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Magnetic safety switch for lathe

cmantunes

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#1
Hello!

I'm looking for a new magnetic safety switch for my lathe with the following characteristics:
1) Preferably dual 110V/220V voltage, dual pole
2) Anti-restart, meaning, if power goes out switch turns off automatically and won't turn on when power is reapplied
3) At least 20 Amps
4) Ideally with a prominent E-stop off mechanism (paddle?)
5) Flush mount

I've been looking everywhere but I'm having trouble finding switches which fulfill all these requirements, particularly the anti-restart functionality. If you know of some, please let me know.

Thanks!

Carlos
 

whitmore

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#12
That's not a magnetic switch.
That's right. the manual (it's on the Grizzly site) says it's a toggle switch. To use a contactor (relay), you
want a (green) momentary-ON switch that turns on the relay, and a (red, big-button) momentary-OFF switch that interrupts the relay coil power. An extra contact on the relay is wired so
that the relay coil is actuated either by green-button OR (extra-contact and NOT red-button).

Red button or power drop unlatches the relay, until the green-button is pressed.
But, the colors aren't what makes the switch appropriate, it's the fact that
it has MOMENTARY contacts that lets you wire the relay for nonrestart operation.

I did this on my bandsaw; it works well, but it takes an electrical enclosure for the relay as
well as a switch box.
 

cmantunes

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#13
That's not a magnetic switch.
As I am separating the switch from the anti-restart device, my (modest) understanding is that I no longer need the switch to be magnetic. Is there a benefit to using a magnetic switch if it is preceded by an anti-restart device? In any case, I ordered a Powertec 71007 (visually similar to Grizzly) whose description says "The POWERTEC power tool switches provide the essential function of safeguarding you and your power-tools in the event of accidental electrical reactivation." I'm not entirely sure I understand what they mean but I'll sure find out once I get the switch, and I'll share here for future reference.
 

WoodBee

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#14
Warning, my experience is with the Dutch electricity system, and may not conform to codes applicable to your situation. This is for illustration of the principle only! Use at your own risk, when in doubt consultant a professional electrician.

I have built several switches for older machines. I essentially used a relay and two momentary switches, one NO and one NC. The relay needs one more switch then the number of phases you want to switch. In my case they are three phase machines, so I need 4 switches. A number of safety/emergency switches can be included by inserting them in series with the momentary NC off switch (and each other).

I am no electrical engineer, so the schematics will not be a up to spec. The switching circuit can be improved by using a low switching voltage and compatible relay.
By pushing the on switch the coil of the relay is activated and the 4 relay switches are closed. Three activate the motor and the fourth keeps the coil activated. If any of the emergency/safety switches or the off switch is activated it interupts this "coil circuit" and the relay switches off. This also happens when the power fails. In any of these cases the on switch has to be pushed to reactivate the coil and restart the motor. Of course any of the emergency/ safety switches need to be closed first.
In my case the three phase breaker in the power line breaks all three phases if only one of them exceeds it's limit. This is necessary because otherwise the coil may stay energized if the wrong phase is switched off by the breaker. This is of no concern when used for a single phase motor.
Any number of variations to this principle can be applied, for example a single phase switch with integrated restart protection that activates a relay to switch the motor (when using a multiphase motor), but the principle will be the same. In this last case the specifications of the switch may limit your possibility to include safety/emergency switches. I personally have always built this from the components because I use cheap used relays and I can adapt the switch to my needs easily.
I hope this helps someone,
Peter
 
Last edited:

woodchucker

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#15
As I am separating the switch from the anti-restart device, my (modest) understanding is that I no longer need the switch to be magnetic. Is there a benefit to using a magnetic switch if it is preceded by an anti-restart device? In any case, I ordered a Powertec 71007 (visually similar to Grizzly) whose description says "The POWERTEC power tool switches provide the essential function of safeguarding you and your power-tools in the event of accidental electrical reactivation." I'm not entirely sure I understand what they mean but I'll sure find out once I get the switch, and I'll share here for future reference.
That sounds like magnetic switch
 

grzdomagala

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#16
Just my 5 cents - my minilathe uses magnetic switch of type KJD17B (or very similiar). Every 2-3 years of infrequent use i must disassemble this thing and file contacts clean or the lathe won't start anymore or contacts overheat and weld plastic parts of switch together. Don't know if the switch is too small or minilathe electronics that bad.
 

whitmore

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#17
Just my 5 cents - my minilathe uses magnetic switch of type KJD17B (or very similiar). Every 2-3 years of infrequent use i must disassemble this thing and file contacts clean or the lathe won't start anymore...
It's possible to look up the KJD17B switch;
<https://www.e-switch.com/product-ca...strial-power-pushbutton-switches#.WNBSHRiZN0c>
It seems to be magnetically latched, and as long as you wire it right (it matters which terminals are power IN and which are power OUT) that means it can trip OFF when power is interrupted. Unlike a relay, it cannot
be powered ON with a low-current button, but it does look like a convenient and useful switch component.
There's KJD17 variants for 50/60 Hz, 120V, 240V... it isn't a one-size-fits-all item.

The Grizzly D4151 switch datasheet doesn't claim any magnetic latch feature.
 

grzdomagala

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#18
The problem may be - it closes (goes to ON position) slow, with the speed of human finger. Typical power switches are constructed to open and close fast - finger force is used to stretch a spring which does the action as fast as possible. That may explain excessive sparking. Opening is fast - when power is interrupted (power lost or security switch trigerred) coil stops pulling the contact armature and spring forces opens the switch
 

cmantunes

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#19
It's possible to look up the KJD17B switch
This company, Kedu Electric, appears to be the manufacturer of these switches. They also offer a KJD30 version which would fully satisfy my requirements. Unfortunately, I can't find anyone selling these anywhere. I'm going to shoot these guys an email and see if they can send me a "sample..."
 
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