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Limit Switch Basics

Posted: Tue Jan 05, 2016 9:07 pm
by ejlatstl
Hi All... Happy New Year,

I've read numerous threads that talk about limit switches, wiring, settings... and so on.

How about this basic question... How are limit switches suppose to function?

And... once I know what a properly functioning limit switch is suppose to do, what is the best way to test that it's doing it?

Am I correct in believing that in the run of a successful tool path... the limit switches should never be triggered?

If anyone knows of a soup to nuts book that covers using CNC machines for a woodworking hobby I'd love to know about it. Mind you, I want to use it as a tool... I'm not interested in building one, or improving one.

Thanks!!

Re: Limit Switch Basics

Posted: Tue Jan 05, 2016 9:41 pm
by Fablicator
ejlatstl wrote:Hi All... Happy New Year,

I've read numerous threads that talk about limit switches, wiring, settings... and so on.

How about this basic question... How are limit switches suppose to function?

And... once I know what a properly functioning limit switch is suppose to do, what is the best way to test that it's doing it?

Am I correct in believing that in the run of a successful tool path... the limit switches should never be triggered?

If anyone knows of a soup to nuts book that covers using CNC machines for a woodworking hobby I'd love to know about it. Mind you, I want to use it as a tool... I'm not interested in building one, or improving one.

Thanks!!

The primary purpose of limit switches is to provide a repeatable reference for (0,0,0) coordinates by slowly moving the machine until the switches are triggered at the beginning of a program.

The secondary purpose is to stop the machine if something goes horribly wrong and any of the axis are about to be pushed past the machine boundaries.

As for easy testing, triggering a limit switch during a run will stop the machine.

Re: Limit Switch Basics

Posted: Tue Jan 05, 2016 9:48 pm
by CastIrony
How about this basic question... How are limit switches suppose to function?

Am I correct in believing that in the run of a successful tool path... the limit switches should never be triggered?
Limit switches should never be hit while performing a job. It basically tells the controller "you're about to crash the machine, abort immediately". There's usually two per axis, one on each side, except the z axis which typically just has one. Since opposing limit switches on a single axis should never be triggered at the same time, they're often wired together to a single pin on the controller.

Homing switches are used to zero the machine's coordinate system when you start the machine, or run a manual homing cycle. Only one homing switch per axis is needed.

Grbl allows you to use some of your limit switches as homing switches, which is very convenient, but can cause confusion when you're learning about them.

And... once I know what a properly functioning limit switch is suppose to do, what is the best way to test that it's doing it?
I havent set mine up yet, but I'm under the impression that Grbl will output limit switch trigger events to the serial console, which should allow you to test that they're wired properly.

Re: Limit Switch Basics

Posted: Wed Jan 06, 2016 12:17 pm
by WillAdams
Excellent discussion, which I need to capture into the wiki at: http://www.shapeoko.com/forum/viewtopic ... 86&p=58325