Machsupport Forum
Mach Discussion => Mach4 General Discussion => Topic started by: dunnitagain on September 16, 2017, 08:31:03 AM
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Just wondering what everyone recommends for Quality Limit and Homing Switches, there will be flood coolant involved . So I Guess an IP67 rating will be required.
I was thinking roller type snap action , that way during a runaway it wouldn't peel them off the machine! going to need atleast 6.
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I use slow acting limit switches from Automation Direct (part #AEM2G43X11-3). I tried snap action switches but had repeatability issues. Tried the slow action style and they repeat quite well.
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Thank You for the Reply , I was curious as to the repeatability as home switches for Mach 4
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My mill is a PM-932 converted to CNC. On startup I home the machine, run a warm up routine, then run the homing routine again. It repeats within .001" to .002".
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Hi,
I used these for home switches, they have proven to be very repeatable, about 0.05mm variance between sessions.
http://nz.element14.com/omron-industrial-automation/z-15gq22-b/micro-sw-roller-plunger-spdt-15a/dp/1500340 (http://nz.element14.com/omron-industrial-automation/z-15gq22-b/micro-sw-roller-plunger-spdt-15a/dp/1500340)
Craig
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We have used both types of home switches that Tom and Craig have used. While the ones Craig uses is more repeatable, we have those types of switches on our Fadal, they are constantly covered in chips and have to be cleaned off before each reference cycle. Although we use the plunger instead of the lever switches from Automation Direct (AEM2G12Z11-3), they are a better option if it is going to be covered in chips and coolant since all the connections are enclosed and sealed inside the switch.
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The best repeatabilty for slow make slow break switches will be using a normally closed contact that you open for location indication. When a contact is landed its position is exactly known. Pushing it open can be extremely precise.
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The repeatability factor for homing this beast has to be with in .0001 tenths , I will eventually use the machine as a powersport block and head machine center . So should I use NO/ or NC/ . Snap or Slow ? Roller Lever or Roller plunger style .
I like both of the AD switches for the IP67. I will use the switches for both Homing and Over Travel . I will mount them under the massive table to help protect them . Will all so use 2
on the Z , to help with tool changer set up .
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The way industrial machines normally get precision homing is to use a two factor
system that consists of a mechanical sensor for coarse, and an index pulse from
the encoders on the servo motors as the fine position. The coarse gets you to
within a half a revolution of the motor, then the system backs up to find the
next instance of the encoder index signal.
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That's what I have with my machine Steve , But the old fadal was strickly software limits . The old Numatix control used the scales for homing the machine cords.
Then the motor encoder for position and velocity feedback. Was a tricky deal. It would over travel and break hell out of everthing!
I want to combine the Switches for Home and Limit Switches for Safety . Just want a good quality accurate outcome.
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Hi,
you will need indexing if you want that sort of resolution/repeatability. Do your servos have index pulses?
I use these as home switches only. They are set to trigger a couple of mm inside the machine boundary and have another six switches for limits,
they need to be reliable but don't need to be repeatable as the home switches and can therefore be cheaper.
Craig
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Your fadal previously homed using the index signal from axis encoders. You needed to park within half a screw revolution for that to work. Much more accurate than a limit switch I would think.
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Hi,
I was in a conversation with another chappy recently on the forum who had all his switches hooked up in series despite having an ESS.
It seemed to me that if you've gone to the trouble and expense of an external controller lots of IO (more than just one port like the UC100 or PMDX-411)
then it makes sense to separate home and limit switches. He had the problem that due to a setup error one switch would remain open circuit and therefore
the machine could not detect a 'home' event when homing a subsequent axis and attempt to crash. It certainly made sense to combine limits and homes
in the early PP days when input pins were scarce but not now. It also means that you can have switches/sensors which are optimised for the task.
You may have noted in the link I posted from the Element14 website that the Omron switches are $32 each. Admittedly that's NZD so works out about $22 USD.
Further an identical switch is made by Honeywell and from time to time they have a bit of a price war, I got mine for $16NZD, or $12USD. Even still I did not
want to pay that for all the switches I required. The roller plunger switches are great for mid axis mounting and activation but mechanically less convenient
for mounting at the end of an axis, where proximity switches are highly convenient.
I also find that having separate inputs for the home switches means that if a fault develops in one switch circuit it does not cause faulty behaviour on a
different axis. If you want index homing you will be required to have separate inputs. The less confusion I suffer the fewer crashes my machine suffers!
