About homing and switches...

[Neale]

Yes, i got the IP-M controller.
Are you saying that i can use one sensor /axis that doubles as limit switch and homing switch or do i need one homing sensor + one limit switch /axis?
Btw im using a driver by the name 2HSS86H.

You can use one sensor on the moving part and two targets, one at each end. In normal operation, this acts as the limit switch but during homing, Mach3 and the IP-M know that it is being used for homing and it does not act as a limit switch. As soon as the homing operation is complete, the switch becomes a limit switch again.

I use Mach3 and IP-M. I use a proximity switch at each end (so two swtiches on X, two on Y, and one on Z as it is difficult to use a limit switch for the bottom of Z travel). It is easy to wire the switches on one axis in series so that upper and lower limit switches are connected to one IP-M input. This means that you can home X and Y at the same time. My machine is configured to home Z first (to get the spindle to the highest point to clear anything on the machine), then X and Y together. This saves a little bit of time on a bigger machine and works well.

If you just use one sensor per axis with a target at each end, then you can still wire each sensor to a separate IP-M input channel, obviously, and get the advantage of homing two axes at the same time.

I had one small problem with my Z axis homing. As I said, my machine first homes Z, then the other two axes. However, these proximity switches sometimes have their on and off trigger points very close together. What this meant was that when my Z axis was homed, the switch was so close to the target that a tiny amount of vibration could cause the switch to trigger accidentally and this often happened while X and Y were moving. The answer is to configure Mach3 so that the home position is very slightly offset from the actual switching position - I use about 1mm. This avoids accidental triggering.

[JW1977]

Ok i see, thanks... It would be interesting to see how you put the switch on Z, do you have any pics that you would like to share?
To use one sensor and two targets... Arent the sensor pointing towards one of them, how can you make it work in two directions?
Would you recommend using one or two per axis? Obviously you choose two, but why?

[routercnc]

To use one sensor and two targets... Arent the sensor pointing towards one of them, how can you make it work in two directions?

Imagine the sensor is mounted to the gantry and pointing down. It moves sideways over the target, not directly towards it. At some point when it is directly overhead it will trigger and stop the axis. If there is a fault it will just glide on past without damage until the gantry hits the hard stop further along.
When it is moving the other way, again it is looking down and will eventually glide over the target at the other end which will trigger the axis to stop. So one sensor, two targets.

It has been shown to be just as effective using a parallel approach as a direct approach. As long the target gets within 4mm (or whatever the rating is) it will trigger no matter how it got there.

[routercnc]

As you are using CSMIO, as Paulus mentioned, this will take the prox sensor 24V directly. It has taken me a while to find my original drawing but for anyone with a cheap 5V BoB this is what I had to do to get it work. Without the diodes it was very unreliable at triggering. With the diodes it is 100%. Maybe these should be fast acting types, but mine were regular types and the homing is very repeatable. Sorry for the side track on the OP.

[JW1977]

Imagine the sensor is mounted to the gantry and pointing down. It moves sideways over the target, not directly towards it. At some point when it is directly overhead it will trigger and stop the axis. If there is a fault it will just glide on past without damage until the gantry hits the hard stop further along.
When it is moving the other way, again it is looking down and will eventually glide over the target at the other end which will trigger the axis to stop. So one sensor, two targets.

It has been shown to be just as effective using a parallel approach as a direct approach. As long the target gets within 4mm (or whatever the rating is) it will trigger no matter how it got there.

Offcourse, thats how its done, thanks!
Well then i have a pretty good idea how to do this for X and for Y.
Im not really sure about Z yet. How do you put the "down" limit for the Z axis?

[routercnc]

Offcourse, thats how its done, thanks!
Well then i have a pretty good idea how to do this for X and for Y.
Im not really sure about Z yet. How do you put the "down" limit for the Z axis?

Lots of people don’t have one since the height of the workpiece and stick out of the tool are variable so there is no obvious place to put it unless you set it up in a variable position to suit each job

[AndyUK]

Lots of people don’t have one since the height of the workpiece and stick out of the tool are variable so there is no obvious place to put it unless you set it up in a variable position to suit each job

I'm planning on putting mine so that the ballnut doesn't impact the BK/BF mount, and so that the carriages don't fall off the linear rails.

[john swift]

As usual
the devil is going to be in the detail
when looking for the solution to problems connecting limit switches to breakout boards
for instance the LJ12A3_4_Z_BX proximity switch does not have the simple open collector NPN transistor output you may expect going by some adverts and data sheets
It has a hidden 10K pull up resistor connecting the output to the positive supply !


with ref to the diagram in post 24

adding the diodes works with the usual TTL inputs found on most BOB's
that requires the switch to either ground the input or go open circuit

some Chinese BOB's have opto isolated inputs with a PC817 input LED and current limiting resistor connected to +10V

the Geckodrive G540 has its opto isolator connected to +12V and has a 2K2 current limiting resistor


so you don't always see 5V across the BOB's open circuit input and ground terminals !

BOB's like CNC4PC's C31 need a positive 24V input
so if you are using proximity switches you need one with a PNP output
to pull the BOB's input up to the positive supply

many Chinese BOB inputs connect to either a 74HC14 or 72HC245 logic
IC input
(see the upper left hand corner and centre of this diagram for the Chinese BOB input
Lower left hand corner for the C31 )



( I prefer N/C switches because a broken wire will be trip the axis limit
with N/O switches a broken wire will not be detected )


John