The likelihood of the failure the other respondent noted being sawdust "turning on" the saw is remote at best. Almost certainly it was a failure in the switch, not the sawdust imo.
Sawdust is far more likely to cause a switch to fail to make contact.
The prime advantage of motor contactors for the small motors of the ilk in home shops is the one noted of fail-safe operation on restart in the (admittedly unlikely) case of power failure.
Second is the low-voltage contacts are more reliable over the longer haul, but again for most recreational/hobby users the demand for direct switching is simply not enough to make it be a real issue.
I use them universally simply because I like the tactile feel of the starter button vis a vis the mechanical switch.
The one other advantage but is only really useful for the DC is that it's much simpler to wire a remote umbilical cord to it for access from afar than it is to wire inline switches.
I designed industrial controls to GM's specs which required NEMA-rated enclosures and conduit that gave reasonable protection from a fork lift, and interconnecting wiring no smaller than 16 AWG for mechanical strength.
Hobby machinery may not be subject to the same level of abuse, but it wasn't initially designed with protected spaces for add-on controls and wiring. I've milled connector openings in a lot of 'cheap plastic enclosures', usually ABS Hammond boxes from Digikey, and seen how easily they fracture. Personally I like metal weatherproof outlet boxes on machines that throw wood or metal chips, or plastic for low voltage and safer locations.
The only machines I own that can feed themselves into the cut are a wood planer and a 4" x 6" horizontal bandsaw. The spring-balanced bandsaw doesn't mind being shut off and restarted in the cut. Would a hydraulically damped bandsaw start safely after power had been out?
That might be a good test just before you replace the blade. If the motor won't start you'll be there to switch it off.
I don't trust my bandsaw to run unattended anyway. jsw
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.
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 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.
This is more of a factor with three phase motors, where a motor once started will continue to run if one phase of power fails, but the current drawn in each remaining winding will be higher, and will be likely to burn out the motor. (And he was going to add a 240 VAC single phase motor to the bandsaw in question.)
A proper motor starter (of which the contactor is a part) includes thermal sensors for the current through each winding, and if any one of the three goes well above the normal full-load running current (as will happen if one phase dies), it will interrupt the power to the motor and stop it before the motor is damaged. And it also protects the windings from burning out when trying to start with one phase of three missing.
And actually, some starters only measure the current through two of the three legs, because if one is out, one of the two monitored will be over-current enough to trip it.
So -- with a single phase 240 VAC motor, the over-current from loss of a phase would not apply.
But the other reasons for using a motor starter/contactor still stand.
Not really. The thermal overloads sense the current in each winding, and generate heat to soften either a wax or a low temperature solder to allow a wheel to rotate and a low-current contact to open. All of the phases have their contacts in series, and in series with the coil of the contactor. Any one of them opening drops the contactor out to protect the load.
With single phase, a slow-blow fuse in series with each hot lead should sufficient, but with three phases, you need to sense any condition of over-current in any single phase, and to drop everything once it is sensed. (Of course, if you are running the three-phase motor from a VFD, that incorporates the functions of the starter (current sensing and automatic shutdown) in the electronics of the VFD.
"DoN. Nichols" fired this volley in news: snipped-for-privacy@Katana.d-and-d.com:
Don, I'm not picking on your answer... I had to respond to someone's, and yours got the prize...
This is the dumbest discussion I've ever heard (short of all the political spew on here).
Even half-quality garbage plastic switches from China are NOT prone to accidentally 'switching on' from accumulations of dust or swarf.
That it's a remote possibility, I won't deny. But such switches typically serve for years to decades without a malfunction in the dusty, dirty, swarf-filled environments in which we use them, and it's dumb to think of the average home craftsman's going to the trouble to re-wire, retrofit, and otherwise jigger-up his equipment with low voltage contactors and safety circuits. That some would or even could is beside the question.
This discussion should be turned to "What's the best-quality switch I can buy affordably that will suit the safety needs of the application." For that, I recommend a good industrial-quality safety-style switch that requires a simple swipe of the hand to turn off, and a positive 'de- locking' action to turn on.
My old (1970s) Shopsmith came with one. When I finally wore it out in the 1990s, I replaced it with the same-quality switch from a US maker (IIRC it was a Square-D safety switch, specifically for table saws). You could knock it off easily, but had to pull the bat out manually to turn it back on -- heavy-duty thing. It lasted more than 20 years of nearly daily use, and the replacement is still on the machine, still working.
David Billington fired this volley in news:52d3e44d$0$1398$ snipped-for-privacy@news.zen.co.uk:
It's simple, cheap, and easy to rig remote switching and sensors for things like safety guards. It requires only bell wire and some thoughtful routing, rather than running live power everywhere a switch is required.
