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Author Topic: M3/M4 spindle reverse safety?  (Read 3487 times)

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M3/M4 spindle reverse safety?
« on: February 19, 2013, 11:16:22 AM »
I'm adding a spindle reverse relay to my lathe (3/4 hp Baldor DC motor w/KBIC-120 controller) equipped with CNC4PC C11 B.O.B. with the 2 relays.

Is there a way in Mach to prevent the system from trying to instantly switch directions if (when) I mistakenly enter an M4 right after an M3 (or vice versa) in MDI or in my code?  KB says "DON'T DOO DAT!"  I should've held out for a deal on one of their controls with the reversing module built-on but I didn't think I'd want spindle reversing at the time.

Obviously I'll do m'best to always put in an M5 and a pause to let the spindle stop before changing directions but I know myself too well to assume perfection. ::)
Milton from Tennessee ya'll.
Re: M3/M4 spindle reverse safety?
« Reply #1 on: February 19, 2013, 11:47:27 AM »
You REALLY don't want to depend on software to protect you from that.  What you need is a hardware interlock, completely independent of Mach3.  If it happens even one time, your motor controller WILL be toast.

Regards,
Ray L.
Regards,
Ray L.
Re: M3/M4 spindle reverse safety?
« Reply #2 on: February 19, 2013, 12:04:53 PM »
OK that makes sense Ray, thanks.   Any suggestions on where to find a suitable schematic?
Milton from Tennessee ya'll.
Re: M3/M4 spindle reverse safety?
« Reply #3 on: February 19, 2013, 12:27:40 PM »
The biggest problem you face is that it is not enough to simply prevent the rotation from being reversed.  You need to ensure that the motor is stopped, or very nearly so, BEFORE it can be reversed.  This means either ensuring that BOTH directions are disabled for a long enough period of time that the motor is certain to have coasted to a stop, or to actually sense motor speed, and not allow the motor to be re-energized until it has stopped.  Using a timer is the simpler approach, but then how do you "tell" Mach3 that the spindle is not running for several seconds?  This approach would require at least two relays, and a timer, either electronic, or a time-delay relay, and custom M3/4/5 macros in Mach3.  Either way, it's not pretty, and I really think you're much better off simply using a manual reversing switch to make it really fool-proof (well, assuming the operator is not the fool....).

Regards,
Ray L.
Regards,
Ray L.
Re: M3/M4 spindle reverse safety?
« Reply #5 on: February 19, 2013, 01:15:13 PM »
Those contactors can be used to create an interlock that will prevent the motor from reversing *while* powered, but will not prevent the sequence:

Motor on CW, Motor Off, Motor on CCW  (or the reverse)

from being executed quickly enough to blow the controller, due to the fact that the motor could still be spinning at high speed CW when the CCW direction is engaged, which is the real risk.  Preventing a change directly from CW to CCW, or vice-versa, can be implemented with just two DPDT relays, such that one relay is disabled any time the other is enabled.  But the CW-Off-CCW sequence, or its reverse, is also deadly to the controller, if the motor is not allowed to spin down during the Off portion.  That can only be prevented by use of speed sensing, whether direct (e.g. - an encoder, speed switch, etc.) or indirect (e.g. - measuring back EMF), or by ensuring the Off state persists long enough for the motor to coast to a stop.

Regards,
Ray L.
Regards,
Ray L.
Re: M3/M4 spindle reverse safety?
« Reply #6 on: February 19, 2013, 01:19:21 PM »
Great links Brett, thanks!

...you're much better off simply using a manual reversing switch to make it really fool-proof (well, assuming the operator is not the fool....).
Hey, I resemble that remark!
Milton from Tennessee ya'll.

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Re: M3/M4 spindle reverse safety?
« Reply #7 on: February 19, 2013, 01:50:30 PM »
No problem, I just got a few last week and they seem quite nice, not hooked up yet though.  Ray is absolutly right as well, I just posted those links to try to help you find some of the bits.  A sensor to monitor the spin and a timer will be needed to do it right.

Brett 
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: M3/M4 spindle reverse safety?
« Reply #8 on: February 19, 2013, 02:35:26 PM »
Well I took the pusillanimous way out.;D  I just copped a brand new KBCC-125R (emphasis on the R) on ebay for $65 + shipping. 

The R models do Forward-Brake-Reverse with a SPDT On-Off-On switch or Forward-Instant-Reverse with only a SPDT switch so surely I can control it safely with the C11's 2 relays.  Being a KB, it's probably bullet-proof as well....dunno 'bout FOOL-proof though. ;)

Now I gotta start figuring where to put the thing since the APRM3 reversing module makes it almost twice as big as the KBIC-120.

Wish I'd had a friggin' clue when I started building this thing!  Oh well, there's always the next one.
Milton from Tennessee ya'll.