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Safeguarding Table Saws |
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Serious injuries can result from the use of portable and stationary
power saws. Accidents may occur if the operator is inexperienced,
improperly trained or if the blade is not properly guarded. The easiest way to avoid injury is to use properly guarded saws and appropriate safety equipment. Avoid loose-fitting clothes and long hair that might become entangled in a power tool and remove rings, watches, neck chains and other jewelry. With hand-fed saws, the operator adjusts the height and angle of the blade. Then, holding the stock, the operator pushes it into the blade. A guide is used to maintain a straight cut at the desired width. At the end of the cutting stroke, the operator either changes positions or pushes the stock past the blade. Self-feed or power table saws are equipped with rollers or a conveyor system to hold the lumber and force-feed it into the saw blade. Injuries can occur if an operator’s hands slip while feeding the stock into the saw, or if the operator holds his or her hands too close to the blades while cutting. The operator can also be injured when removing scrap or finished pieces of stock from the table. Solutions: Enclose the portion of the ripsaw and crosscut saw above the table with a self-adjusting guard. The guard must adjust to the thickness of the material being cut and remain in contact with it. Hinge the guard so that the blades can be changed easily. Use a push stick for small pieces of wood and for pushing stock past the blade. Additional safety measures: Keep hands out of the line of the cut. Attach a brake to the motor’s arbor to stop the saw from coasting after the power has been cut off, or have the operator remain at the saw station after the motor is shut off, until the blade stops turning. Injuries can occur if the operator makes contact with the blade under the table or with the power transmission apparatus (if not enclosed). Solution: Always guard the portion of the blade below the table. Protect operators from possible contact when reaching under the table. Always guard the power transmission apparatus (belts, pulleys, chains, sprockets, etc.). Kickbacks occur when the blade catches the stock and throws it back toward the operator. Kickbacks can result if the blade height is not correct or if the blade is not maintained properly. Kickbacks are more likely to occur when ripping, rather than crosscutting. Kickbacks also can occur if safeguards are not used or if poor-quality lumber is cut. For ripsaws, use a spreader to prevent material from squeezing the saw or kicking back during ripping. Use anti-kickback fingers to hold the stock down in the event that the saw kicks back the material. Additional Safety Measures: • Use the proper blade for the cutting action. For example, do not use a crosscut blade for ripping. • Operate the saw at the speed specified by the manufacturer. • Leave sufficient clearance for stock. • Guide the wood to be cut parallel to the rip fence to minimize the potential for kickback. • Avoid crosscutting long boards on table saws. Considerable hand pressure is required close to the saw blade, and the boards create a safety hazard to other people. • Stand to the side of the saw blade to avoid injury due to kickback. • Properly support all pieces of stock, including the cut and uncut ends, scrap and finished product. Designed for Safety To minimize saw related injuries and the costs associated with them, the most advanced table saws in the world are being equipped with a patented, award winning safety system that detects the difference between wood and skin. When someone accidentally contacts the spinning saw blade, SawStop’s built-in protection automatically stops and retracts the blade within five milliseconds. In most cases, such an accident would result in just a nick, instead of the devastating injury that would likely occur on an ordinary saw. SawStop saws are not being promoted as a replacement but as a supplement to machine guards. More than 400 individuals have filed a petition with the Consumer Product Safety Commission to integrate SawStop’s safety technology into all newly manufactured table saws. SawStop has documented 25 finger and or hand “saves” in real industrial and educational environments. In most cases, the accident victims did not even require a bandage. How it Works The safety system is made up of two different but integrated components…a detection system and a reaction system. Here’s how the detection system works. The saw blade has a low voltage current (about three volts) that constantly runs through the blade when the blade is spinning. This current is constantly being monitored by the brake cartridge, which is mounted directly underneath the blade and held in place with two pins and a locking key. If the key is not locked into place, the saw will not operate. The brake cartridge is made up of several different components. There is a micro-processor, a small steel fuse wire, a spring under 150 pounds of compression, and a soft aluminum brake pawl. The next thing to understand is that the human body is an excellent conductor of electricity, and wood is not. That said, when wood is touching the blade, there is no absorption of electricity into the wood so the saw operates as it should and cuts as normal. The electrical charge in the blade remains the same. However, if you were to touch the saw blade while the saw is running, your body, being conductive, absorbs some of the blade’s electricity and the voltage level drops (decreases) on the blade. Here is where the brake cartridge comes in. If you touch the blade, the microprocessor in the brake cartridge recognizes the drop in electricity and sets off the reaction process. A message is sent by the computer chip that initiates a quick burst of electricity, which burns through a small fuse wire. The fuse wire is holding back the spring. Once the spring is released, it pushes or jams the brake pawl up and into the saw blade. As the teeth of blade dig into the brake pawl, another reaction occurs. Integrated into the arbor assembly is a quick release mechanism. Because the momentum of the blade grinding into the brake generates so much force, the residual energy breaks the quick release mechanism free and the arbor assembly pivots down. This pivot action drops the blade below the table top and the blade disappears. In addition, the saw motor stops immediately. The combined actions of the blade stopping, the blade dropping and the motor shutting down all happen in less than five milliseconds or five one thousandths (5/1000’s) of a second. FSM |
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