I have recently learned that any tool with a magnetic start switch can be started without actually striking the on-switch directly.
This information probably comes as a shock not only to woodworkers, but to the tool manufacturers as well. Make no mistake, this is not intended to criticize tool manufacturers. This merely reveals an inherent problem associated with many magnetic starters.
Where this all Began
Let me start by explaining how I came to this observation. A fellow woodworker had stopped over to my shop one Saturday to glue up some pine panels. While we were working at the far end of the shop, a board fell against the tablesaw. We watched in amazement as the saw started up on its own, even though it didn't look like the board had struck the on-switch. Puzzled; we went to the saw to investigate, and found a track in the dust on the saw's power cord where the board had struck. We just shrugged it off and went back to work.
Later that day, I started to wonder about this further. So I went back to the shop, picked up the board, and struck the lower corner of the plastic case of the starter. The saw started up, and I was positive that I had not struck the on-switch itself. This was really odd.
Suddenly, I started putting the pieces together. Several times in the past, my planer had turned on when I inadvertently banged into the starter with a sheet of plywood. I rapped on the junction box containing the magnetic starter. The planer started. Next, I turned to the shaper. I banged on the sheet metal pedestal next to the starter. On the third try, the shaper started. So I grabbed a screwdriver and opened the junction box of the planer's magnetic starter. With the actual starter exposed, I rapped on the starter's face with the butt-end of the #2 phillips screwdriver. The planer again started.
How Can This Be? What is a Magnetic Starter?
Magnetic start switches are different from normal on/off switches. They are used on medium and larger power tools as a safety device. Among other features, they prevent the tool from remaining in the "on-state" after it is unplugged or a power loss occurs. (To test whether your tool has a magnetic starter or not, turn the tool on, unplug the tool, and if the tool begins running when you plug it back in, then it is NOT a magnetic starter.) A Magnetic starter uses a spring loaded, self-sustaining relay.
What this means, is that when you press the ON button, you physically push a set of contacts together which starts the tool. At the same time, there is another set of contacts which supplies power to a small magnet. This magnet "holds" the contacts together until either the power gets turned off, or you physically separate the contacts by pressing the OFF button.
The key to this, is that there is a small spring which keeps the contacts apart unless the magnet (called a solenoid) pulls them together. This is not a very strong spring, otherwise the magnet couldn't hold the contacts together when it needs to.
Let's look at an analogy. The springs in your car are strong enough to support your weight when you get into the car, but if you jump on the back of the car, your own body weight could compress the springs to the point that the car "bottoms-out". The same is true with a magnetic starter. While the spring is strong enough to hold the contacts apart, if jarred, they will bottom-out, the motor will start, and the relay will be energized-holding the contacts together. That is, once contact is made, the relay will remain active even though the actual on-switch button was never pressed: The tool will run as though the on-switch had been pressed.
Placement of the Switch Body
Each tool in my shop required a different amount of force to start. The planer was the easiest, the table saw was next, and the hardest (best) was my jointer. This all relates to how the starter control is mounted. The starter control on my planer is mounted on a thin bracket on the side of the planer body. The flex of this bracket allows the starter control to literally bounce upon slight impact (even the butt-end of an eight inch screw driver was enough force to start the planer). The table saw control is mounted via a short nipple to the bottom of the fence rail, which also has some bounce. The shaper is surface mounted to the cabinet pedestal, but there is still enough flex in the sheet metal. The best tool, was the jointer. In this case, the control was also mounted to the pedestal, but it was near the corner of the sheet metal, which is very resistant to flexing and bounce.
It is the bouncing of the switch body which permits the relay contacts to touch , which starts this whole thing rolling. If the switch was mounted on a rigid platform, then, like my jointer, it could not be jarred hard enough to cause the initial contact. What's important to remember, is that regardless where your switch is located, there is always the potential for any tool, magnetic start or other, to start unexpectedly in the presence of a sharp jolt. The next time you loosen the arbor nut on the table saw, just think how simple it would be for your knee to hit the edge of the starter control.
Be safe, and always unplug the power cord when doing any type of maintenance on a power tool.