade begin to enter the panel (Photo 2). But as the cut progresses, the feed force from my thumb at the rear corner causes the leading end of the panel to meander toward the blade, pressing against the rising rear teeth, which start to lift the panel from the saw table (Photo 3).
Then all of a sudden–wham! The panel rides fully up onto the top of the blade, where the teeth now find traction on its underside and begin to propel it backward (Photo 4). Looking at it more closely, we can see how the panel begins to spin as it twists away from the fence (Photo 5). It continues spinning and hurling at a fierce speed toward where I would normally be standing (Photo 6). This all happens too fast for reaction, as emphasized by the fact that my hand is still in play even as the panel begins its flight across the room (Photo 7), ultimately hitting the rear wall 16 feet away from the blade (Photo 8).Finally, a look at the underside of the panel shows the arched scar that recounts the spinning journey over the top of the blade (Photo 9). This underscores the fact that if your hand is anywhere near the blade during the flash of kickback, it’s obviously in danger of being deflected into the blade. There’s no reason to ever put yourself in this kind of danger. Use a splitter or riving knife!
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