VFD spindle control safety

[Neale]

No, I never turn off the VFD during tool change. Would probably double tool-change time, waiting for VFD to power down/run up. I'm with Doddy - life's too short...

[Kitwn]

A little less certain that powering off the VFD, you could incorporate a switch in the control cable from the computer to the VFD preventing it being remotely activated by accident. This doesn't protect you against a control fault in the VFD itself. The danger then is that you forget to switch it back on and dig a non-rotating tool into your workpiece. How you protect against that is up to you.

Kit

[Neale]

I realise that I have to trust some amount of hardware and software. However, I am more concerned about triggering the "start" function by accident, which would move the spindle to the next position and then run up the spindle. So, I would get some warning as the machine would start to move from the tool change position, allowing me to let go the spanners. In addition, my F360-generated gcode seems to need the start button to be hit twice - first moves spindle to tool change position, second moves it to start position before turning it on.

I control the spindle by supplying a signal to the VFD control input, so a loose connection would stop it running rather than start it. There are other possible failure modes, of course, but these seem to be less likely and the associated risk is less.

In addition, I am the only user of a hobby workshop so my approach to risk is different to a commercial workshop. I'm now curious to know how commercial machines address this kind of issue

[Doddy]

There will be interlocks on a commercial machine. I was contemplating my little SX2.7 but realised that I'd previously removed the swarf guard from around the spindle - of course its a requirement to move this to change tools and this provided complete isolation from the incoming supply - I expect any commercially available machine will have similar interlocks. God help you within a commercial environment if you defeat such an interlock and the HSE get involved.

[Clive S]

Also you can factor in that there is little to no torque on start up of the spindle with these type of spindles .

[magicniner]

If the VFD is not of exceptionally poor design you will be able to hard wire an E-Stop switch directly to the VFD data inputs and configure the function in parameters if required, when activated this will over ride any other signal trying to power on the spindle until the E-Stop switch is reset.

[m_c]

Also you can factor in that there is little to no torque on start up of the spindle with these type of spindles .

This.
Unless you have a spindle with lots of torque (I.e. milling machine/lathe spindle that's been geared down from a multiple KW motor), then holding the spindle with your hand will be enough to stop it from spinning up.

Plus most VFD manufacturers advise against regularly power cycling VFDs.


If you were worried about damage, I'd put the risk of crush injuries on any reasonably sized/powered CNC, far higher than a spindle starting up when you've got your hands near it.

[Neale]

If you were worried about damage, I'd put the risk of crush injuries on any reasonably sized/powered CNC, far higher than a spindle starting up when you've got your hands near it.

Probably right. Thinking about it, I've stalled my 2.2KW spindle by trying to push it too fast/deep through steel - with a 4mm cutter. That was at around 5K RPM - no torque worth talking about. A spanner on a spindle would easily hold it.

[Tangoterrier]

I have done the non-rotating tool trick a few times with my previous machine!!

[Kitwn]

I have done the non-rotating tool trick a few times with my previous machine!!

Haven't we all! My normal process now is to add 8 second delay to the G-code after starting the spindle to make sure it comes up to speed. As the same relay starts the dust extractor I cannot hear the spindle start but keep a wary eye on the VFD display to make sure it doesn't stay at 00000. 8 seconds is enough to realise I've done something stupid and stop the cut before it starts.

Kit