Limit/home Switch Repeat Ability In Home Cnc

[Metal]

So I'm putting together my bridgeport, going to probably get the oiler and knee on this week/end.

It's time to start thinking about the CNC stuff I'll be putting on it.

On my previous cnc project, I used kind of garbage amazon switches which were a pain in the butt to setup.
On this one, I'd like .0005 or better repeatability , Ideally I'd like to be able to have my vise jaws a known distance from all three axis's zero so I can get something remotely sort of resembling a production run without having to edgefind every time I move the part

So anyways, with some googling, I havent really been able to find a whole lot of information on specifics, It appears that long-arm mechanical switches are the way most people go for this sort of thing, I'd prefer to make a little box with roller switches in it, and have the mill axis interact with that rather than directly with the switch (for cleanness mostly)

What switches are you folks using and how accurate are they?

 

[JimDawson]

I have never seen a limit switch that would hold that kind of repeatability. They may exist but are not common. Use slow acting switches rather than snap action, they are much more repeatable. I normally use a direct acting, roller actuator type switch, and actuate it with a 15° cam.


On the mill I don't use home switches, only overtravel limits. When using the vice for multiple parts, I normally edge find the jaws, then offset the zero to where ever I need it to be, rather than set a zero from fixed machine coordinates. I also set a ''parking position'' where the Z is retracted to a safe position, then the X/Y moves to the parking position at the end of an operation. This is set up somewhere so it gets the tool off of the work to facilitate a tool or workpiece change or to allow room to take a measurement. The parking position can be anywhere in the working envelope of the machine. During the run, you never lose your zero unless there is a power fail or in the case of open loop steppers, lose steps.

On the router, I do use home switches to set the initial zero position for the X/Y. This just parks the table with the tool centered on the top right corner. But I am not interested in extreme accuracy at that point, it's just a starting place. I normally makes no difference if the start point of the work is +/- 1/16 or so because I always leave excess material for removal.

 

[Metal]

well, it also helps if I dont move the decimal point too far! I was actually looking for .005 accuracy

The closest thing ive found has been these, which are probably doable, but I'd prefer not to trust my meat fingers soldering together something that will prevent a crash (theoretically) as I'm actually a celebrity, the worlds worst solderer.
http://www.cnczone.com/forums/open-source-cnc-machine-designs/101878-cnc-forum.html

otherwise I've found a few proximity sensor kits, which are in the hundreds of dollars, i'd rather not bite /that/ bullet, but i have seen some clever enclosures that combine the +/- switch into one so you'd only need 3

 

[JimDawson]

0.005 is proabaly in the realm of possibility. Depending on how your machine as configured and what software you are using, 3 switches makes the most sense. That is exactly how my machine is set up. One switch on X, one on Y, and one on Z. The switches are mounted in a fixed position, and there is a cam at each end of the travel that actuates the switch for the X & Y. On the Z, the switch is buried in the quill stop plunger system and uses the stock quill stop hardware to actuate. The switches are wired active low (normally closed) so that in case of a wire break it looks like a tripped switch to the computer. I would not use any electronics in the limits, just standard limit switches. They are pretty bullet proof.

If you are using Mach3, you can tell it to use the travel limits for a home switch when homing. Other software I don't know about. Again, on a mill, I don't see a need for a fixed home position. To get extreme home accuracy, it is common to home to a switch, then move to the encoder index pulse to set the actual home position. This takes the switch accuracy out of the equation and provides a very repeatable position.

 

[Metal]

Yeah that is how I have my current cnc wired, except with 2 switches per axis since they were cheap.

I'll poke around in mach3, I was "hoping" to have a higher jog speed which ramps down to more reasonable/accurate speed after pressing a first switch and approaching the home switch so that homing doesn't take quite so long, but some googling i'm not finding anyone that has done it that way (mach3 by default trips the switch, overruns a small amount and then backs off until the switch unsets, where it sets home, i'm not sure overrunning with a bridgeport like that and all that inertia is a good idea, but i'd also like finding home to not take /forever/ as the huge table makes its way over to it.

my concern overall is that with this conversion I'm not going 100% cnc, I'm leaving the handles on, so a handle bump might throw the whole thing off (and backlash concerns) so having an accurate home would be nice, a known rest position could also work if I wasn't so concerned about something moving on me.

 

[JimDawson]

There is a way in Mach3 to get it to rapid to a position near home, slow down, and then find home. There is a setting in there somewhere, I have done it, but I don't remember exactly what the setting is. This only works if your DRO is pretty close to the real world, for instance, it won't work on a cold start.

 

[jbolt]

I'm using proximity sensors from Automation Direct on my mill and router. Repeatablity is within 0.001". Took me a while to get it right. What I learned is the sensor targets need to be at least the size of the sensor face or larger and the gap set to within the specification of the sensor. They work best when the sensor face moves at the target but this would require two sensors per axis. When using a single sensor that moves across the target from the side I found I get the best results when the target has a sharp straight edge vs round(ed). I use one switch for each axis with two targets. Other things that help are having very low or no backlash, and on my mill having the dovetail gibs properly adjusted and use quality way oil. The sensors/targets need to be protected from swarf, coolant etc.

 

[mksj]

Agree with Jbolt, I have been using the AutomationDirect proximity sensors as a VFD electrical stop, repeatablity is better than 0.001" at the same feed rate, but the stopping point will change if the feed rate is significantly changed. They need a metallic, preferable steel stop to be parallel to the sensor face for maximum sensitivity. AD has various P sensors, most are under $50. I have been using, PFK1 series, 8mm range, shielded, and a high sampling rate of 1.5Khz, either the PFK1BP-3H or PFK1BN-3H. I use mechanical limit switches for some applications, they do not have the repeatability and reliability of a P sensor.

 

[Metal]

Thanks for the info!
for my research what breakout board are you using ? since parallel voltage is 5.5v at best and these take 10v at minimum I'm assuming either you're using something more advanced than I used previously or there's another board inline with the switches.

 

[RJSakowski]

So I'm putting together my bridgeport, going to probably get the oiler and knee on this week/end.

It's time to start thinking about the CNC stuff I'll be putting on it.

On my previous cnc project, I used kind of garbage amazon switches which were a pain in the butt to setup.
On this one, I'd like .0005 or better repeatability , Ideally I'd like to be able to have my vise jaws a known distance from all three axis's zero so I can get something remotely sort of resembling a production run without having to edgefind every time I move the part

So anyways, with some googling, I havent really been able to find a whole lot of information on specifics, It appears that long-arm mechanical switches are the way most people go for this sort of thing, I'd prefer to make a little box with roller switches in it, and have the mill axis interact with that rather than directly with the switch (for cleanness mostly)

What switches are you folks using and how accurate are they?

I was concerned about poor repeatability with the Tormach mechanical limit switches. I installed an optical homing system and can routinely get +/- .ooo1".