Homing with probe input

Hi all, 

I am using my hobby or DIY milling machine. I get more and more used to the machine and achieve really amazing accuracies, if you consider what type of machine I have. Most of it is due to the fact that I know what my machine is capable of. 

I do not have encoder feedback, means I run the machine in an open loop set up. The machine only now’s by the software where it is after I home it. I am using a probe input and touch off directly on my workpiece. Even finding a center of a hole works pretty well. 

Now, I do not trust the home switches I have on my machine. They are pretty cheap and they are probably relasing the trigger signal with quite some tolerance. 

I wonder if there is a better way of getting my machine back to the same spot after I switch the stepper drivers on and off. 

E.G. I touch of at a workpiece and start working on it. Before I am finished, I decide to stop and continue work the next day. Normally the next day I would switch the machine on, home it and touch off at the workpiece again. 

What I would love is either:
I home the machine and after homing I probe on a fixed installed touch off block or so at the machine with the probe input and from there I run load somehow the next machine operation without touching off at the workpiece again. 

OR 

Instead of homing to a limit switch, I would like to find home and once the a limit switch is hit I reduce the homing speed until the machine touches a mechanical stop which is connected to the probe input and homes.

Maybe there is an even better way of doing that. Would love to hear your solutions. 

Mike

Inductive switches? The react on metal. They only cost a few dollars, one for eatch motor. See the PrintNC.

That‘s what I am currently using, but they are not accurate enough. My machine has a minimal step of 0.0125mm which I can measure with a gauge. I would love to have the machine after power cycle and homeing +/- 1 Step = +/- 0.0125mm of where it was before.

I could reduce the home search velocity super slot and make a electrical contact between the portal and a fixed end stop, but that would increase the homeing cycle quite a bit.

Is it possible to set the homeing cycle in a way that the machine searches fast for home hits a limit switch and for the final home move (the slow move) is then searching for another input and ignores the first limit switch?

mBender wrote:

Is it possible to set the homeing cycle in a way that the machine searches fast for home hits a limit switch and for the final home move (the slow move)?

 

Yes. (in your INI file)
[JOINT_n]
HOME_SEARCH_VEL = 5.0
HOME_LATCH_VEL = -0.5

... ... then searching for another input and ignores the first limit switch? << not sure. Fwiw, with the above INI fast search & slow latch, I get +/-0.005mm repeatability with this world's cheapest, simplest and dumbest $1.99 mechanical limit switches. 

More importantly; regardless of the 'switch' you'll use, whether it's engineered to land on Mars and come back to earth, it will inevitably require signal conditioning for accuracy and integrity. We normally buffer this hardware to solve this problem.

e.g. a fast RC filter (R1k & C1uF) and a Schmitt-Trigger (SN74LVC1G17DBVR).

tommylight wrote:

Inductive switches will shift with temperature changes, a lot, some i have in use from 15-45C shift by 1.2mm !!!
Optical ones from old photocopiers are much better as they are inside of housings so no scattered light can mess with them.

 
What proximity sensors are you using? I use Turck 2mm sensing distance. These are so common that they often pop up on Ebay as NOS for $10ea. No idea where in the world you've seen a 1.2mm shift, those have to be the absolute worst sensors ever made or they had a huge sensing distance. Turck rates these @  2% repeat accuracy of full scale with a 10% temp drift, but the operating range is -25C to +70C.

I just grabbed a random datasheet here but I use a similar switch.
www.turck.us/datasheet/_us/edb_1003620_eng_us.pdf

In practice all 3 of my axes use them and repeat within 0.001". I don't do any super precise work and my experience is based on from when I first home the mill that's in my basement to after I've been running it for a while and something screws up and I have to re-home with a part partially machined. I installed these when I built it and it's been one of the few things I've never had to worry about.

As a side note I also use 5khz NPN inductive proximity switches as encoders on gearteeth and holed disks. This is standard practice on alot of machinery, you're not going to get away with doing that if they're as bad as you say.

Well thats not really a good representation of what you would add to a machine tool if you were being remotely serious. For one, the shortest sensing distance possible is the biggest accuracy thing. For 2 most can sense non-ferrous metal but they do so poorly, this is generally not an issue on a cast iron mill. For 3 you need to use shrouded prox switches so they don't pick up radial stuff. And 4 you need to buy a reasonable brand like Turk or Telemechanique. Turck's are what I use for Limits/homing, telemachanique is what I use for pulse counting since they're a bit better but cost more.

Now that I think about it I have a Telemachanique in the head of my milling machine as a spindle index. I use this for rigid tapping.