VFD spindle control safety

**Neale** · 01-08-2019

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** · 01-08-2019

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** · 01-08-2019

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

**magicniner** · 01-08-2019

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** · 02-08-2019

Originally Posted by 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 .

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** · 02-08-2019

Originally Posted by m_c

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.

**routercnc** · 02-08-2019

I’m in the minority here and I start and stop my spindle manually through large push buttons on the control box main panel. These are linked back into the VFD inside the control cabinet through the built in terminals under the front cover.
Same goes for the speed control which is a potentiometer on the control box main panel linked back to the VFD.

Some jumpers in the VFD have to be set to use external controls.

This means the PC is not in charge of the spindle at all except for an e-stop where it will stop and inhibit the spindle running, again using terminals on the VFD.

I’ve never had the spindle come on when changing tools, and have never forgotten to start the spindle before hitting cycle start. In the same way that I have never tried to drill a hole on a drill press without starting the motor! It’s a preference thing and I just like to use it that way, plus it feels a bit safer.

**Doddy** · 02-08-2019

I've had my G&T for the night so am feeling somewhat ambivalent. The OP questions safe operation - and he's correct to do so. I hold a view which I think is shared by some that, gauging an acceptable level of personal risk against convenience that I/we are prepared to cut some corners. But, please understand that doesn't make it safe. I wouldn't pretend for a second, or promote that nipping up a cutter in a spindle with your hands/body/spanner anywhere near is "safe" in the event of either hardware or software failure.

Right!, where's that blasted cat?