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View Full Version : Spindle on/off under CNC software control - safety worries!



HankMcSpank
18-12-2012, 05:42 PM
I quite fancy the idea of my CNC software turning the spindle off automatically....what I'm not keen on is the concept of it turning on again while I've still got my hands on the collet holder!

So how is this addressed in practice? (I'm sure it's not just me that it alarms!) ...the thought of a simple relay to control the spindle on/off doesn't seem that robust....I'm figuring there needs to be some form of time delay (sustained condition for X seconds) before the spindle can be allowed to start?

m_c
18-12-2012, 06:44 PM
If you're really concerned, build in an interlock of some kind, which can be something as simple as just switching of the spindle power when needed.

JAZZCNC
18-12-2012, 07:22 PM
So how is this addressed in practice? (I'm sure it's not just me that it alarms!) ...the thought of a simple relay to control the spindle on/off doesn't seem that robust....I'm figuring there needs to be some form of time delay (sustained condition for X seconds) before the spindle can be allowed to start?

Doesn't work like that Hank. Works in 2 ways.?

If doing a tool changes in Job then the M6 tool change macro code stops the G-code and the spindle via switching the relevant output state controlling the spindle relay. It then waits until you press cycle start again often pausing for a second or two so the spindle spins upto speed before continuing on thru the g-code. It's not a time based operation at all.

OR. . . You can manually stop/start the spindle often Via a screen button or thru MDI using M3/M5 G-codes which does the same has screen button and switchs the relay with it's output signal.

If you really don't trust the relay or system then just do like M-C says and put an in-line isolator switch. Doesn't need to be on power lines it can be a simple on/off switch breaking the signal to relay so the NO relay can't be switch state.!

In practice it's not a problem and I've never had a false start or relay switch state.

HankMcSpank
18-12-2012, 07:33 PM
I guess ultimately then, it's how much do you trust the relay (or more so, the relay contacts) that controls the spindle startup! (I'd figured there'd be a little more 'conditional' scenarios involved before a spindle could be allowed to start)

The problem with an isolating switch, is forgetting to switch it back to the 'enable' position & then walking off (or forgeting to isolate it in the first place!)....but far better that then having my hands turned to mincemeat by a spindle relay contact closing while my hands are still on the spindle!

JAZZCNC
18-12-2012, 07:59 PM
I guess ultimately then, it's how much do you trust the relay (or more so, the relay contacts) that controls the spindle startup! (I'd figured there'd be a little more 'conditional' scenarios involved before a spindle could be allowed to start)

Not really the relay contacts because it's a NO relay and when signal drops power to relay coils the contacts break naturally ( Unless faulty in which case it won't stop anyway.!)
It's the control software or more accurate the signal state you can't trust and in extreme case's it's possible it could switch state thru noise interference.!!

You could build into the Macro some conditional checking like checking the state of a input. This could be done by using double throw relay which sets a spare input used has a NOT-READY-NO-GO
This is one of the reasons for using a M6 tool change macro. It's in this macro where you'd put code for things like auto tool changers etc and also for doing safety checks like this if needed.! (could even turn kettle on for you for quick brew between changes. . Lol)

HankMcSpank
18-12-2012, 08:05 PM
Not really the relay contacts because it's a NO relay and when signal drops power to relay coils the contacts break naturally ( Unless faulty in which case it won't stop anyway.!)
It's the control software or more accurate the signal state you can't trust and in extreme case's it's possible it could switch state thru noise interference.!!

Exactly, I realise the relay contacts in play will be NO ...but it's the software/circuit that drives the coil I worry about...this is why I figured some condition dependent time delay would be brought into play here on the relay circuit ...therefore if a transient occured (in software or the control board), the spindle wouldn't kick into life straight away ...ultimately, I kinda seek a beep-beep-beep (like bin truck reversing sound), a couple of seconds before the spindle starts ...that way if I hear beep-age, I know to get my hands off the spindle sharpish!

JAZZCNC
18-12-2012, 08:31 PM
.ultimately, I kinda seek a beep-beep-beep (like bin truck reversing sound), a couple of seconds before the spindle starts ...that way if I hear beep-age, I know to get my hands off the spindle sharpish!

Sure it's not that difficult to implement if your that concerned but to be honest IME not really needed and something along the vain of my last suggestion would be easy to do and pretty much fail safe.

ptjw7uk
18-12-2012, 08:40 PM
Thats the reason all factory based cnc machines have to be interlocked!
Although I have seen some bypassed but not to be advised though!
Most problems arise from the ID ten T doing the controlling!

peter

m_c
18-12-2012, 11:11 PM
If you're worried about not switching something back on, for the interlock, use a switch with an extra contact, or wire in an extra relay, and have it connected to an additional input, then set up the controller to prevent it from restarting until it sees the additional input, and flagging up if the interlock isn't closed.

JAZZCNC
18-12-2012, 11:32 PM
If you're worried about not switching something back on, for the interlock, use a switch with an extra contact, or wire in an extra relay, and have it connected to an additional input, then set up the controller to prevent it from restarting until it sees the additional input, and flagging up if the interlock isn't closed.

Isn't that just the same has what I said in post #5.?

m_c
19-12-2012, 04:18 PM
Maybe :rolleyes:



(It's been a long week!)

Christian Knuell
29-12-2012, 04:54 PM
Hi,

a time delay is quite hard to implement - and you'd also need something like a beeper telling you the timer started - otherwise you'd be facing the same trouble just a few seconds later.
Voltage spikes due to interferences are no problem - a mechanical contact will not act at all as the energy is much to low (and even if - it would fall back immediately) and a solid state relay will just switch on for one cycle and then stop again.

If you really worry about it put a switch into the control or power line as suggested before. However in case of a spindle with VFD: never put a switch between VFD and spindle - you'll kill the VFD if you accidentially switch while running.

Christian

cropwell
29-12-2012, 05:20 PM
In my control box I have a 3 position switch - Auto (which goes thru the relay), Off and Manual. My greater concern is that LazyCam sometimes doesn't generate the M code to start the spindle, in which case I have to hit the E-stop before the tool breaks.

Wombilly
12-02-2013, 02:47 AM
as an ex employee of www.emachcnc.com (http://www.emachcnc.com) we used to use m3 to turn on spindle and m5 to turn it off, the output from the controller switched a relay, for added safety we supplied the inverter via a contactor, whose coil was switched via a key switch, as far as the HSE were concerned, as long as the facility to remove a tool ( key ) to make safe the spindle, it was deemed safe to handle.