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-   -   VFD & motor spindle safety switch (

Hans Wed 09 December 2009 20:30

VFD & motor spindle safety switch
When using a standard router we can add a router on-off switch near the router in order to turn power off (for safety reasons) to the router when changing routerbits.

My VFD and a 2.2K water cooled spindle are on its way.

I would like to have the same safety feature and put a "switch" between the VFD (3 wires UVW) and the Spindle. Using a 4th push button switch next to the e-stop/pause/resume.

I came up with 3 options:

1. For the 4th push button I could use a spring return push button with 3 NC contacts blocks. They are 6A 230VAC. Is this enough?
Concidering the high frequency/current generated by the VFD.


2 Use a 3 pole NC relais in the control box that is disactivated by the 4th push button on the gantry.


3 Not a good idea due to the high frequency/current between the VFD and the motor spindle. Or for other reasons . . .

Any suggestion ?

Gerald D Wed 09 December 2009 21:56

The proper device would be a motor disconnect switch (example) mounted right near to the spindle. You can get them supplied with their own small enclosure.

However, this switch must never be used under load . . . . . a VFD does not take kindly to current being switched between the VFD and spindle. Use it only as a safety device to ensure that a dead spindle stays dead while you change the cutter

KenC Thu 10 December 2009 02:56

Garald is right, again....

Industrial practice is to connect the VFD to the Motor directly, nothing should be install betweem the two except for the inductor which filter out the harmonics & RFI.

If you wish to connect any switch, power correction capacitor, line filter, fuse, relay, e-stop... etc etc etc any thing which you fancy, do it before the VFD.

Thought you might be interested to know.

Hans Thu 10 December 2009 03:07


Conclusion: it is better to switch the VFD off when changing router bits.

Than I'll use this one


javeria Thu 10 December 2009 06:57

Hans - we regularly change the bits when the spindle has come to a standstill, its also not a good thing to switch on/off a vfd very often.

if inbetween a program(feedhold) you want to put the spindle off - use the on/off wire which is connected between the BoB and the VFD, better still - on MDI tab on Mach put a M5 command and the spindle will stop, then again you can use a M3 to start the spindle again and then resume the program.


Richards Thu 10 December 2009 07:14

The safety interconnect that Gerald posted is basically fail-safe. As he pointed out, it is only to be used when the VFD is idle. That is the condition that you would always have when you're changing a cutter. The interconnect would open all three conductors of the 3-phase lines going directly to the motor.

If the VFD is idle when the interconnect switch is opened, no damage could occur to the user or to the electrical components. If the switch is closed before the VFD becomes active, everything will work as expected.

For the $25 to $100 that a switch and enclosure would cost, it seems like a small price to pay to keep your fingers from being cut off.

Gerald D Thu 10 December 2009 08:16

I am starting to believe that a switch at the spindle (or router) should be mandatory. It is not good enough to have it a few yards/meters away because then it:

a. won't be used, or
b. could be activated by someone/something else (human, pet or electrical fault).

The only risk of having the switch at the spindle is that the switch can be turned while the VFD is energising the spindle, (but that would be stupid), and the VFD may be damaged. But, you need to weigh up this risk with the safety of your hands.

A little story: one day our workshop power tripped out and it took me a while to find the tripped breaker. During this time, Eric saw that the machine was stopped and proceeded to change the bit. I reset the breaker without thinking what might happen at the machine. We happened to have a faulty charge pump and the rebooting PC started the spindle. Eric was okay, but we both got a shock at how bad it could have been. An isolator at the spindle would have prevented the potential accident.

KenC Sat 12 December 2009 23:04

No intention to create more horror stories but rotating machinery are really dangerous.
In my last job, we have screw conveyor accidents which chew up limps and kill... They don't have to be big, a 30cm diameter conveyor turning at 27rpm on a 1/4 hp motor can do a good job to mince up a healthy limp in a matter of seconds... Even with magnetic contactors, inter-locking safety relays, removed fuse, there will be some smart Alek who will by-pass all the safety precautions "for-convenient-sake". Can you imagine the consequences if these safety precautions were not in-place.

Schaffler Wed 10 March 2010 13:02

Fail Safe Switch
The DME Factories Inspector calls for an isolator at the motor and that the relevant plant engineer has made sure all his personnel have undergone sufficient lock out training and follow a standard procedure when working on a machine. Generally they lock the switch in the off position before removing any guards etc.

I really like Gerald's placement of the switch between the VFD and Spindle and as much as I'd hate to pay the expense, It's much cheaper to buy a new VFD than fingers.

Just because we are home hobbyists does not mean that we should neglect the wisdom in factory and mine safety standards. Each time we approach a hazardous situation we need to make sure we are safe. On an average day its 35DegreeC in my workshop. I wear jeans, denim jacket, full face shield, earmuffs and dust mask as a minimum. Welders gloves or "Ninja" gloves when necessary.

