The Home Machinist!

I just got done setting up the X2 for CNC operation. However I would like to now add limit, and possibly home switches.

The game plan is to make little steel brackets to secure two limit switches in opposing directions. Then have angle iron to press the switches. (1 set for each axis)

Now a couple of questions come up in this.

1 - There is going to be 6 switches, and I plan to wire them together with 22 gauge wire. Now I am not familiar with electronics, but would this create a problem using a couple of feet of wire connecting these switches in series? Such as creating too much resistance, and if that is the case could the resistor be swapped to to make something work?

2 - Now most limit switches I see use lever switches, why is that? I just see it as closing/opening a circuit so any switch would work. I plan to use these momentary NC push switches.

3 - Home switches would be nice, but I haven't the foggiest as how I would implement them. How would you wire it, how do you get it to be activated at a acceptable tolerance of ~.001"-.002", how can the switch be activated from either side?

Thanks all for reading.

Most people use snap action switches which are fairly precise
in when they switch from NC<->NO where as the one you
depict will be more vague as to when it switches and require
a motion of 30-60 thousandths to engage. Snap actions are
available with rollers on the ends of the levers so the actuator
can trip it from either direction. The wire resistance will be
immaterial unless you are talking about yards of the stuff.
I suggest stranded rather than solid, less likely to break.

You have to be careful about the amount of "over travel" available or you
may MASH the switch. :-) that is wy most of the limit switches are of the
sensitive sort, ( Micro switch is a trade name but most electronic people
refer to all of them by that name) The basic switch is normally enclosed
to reduce the contamination problem. The actuator can be of a wide
variety depending on the situation. Rollers, plungers(with overtravel),
levers, etc.
I suggest you look at a few different machines to get some better ideas
before you start cutting metal or ordering parts.
...lew...

I tend toward the magnetic pickup type of proximity switches, so that I CAN intentionally overtravel if I want to (for maintenance or those once in a while LONG PARTS).

As for the exactness of the switch, there are a couple of schools of thought on that.

If / when I am designing / building a machine that requires very exact home positioning, I don't rely on the switch to pick the point.

I write a routne that looks for the switch to trigger, THEN look for the Z reference pulse on the motor and use THAT as my home position. This way, of the switch wears / is bumped out of position, it makes no difference since it is the Z pulse that sets HOME.

In answer to your other question, no wire length typically isn't a problem.

DO wire your switches so that if the wire breaks, the circuit fails-safe and the machine stops. Nothing in the world worse than THINKING that you have a switch when you don't...and normally open circuits can fall into this category.

It isn't as important with the HOME..because if you run past it, you still have the LIMIT as protection.

I just finished my first CNC (mill) conversion using up just about everything in my junk drawer to save some $$. I had 6 Omron switches that have a roller on the end that cycled when switched from either direction. I set up my limits in series and placed one switch in the center of each axis. The wire size is not as important as you might think as you are only channeling 5V most of the time. I used the other 3 switches as dedicated home switches. http://i2.photobucket.com/albums/y29/Ho ... isc002.jpg

Glad you have it working....the first one is always a challenge...

Wire size for a TTL signal can be phone (24 gauge) or less, it really matters not since you are only carrying 10 ma or so.....

Builders use big wire only from the standpoint of mechanical strength and resistance to being damaged by 'outside influence'....

If you just put the two extreme limit switches in series as NC switches then
when the limit is reached how will you reverse the travel? I think it is normal
to have each limit switch configured to stop travel in only that direction.
Then the reverse direction still works. Without the technical info. on the
drive I can't be more specific. But surely you see the problem.
...lew... (retired electronic engineer)

Lew, In Mach3 you can set your machine up to either "automatic limit switch override" or manual. Once a limit is hit, it requires the operator to jog off the switch to correct the trip. It is the home switches which are programmed to back off X inches after the switch is made. Another feature of the program is to set soft limits in which you can tell mach3 where your limit switches are and set a distance at which the machine slows down before tripping one. -Mike

OK I see this handled in some sort of software. I'm a hardware sort of
guy. :-)
...lew...

Food for thought.....

From an industrial standpoint, based on many years of bad experiences, I NEVER rely on software-only solutions for safety / limit switches.

Point in fact, many industrial standards (some country standards, like in the EC), specifically FORBID using software only monitoring for safety and limit switches.

Software limit is OK to stop the machine BEFORE the switch is actually hit, but the switch itself must be separate from the logic control...what if the machine goes crazy and the software ceases to function, thereby not controlling anyting...and the machine runs away?

The most you can have is a fail-safe relay (loss of power causes the machine to stop).

Having multiple switches in series with a manual bypass to 'get off' the switch sounds like a good idea...as long as when you bypass the limit you are SURE to move the machine in the correct direction and not continue in the WRONG direction.

It is a very easy mistake to make, so take care.....

Separate circuits allow you to move the machine ONLY in the opposite direction to the tripped switch...which in itself is a safety feature.

The world of machinery has to be designed for the least common denominator of person at the controls, otherwise the lawyers get involved and machine makers go out of business.

I once saw a C-M Sabre vertical mill attempt, emphasis ATTEMPT, to make a tool change while the spindle was running at 2000 or so RPM. A software bug failed to report to the control that the spindle was running and out came the tool-arm....