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Author Topic: Best way to wire my home and limit switches?  (Read 6522 times)

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Offline jevs

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Best way to wire my home and limit switches?
« on: January 16, 2011, 07:20:19 PM »
My machine has a limit switch on the X and Y axis that is hit by a dog at each end of the table. There is also what I guess is a home switch that is slightly in from one of the limit dogs.
Should I uses the home switches at all? This is a VMC bed mill with tool changer.

My Z-axis is a whole other issue. There are two switches in series for the Z limits, but the top one is after the tool changing operation. The Z axis also has a home switch that designates the top of the "machining area". Above this tool changing occurs.
So, how should I wire this? Should I just rely on Mach 3 and a soft home limit for this home position, or try to use the switch somehow?

There are also 2 other switches above the home position and before the top limit switch as follows:
-A N/O switch that gets closed anytime the Z axis goes above the home and into the tool change area.
-An N/C switch that opens when the Z has gone up far enought to change the tool.

Here is a visualization of what occurs on the Z axis.
--- Over travel limit switch
|
--- Tool Change Zero switch (should never really go past this)
|
| Rotate To next tool
|                              Tool change occurs in this zone of z-axis movement
| Drop Turret          
|                                
| Release draw bar
|
--- Tool change area switch is closed above this and stay closed until it comes back down past
--- Home limit switch
|
|
|
|
| Machining area of Z axis
|
|
|
|
---Over travel limit switch

I realize I am going to have to do some sort of macro for the tool change, but how should I wire this and which switches should I actually use? There are a couple risks. One is that you don't want the spindle motor to be on during the tool change area. Also, I am not positive yet, but damage may occur to the tool change rod that rotates the the turret if you go past the tool change zero switch.

I am drawing the schematics for the machine now, and the last thing I need to figure out is the wiring of all these switches.

See attachment for a visualization of the switches. You can also read my post on determining if I needed a PLC or not here:

http://www.machsupport.com/forum/index.php/topic,17091.0.html

I have determined that a macro should do, but I  don't know how to do it yet. Right now I just want to get it wired correctly.

Thanks

Offline Hood

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Re: Best way to wire my home and limit switches?
« Reply #1 on: January 17, 2011, 03:12:25 AM »
Your X Y and Z limit switches are likely Normally Closed switches and are often linked in series on machine tools, some I have seen are just in series for each axis, some are all axis.
In Mach you can have them any way you wish but the normal way, which is to conserve inputs, is to have all home and limit switches Normally closed and in series.

Now your machine, if the limits are normally closed then I think I would just wire all axis limits in series and just use a single input for all axis limit switches.
Home switches could either be removed and the limits set up to home switches or you could leave them in place and use 3 inputs, one for each axis home.
For the Z and the toolchanger you are going to need to keep the home switch I think as you will need that for your tool change macro, you could of course do without it and use machine coords but personally I would keep it.

On my machines I have servo drives that are capable of homing to switch then index and for that reason I have separate home switches but all of my limits between axis are wired in series and just use one input to Mach.

Hood

Offline jevs

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Re: Best way to wire my home and limit switches?
« Reply #2 on: January 17, 2011, 08:27:37 AM »
Attached is the schematic I have drawn up for my machine. As you can see I have it drawn with just the limit switches going to the I/O board (Machmotion).

I was thinking to do it this way and just forget about the homeing switches on the X and Y. What do you think?

As you can see at the bottom left, those are all the switches currently not designated to go anywhere. Can I use more than 3 inputs? If so which other switches will I need to get my tool changer working?

I am not sure that I like having all the limit switches on one input, is this a normal practice? According to the Mach3 directions and Machmotions directions for their I/O board I should hook all three axis seperatly. This machine is 4500 pounds and there is a lot of mass moving around on that bed mill (not sure this makes a difference). Mach gives different instructions based on the size and speed of the machine.  In part 4.6 of the manual.

