Parking with "jog to home" machines

[Michael M]

I've got Linux CNC10 on my Tree 425. This is a "jog to home at startup" machine, so Y is usually just short of the minus limit switch, Z similarly at the plus switch, and X is in the middle of the travel (these are the standard positions for the Tree with Dynapath control).

I've got a project that will take me several days/restarts to do. Park returns the axes to 1/4 motor revolution (2000 lines) of XYZ home positions which is probably very useful for machines that seek home off the limit switches/index pulse but not so useful on my machine. Since I've got a knee mill instead of a bed mill I don't want to have it sitting with the table hanging the maximum amount off the knee.

What I've been doing is after homing (if I'm going to need to restart with the same coords) is I go to WCS #18 and zero all axes right after homing. Then I switch to WCS #1 for the part environment. Then instead of parking I can switch back to #18 and MDI G1 X0 Y0 Z0 to try and return to the 0 absolute positions. I've noticed that sometimes when I do that and then check in the PID screen an axis may be a line or so off of zero. That is probably not an issue since it is far beyond the movement resolution of the mill. But knowing it is slightly off nags at me.

M25 returns to Z home. Is there a similar but undocumented command to return X and Y to their home positions (absolute zero in PID)? Or is there some other command that I could use to get back to the true home positions, and not just "very close"? Maybe some way to "park" and then command "move 2000 lines"?

FWIW, I've noticed the Centroid MPG, even at .0001" resolution, is too coarse in the jumps to hit that zero line position. MDI tends to get closer than I can with the MPG or jog buttons.

My WCS #18 method is very likely good enough, but I thought I'd ask in the hopes of moving 99.99998% to 100%.



cheers,
Michael

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[cncsnw]

I don't know that it would make any difference in the final position, but it would be simpler to use G53:

G53 X0 Y0 Z0

[Al_The_Man]

Yes G53 always uses machine zero as a reference and can be entered with a certain position relative to machine zero for each or any axis if so wished .
Al.

[Michael M]

Thanks, I'll look at G53. I have to admit that I'm not very conversant with the fine details of G/M codes, so when I read things in the Centroid manual they do not always make much sense to me. After reading your comments and then reading the manual on G53 I find myself thinking "oh, is that what this is talking about? Who knew?"

I have the same problem with RhinoCAM and what is to me arcane Rhino stuff. I hear them talking, but it is mostly white noise because I lack the extra 30-80% of knowledge (hours of classroom instruction from someone who really understands the subject and can clarify things) they seem to presume the reader must have.

It is kind of like the job announcements where they offer an entry-level position that requires 5 years of experience.

Or maybe "what dogs hear"



blah blah G53 blah blah 4th dimensional NURBs intersection blah blah blah 13 degrees of freedom blah blah blah.



Perhaps the problem is too many programmers writing the manuals, instead of getting technical writers in the middle to say "what do you think a person who hasn't been doing this for 40 hours a week for 3 years will comprehend when you tell them this?"


And yes, as someone who has done technical training in the past I'm aware of how difficult it can be to do a good job of it, especially with naive users. But I do find myself wondering at times if companies might not make more money by writing very clear and explicit and understandable documentation up front and then not need to spend quite so many hours of tech support on questions from people who if they had a clear explanation in the documentation might have figured things out on their own without needing to call for help. Adding another couple of sentences of explanation to the manual might eliminate 30 hours a year of telephone support costs.

I saw mention in the manual about the M25 command to go to Z home, but I'm pretty sure I didn't see anything in my readings about going to X or Y home, at least not in terms I was looking for. A sentence added to the M25 command along the lines of "if you want to do something similar for a different axis use G53 and the 0 coordinate for the specific axis" would have given me an answer.

Not that (presumably) anyone here was involved with writing the documentation, but it would be a help if someone at these software companies (Microsoft has lots of problems too, and a LOT more money to put into hiring good tech writers and then test the info with users) could pay a little more attention to trying to make it easier for the the people they are selling the product to. I don't expect much in the way of documentation for $50 software, but when I spend $1-4-20K with a company it would be comforting to feel I've got a fighting chance of actually using the product with the support documentation that comes with it.

cheers,
Michael

[Al_The_Man]

Pick up a copy of CNC programming by Peter Smid, gives excellent examples of all G codes.
Al.

[109jb]

M25 is not a standard RS274 G/M code, so must be an added M-code for your specific controller.

Another opton you have is using G28 or G30. These codes move the machine to a user defined position. What you do is position the machine to the desired position using jog or MDI commands, and then invoke a G28.1 or G30.1. this sets that location as the position to move to. Then, later on, if you want to go to the point defined by a G28.1 command, you simply invoke G28 and the machine will move to that previously defined position. G30 will do the same by moving to the position defined by G30.1

With the G28 and G30 commands you can also modify the motion with an intermediate point

G28 - goes straight from the current point to the position defined by G28.1

G28G91X0Y0Z1 - Moves up 1 unit first and then rapids to the predefined position.

[Michael M]

G53 does what I was looking for, so thanks very much for pointing that out! It is Sharpied on the side of the console so I won't forget.

I'll have to read up on the G28 stuff and see what I can learn.

I bought a copy of Smid's book when I had my Tree 325, but I have not been able to find it. I may have loaned it to one of two people, but they both deny having it so it might be hiding here within arms reach (things tend to do that to me). I may need to buy another copy, but if I do I'm sure I'll shortly thereafter find the old one.

I have/had the 2nd edition and I see he has a 3rd edition out. Does anyone know if there's enough new information to justify buying the newer version over an older and less expensive edition?

cheers,
Michael

[Al_The_Man]

I have the 3rd edition so I could not say, one of the first places I try for books is AbesClearing house often get great deals there.
Al.

[cncsnw]

Yes, you could use G28 or G30 to move back to your machine zero position as well.

On a Centroid control, there are no "G28.1" or "G30.1" codes. Instead, you set the return locations by entering coordinates on the return points menu:
F1/Setup -> F1/Part -> F9/WCS -> F1/Return.

Until you change them, all the return points default to 0.0 in machine coordinates, and so G28 or G30 will go to machine zero.