On 2014-01-12, Mike Marlow wrote:
DaveC wrote:
The reason to use a contactor instead of just an on / off switch is
that the insides of the switch can get full of saw dust and turn on
the saw with the switch in the off position. I've seen this happen!
Regular AC switches are not sealed well enough, but you can buy
sealed switches with lower current ratings which is enough to engage
the contactor.
Shaun.
Best answer yet.
Yes, we all know about "during a power failure" but many times this
won't be an issue (actually the friend would appreciate the saw
continuing the cut after power comes back!), but the "auto turn-on"
feature )c: is one I hadn't thought of.
Thanks!
It may not get you much mileage with your friend. Despite the previous
poster's claim to have "seen this", you really have to consider the
likelihood of such a thing. Wood is an... insulator.
A) Note the newsgroups to which this is cross-posted:
sci.electronics.misc
sci.electronics.repair
rec.crafts.metalworking
rec.woodworking
And while wood may be an insulator, horizontal bandsaws (which I
remember from the earlier articles in this thread, and which I
use) are more commonly used to cut metal. And metal swarf is
certainly conductive.
B) Toggle switches typically have a spring-loaded actuator which
bears on a metal plate which rocks back and forth to bridge
contacts or not bridge them.
Even a buildup of wood sawdust or plastic dust *could* trap the
metal rocker in a position to keep the bridging happening, even
when the toggle is in the "off" position.
Though he may have
"seen this", do you think your friend is going to believe it to be a real
enough possibility for him to consider? There are millions of simple
switches in shops all over the world - and they work.
And most of those switches are not located near the point of
generation of metal swarf as they are on the typical inexpensive
horizontal bandsaw. The airborne sawdust in a wood-working shop is not
likely to be a problem. However, metal chips dribbled from the blade
(after going almost a full turn around the path of the blade) are a
possibility, depending on the construction of the switch. (And, they
could also conduct from the switch terminals to the chassis of the saw,
if they got under the frame.) Certainly a sealed limit switch (more
commonly found on serious machine tools) would be free of that
particular problem. They are a small MicroSwitch module inside a metal
housing with O-rings to keep out oil as well as chips. They are
operated by a cam, also sealed, or an external plunger -- operating
through a sealed path. (Honeywell is one maker -- and they *are* quite
expensive, but the best thing where swarf will build up.) The wiring
comes in through a fitting which normally has a tapered gasket which
compresses around oil-proof wiring, so there is no path for the chips in
through there either. And yes, the are best used at low voltages and
low currents. I recently re-designed the limit switches in a conversion
of a CNC milling machine, and needed to get switches of the same
physical mounting but which had two separate circuits depending on which
direction they were operated in.)
It would probably be
hard to convince your friend their is a real risk here. For good reason...
It is at least a *possible* one. Now, if the power is fed to
this through a wall-mounted disconnect switch, so you could stop the
bandsaw without having to unplug it (which might be difficult to reach
with the saw still running), that would be less of a problem. It would
be nice to have a warning LED near where you would reach to change the
clamping of the workpiece, so you know the saw is still running,
especially of other noisy tools are being run at the same time in the
shop. A horizontal bandsaw is typically fairly quiet, unless it is
cutting relatively thin metal.
Enjoy,
DoN.
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