One limit switch per axis

I've not seen this mentioned but sometimes it's hard to find specific info here. I thinking about using one limit switch per axis, so take the Z axis for example, you mount a switch at the mid travel point on the Z 'fixed' backplate, then you position 2 strikers near each end of the Z 'moving' plate, these would have slotted brackets to allow adjustment. Now when the Z axis reaches either end of it's travel one of the strikers will activate the limit switch. Obviously same goes for X and Y and the switch could be mounted on the moving part instead if that was more suitable.
Any opinions about this ?

First it depends on the encoder on the axis -Absolute meaning it knows were it is at power on or the other that requires the machine to be sent to home position/reference at start up ----these have to be very accurate switches and usually are plunger type -------Home position is found using the switch but a seperate dog is used for plus and minus over travel protecting the ball screw ends from being crashed into

I won't be using encoders, absolute or incremental, I'm just talking about physically activated switches or proximity type. I will be using separate switches for homing mainly because I will have a slaved stepper that I believe needs it's own homing switch for squaring up the gantry.

Single switch per axis is fine.
Often the number of switches per axis is more to do with convenience than anything else.

We used this method on overhead cranes and other reciprocating machinery, it's just that whenever limits are mentioned in regard to cnc machines they always talk about 2 switches per axis.

Yes it works fine many milling machines use this method.

Edit: You can still wire them in series has well.

That's the method I'll use then, I thought I would get the electrical drawings done, get some stuff ordered, put it together, while waiting for items from China.
I'm going for an Ethernet Smooth Stepper, probably AM882's, but still pondering the best bob for this combination.

Usual CNC setup with homing required machines
Move to Zero
Plus soft limit--- where the servo is stopped or held mode is plus 5mm and comes from the axis control
Hard limit activated by a switch linked to the Estop circuit is plus 10mm
The same goes for the Minus side
Wither its two switches or two position dogs really doesn't matter but they are there to protect the machine and zones
For recovery from these usually needs an over travel release button on the operators panel and sometimes keyed so proper- questions are ask to why its there in the first place

Quote:

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Originally Posted by Ulsterman

Normally wired open given you a warning if the switch is sticking or failed --remains in Estop with alarm

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Not being pedantic here but it's Normally closed switches not open.! Then if wire breaks you get warning, switch gets broke when it's triggered.

The failure is usually caused by the switch never being used and then they stick down in the on position not the wire being broke ---------So when you release the machine off the switch it is stuck on the on position -so normally open would be a good idea ?

If I was using servos and encoders I would be using the absolute type and as you say soft limits would be my choice with hard limits providing final limits. As I'm using stepper motors this is not an option so while I could fit encoders I've decided to just fit switches. As you say, once these switches have been activated there must be a way to recover and I admit I have not investigated this aspect yet but I'm sure there is a simple solution such as an override button as you mention. With regard to limit switches these are always in the normally closed mode and are opened upon activation, this means that if the wire is broken or the connection fails the system will stop.

Removed by me

Just ignore, I seem to have posted twice.

All safety systems, and limit switches are just that, should ALWAYS fail safe. This means that a failure if the wiring or of a connector MUST result in a fail condition. Being stuck in e-stop nay be inconvenient but no system should ever be designed that cannot validate that it's safety mechanisms are working therefore continuity of limit switches and their wiring is a fundamental pre-run test. It's also why e-stop and limit switches should interlock directly on motor power and not rely on software intervention. You should ALWAYS require some manual action to over-ride e-stop or limit switches, usually by holding down a momentary contact switch while cycling power or performing a software reset action.

Quote:

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Originally Posted by Ulsterman

The failure is usually caused by the switch never being used and then they stick down in the on position not the wire being broke ---------So when you release the machine off the switch it is stuck on the on position -so normally open would be a good idea ?

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No that's unsafe and it's very common for a wire to break or come disconnected in which case the machine E-stops.
Doesn't matter if the switch is stuck after being tripped because E-stop is still affective it just means you have to find the faulty sticking switch which you would want to do anyway.

Regards the Limit over ride then I use Momentary button with Buzzer warning it's overridden.

Thanks jazz your right should be that way so always a break in power detected for these and Estops -sorry for the mix up

Quote:

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Originally Posted by Ulsterman

Thanks jazz your right should be that way so always a break in power detected for these and Estops -sorry for the mix up

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No problem understand and like said was being pedantic but it's small things like these that new folks pickup on and see from other post's that your knowledgeable so take that it's correct and use it.!!. . . I see it often with helping new users so it jumps out at me and wasn't trying to put you down just clear it up.:ambivalence:

Hi

Yes there's 2, but very easy to install

Something like this ?Attachment 10283

Ray