The Home Machinist!

What do you think of using limit switches for security alarms/windows/etc. for limit switches?

http://www.ebay.com/itm/5-Set-Recessed- ... Sw-7RVCm9F

Thanks, Mike

I looked into this a long time ago, and what I learned was that switches are better for preventing disasters than for accurate stopping.

SteveHGraham wrote:I looked into this a long time ago, and what I learned was that switches are better for preventing disasters than for accurate stopping.

A repeatable machine zero is a pre-requisite for automated CNC, this is achieved by.........

Accurate Home Switches

actually...accurate HOMING is done with the Z PULSE (only one pulse / revolution) on the encoder....unless you have ABSOLUTE ENCODERS.

I would never rely on hitting the switch to 'find zero' if you intend to ever shut the machine off and return to continue running with the same programming origin.

Use a PROXIMITY LIMIT SWITCH and have it out in the open where you can SEE / CLEAN it every time you start the machine.

1> run to the limit switch IF the switch is OPEN.
1a>If the switch is CLOSED, do nothing until it is clear. Manually move the slide or check the switch.
2> Stop when you get there (switch closed). I have my machines set to go 1/2 additional revolution after the switch goes CLOSED.
3> back off until the switch clears (switch open)
4> stop at the next Z PULSE found.
5> set this point as 0 for that axis

the switches you show from ebay are OK for letting you know someone has opened a door or window. Otherwise, they have no place on a machine tool.

I agree with what Bill has written but there are lots of limitations with home built CNC.

Most home CNC, Mach 3 for example, does not allow you to do anything other than monitor a normally open or normally closed circuit for limits. This is common in "home" CNC software and its associated breakout boxes. With that limitation, the best you can do is buy good switches that have repeatable "snaps" to them. I spent about $25 a switch for my sealed "liquid tight" limit switches and I'm very happy with them. However, I am running them on a lathe and the only critical position is the X axis as the Z gets set for stock length change. (I make bump stops in the CNC code to set the next piece of stock up to so the zero is the same from one part to the next in a run)

Still I do have limits on all four ends of travel so I don't crash the machine. And yes, I have hit them more than once! Well three out of the four, LOL

On higher end CNC Bill has the right approach. Use encoder pulses and you will have exact location every time. The rest of us, well we ain't making piano's.

Dave C.

this is the switch configuration on my 'home mill' 20 years ago.

I don't know about 'high end'...but guess it is higher end than Mach 3.

Seems a little silly that a system would start out not giving you a repeatable 0,0,0 point.

Pianos? those things with the black keys painted on?

The "we ain't building pianos" is an old Carpenters by line when challenged on the quality or closeness in fit up on the job site.

On the other hand, the best carpenter is still a poor machinist!

Just saying!

Dave C.

Have you looked at the insides of a piano lately? Tough to say they would be poor machinists.

They seem to do things with wood that would have me scratching my head for a week.

Ok Bill, I think you misunderstood my reference. Yes, Piano makers do some fine work and are indeed craftsmen "craft persons". While the General carpenter who tells his boss "Hey, we ain't building pianos" is basically saying his work is good enough for whatever they are doing.

So back to the topic at hand about limit switches. Some of our machine are "good enough" for what we are doing even though they don't have relocatable 0-0-0 by their limit switches.

I'm still having my share of fun with my machines!

Dave C.

Those are magnetic "reed" switches, and they are not repeatably accurate as regards the operational distance to trip.

Dave_C wrote:Most home CNC, Mach 3 for example, does not allow you to do anything other than monitor a normally open or normally closed circuit for limits. This is common in "home" CNC software and its associated breakout boxes. With that limitation, the best you can do is buy good switches that have repeatable "snaps" to them.

To use Mach3 homing to the fullest on the X-axis of my lathe, I use a standard microswitch in conjunction with an opto sensor on a slotted disc on the other end of the double shaft stepper. The microswitch is wired in series with the opto signal output that feeds into the BOB. It is adjusted so that the N.O. switch closes just before the axis reaches zero & the next "on" opto signal then gets through to Mach which stops, backs off slightly 'til the opto signal turns off. At that point I click the "Zero World X" button, voila...done! (Mach can be set to "Auto Zero" but I like to do it manually.)

This gives a VERY accurate & repeatable position to set as the X-axis zero point which is critical on a lathe's X-axis for setting up an accurate tool table. I use the homed zero point as Tool "0" and reference all other tool offsets from that. If I have a crash or have to restart Mach for any reason, I can set the "Tool No." box back to 0, click the "Set Home X" button & a within few seconds I know the axis is back to as near perfect zero as I can measure. It's been working that way for over 2 years & I love it. I'm confident that any of one the 17 QC tools will produce a diameter within a thou or 2 depending on other factors like material type, speed/feed etc.

If the job is a first run, I home the machine, choose a tool, set the "X Wear" box in the "Tool Table" screen to a negative couple thou; run the part; mike it and tweak the tool wear to get it perfect. You can bet it will produce many parts after that which are very close to exactly the same size regardless of how many times you have to restart.

I use another opto sensor on a permanently mounted spring loaded probe on the carriage to home the Z-axis to the end of the work & use the same Tool "0" method to set all the tools' Z0 point.