Thor, or should that be Zeus?

[Chaz]

Ok, fundamental question.

Is it normal or acceptable to use the same switch (mechanical or proxy) for both home and limit? I dont understand how the machine would know if it was simply sent home or it actually needs to stop as its a limit. Is there logic that says 'ref all' and it goes home but if its hits the home limit during any other movement (manual or programmed) that the machine will then stop?

[Desertboy]

Ok, fundamental question.

Is it normal or acceptable to use the same switch (mechanical or proxy) for both home and limit? I dont understand how the machine would know if it was simply sent home or it actually needs to stop as its a limit. Is there logic that says 'ref all' and it goes home but if its hits the home limit during any other movement (manual or programmed) that the machine will then stop?

I know how it works on grbl (Arduino cnc solution) when it's doing a home it ignores the limit so doesn't go into shutdown mode. If it's not homing then it treats it as a limit switch and shuts everything down until reset G code is sent.

I'm sure Mach 3 treats it the same way (I think LinuxCNC does)

If I were you though I would fit another set of limit switch further back that trigger an E stop as well as extra protection! I can maybe help you source high quality IP64 rated microswitches I have a friend who services and repairs 4 poster lifting ramps for steam cleaning bays I'll give him a bell see how much he would charge for new ones if they're cheap enough I'll use them lol.

[Clive S]

Ok, fundamental question.

Is it normal or acceptable to use the same switch (mechanical or proxy) for both home and limit? I dont understand how the machine would know if it was simply sent home or it actually needs to stop as its a limit. Is there logic that says 'ref all' and it goes home but if its hits the home limit during any other movement (manual or programmed) that the machine will then stop?

Chaz I think you are getting bogged down with all this. When you home the machine the software knows that you are homing and therefore treats the switches as (homes) when the machine has homed the software then treats the switches as limits.

In your case you will have plenty on inputs so why not keep them separate. (it is also OK to use just one switch for home and one end limit)

[AndrewMawson]

I agree with Clive in his post above (Hi Clive !)

However my personal preference on big powerful machines is to have the home switches inside the boundries set by the limit switches, and have the limit switches trigger powering off of the drives. Also to have the limit switches within the boundries of mechanical stops that limit travel before ball screws over travel.

With a big servo driven machine like my CNC Beaver Partsmaster Mill, or the Traub lathe I rebuilt last year (now sold), the servo drives are a few killowatts, so if they run away damage WILL be done without limits.

However that said, my CNC Plasma Table is set up sharing home and limit switches driven by Mach3 - it's how the chap who originally built it made it, and it wasn't easily altered when I re-built it. But the drives are steppers and only Nema 46's so in the worst case if it runs away not much breaks !

[Chaz]

Chaz I think you are getting bogged down with all this. When you home the machine the software knows that you are homing and therefore treats the switches as (homes) when the machine has homed the software then treats the switches as limits.

In your case you will have plenty on inputs so why not keep them separate. (it is also OK to use just one switch for home and one end limit)

Thanks. The penny dropped, just never knew this before.

[Neale]

Clive has just answered the home/limit question - Mach3 knows what you are doing so treats the switch inputs accordingly. There is a minor gotcha with this. If you use a switch with effectively no offset between on and off positions, you can get problems (to be more accurate, I had problems) with this. Z homed, then X and Y started moving. The Z switch had swapped from homing to limit function, and the slight vibration caused the Z switch to trip and stop the machine ("limit switch event"). Fortunately, the IP/M has the capability to move the axis very slightly away from the home position immediately after homing which fixed the problem.

The limit on number of inputs only applies if you are using a parallel port. That statement on the Mach3 config page is a bit misleading. There are only five input pins on a parallel port as all the others are output only, so the limit is a physical one and nothing to do with Mach3. You will be fine with the IP/A and are free to use all the ports. I doubled up my switches (combined upper and lower) mainly to reduce the number of wires needed and I still have some unused inputs on the IP/M.

Don't worry about voltages. Desertboy was talking about Arduinos and they are definitely 5V only. The IP/A will be very happy with 24V signalling, and in fact this gives much better noise rejection. The CSMIO kit is built to use the industry standard 24V signalling. No pull-up/pull-down resistors needed, but watch the wiring instuctions carefully so that you get it right. Less flexible motion controllers only have one input pin but there are two per channel on the IP/A. There are plenty of examples of how to wire NPN n/c proximity switches in series if you do a Google search - I have had four in series working reliably in testing although in practice I only wire them in pairs on my machine.

[Chaz]

Clive has just answered the home/limit question - Mach3 knows what you are doing so treats the switch inputs accordingly. There is a minor gotcha with this. If you use a switch with effectively no offset between on and off positions, you can get problems (to be more accurate, I had problems) with this. Z homed, then X and Y started moving. The Z switch had swapped from homing to limit function, and the slight vibration caused the Z switch to trip and stop the machine ("limit switch event"). Fortunately, the IP/M has the capability to move the axis very slightly away from the home position immediately after homing which fixed the problem.

The limit on number of inputs only applies if you are using a parallel port. That statement on the Mach3 config page is a bit misleading. There are only five input pins on a parallel port as all the others are output only, so the limit is a physical one and nothing to do with Mach3. You will be fine with the IP/A and are free to use all the ports. I doubled up my switches (combined upper and lower) mainly to reduce the number of wires needed and I still have some unused inputs on the IP/M.

Don't worry about voltages. Desertboy was talking about Arduinos and they are definitely 5V only. The IP/A will be very happy with 24V signalling, and in fact this gives much better noise rejection. The CSMIO kit is built to use the industry standard 24V signalling. No pull-up/pull-down resistors needed, but watch the wiring instuctions carefully so that you get it right. Less flexible motion controllers only have one input pin but there are two per channel on the IP/A. There are plenty of examples of how to wire NPN n/c proximity switches in series if you do a Google search - I have had four in series working reliably in testing although in practice I only wire them in pairs on my machine.

Awesome, thanks.

Suppose now I need to decide if its proxy and mech or only proxy. I could do mech as primary home / limit and proxy as limits via E Stop circuit.

[Chaz]

My drive will likely support limits too although Id have to undo some of the wiring already done to accommodate this.

[Davek0974]

On my IP/A setup i used mechanical limits and mechanical homes, two limits and one home per axis.

My limits are a high power circuit (24v contactor circuit as opposed to a logic circuit) as they are wired to a PILZ relay for the safety circuit which cuts all power to the drives if tripped - I really did not want to risk runaway blowing the end brackets off my table :)

The homes are inboard of the limits and i use home-offset to set the locations.

It all works well here.

When using mechanical switches, get quality ones with Gold-flashed contacts, these will not degrade under logic levels, normal switches rely on an arc to keep the contacts clean - these will fail pretty fast at logic voltages.

[Chaz]

On my IP/A setup i used mechanical limits and mechanical homes, two limits and one home per axis.

My limits are a high power circuit (24v contactor circuit as opposed to a logic circuit) as they are wired to a PILZ relay for the safety circuit which cuts all power to the drives if tripped - I really did not want to risk runaway blowing the end brackets off my table :)

The homes are inboard of the limits and i use home-offset to set the locations.

It all works well here.

When using mechanical switches, get quality ones with Gold-flashed contacts, these will not degrade under logic levels, normal switches rely on an arc to keep the contacts clean - these will fail pretty fast at logic voltages.

How do I know if they have gold flashed contacts? Quick look at RS doesnt seem to show that level of detail and their details are normally fairly generous.