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Limit switches
« on: December 16, 2017, 04:23:31 AM »
Ok, I'm sorta confused by 3 limit switches for each axis (HOME, LIM-, and LIM+).  I have limit switches on my X and Y axis (no room on Z without loosing Y travel) and I'm wondering if I use LIM- and LIM+ or HOME and LIM+.  Can someone explain how limit and home switches work in MACH4?  I come from 3D printing background where HOME and LIM- are the same thing.

Thanx,

Rob.
« Last Edit: December 16, 2017, 04:25:42 AM by RBeaubien »
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”
Re: Limit switches
« Reply #1 on: December 16, 2017, 03:30:46 PM »
Hi,
you need all three switches. There are means to cause one switch to do two jobs and many do to economise on switches. In the early days of Mach
when everyone had a parallel port with only 5 inputs it was necessary to be a bit crafty to combine switches together. You have a PoKeys board with
lots of inputs so you don't have to do that nor would I recommend it. Combining switches can be confusing and if for instance you require a switch to behave
as a home switch then Mach will ignore it as a limit switch while homing is in progress. So just when you are likely to need it you suddenly have no limit switch,
not good.

The limit switches are at each end of the axes and really are safety devices. Provided they work their quality is not critical. Further if you have properly homed
(referenced) your machine and have correctly configured soft limits the limit switches will never be operated.

The home switch is a different matter. You will reference your machine on every start up and sometimes several times throughout a session. To have a repeatable
home position requires a top quality switch. Note that while its common to put the home switch at or near the end of an axis you don't have to, in fact the roller
plunger switches I'd recommend are best placed within the axis extents, where is a matter of mounting convenience.

I have three roller plunger switches as home switches, one for each axis and each on a dedicated input. The limit switches, all 6 of them are all hooked in series
and are monitored by one input on the BoB. If an over limit excursion occurs Mach will know that its happened and shut down but it won't know which axis
or whether its a positive or negative over limit. Given the number of inputs that I have to spare that's a bit silly....still there is 'nothing so permanent as a temporary
solution'.

If it comes to it don't worry about limit switches for the Z axis, if you just can't fit them in you can't fit them in, but you absolutely need a decent home switch.
If you can't reference the Z axis soft limits are meaningless and you WILL crash your Z axis.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: Limit switches
« Reply #2 on: December 16, 2017, 05:41:48 PM »
Thanks for the description.  I'm using optical switches and they should be pretty accurate so I figure on sharing LIM- and HOME for X and Y.  It sounds like I might need a Z- home switch, but I was going to use a XYZ touch plate/corner finder for my homing functions (that's if I understand that device/function correctly).  My home would be the corner of my material in that instance.  Does that sound right?
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”
Re: Limit switches
« Reply #3 on: December 16, 2017, 06:21:03 PM »
Hi,
homing establishes the machine co-ordinate 0,0,0. What your proposing for the Z axis will result in uncertainty in the location of that position.

Lets say that you home your z axis by touching off and it happens to be 50mm lower than the extreme uppermost of travel. If your softlimt is -50mm
then all is well. If a program/MDI or jog attempts to drive the z axis higher than the -50mm machine co-ord limit it will stop and prevent a crash.

Now imagine the next session of Mach and you have a different tool fitted. Now your z axis machine zero may be 45mm below the extreme uppermost of
travel. If the softlimits are still -50mm then an upward MDI of 49mm would crash your axis.

The point is that for your softlimits to provide any protection the zero location MUST be repeatably defined. Quite frankly if it comes to a choice between
limit switches and home switches I choose home switches. It took nearly a year before I got around to fitting home switches and had suffered a number of
crashes. Since I fitted them I've had one only when I forgot to reference the machine....dummy! Fitting good home switches has proven to be the single best
thing I've done to improve the robustness of my machine operations and even better can come back to the machine a couple of days later and carry on the
same part from where I left off. With good home switches you can get away without limit switches. Without good home switches you'll need good limits
because you're going to crash into them a lot.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: Limit switches
« Reply #4 on: December 16, 2017, 06:24:32 PM »
so best course of action would be to use HOME and LIM+ for X and Y AND install a Z Home (presumably at the top of travel)?
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”
Re: Limit switches
« Reply #5 on: December 16, 2017, 06:32:27 PM »
Hi,

Quote
My home would be the corner of my material in that instance.  Does that sound right?