Craig
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Yes!!!! This is one of my pet peeves. Eliminate the one switch wonders with extreme prejudice! It just makes life sooooooo much better. Life is short. Why let one switch wonders ruin it? LOL Buy a few $13.00 switches and be happy!
To the OP, those limit switches that Craig pointed to have been used on many many CNC machines. My Matsuura MC500 has them on it. They are mostly used in center mounted positions with ramps to engage the switches at the ends of the table/stage. These ramps eliminate the issue of a runaway crunching the switch. Meaning that the plunger of the switch is mounted perpendicular to the movement of the table instead of inline with the movement. The table has the ramps mounted in inline with the direction of movement. The ramps have a starting slope, a slew, and a trailing slope and are of a height that will never be more than the travel of the switch plunger. This is a solid configuration that prevents damage and naturally keeps coolant away from them as they are usually well under the table. They can be enclosed in boxes (and most are) to further limit coolant ingress. I call these "the $13.00 switch" because as Craig noted, that's about what you pay for them.
On thing though... do not run mechanical switches on logic level voltages (5v or less). Use 12v at a minimum with 24v being preferable.
The prox switches are pretty nice since a lot of the time they are already pretty much coolant proof. And if you are wanting to switch at logic level voltages, these are the better choice.
Steve
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Here is a link: http://www.ebay.com/itm/OMRON-Z-15GQ22-B7-K-Snap-Swch-15A-SPDT-Panel-Mt-Roller-Plngr-/321485205824?epid=691253239&hash=item4ada02f140:g:rjsAAOSwtfhYtaK9
You can find them cheaper. But they may not be "real" Omron, but copies. You said "quality", so I would go with the real deal from Japan. So ~$115.00 you can get 8 switches and have limit+- and home for X and Y, and limit+ and home for Z.
That model may not be what you want. They make "drip proof" designs that may have better coolant protection.
Or... get the real thing used. http://www.ebay.com/itm/OMRON-LIMIT-SWITCH-4VBD4-1N-10A125-250VAC-4-POSITION-/162653739387?hash=item25deeb197b:g:XwcAAOSwYlRZLu26
That will do one axis. It has 4 switches for limit +, limit -, home, and slow down. The slow down switch can be used for high speed homing that automatically slows down a certain distance from the home switch so that you don't have to run a 40" travel at 1 IPM (if your motion controller has that option). :)
Using the index pulse for repeat ability is the only way to go, IMHO.
Steve
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Thanks Steve , Just ordered 10 of the Switches , I needed 2 more for home switches on my A&B . This machine has several Hall Style Prox on it with 5V .
So your saying I need to put a 24v Powersupply on for the home and limit switches ? Does the higher voltage just help with the switch lag?
Thanks Again for the Knowledge Everyone!
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24V is far less prone to having electrical noise issues.
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The caution about using 5 volt logic signals with common industrial limit
switches is based on the type of contact material used. Typically these
switches are designed for use with 24 VDC or even 120 VAC and use
contacts made with palladium alloys. This material withstands arcing
well, and also depends on arcing to burn oxidation and contamination
off of the contacts. A 5 volt logic signal does not provide enough
energy to perform this burning/cleaning action and the switch can
develop intermittent high resistance at the contacts if used with
low level signals.
There are mechanical switches designed to work with low level signals
such as 5 volt logic signals. They typically have gold alloy contacts
which do not oxidize. For industrial use they should also be sealed.
Because the gold is applied as a very thin layer, it can be damaged
by high level signals that cause arcing.
PMDX sells a micro-switch with contacts suitable for 5 volt logic or
24 volt logic. They are not suitable for 120 VAC control signals nor
for controlling the coil of a relay. These switches are not fully
ruggidized, but are sealed against liquids, have a roller follower,
and are a bit more durable than typical "micro-switches". See:
http://www.pmdx.com/Limit-Std
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Hi smurph,
This is one of my pet peeves. Eliminate the one switch wonders with extreme prejudice!
Whoa there smurph, we're talking CNC position switches not molesting your missus! LOL. Must say the number of people who pursue the strategy of
minimising input pins bemuses me also.
Another thing that bemuses me is that so many CNCers go to extreme measures and expense to get spindle speeds as close to perfect as they can.
While there may be a few who need genuine speed precision for the vast majority its unnecessary. The overwhelming majority of my milling can be
done by setting the speed manually at the start of a job and leave it like that until the jobs done three hours and eight procedures later. Oh, and I don't
have any left hand cutting tools so being able to reverse the spindle means what again?
When I think about the time and money I wasted programming to get Modbus control of my VFD when what I actually needed was better coolant pumps or better
yet thru spindle cooling.....
Craig