Just a quick silly question for Tormek

[JustADudeInSweden]

Hey!

I'm a noob to these kinds of machines, and I was wondering what's the logic behind having an On and Off button on the Tormek instead of one button that does both on and off?

The one button option seems more convienient and should/would be cheaper to make... right?


(Just curious)
Thanks!

[SparkyLB]

It might sound trivial, but a button that does double-duty is less safe.  With some "fixed" rotating machinery it is somewhat customary to have two independent buttons.  The buttons are electrically mechanically interlocked, so the operation of one button is not only single action, but also negates the action of the other.  Much like a drill press the user has some peace of mind with that intuitive contingency should the unexpected occur, and a second to think is a luxury.  Add edged tools to the mix, and the safety it adds is more significant. 

[RichColvin]

Isn't there a European safety requirement also?  I thought that it was something to do with when power is lost and then restored, the device doesn't automatically re-start.

[SparkyLB]

That's a great point, RichColvin.  I'm in the US.  I know the National Electrical Code (applicable to North America but used elsewhere voluntarily or by preference) states when an overload device causes a motor to stop, it is generally restarted manually, but is permitted by exception(s) to restart automatically.  (see Art. 430). 

By design, if a Tormek's thermal overloads caused it to shut down, the power button would have to be pressed to restart it.   

[tgbto]

I know for most (all?) of my rotary equipment you either have rotary switches (so there is no ambiguity what you need to do at any time to interrupt the power supply) or double push-buttons. In the latter case, the ON and OFF switch work very differently (and the safety manual of the Tormek seems to confirm it is the case with Tormek as well) :
- The ON pushbutton triggers a magnetic switch by powering a solenoïd. This closes two switches that connect mains power to the machine. Once the machine is powered the solenoid remains powered and so power is maintained. Pushing again on the ON switch does basically nothing as the solenoid is already powered.
- The OFF switch interrupts the power supply to the solenoid, so the machine is disconnected from mains

Such switches usually include an overcurrent and/or differential protection as well, so that when the protection is triggered, power is removed from the solenoid, and the machine is turned OFF. I'm not familiar with the "U<" symbol on the switch, my guess would be differential protection.

All in all, you can't have such a switch with a single button as at a moment in time you wouldn't know what pressing on it might do : Is the machine off so i'll turn it on, or is the motor somehow not turning but the machine is powered and i will turn it off ? That plus the fact that if the switch was bistable, you'd keep supplying power to the solenoid, overriding the protection.

[JustADudeInSweden]

Thanks! It's fun to khow how even such a small design of two buttons has a bunch of history and regulations behind it 

[highpower]

U<  designates an under-voltage protection relay.

I notice the ON button is guarded so that it takes a deliberate aim with a finger to turn the machine on. That would help to prevent accidental starts if something falls or is dropped on top of the switch area I suppose. The OFF button has no guard making it more likely the machine would get turned OFF in the same situation or just easy access in case you should need to turn the machine off quickly.