[Source] Limit Home Switches For Mini Cnc

Dracen knights

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ok so as some of you might know I have a Taig cnc mill Currently with 4th axis and a sherline 4400 cnc lathe. Now I'm in process of building a auto tool changer (ATC) and I am getting to the point I want/need to setup home and limit switches. so I'm looking for tips, info and sources for the parts and setup...
Now I have a G540 controller and I'm using mach3. any help or links to parts and info sure would be great... Thank you.
 
Typically, a "micro-switch" with a roller tip is used for limits. Whether you need a fully "sealed" switch depends upon if you intend to use coolant or not. The variety of switch with 3 terminals lets you connect to the Normally Open (NO) or Normally Closed (NC) side of the switch as your electrical design requires.

The cheaper unprotected type switches start on Ebay for less than $10 for sets of 3-5 switches, such as auction #281125076779.

If you want long-term protection from flying chips and lubricant, then the same switches start at ~$5 each, such as Ebay auction #281370390753.

Either type switch is typically mounted on a flat strip of sheet metal that can be adjusted back and forth, so that the location of "stop" has some final adjustment. You definitely want to use the roller type switch, rather than the plunger type, because you don't want the switch broken off in case the machine over-runs its "stop" position. And remember that due to momentum, the table may not stop in the exact same position each time. Don't confuse the table's "hard stop" position with the limit switch position. The limit switch should be hit well before the hard stop. The hard stop is only encountered in the case of "run-away" machine. (Hopefully never.)

On the machines my company builds, 24V is used at the limit switches which controls the coil on a contactor (power relay). All the limits are connected in series. If any limit switch is hit, then all power is cut to the motors. This means the table has to be manually backed out of the switch before the machine is restarted.

We use reed switches for "home" detection, since they typically have the same (or closer) physical location regardless of direction of travel. Again, inertia of the table plays a roll. If we need to find an absolute "home" with a high inertia system, then our machines approach from one direction only, and in a special low speed crawl.

Take all this with a dose of realism. Our machines can have a ton of steel moving 10+ feet overhead and human life is at stake.

Hope this helps.
 
ok so as some of you might know I have a Taig cnc mill Currently with 4th axis and a sherline 4400 cnc lathe. Now I'm in process of building a auto tool changer (ATC) and I am getting to the point I want/need to setup home and limit switches. so I'm looking for tips, info and sources for the parts and setup...
Now I have a G540 controller and I'm using mach3. any help or links to parts and info sure would be great... Thank you.

One issue with using microswitches for referencing the home position is that swarf can build up on the ramp or roller. On my Tormach, I have found that it is possible to have as much a .003" deviation in the actual homed position. My version of Mach 3 has a "Verify" button on the MDI screen to check if the home position has shifted. The switches should be mounted in locations to minimize that sort of contamination and consideration given to providing shielding for the ramps and switches.

I installed optical homing sensors on the Tormach. The x and z axes perform well with repeatability to within .0001". When using heavy coolant flow, I have a problem with the y axis because of the location I chose for the sensor. Under heavy flood cooling flow, the coolant running off the bed wicks into the sensor and occasionally triggers the software E-stop. Because of that, I have disabled the sensor and am using the microswitch for the y axis. The problem can be solved with a redesign of the sensor shield and is on my to-do list. Aside from that issue, it also performs well.
 
good info guys thanks... I was leaning towards doing the roller type switches but was not sure they where 100% needed. Now I guess thats resolved.. but I might try to find optical switches as I like that option.
 
You'll probably find a wider range of choices at Digikey.com.
 
Prox switches would get my vote also.
 
I'll have to look into the proximity switches.... I have 2 right now one for each cnc spindle/chuck tachs.
 
Proximity (prox) sensors are pretty good for limit switches. But they're not nearly as precise in detecting position as other types of sensors, so they might not be of much use as home switches. A prox would not be a good home sensor for a stepper driven system.

Then again, if you're using a hard stop to define your travel limits (such as with air cylinder driven motions), a prox is a great way to detect that "it has arrived," either at the "home" end of travel or the "limit" end.

And as always, be aware of potential failure modes. Others have mentioned ways that mechanical switches or photo sensors can be fooled or fail. Metallic swarf can fool a prox into giving a false positive signal (though a tool changer probably won't see a lot of swarf).
 
ok so I got some limit switches and will be doing my first setup with the ones I have now and will later upgrade them to the optical switches recommended on here as well (thanks RJSakowski) for all the info so far.... Now I have another kind of out there question...

I have a tach with a proximity switch or sensor which ever you would like to call it. now there is no place on my spindle that will allow the full sized magnet to be attached without loosing one speed or more. now if I can trim the magnet down IE narrow it into a rectangle of sorts then I would be golden so do any of you have experience doing something like this? I would hate to cut it down and find out I needed a new magnet.

This is the exact one I got. http://www.ebay.com/itm/271729383222?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

I'm a big fan of green so I wanted green numbers anyway if you have any info please let me know... from what I have found the tach is pretty good. I don't think I need to be exact on this just pretty close would be good enough. anyway anyone tried cutting down the magnet on one of these.
 
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