Low voltage control confines the high voltage to the control box and the motor, so it's less exposed to physical damage. There isn't much if any benefit if you have only a single short run to the power switch and the motor draws less than the switch's rating, for the cost of a
24V transformer and a larger control enclosure.
I can't remember ever seeing a contactor and low voltage controls on consumer power tools although it's pretty much standard for industry.
The difference may be the risk of forklift accidents, since humans can't crush conduit or puncture sheetmetal.
"Why on earth would frind want the saw to turn on after a power failure"
which OP claims "the friend would appreciate".
When the power is turned off, the material being sawed, tools, fingers etc. could block the saw. I very much prefer the saw does *not* start by itself.
In the circuits I've seen, the ON is a "no", normal open pushbutton, which energizes the contactor. An on-switch on the contactor in parallel with this provides current to the contactor. The OFF is a "NC", normal closed pushbutton, removing the voltage to the contactor. All kinds of safety switches could be wired in series, all required to be closed for the saw to run.
Though some which are made to mount behind a trim plate in the wall (typical home light switch) may be more open to a buildup of swarf inside it. I've seen various failures in home electrical hardware which I would not have expected.
Among those, there was an outlet which failed during a nearby lightning strike. The form of the failure was a breakdown in the hole into which a drive screw went to hold the ground strap which mounts it to the outlet box. It was *supposed* to be a blind hole, but they had used a slightly too long drive screw, or a chip was under it, and there was a little of the inside surface which broke away.
Still -- normally not a problem. However, the lightning strike introduced a high enough voltage so the gap to the nearby hot lead was bridged, and the subsequent arc, both partially melted the "hot" inside the socket, and welded it to what was plugged into it (one of those duplex to six outlet plates) -- *and* filled the vicinity with carbon dust.
When I got home from work and discovered the power loss in the living room, I went downstairs and switched on the breaker. Five second delay and BZZZZTTT-CLICK. A repeat got the same results. I then walked around the house (old wiring, given breakers showed up in a lot of outlets around the house.
I then walked around the house, until I smelled the burnt Bakelite. Of course the outlet was behind a bookcase which I had to empty and move to get to it. Then I discovered that the multi-outlet adaptor would not unplug until I applied a lot of force, and that left one pin of the adaptor in the socket.
I then removed the socket, and replaced it with a new one -- and just had to take it apart to see what had happened.
If the Bakelite had not flaked around the drive screw, it would not have broken down -- and the failure would have been somewhere else at an even higher voltage -- or maybe not.
So -- I don't *depend* on any commercial electrical hardware for the home to do what it should do.
And -- once I needed a toggle switch which did not introduce vibration when it was operated to put in the mounting plate for a turntable, so I could cue a record, and then switch on the motor at the appropriate time. At that time, you could get "silent" light switches which contained a puck of mercury in glass and metal end caps. It was designed to work in a vertical orientation, but it was possible to take the switch mounting plate off, and file different notches into the toggle handle so it would hold the puck for horizontal operation. Now, that switch was *not* designed to keep swarf out, so it would have to depend on the decorative plate which goes over it. Granted, no normal person would mount it as I did -- and I never had swarf near the turntable, so it was no problem. But I do remember that switch, and how easy it would be for swarf or sawdust to work its way in. Sawdust would just make it take a bit more force to operate. Metal swarf would bridge the ends of the puck and leave it on full time.
Granted -- most of the horizontal/vertical bandsaws use a normal bat-handle toggle switch, but there are variations in quality there. The best have a seal around the ball on which the bat rotate, and have sealed wire entrance on the back. Cheap ones either have terminals on the back (which you screw, solder wires to, or tabs for (hopefully) insulated push-on crimp terminals -- but still metal swarf could bridge that to the frame.
Given that he is already "jiggering up" the bandsaw -- replacing the single phase 120 VAC motor with a higher horsepower single phase 240 VAC motor, you introduce another problem. Ideally (given USA wiring, where 240 VAC is really two 120 VAC wires 180 degrees out of phase, so they produce 240 VAC between them), you want a switch which interrupts both sides of the power -- so a winding failure in the motor does not leave some part of the system perhaps floating at 120 VAC instead of near ground as it should be. Now, -- in the UK you don't normally have
240 VAC with a grounded center tap. Instead, one side of the 240 VAC is grounded.
*And* -- one which interrupts both sides of the line, since he is putting in a 240 VAC motor.
Good enough. Remember that the motor is being changed here, so at least some change in the switch is called for.
