Right procedure to change a bit on a spindle during a job?


I don’t currently have a VFD/Spindle.

With a router you have the physical on/off switch to make sure power doesn’t turn on when you are changing the bit, even if your router is controlled by the Onefinity controller (pin 15).

What’s the equivalent with a HUANYANG HY-Series VFD and spindle? How do you ensure the VFD doesn’t spin up the spindle when you are changing a bit? You could kill power to the VFD, is that the only safe way? Does that cause problems with the Onefinity controller (i.e losing communication with the VFD)? Does it cause any other issues (more times power down and up of the VFD)?

I’ve seen online where somebody put a 3 contact switch on the spindle side of the VFD but generally you are told not to put a switch on that side. I assume he’s taking the chance that he’ll always flip it only when the spindle is stopped.

I saw PWNCNC used to have a button (“Running Pause”) to enable and disable the communication from the Controller but have removed it in their latest enclosure. That seemed like a good way of doing the bit changes more safely.


Hey Atroz,

I think this was already asked before :slight_smile:… Ah here is the answer:

Thanks. I saw those buttons (and that response before) and thought of that but it still seemed like it was under electronic control vs a physical power cutoff. I.e. If you hit stop on that and put your wrench on the collect, there’s still a possibility that software could switch it back to run on you isn’t there? I guess you are just putting your hands in harms way in the understanding that it is unlikely a command (including noise (bit flip) on the wire that happens to be the modbus command) would go to the VFD to go back to RUN mode. Am I right to assume that even if it went to run mode it would be unable to spin up without additional commands?

Hey Atroz,

what you are saying, could it be that if the authors of IEC 60204-1, which is an industry standard used in the entire world’s machinery industry for safety of machinery, sell you a “Safe Torque Off (STO)” as not safe?

A noise or bit flip is not likely to be that smart that it acts as a RUN command.

Furthermore, unlike a universal motor like the Makita hand trim router, which is switched on by only one single contact, a VFD will produce the three phases to let the induction motor (=spindle) run only if its microprocessor switches six IGBTs in a smart way.

But yes, if you press the RUN button on the VFD or if you send the RUN command via ModBus while you have your hands on the bit, then the same happens as if you switch your Makita hand trim router on while doing this. This is why safety standards at same time require you to set up a CNC machine in a way that it immediately enters the “Safe Torque Mode (STO)” in the moment you open the door of the CNC enclosure. If you read the manual of my spindle (and I assume that you will read yours when you bought your spindle), there it says, you have to run the tool in an enclosure. And that makes sense. The debris of a broken bit can always fly around and you don’t want it sticking in your body.

I guess I’m a bit old school where I was taught to disconnect the power before changing bits/blades. This trusting the software thing leaves me a bit uneasy but you are right, it is the standard.


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Hey Atroz,

on Omron MX2 / Hitachi WJ200 and some other VFDs for industry, you can additionally use their emergency circuit capability. These industry VFDs have ISO 13849-1 Safety Inputs with which you can put the VFD into an emergency stopped mode. This will prevent the spindle to run even if you press RUN ;).

Possibly you can use some of the programmable inputs of cheap chinese VFD to have the same functionality.

Usually such Safety Inputs are wired to an ISO 13849 Safety relay such as a Omron G9SE (an example wiring is shown here), but you can also enter the Safety mode whithout such a relay, just by connecting a switch to the VFD.

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Quickly searched it for you :slight_smile::

  1. Emergency Stop circuit on VFD ← Here it is

  2. Risks from rotating parts and flying debris:

¹) by ensuring “Safe Torque Off (STO)” mode on the VFD

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I do intend to add an emergency switch (have one in my amazon cart) and I do have my CNC machine in an enclosure. No switch on the door though.

Hey Atroz,

if you search for “Emergency Stop” in the forum, you will notice that there are two very different kinds of Emergency Stop or Poweroff. For the one which we discussed above, which is described here and which means to prevent the spindle to run in case you open the enclosure door or during a bit change (or simply quickly halt the program to prevent it doing something with the workpiece you did not intend), you don’t forcibly need some special switch, in fact you only need a pushbutton (see circuit) and also a reset button to release the emergency state afterwards. Note that such a circuit can be wired to integrate the “estop” mode of the Onefinity Controller. A smart Emergency Stop mode, with no power cut off, just everything halted safely.

The completely different “Emergency Power Off” means, in case of fire, you quickly want to separate the whole installation from electricity. This is usually a big red/yellow button near the door of the room. Since such a total power cutoff does not take into account the state of the machine (spindle may still have been running), it’s not the ideal way to switch the spindle off and it can do harm to your VFD in case the spindle still runs. But, of course, this is a type of Emergency where the state of your machine doesn’t matter that much anymore, you just want the firefighters to be sure that the installation is cut off from electricity so they can spill water on it to prevent the building burn down.

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Yes, I’m talking about the first type being in my cart. A push button that latches to unlock, connects in to the “estop”. The Onefinity one is also easily accessed and I’ll put another on the VFD. 2 switches because of 2 circuits (110V and 220V). I’m the only user of the machine.

Hey Atroz,

just an addition, I think you already know that – at least when I looked at the circuit the last time – the big red/yellow button on top of the Onefinity Controller case is NOT wired to enter the smart “estopped mode” which will make sure the spindle is also stopped (or the “tool-enable”-wired router), but instead just cuts power to the Onefinity Controller at the input of the internal power supply (and leaves spindle running!). If you want to use the true “estopped mode” of the Onefinity Controller, either you got to have the Emergency button wired to pin 23 of the 25-pin I/O port and ground (as described here), or you use the red/yellow Emergency Stop icon that is shown on the top right of your display to enter this mode.

What is important to know (possibly also for others finding this thread later), is that there is a standard way to put a VFD in a safe mode to be able to change the bit, and it’s not powering the VFD off. It’s not correct to power off the VFD for this and it’s also not needed.

Furthermore it can harm the VFD if you constantly power it on and off. In the manual of my VFD it says:

Do not turn on the power and then turn it off again more than once every 3
minutes. Doing so may damage the Inverter.

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How did you discover this information?

Hey Dave,

do you mean how the Emergency Button on the Controller box is wired? Onefinity stated here :slight_smile: