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December 13, 2018, 14:33:35 pm


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Author Topic: Many False Starts - Mach3 or LinuxCNC - Most Repeatable Homing Switches  (Read 292 times)
Bob La Londe
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« on: November 22, 2018, 16:15:59 pm »

I know there are more specific forums to ask this, but the members of this forum seem to be much more balanced and practical in their views.  

I'm always buried in projects.  One I have had on the back burner and been thinking about for some time is a bridge mill.  I've played back and forth between a moving gantry or a moving table.  

A moving table has of course the advantage of a slightly more rigid setup, single motor, & single drive.

A moving gantry or bridge has the advantage of greater work envelope compared to foot print.  

I am leaning towards the moving gantry or bridge.  I think I have a pretty rigid setup figured out using some pretty decent heavier than average NSK linear rails Lloyd pointed me to when they were on sale at one of the surplus resellers.  Since 90% of my paying work is with 6061-T6 aluminum I believe it will be adequuate.  For light steel work I do have the mills and for heavier steel milling I still have the Hurco.  Well, that and I have done aluminum milling with a much lighter CNC router (bridge mill) with moving gantry.  

With a moving gantry on a machine I hope to do modestly heavy aluminum milling it really needs to be driven on both sides of the bridge.  The options seem to be dual screw or dual rack and pinion.  Since the machine is not going to be huge for sheet goods and I hope to keep repeatability under 0.001" I am leaning either towards hand packed (balls sized to minimize backlash) ballscrews or double nut ballscrews.  I already have a stack of double nut ballscrews on the shelf I picked up another time I found a deal on a surplus closeout, so out of economy that's probably what I will use.  

So far I have written a few things I am sure will trigger reactions or feedback, but please bear with me.  I have not gotten to the meat of my quandry just yet.  

Assuming a dual screw drive on the bridge I have had some little worry about drivng them and keeping them sychronized.  For larger rack and pinion machines they mostly have dual motors with one slaved to the other.  In Mach 3 that is done in software.  Many setups then just cut measurement features and adjust the limit switches so that the features come out square after homing the machine.  Sounds a little hectic, but if you used closed loop steppers or decent servos and take advanatge of the trouble outputs to generate stop condition it seems like it woud be fairly reliable.  I don't see why the same thing wouldn't work with dual screws.  Of course it needs to be "slack/square" to begin with.  

Some time back I asked what is the most accurate switch to use for a limit/homing switch.  The answers were all over the place with some people saying, "it just not good enough".  My Hurco mill uses prox sensors and it repeats+/- about 0.001".  Often close enought to restart a job, but I always use an edge finder if its important or a probe.  That gets me under 0.0005".  Anyway +/- 0.001" is not good enough for this application IMO.  

The other often suggested (sometime vehemently) method of pushing dual mechancial drives is a single motor and driver with a pair of timing belts.  I do not like this at all.  It puts to much hardware out in the open for my taste, and with the design I have in mind putting it under the bed is not really an options.  It would also make it problematic, because I intend to run flood coolant with a large under bed drain opening.  

Now with all of that I know how to do it with Mach 3.  What about with LinuxCNC.  The only LinuxCNC application I have any experience with is the PathPilot proprietary version sold with Tormach Mills, so I am at loss to know how to tackle it with that.  


« Last Edit: November 22, 2018, 16:19:03 pm by Bob La Londe » Logged

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kvom
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« Reply #1 on: November 22, 2018, 19:22:13 pm »

I converted my bed mill to PathPilot from Mach3.  It did take a fair amount of reading of the LinuxCNC online documentation to know how to figure out the Tormach configuration and modify it to suit.  I had a new PC built to run it, and had to get a Mesa control card.    Now that I've had it for a year or so I really prefer it over Mach3.  Everything is on one screen, and I have faster rapids than was possible with the parallel port.

One of my limit switches is inoperative, but since I zero on the stock for each job I don't need it.  I set soft limits in the config, jog to the center of the table at startup, and reference the axes from there setting the max movement of all axes.

The main operating difference from Mach3 is that I cannot jog during tool changes.  So I have to stop the program, change and zero the next tool, and then set the "run from here" line to continue.
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Dragonfly
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« Reply #2 on: November 22, 2018, 19:39:44 pm »

If you haven't worked with Linux or have only general user experience with it you'll be perhaps disappointed.
The whole specific setup is done by manually editing text (HAL) files. With some weird logic from the developers. I guess they haven't seen much real working machines or don't care about the real world.
And they have failed greatly in making LinuxCNC user friendly with an easy straight-forward GUI setup. There is no such except a very basic "Step Config". Mach3 is light years ahead in that respect. Once configured it seems to be working better because the real time machine driver is compiled into the Linux kernel.
Both programs though use the now obsolete parallel port.
A friend of mine told me that there are quite affordable price wise Chinese stand alone controllers and he has ordered a 6 axis one. According to him they are based on Linux or even derived from LinuxCNC. I will ask him for a link.
I am quite curious and waiting for the arrival of the ordered device to hear his first impressions.
About squaring the gantry, in Mach3 two motors can be configured as master and slave axes so they work synchronously. And during homing there is a way to decouple them and each will home to its own homing signal.
There is a hardware solution for simple steppers where the two ballscrews are connected by a timing belt and using common step/dir signal sent to both drivers. This way the two motors 'help' each other.
« Last Edit: November 22, 2018, 19:41:29 pm by Dragonfly » Logged

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RickinBeachcrest
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« Reply #3 on: November 27, 2018, 03:08:02 am »

Hi Bob,
I made some Hall effect home switches.  Here is a link to a very in-depth thread that describes them.

https://www.cnczone.com/forums/open-source-cnc-machine-designs/101878-cnc-forum.html

The parts are incredibly cheap.  I built a couple extra in case one of the originals failed. Several years later, the originals are still working great.  I guess they wouldn't work so great if you were cutting ferrous material since a magnet is being used.  Not an issue for me since I cut wood 95% of the time and plastic the other.

 


* home switch1.JPG (35.99 KB, 640x480 - viewed 27 times.)

* dscn0431.JPG (33.54 KB, 640x480 - viewed 25 times.)
« Last Edit: November 27, 2018, 03:12:46 am by RickinBeachcrest » Logged
Bob La Londe
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« Reply #4 on: November 27, 2018, 16:52:39 pm »

Rick,  That's a good read.  You probably know by now that when micro stepping with stepper motors the step sizes are not uniform.  I have heard they can vary by as much as 50%.  I think your measurable error is more than well enough explained. 
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Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com
Dragonfly
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« Reply #5 on: November 30, 2018, 16:52:30 pm »

Bob,
here is a link to the Chinese manufacturer of stand alone control units
http://www.xcmcu.com/index.php/list/index/g/e/id/5.html
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