Perhaps one should also "dock" the spindle in such a way that it fits into a clamp/spanner that will lock the rotor. That way you'll be free to loosen the nut and replace the router bits with both hands. I've thought about building a spindle using an industrial sewing machine motor as it's motor has a clutch which has to be engaged before you operate it.

Personally I would like to use an auto changing spindle ASAP and replace my current spindle.

Travish Wed 10 March 2010 14:08

I'd like to raise a question using a spindle,

By adding a motor disconnect switch right near to the spindle, and one turns to switch off opening the circuit, does his/her tool change then forgets to close the switch and tries to re-start the spindle? Is this bad for the VFD with no load to it?

KenC Wed 10 March 2010 23:33

Travis, I don't have an answer, the manuals I have discourage any magnetic switch/relay between the VFD & spindle/motor. If you really want a switch you can use a non-fused switch.

Just another thought, with all the hassle of what can be use between the VFD & spindle, why don't we just switch off the power to the VFD when we are doing anything on the spindle? That should be safe, right?

Gerald D Thu 11 March 2010 00:28

A VFD can be safely started with no load (disconnected motor/spindle).

MetalHead Thu 11 March 2010 04:59

I think the thought here is around the fact that most VFD setups tell you to not break the path from the VFD to the motor. My Mill (Industrial Hobbies) has a reversing switch in the path between the motor and VFD. If you turn the switch off while the VFD is on , the VFD will error and you have to reset the VFD to get it all going again. So I am not sure a spindle disconnect near the spindle will work.

But you may be able to hook to one of the Aux ports on the VFD and "pause" the spindle with a button that you place near the spindle.


riesvantwisk Thu 11 March 2010 07:35

Are CNC machine not suppose to have a safety system with a gate to access the moving parts?

To access the spindle you have to open the gate. this gate can then switch off a circuit that prevents a machine from operating.

In our case it would prevent the machine from moving and turning on a spindle, but will keep the stepper motors engaged.

Obvious you can jump on a machine without opening the gate, but then this is really the operators fault.

I can imagine in the case of the MM having a wire of some sort on both ends that need to be closed before the machine can be operated.


Schaffler Thu 11 March 2010 08:13

Ken, I like your answer and is the method I use. Yes, I know it "shortens" the life of the VFD but the useable lifespan you'll get out of it is more than enough.

I also think it might be an answer to use a tool holder to hold the bit when you put it in the collet. That way your fingers are not near the bit anyway.

Personally I like to have some sort of mechanical lock in place and even when teaching my electricians to build control panels for 3 phase systems I insisted on mechanical interlocks for the star delta systems.

Generally I setup my machine before I switch it on. So the bit is in place and everything when the power comes on. So I don't switch my VFD on and more or less and I almost never change bits. I don't seeing this being very different for most hobby processes.

In a business situation, I would rather buy a spindle that has tool changing abilities. They have a pneumatically operated draw bar that holds the tools. Entry level spindles like these cost twice the price, but the time saved in changeover will most certainly pay for itself within the year.

Travish Thu 11 March 2010 09:54

Quote- "A VFD can be safely started with no load (disconnected motor/spindle)."

Thanks Gerald, that makes and keeps things simple and effective.

Fox Wed 31 July 2013 03:34

Did anyone actually implemented the feature discussed here, and how ?
I mean it makes sense safety wise... but I think I have not seen anyone actually do it in all the pictures I have seen of controlboxes and machines (because of the issues discussed) ?

Mrayhursh Fri 30 August 2013 15:32

Big Red Kill Switches
On the Mechmate plans it shows the Kill Switch. As I see these switches on the plan it only shows the "Big Red Kill Switch" and not the Stop or the Start button. Am I looking at it wrong or does some one have a diagram? Also is it recommended to have one, two or three of these sets of switches... Thanks

KenC Fri 30 August 2013 20:46

The "Big Red Kill Switch" A.K.A E-stop. full name Emergency Stop.
If there is any mandatory item on ANY MM, this will be it.

Start is actually a botton wired to Mach3 Start
Stop is a ctually a botton wired to Mach3 Pause.

Mrayhursh Fri 30 August 2013 22:58

Thanks Ken,
the START would be NC in series.
the STOP would be NO in series.

Oleks Tue 24 December 2013 04:47

1 Attachment(s)
If spindle equipped with any kind of Over Heat switch AND - VFD configured properly (disables VFD without delay time)
a simple switch can be added in series.
Also any "disable" (reset, start, brake - whatever your VFD got) signal can be disconnected - just put two additional control wires from VFD to spindle.

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