[phantomcow2]

I dont entirely understand how limit switches work and what types there are. So are there any good online stores that have various types of switches that I can look through and hopefully learn?

[PEU]

I won this auction: http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7546658549, and planning to install them this week. Switches are super sturdy.


Pablo

[phantomcow2]

So ebay has a section just for limit switches, that is helpful.
Would switches like these be sufficient for a small milling machine?
http://cgi.ebay.com/10-Honeywell-Mic...QQcmdZViewItem

[widgitmaster]

http://www.mpja.com/listitems.asp?dept=64&main=117

http://www.alltronics.com/switches.htm

http://www.action-electronics.com/gcmicro.htm

http://www.surplussales.com/Switches/SWPressMic-4.html

http://www.electronicsurplus.com/dictionary/cat2105.htm

http://www.sciplus.com/category.cfm?subsection=14

[Al_The_Man]

I would consider those too fragile for exposed conditions, one of the best I have found is Honeywell 914CE series, they are hermetically sealed and come wired with a 12ft sealed cable, they are compact with a roller actuator and they can be stacked easily.
There is an Omron equivalent also and they come up on ebay regularly.
Al.

[phantomcow2]

THanks for the links.
So basically you mount switch where you need it to be mounted, and mount some other contact where ever you need the axis end on the table iutself, so the mounted piece hits the switch.
What about something like this guy did?
http://www.balbots.com/article_info.php?articles_id=10
He ysed hall effect sensors. THe part # he gives for digi key shows about 1 dollar each.

[widgitmaster]

Here is an example of how I used one micro-switch to limit the travel in both directions of the x-axis
The springs keep the trip pin in the center, there is a little divit in the pin where the limit switch sits, it is normally open, and is grounded on contact.
Eric

[philba]

I'm a cnc newbie (though not to electronics). I've always wondered why people use contact sensors for limit switches. It seems like you need to carefully tune the location of the switch to prevent over-travel from smashing it. I would think a Hall Effect switch (using a magnet), photo interruptor or other contactless switch/sensor would be less prone to damage.

This isn't an oblique criticism - I would really like to understand this issue.

[ger21]

A lot of machines use ramps to activate the switches, so the machine will slide right past the switch without hurting it.

[philba]

A lot of machines use ramps to activate the switches, so the machine will slide right past the switch without hurting it.

That makes sense, seems a little trickier to make, though.

What about over travel? I'm not yet done with my first machine but am at the point of trying to figure out where the Limit (home, too, I think) should be. I assume it's specific to my machine but I don't want to be over conservative yet want to make sure the carriage stops before it smacks a hard limit. Figuring that out is a real puzzle to me. Guess I'll make it adjustable - I've got a bunch of photo interruptors that I'll try first.

[phantomcow2]

I was htinking that a mechanical switch could break or something, which is why the hall effect system like i posted a link to appeals to me. The only thing is, i dont know how to put it all together to work with my HobbyCNC board.

[ger21]

One thing to keep in mind, if you're worried about switches getting broken. If you're using Mach3, you set up soft limits (software limit switches) just inside your limit switches, and the machine should never hit the switches. The switches then become basically backup switches, just in case. of a Mach3 error.

[phantomcow2]

WEll I am using TUrboCNC. I dont suppose that has a similar thing does it?
Some day I might give Mach3 a try...someday

[philba]

I was htinking that a mechanical switch could break or something, which is why the hall effect system like i posted a link to appeals to me. The only thing is, i dont know how to put it all together to work with my HobbyCNC board.

HE devices are pretty easy to work with. The ones from the link are typical. You need to apply +5 and gnd to the appropriate leads. The output lead needs to be pulled up to +5 via a resistor (10K usually will work). The HE device will pull the output low when it get's close to a magnet. So, the output with the pull up will be high when not at limit and low when limit is reached. I think that board has the limit pullup already so you should be fine.

as I recall, your board only has one limit input. To use more than one, you will need to "wire OR" the limit switches - run all the limit switch outputs together. Any one of them can pull the line low and stop the machine. You can even mix physical switches with HE or opto-interruptors as long as each of the switches, when closed, pulls the output to ground (low).

By the way, some HE switches are "latching" . That means they won't reset until you apply the opposite magnetic field or cycle power. Not a big deal and probably good for safety.

Phil

[phantomcow2]

Thanks for that reply, i think i might almost understand it.
So for a hall effect system.....
mount a small magnet on the end of travel. Mount the sensor in some way (the bal bots guy used a brass tube, i assume this is what hes doing) so it will react to the magnet when the time is right.
The hall effect sensor itself will have wires attached, correct? Wires that will got gnd and one of the terminals of the board. What do you mean by +5? Does that mean +5volts?
ALl the wiring is done to this part of the board:

[philba]

+5 = +5V. The HE sensor has (usually) 3 wire leads: Vcc (+5V), Gnd and Vout. The Vout is the sensor value and goes to the limit input. +5V and gnd go to the same named terminals on the controller. I don't recall if the hobbycnc board supplies +5V out but would be suprised if it doesn't. YOu should get the datasheet for the device - usually available online - it will tell you which leads are which (AKA "pinout").

Yes, I would mount the sensor stationary and the magnet on the axis shuttle (or what ever its called). You might want to mount the sensor on a small piece of perf board. I'd use shrink tubing to protect the leads (metal chips could short). You can get adhesive wall shrink tube that will really protect the metal leads. I'm also thinking of dipping mine in epoxy - bullet proof.

You can test the sensor once you have it wired in by using a DVM and measuring the voltage at the Vout pin. Magnet present - 0V, magent absent - +5V (actually, anything over +3V would be OK).

Phil