For special switches, somewhere I still have a switch designed for reversing a single phase motor. It runs in either direction (reversing two of the three circuits), but it enforces a pause when switching from one direction to the other -- since running single phase motors can't be instantly reversed just by swapping two wires. They have to be allowed to slow down enough to come to a near halt (enough so the centrifugal switch closes to enable the start winding for the reversal.)
So -- yes with the right switches -- no problem. However, if you want the weight of the arm of a horizontal/vertical bandsaw to switch it off (at least he 4x6" ones) -- you want a switch with not much operating force -- which is less likely to be capable of switching both sides of the line for the 240 VAC motor. (I'm still not sure why he feels the need for a larger motor -- perhaps it was one of those Chinese import motors which are almost all empty housing, and very little frame, made to *look* like a bigger motor, but bound to burn out with any serious use. My 4x6 HV bandsaw came from MSC, and apparently they spec'd a better quality motor -- and that one is still running, and does not get hand-burning hot with a long cut. :-) So perhaps all he needs is a motor which really is the nameplate horsepower on what he had -- and for that, a 120 VAC motor should be sufficient and the original switch might even do well. (I'll have to look under the base of mine to see what the switch looks like there. :-)
Routing the wiring around the machine tool to provide both the "stop" switch at the end of cut condition, and multiple "stop" buttons within convenient reach if something goes wrong, plus more than one start button as well. With no high voltage on those (if design makes the back easy to contact). You can use smaller gauge wire to route it where you need it without adding stiffness to the machine's movement. (I would like stop and start buttons on the moving arm of the H/V bandsaw, so I don't have to bend over as far to reach the power switch.) But this is just me. :-)
Even the Chinese 12x40" lathes have relays and a transformer to provide the control voltages at 24V instead of the 120 or 240 VAC (the latter more likely for that large a lathe). And -- the wiring might be for 480 VAC as well, depending on where the machine will be installed. Just move some jumpers to change operation from 240 VAC to 480 VAC, and the control buttons don't get any more voltage than 24 VAC.
I discovered how this was set up when I helped a friend convert his to a three-phase motor with a VFD. Even added a "jog" feature to enable rotating the chuck until the proper wrench socket is facing out. :-)
And the VFD actually uses even lower voltages -- 5 VDC or at worst 10 VDC for all the signal leads.
Wood sawdust packed in the switches? What kind of switches? Perhaps the standard wall switch?
Did you take them apart for the fun of finding out what the failure mode was? (Detail photos would go a long way towards settling the debate ranging here. But at least is is about *metalworking* (and woodworking), not politics.
O.K. I've not experienced that on mine -- but given some of the motors on the import H/V bandsaws, that is a possibility. And *that* is a good reason for replacing the motor with one which delivers the nameplate horsepower, instead of being mostly empty air in a too-large housing trying to look like an adequate motor. :-) I've read the reports of motors getting so hot that they burn the hands of the user, but mine never gets that hot. MSC got a proper motor put into it, apparently.
"DoN. Nichols" fired this volley in news: snipped-for-privacy@Katana.d-and-d.com:
Don, I'm not picking on your answer... I had to respond to someone's, and yours got the prize...
This is the dumbest discussion I've ever heard (short of all the political spew on here).
Even half-quality garbage plastic switches from China are NOT prone to accidentally 'switching on' from accumulations of dust or swarf.
That it's a remote possibility, I won't deny. But such switches typically serve for years to decades without a malfunction in the dusty, dirty, swarf-filled environments in which we use them, and it's dumb to think of the average home craftsman's going to the trouble to re-wire, retrofit, and otherwise jigger-up his equipment with low voltage contactors and safety circuits. That some would or even could is beside the question.
This discussion should be turned to "What's the best-quality switch I can buy affordably that will suit the safety needs of the application." For that, I recommend a good industrial-quality safety-style switch that requires a simple swipe of the hand to turn off, and a positive 'de- locking' action to turn on.
My old (1970s) Shopsmith came with one. When I finally wore it out in the 1990s, I replaced it with the same-quality switch from a US maker (IIRC it was a Square-D safety switch, specifically for table saws). You could knock it off easily, but had to pull the bat out manually to turn it back on -- heavy-duty thing. It lasted more than 20 years of nearly daily use, and the replacement is still on the machine, still working.
LLoyd
The AC switch that was used on the table saw that became permanently turned "ON" was a standard house hold light switch that wasn't sealed. The saw dust accumulated behind the contacts inside the switch casing. When the saw was wheeled in and out of the garage, the saw dust pushed the contacts closed. When I cracked open the AC switch it was literally stuffed full of saw dust. I replaced the switch and covered up all openings in the metal box that the switch was mounted in.
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