Offline Hood

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Re: Best way to wire my home and limit switches?
« Reply #3 on: January 17, 2011, 08:51:56 AM »
Having 1 or 3 or 6 inputs will make no difference, when a limit is seen Mach will E-Stop. Really though you are still relying on software and if there is a possibility of damage being done if the software malfunctions and the axis hits the end of the screws then you should really have hardware limits, in other words these limits at the very least take the drives enable away.

 I have two largish machines, a 2Tonne (4400lbs) mill and 3Tonne (6600Lbs) Lathe and I just have the limits wired to Mach in a single input That was the way the limits were wired on the previous controls on my machnes (Heidenhain 145 and GE 1050) but also there are hard stops (metal blocks) and large ballscrews so the drives would trip with over current before damage was done if  the software didnt act properly.

E-Stop however is another matter, you should never rely solely on a signal to the software to E-Stop, I have a signal from E-Stop button going to Mach but also the second contact on the E-Stop cuts the E-Stop string on all contactors and drops out the power to the drives etc.



Hood

Offline Hood

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Re: Best way to wire my home and limit switches?
« Reply #4 on: January 17, 2011, 08:59:41 AM »
Oh and regards the Home switches, you need some means of referencing the machine, either via accurate home switches or if your drives support it Index homing in the drives. Some drives can also set home with a motor current value, in other words you drive the axis against a stop and if it reaches a set current the drive stops the motor and sets the home position, this however would be tricky as you would have to have the stop positioned before the limits so not really an option if you have limits.

Other option is if you can accurately position the axis (align marks etc) then you can just tell Mach it is home, but obviously it will only be as accurate as your manual positioning is.
Hood

Offline jevs

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Re: Best way to wire my home and limit switches?
« Reply #5 on: January 17, 2011, 10:42:30 AM »
Having 1 or 3 or 6 inputs will make no difference, when a limit is seen Mach will E-Stop. Really though you are still relying on software and if there is a possibility of damage being done if the software malfunctions and the axis hits the end of the screws then you should really have hardware limits, in other words these limits at the very least take the drives enable away.

 I have two largish machines, a 2Tonne (4400lbs) mill and 3Tonne (6600Lbs) Lathe and I just have the limits wired to Mach in a single input That was the way the limits were wired on the previous controls on my machnes (Heidenhain 145 and GE 1050) but also there are hard stops (metal blocks) and large ballscrews so the drives would trip with over current before damage was done if  the software didnt act properly.

E-Stop however is another matter, you should never rely solely on a signal to the software to E-Stop, I have a signal from E-Stop button going to Mach but also the second contact on the E-Stop cuts the E-Stop string on all contactors and drops out the power to the drives etc.



Hood

My machine also has some large blocks that would bang stop the machine on the X and Y if it did get past the limit switches. The Z axis would just bottom or top out and be metal on metal as well. It is captured so it could not go farther.

As for my E-stop, the limit switches will kill the drives, through the I/O board, but if you hit the actual E-stop button, it will kick off the 50 amp contactor and kill power to all drives and the VFD. So, that is about as safe as I know to do it and what machmotion recommends.

Offline jevs

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Re: Best way to wire my home and limit switches?
« Reply #6 on: January 17, 2011, 10:46:55 AM »
Oh and regards the Home switches, you need some means of referencing the machine, either via accurate home switches or if your drives support it Index homing in the drives. Some drives can also set home with a motor current value, in other words you drive the axis against a stop and if it reaches a set current the drive stops the motor and sets the home position, this however would be tricky as you would have to have the stop positioned before the limits so not really an option if you have limits.

Other option is if you can accurately position the axis (align marks etc) then you can just tell Mach it is home, but obviously it will only be as accurate as your manual positioning is.
Hood
I thought I read somewhere that to set home, you hit the limit and then back it off just a tad and tell the machine that is home? This is just from memory...

Can I use seperate inputs and tell mach they are home switches and not limits?

Another option I guess I could do is hook LEDs to the original home switches and just run the motor up slow until the LED comes on and tell mach that is home. I could just put LED's on the panel permanently for this indication in order to "recalibrate" periodically. Any need for this?

