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2 Questions
« on: December 25, 2008, 08:22:43 PM »
I was trying to run a test prior to cutting my first piece and I realize that I don't understand HOME. I figured I would design a small 3" x 5" rectangle, jog to the middle of the table and run the file in the air w/out the router on, but when I got there and tried to change the X, Y, and Z absolute home zero to that new location, I couldn't do it. I first clicked Machine Coordinates, then zeroed the axes, then clicked Ref All Home, but the Abnormal Condition light started blinking and when I checked the Diagnostics Page I found that the old home position hadn't changed. What should I have done?
Also, whenever I close Mach 3 I get a Fixture Save? query, even when I haven't loaded a Gcode file. What kind of fixture are they referring to?
Thanks again, Bob
Re: 2 Questions
« Reply #1 on: December 25, 2008, 09:13:00 PM »
Bob,

    If your machine does not have home switches, you don't need to concern yourself with the whole concept of home - It's of no value to you.  Decide where you want zero to be for the job you're running, put the machine there, and zero the user coordinates.  Don't worry about what the machine coordinates are, they're not important.
   Home is only useful if you have home switches, and it's primarily useful for quickly re-zeroing the machine after power-loss, a tool-breaking, etc.  Without home switches, you just have to manually re-zero to whatever you chose as your user coordinate zero point if something goes wrong.

Regards,
Ray L.
Regards,
Ray L.
Re: 2 Questions
« Reply #2 on: December 25, 2008, 09:14:44 PM »
Oh, on the second question - it's asking if you want to save the offsets for the fixtures defined by G54-G59, etc.  Again, without home switches, there's really no reason for you to care.  Save them.  Don't save them.  Doesn't really matter.

Regards,
Ray L.
Regards,
Ray L.
Re: 2 Questions
« Reply #3 on: December 25, 2008, 09:26:29 PM »
Thanks again, Ray, and Merry Christmas.
All the best, Bob

Offline jimpinder

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Re: 2 Questions
« Reply #4 on: December 27, 2008, 03:36:55 AM »
Bob -

The machine needs to know where it is before it starts. It keeps it's position in Program Co-ordinates which are displayed when the program co-ordinates led is lit. To this it adds all the various offsets for programs and tools etc, so the tool arrives at the right place at the right time.

On commercial machines, the first job of the day is to "home" the machine - done in Mach by selecting the various home switches. The machine will move to a position that it knows is 0.0.0 in machine co-ordinates and set the DRO's accordingley. It is now "home" . The home position is probably not any good for machining from, and will certainly not bear any relationship to the 0.0.0 position the program is written from - so each program then has an "offset" written into it, which brings the machine to the correct position to start work on that program.

If you do not have home switches fitted and activated, Mach has been written so that your machine co-ordinates will zero at any position.

You should, therefore, move your machine to a suitable position to start your program, and this is normally (but not necesarily) the 0.0.0 position of the program you are about to run.

The safest way to proceed is then to press "machine co-ordinates"  so the button is lit. The machine is now displaying "machine co-ordinates" and then press "RefAllHome" This will zero the machine co-ordinates. Then press the "machine co-ordinates" again, the led should go out. The DRO will display "program co-ordinates" This could be 0.0.0 or could be a random set of numbers from your last job.

Zero each DRO using the zeroX, zeroY and zeroZ - the DROshould again display all zero.

You now have the Machine Co-ordinates and the Program Co-ordinates all singing from the same hymn sheet, and you can start your program run.

Why so much fuss - if you do not have both sets of co-ordinates in a fixed relationship to each other (here they are identical - which is the simplest relationship - i.e. the offset is 0.0.0) then any offsets in your program will not necessarily work correcty, and you will be left wondering why.

There is nothing wrong with starting like this, I have home switches on my lathe, but for some "one-off" jobs, it is easier just to get on with it rather that worry about all the offsets using the homing switches. Fitting home switches is something you can tackle later - if you feel then need to do so.

If you look under Config/Fixtures you will see a fixture table. This has from G54 to G58 then another 250 or so from G59P7 upwards, which allows you write a lot of programs, each with it's own start point. If you were in a long run, and closed overnight, when you came in the morning you would zero your machine and press "GO". The machine would pcik up the offset from the fixture table, move to the correct position for that program and get on with it, without further ado. Some porgrams have the same series of instructions, working from different "offsets" if cutting say, patterns out of a large area of material.

These offsets mgiht change during the day, so when Mach shuts down, it saves all the current fixture list, so it is ready for the next day.







 




« Last Edit: December 27, 2008, 03:48:33 AM by jimpinder »
Not me driving the engine - I'm better looking.

Offline RICH

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Re: 2 Questions
« Reply #5 on: December 27, 2008, 11:37:41 PM »
Jimpinder,
Very good write up ( as usual ) with the why part included. One worth saving. :)

RICH
Re: 2 Questions
« Reply #6 on: December 28, 2008, 12:06:41 AM »
...if you do not have both sets of co-ordinates in a fixed relationship to each other (here they are identical - which is the simplest relationship - i.e. the offset is 0.0.0) then any offsets in your program will not necessarily work correcty, and you will be left wondering why....

If you don't have home switches, under what circumstances will it matter what the machine coordinates are?  You shouldn't be running any G-code that cares....

Regards,
Ray L.
Regards,
Ray L.

Offline jimpinder

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Re: 2 Questions
« Reply #7 on: December 28, 2008, 05:14:10 AM »
Ray - Yes you are right, and I could not think of circumstances where you would use offsets., but never-the-less I feel it is a good idea to understand the relationship between machine and program co-ordinates, when it comes to using them.

You could, of course, make your own "home" without having  switches fitted - i.e. a place where you can put the table with some repeatable accuracy - and then zero the machine co-ordinates.

I must admit, I am in a quandry on the lathe. I home my machine - but where then to move to. x axis is down the centre - that is easy, but the z axis ???

Do I have 0 position at the chuck - (if so is it right at the chuck, or about 10 mm off, so a parting tool can get in behind), or is it at the opposite end of the workpiece. That one I will have to ponder.

At least the X axis is fine, and my centre drilling fixture works a treat.

Dom't worry Bob, just an old man ruminating at Christmas - and the best to you all in the New Year.


Not me driving the engine - I'm better looking.

Offline Hood

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Re: 2 Questions
« Reply #8 on: December 28, 2008, 05:41:29 AM »
Jim
 for the lathe I would say home position of X fully away from the centre would be a better option. Z is a trickier proposition if you have a tailstock.
 It could be argued that a Z home is not really needed as you mostly zero to the stock end but I have one on mine as it was there and I do use it.
My X axis is actually not fully away from the centre in the conventional respect as I have both front and rear toolposts so if it was then the front post would be at centre, instead mine is positioned so that both front post and rear turret are equal distance from the centre. 
 My Z axis home switch is moveable so if I wish I can have a home at any position but I tend to just have it in a fixed location.
The major benedit to having home switches is so that you can also set up softlimits but for a lathe softlimits are really not much use as you can have tools sticking out in all directions  and thus could smack one into a chuck and still be well within the axis limits.
Hood

Offline RICH

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Re: 2 Questions
« Reply #9 on: December 28, 2008, 08:52:22 AM »
Hi All,
I don't have any limit switches on my mill but may add them someday. Attached is a file which describes
setting up homing without limit switches and may be of value if someone wants to fool around some.
RICH