No. The corner of your material is 0,0,0 in WORK CO-ORDINATES. Homing is about defining a location 0,0,0 in MACHINE CO-ORDINATES.

Note that the 0,0,0 home location doesn't have to be in one corner or at the end of travel. It can in fact be anywhere within the machine boundaries.
Its normal to arrange your machine home to be at one corner of the machine limits because it causes less confusion for human operators.
What's important is that every time you home your machine it goes to exactly the same spot. With my roller plunger microswitches I can get to
within 0.02mm. If I had indexing on my steppers I could use both the home switches and index homing to get within 2um!

In the first instance FORGET limit switches, work on the best, most accurate, and most reliable home switches you can manage.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: Limit switches
« Reply #6 on: December 16, 2017, 06:40:44 PM »
I think you read the wrong message.  :-)  I've already moved past the  Zero Plate.  So best course of action would be to use HOME and LIM+ for X and Y AND install a Z Home (presumably at the top of travel)?
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”
Re: Limit switches
« Reply #7 on: December 16, 2017, 07:19:12 PM »
Hi,
I wouldn't be concerned with limit switches initially. Home switches are important though. Your proposal to use two switches each for x any y axes will be fine.

You still need a z axis home switch. If you could arrange it to be near the top of travel all well and good but its not essential.

If for instance you had to mount your Z axis home switch 20mm lower than the extreme top so that it fitted or did not otherwise foul or restrict the axis
movement. Once you've homed your machine the machine coordinate of the extreme top would be 20mm, and the bottom would be -80mm if you had
100mm of travel say. Your soft limits would be 20 and -80. I know it sounds a bit weird to have your z axis machine zero inside the boundaries, 20mm
short of the top in this example, but so what? Its not really that you use machine co-ordinates, Mach does not you. When you are working at the machine
you'll be using work coordinates.

Another alternative that may well assuage your misgiving about having the machine zero at other than an extreme of axis travel is to use the Home Offset
provision of Mach. Again with the example above with the home switch 20mm short of the top you can set the <home offset> setting of -20mm.
Then the machine would home as normal stopping on your Z axis home switch (20mm short of the top) but instead of the machine coordinate being set to zero
it would be set to -20mm. If you jogged until the machine coordinate of the Z axis was zero you would be at the extreme top of travel. The advantage is that
the top of travel is 0 and logical for us humans but your switch is at 20mm below that but that turned out to be a much better place to put the switch.

Craig
'I enjoy sex at 73.....I live at 71 so its not too far to walk.'
Re: Limit switches
« Reply #8 on: December 16, 2017, 07:27:20 PM »
My only puzzle is how to put a home switch at some other point than a limit switch.  I can't picture it.  I don't have any way that I know of to put a home switch somewhere in the middle of travel that a) does't block travel of that axis, and more importantly b) how it would be active ONLY at the home point and not at any point to either side.  Doesn't make sense to me.  I understand the logic you are talking about, it's the physical implementation I don't understand.  Pictures would speak a thousand words.  :-)
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”
Re: Limit switches
« Reply #9 on: December 16, 2017, 07:35:16 PM »
My only puzzle is how to put a home switch at some other point than a limit switch.  I can't picture it.  I don't have any way that I know of to put a home switch somewhere in the middle of travel that a) does't block travel of that axis, and more importantly b) how it would be active ONLY at the home point and not at any point to either side.  Doesn't make sense to me.  I understand the logic you are talking about, it's the physical implementation I don't understand.  Pictures would speak a thousand words.  :-)
Here are the switches I installed.

Sent from my SM-G955U using Tapatalk
- Robert Beaubien
- Drone Plastics
-
“Dear Algebra, Please stop asking us to find your X.  She's never coming back and don't ask Y.”