I am sure I may be asking some dumb questions, but I am trying to educate myself ahead of time. This may be more clear to me as I actually get the machine working. Right now I am just waiting on parts and preparing.

I am going to call Machmotion and see what they recommend today.

Offline Hood

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Re: Best way to wire my home and limit switches?
« Reply #7 on: January 17, 2011, 11:01:37 AM »
..... if you hit the actual E-stop button, it will kick off the 50 amp contactor and kill power to all drives and the VFD. So, that is about as safe as I know to do it and what machmotion recommends.

Yes thats the way it should be, not sure if you also have a signal to Mach or not but often a good idea as it will also stop Mach if you whack the E-Stop. Although its no big deal if Mach continues it is often easier to have it stop as you know where you were in the code and also it may make it easier to work out where things went wrong.



I thought I read somewhere that to set home, you hit the limit and then back it off just a tad and tell the machine that is home? This is just from memory...

Sort of, if you have the limit switch also defined as a Home switch then when you home Mach will move the axis to the switch, when it opens Mach will reverse the axis until the switch closes again and then that will be set as Home by Mach.

Quote
Can I use seperate inputs and tell mach they are home switches and not limits?

Yes, the benefit of that is you can have the home switches at any position and Mach will only pay attention to them if you are actually homing. This on a lot of machines is not needed but on large machines it may save time as you could have the switch positioned closer to your normal working area. Another time when it is handy is on a lathe similar to mine, first the Z is 1.5M long so dont really want to have to travel to the end of the axis each time I home, the X on my lathe has a front and rear turret so I dont want either to have to travel across the centre line whilst homing so the home switch is positioned so that the two turrets are equidistant from the centre line.

Quote
Another option I guess I could do is hook LEDs to the original home switches and just run the motor up slow until the LED comes on and tell mach that is home. I could just put LED's on the panel permanently for this indication in order to "recalibrate" periodically. Any need for this?
I would use switches rather than that method but yes it could be done.

Quote
I am sure I may be asking some dumb questions, but I am trying to educate myself ahead of time. This may be more clear to me as I actually get the machine working. Right now I am just waiting on parts and preparing.
Very wise thing to do, questions are never dumb if you are not sure and often you will get many different answers which can give you a better and more informed route on which to proceed.

Quote
I am going to call Machmotion and see what they recommend today.

Probably a good idea as they will know their hardware.

Hood

Offline jevs

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Re: Best way to wire my home and limit switches?
« Reply #8 on: January 17, 2011, 11:24:03 AM »
Quote
Yes, the benefit of that is you can have the home switches at any position and Mach will only pay attention to them if you are actually homing. This on a lot of machines is not needed but on large machines it may save time as you could have the switch positioned closer to your normal working area. Another time when it is handy is on a lathe similar to mine, first the Z is 1.5M long so dont really want to have to travel to the end of the axis each time I home, the X on my lathe has a front and rear turret so I dont want either to have to travel across the centre line whilst homing so the home switch is positioned so that the two turrets are equidistant from the centre line."

Can I use inputs for these home switches besides the 3 that I am using for the limits? I like the idea of sticking to the original switch configuration as much as possibe.

Also. the home switches will make the tool at the back right of the table (see attached Excel sheet), is this a normal home position for a modern machine? It seems like everything would be moving in the negative direction from that point during operation? I know nothing about this as I have not had a CNC or even a DRO on my little "toy" mill.

Offline Hood

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Re: Best way to wire my home and limit switches?
« Reply #9 on: January 17, 2011, 11:42:13 AM »
Yes you can use separate inputs for the Home switches.

The machine can move to any position you like, for example my mill moves table right and forward for Home position, so tool is back left corner. I just set a home off value in Homing and limits equal to the value each axis would need to move to get the tool at lower left of the table. So when I home although the home is set the Machine Coords DROs actually show the position the axis is away from the true machine zero.

Hood