Happy New Year to all. Your intrepid moderator Jeff directed me to the recent topic on SawStop and asked if I could give a post some additional information/explanation - so hear goes. For those of you who want to see it in action, we have videos demos on our website
www.sawstop.com.
In summary, SawStop is a active guarding system that can tell the difference between the material being cut and the person operating the machine. Although it is applicable as some level to almost all woodworking machines, I will explain how it works on a tablesaw - which was our first prototype. In the tablesaw, the blade is electrically isolated from frame of the saw by placing plastic bushing inside the bearings around the arbor. A high frequency electrical signal is induced on the blade and constantly monitored. When you are cutting wood, the signal is generally unchanged because wood doesn't conduct electricity. However, the instant your hand comes into contact with the blade, your body absorbs some of the electrical charge from the blade reducing the size of the signal on the blade. When the circuit sees this reduction, it sends a signal to an extremely fast brake system that stops the blade in about 1/200th of a second. In most cases, that means you get a small cut instead of potentially loosing a finger. Before I explain how the brake works, let me say that the detection system can be any conductive surface that can be isolated from the rest of the machine - i.e. a band on a bandsaw, a guard, a metal plate near a dangerous area etc. Using the blade, however, offers the benefit of allowing the system to work even if the user is wearing a glove because the blade cuts through the glove and the system still trips when the blade contacts the finger.
On the tablesaw, the brake is a block of plastic or soft aluminum that is jammed into the teeth of the blade. The block is about 6" long and pivoted at one end located about 2" away from the edge of the blade. The other end located near the teeth and constantly pushed toward the teeth by a strong spring. A small fuse wire holds the brake in place against the force of the spring. When contact is detected, a large current is released through a small section of the fuse wire. The current surge burns the fuse wire in about 50 millionths of a second, releasing the brake to contact the blade. Once the brake contacts the blade, it is self locking because of the location of the pivot and the shape of the pawl. The brake, spring and fuse wire are all located in a cartridge that is easily replaced after firing
The specifics are different on each machine - for instance on a bandsaw, we cut the blade just below the table, or on a pneumatic upcut saw, the driving cylinder is quickly reversed to retract the blade without stopping the rotation.
Overall, we estimate that the system will add about 15% to the price of machines to which it is added. For the tablesaw, we estimate that the cartridge will cost about $50 to replace along with new blade. Other than that, there is no damage to the saw.
Until recently, our focus has been to license the technology to machine manufacturers, rather than building something ourselves. Part of the reason is that, in most cases, you can't retrofit the system, so the system needs to be installed at the time of manufacture. Also, we wanted to make the system available to all manufacturers so that it (hopefully) could be on all machines. Unfortunately, most of the manufacturers seem to be more interested in dragging their feet than protecting their customers. As a result, we are in the process of designing a tablesaw (similar to a Delta Unisaw or Powermatic 66) which we hope to have available by August 2002.
One manufacturer that has been sufficiently concerned with the safety of their customers to adopt this type of system is Precision Products -
www.heavychop.com. They have licensed the patents behind this technology for their products and you can purchase a pnuematic chop saw with an this type of active guarding system from them.
If people have more questions, I would be happy to answer them as best I can here or via email. Thanks for your interest and to Jeff for inviting me to post.
Steve
