Popular Woodworking 2008-11 № 172, страница 40

offsomeihingand head inanydireciion.and boards w il h some sign i fica ni widl h i hai kick back can pivoi sideways as well.

1 believe thai mosi woodworkers don't understand the cause and cure of kickback, so let's take a look at both.

The Mechanics of Kickback

Kickback occurs when a piece of wood is unexpectedly thrown back toward the operator. The lifting and throwing force of kickback starts ai the back of the blade, not the front.

To get a better understanding of how this force is created, unplug yoursaw and mark any tooih w ith a fell-lip marker. Now rotate that tooth to where it just starts lo rise above the table at ihe back of your saw's throat plate.

Ifyou follow this one tooth as it rotates it will give you a better idea how a piece of wood is lifted and thrown forward. When the blade rises through the table at the back of the saw it has an initial vertical lift. As the blade continues to rotate and reaches the top of its arc, the vertical lifi begins to transform into a horizontal thrust. By ihe time the blade moves from the top of the arc back down toward the saw's table, the horizontal thrust transitions back tovertical. By nowall the force is moving down towards the table on iis way to doing il all over again.

To help students understand how much lift a rotating blade has, 1 move them over to i he 20" disc sander (unplugged, of course). As the sandpaper disc rotates, you can safely sand wood on the side that is rotating down toward the table. But move ihe wood to the liftingside and the piece is immediately lifted up. A table saw blade works in a similar fashion.

The Causes of Kickback

OK, so we know thai kickback occurs at the back of the blade, but what are the series of events that can cause ihe siock to contact the back of the blade?

The power of kickback.

One of the magazine's former editors experienced a kickback on the table saw that resulted in the board flying back and striking a bench chisel, bending it.

Up, forward and down again. If you follow the marked tooth in the series of photos above, you can see how initially the tooth lifts the work up. Then it pushes it forward. And finally it pushes it down.

By far t he most com mon cause is bindi ng or pinching. This happens when a piece of wood becomes trapped between ihe rotating blade and a stationary objeci, such as the fence or the guard. The following is a list of reasons that stock can kick back:

1. Crosscuuing a piece of wood with the miter gauge on the left side of the blade while the fence is being used as a stop on the right-hand side of the blade - blam-o!

2. In some cases kickback iscreated ifthe saw kerf closes around the blade.

3. Make sure ihe fence is parallel lo ihe blade. Ifthe fence is toed inward toward the blade, it can cause the wood to come in contact with the back edge of ihe blade.

4. Cutting twisted, distorted, knotty, crooked or springy wood.

5. Freehand cutiingor cutting wood that is not flat on the table, such as round stock.

6.1 .osi ng control of t he work or leu i ng go of ihe wood at the same lime it's in contact with the saw blade.

7. Not followingthrough when ripping, or stopping before the cut is complete.

8. Intentionally or unintentionally allowing the wood to "drop" on top or to the side of the back of the blade.

9. Backing out of a cut.

10. Improper setup of ihe machine's guards, fi xtures or hold-downs.

11. Applying the entire pushing force toward the off-fall or free section of ihe work instead of pushing toward the fence.

The Cures for Kickback

Use the guard. The best cure forkickback isio deny the wood access to ihe back of the blade. A splitter, especially a rivingknife, is by farthe best protection you can give yourself.

Double-check all yourseiupsand plan your cuts carefully. Be aware of all the pinch points or bindings that could be created duringany cut and plan your work first. And although ihis isn't a cure for kickback, there are blades that have anii-kickback fingers behind the teeth that limit the chip or depth of cut. These blades are sometimes sold as anti-kickback blades, but they do not eliminate kickback.

Now let's look at two important devices that can help you avoid injury, but only ifyou use them properly.

Featherboardsand Push Sticks

Dr. Jeffery Greenburg, a brilliant hand surgeon and preuy fair woodworker, told me that he has never treated a paiient whose injury occurred because of a push stick or any other safety device. As a matter of fact, a common reason accidents occur is from not using a safety device. Push sticks and feaiherboards are easy to make, inexpensive, can be made 10 any size and are expendable.

There are a lot of articles, drawings and details written on how to make elaborate push siicksand fcatherboards, but most professionals make the simpler types for two reasons: They arc easy and quick to make, and when

64 ■ Popul ar Woodworking November 2008