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Author Topic: Limit Switches Hard and Soft  (Read 1427 times)

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Limit Switches Hard and Soft
« on: February 22, 2018, 03:27:05 PM »
Hello all,
I have my Hard Limit Switches glued to the chassis and wired in series and when any of the limit switches are contacted the machine stops. X travel is 50.0000mm and my Y travel is 60.0000mm these numbers are when the machine just moves the switch.
Now how do I set the soft limits? On the X travel do I start at 50.0000 mm enter this in max and 00.0000 in min and hit enter.

Best Regards and God Bless
Jim G
Cushing, WI

Offline Davek0974

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Re: Limit Switches Hard and Soft
« Reply #1 on: February 22, 2018, 04:45:57 PM »
I always start a few mm away from the switches and see how it goes -its easy to change.

Yes, 0mm is usually the homing end and the other would 50 or 60 in your case, make sure the table is in the middle area, set the values then turn on "soft limits" and try jogging into the end, a little fine tuning and thats it.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Limit Switches Hard and Soft
« Reply #2 on: February 22, 2018, 09:49:27 PM »
I just cannot seem to get it through my head, I have been watching You Boob and it all seems so easy although each person has their own way.
Re: Limit Switches Hard and Soft
« Reply #3 on: February 23, 2018, 12:35:51 AM »
Hi,
soft limits rely on the machine being referenced or homed. How does your machine do that?

Craig
My wife left with my best friend...
     and I miss him!
Re: Limit Switches Hard and Soft
« Reply #4 on: February 23, 2018, 11:42:19 AM »
Settings (ALT6) Tab
X axis, I told the machine to move 20 mm and all I got was 10 mm  pondering this huge discrep the machine must have moved the X only 10 cm .
I do have a steel flat ruler taped to the bed so as to get excellent readings

Offline Davek0974

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Re: Limit Switches Hard and Soft
« Reply #5 on: February 23, 2018, 11:59:46 AM »
Sounds like your steps-per is out maybe, if its only moving 50% of the instructed distance, you need to do some tuning before messing with soft limits.

As Craig said above, you cannot use or set soft limits until the machine is referenced or homed.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Limit Switches Hard and Soft
« Reply #6 on: February 23, 2018, 02:15:40 PM »
Hi,
you are trying to run before you can walk.

If you can't jog or MDI accurately 10mm say then who cares about whether the limit switches work? Until you can home your machine then
soft limits are meaningless.

Start by reading:

http://www.machsupport.com/wp-content/uploads/2013/02/Mach3Mill_Install_Config.pdf

Get your axes moving the correct distance by calculating or measurement to set the 'Steps per Unit' setting in the motor tuning page.

Craig
My wife left with my best friend...
     and I miss him!
Re: Limit Switches Hard and Soft
« Reply #7 on: February 23, 2018, 09:27:32 PM »
Hi wisconsinjimmy,
are you a country person? I am and am therefore comfortable with rural analogies.

Lets say you have an automatic tractor and you wish for it to plow a rectangular paddock. You could build 'limit switches' on to the tractor so it knows when it
hits the fence but really its too late, its hit the fence, the switches may stop it from wrecking it but the switches haven't helped it navigate around the paddock
to do its job.

Imagine that somewhere in the paddock there is one rock which pokes out of the ground. If you manually drive the tractor to the rock and then program the tractor
'you are at the rock', and the boundaries of the paddock are 210 yards to the North, 65 yard to the South ...etc. Now the tractor knows where it is and if you let it go to
work it will know where it is based on the distance and direction its travelled since it was 'at the rock'.

If you had mistakenly programmed the tractor to plow out to 250 yards North of the rock the tractor 'soft limits' will say NO, the boundary is 210 yard North, I can't
go where you want me to go so I'll stop.

That is the essential meaning of soft limits. It absolutely relies on a place 'at the rock'. That place must be exactly the same everytime the tractor is required to plow
that paddock because only then do the boundaries make sense.

For this reason I always argue that home switches ('the rock') are more important than limit switches. It seems that a lot of people disagree with me but I happen
to know some very experienced CNCers who do agree and don't even bother with limit switches. Good reliable home switches and soft limits are the way to go.

Craig
My wife left with my best friend...
     and I miss him!

Offline Davek0974

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Re: Limit Switches Hard and Soft
« Reply #8 on: February 24, 2018, 02:22:50 AM »
I fully agree limits are pointless on a machine with steppers as they will stall if soft limits fail or were turned off, but on a servo driven machine i would prefer hard limits wired to the safety circuit not the controller - if an axis runs away it will kill the power.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: Limit Switches Hard and Soft
« Reply #9 on: February 24, 2018, 02:39:18 AM »
Hi Dave,
I'm not decrying the use of limits but rather that homes switches are more useful to actually do a job.

I am lucky enough to be able to sail keel boats regularly as part of a race crew, I guess the pic which is attached to my posts rather suggests that the sea is the best
recreational resource we have. Anyway people often ask about safety equipment like VHF, EPIRB and a liferaft. All of those things come into play when you've already
sunk the boat....I call safety those things that you do to PREVENT the sinking of the boat in the first place! Things like up to date charts, knowing how to read them, how to
navigate WITHOUT GPS, what techniques and sail trim you can employ when or if a gale strikes....you know the skills of sailing!

In like manner I consider limit switches to be safety devices of last resort and home switches the first port of call to avoid crashing into the limits.

Craig
My wife left with my best friend...
     and I miss him!