My opinion, home  (limit)  switch's are as important as an E-stop.

Thanks, that's my opinion too. Although, in this case, the disaster would be a small one, this is just a tiny little Sherline.
Still, I haven't really looked at Mach's homing setup yet, because I can't apply it to anything. I'll get to the switches this week.

There are several posts on the forum about this.

The machine keeps it's position in machine co-ordinates. These are displayed on the DRO's when the machine co-ordinates button is pressed and the led surround is lit.  In theory, the only way to zero the machine co-ordinates is to have homing switches and home the machine. However, when Mach was written, if you do not have home switches and they are not activated on the Ports and Pins, RefAll Home will zero them anywhere. You could, therefore, as Hood says, place your table in any position and home the machine co-ordinates. How accurate this would be depends on you.

The "home" position is probably no good for machining, and certainly will not be the start position of your GCode Program. For this you use offstes - G54 to G58 and then another 250 or so under G59 - plenty to go at.

The idea is that you wake up in the morning, and in a stupor, press the RefAllHome button. The machine then moves to it's "home" position. So the machine is now happy - it knows where it is. As Hood says, you cannot rely on it being in the same state as when you switched it off.

Having now woken up, for the first run, you then need to jog your table to the 0.0.0 position of your program. If you input G55 on the MDI line, then change to the program co-ordinates (by pressing the machine co-ordinates button - the led goes out) and zero each axis individually, then G55 is set to the start co-ordinates of your program. You can check this on Config/Fixtures. Run your program.

The next time, before you start, insert a G55 command in your GCode at the begining, followed by a G0 X0Y0Z0.

Start up, and "home" the machine. Run the program, and the first thing it will do is get the offset from the table, then move to the X0Y0Z0 position of the program, and then run the program.

The offset table is saved when you close down, and reactivated when you start up, so once set up, it can be used as many times as you want.

If your case, with a fixed position for your work piece, once "home" switches are fitted, the rest is easy.
As Hood says, optical switches are probably better than fixed mechanical switches. I use lasers shining onto detectors, and these are accurate to several 1/10ths of a thou, and have the advantage - if I accidentally overrun doing something manually, I don't plough through them.

Jim Pinder

Yeah, I don't know what Mach is doing with the offsets. Since they have no steady Machine Zero to reference from, I feel like I'm getting different results with the offset buttons every time I hit one.
My earlier problem was that I thought the button marked "GOTO Z" was trying to return the mill to Machine Home.

The 'Abnormal Condition' blinking light is annoying, the edgefinder diagram on the offsets page seems like a waste of space, as are the TWO Tool Offset buttons with 'Gauge Block Height' settings. Those last two features seem like they're aimed at non-machinists.
I'd rather have the Work Offset and Tool Offset lists onscreen.

When you do "Go To Z", it send all axes to their *user* coordinate zero positions.  This has nothing to do with the machine coordinates, and it makes no difference whatsoever where machine zero is.  Machine zero is useful *only* for referencing a vise, fixture, etc. to an absolute position on the machine, so you can quickly and accurately return the user coordinate zero to an exact physical location on the machine after a power loss, position loss, etc.  It is a convenience that saves setup time.  Nothing more.  Once you're actually machining, unless you're writing some very "incorrect" G-code, the location of user coordinate zero relative to the machine coordinate zero is totally irrelevent.  All your G-code should be working only within the user coordinate system(s) as defined by one or more of the fixture offsets (G54-G59).

Regards,
Ray L.

Quote from: edvaness on December 30, 2008, 12:36:30 AM

My opinion, home  (limit)  switch's are as important as an E-stop.

Thanks, that's my opinion too. Although, in this case, the disaster would be a small one, this is just a tiny little Sherline.
Still, I haven't really looked at Mach's homing setup yet, because I can't apply it to anything. I'll get to the switches this week.

If you don't have home switches YET and you using steppers, simply place a dowel pin between a stop and the table and jog till it jams and the motor slips, do this in the same place everytime for all axis and press reference all
this proves to be repeatable to within 1 motor step size
Fit a clock and try it, you will be surprised
PS when the motor starts slipping stop movement within a second or so (try to keep this time the same and use the same slow jog every time)
thanks Friedrich

softselect,

NO-NO-NO-NO   

YOU DO WHAT?

Ed

Quote from: vlmarshall on December 30, 2008, 06:16:53 AM

Quote from: edvaness on December 30, 2008, 12:36:30 AM

My opinion, home  (limit)  switch's are as important as an E-stop.

Thanks, that's my opinion too. Although, in this case, the disaster would be a small one, this is just a tiny little Sherline.
Still, I haven't really looked at Mach's homing setup yet, because I can't apply it to anything. I'll get to the switches this week.

If you don't have home switches YET and you using steppers, simply place a dowel pin between a stop and the table and jog till it jams and the motor slips, do this in the same place everytime for all axis and press reference all
this proves to be repeatable to within 1 motor step size
Fit a clock and try it, you will be surprised
PS when the motor starts slipping stop movement within a second or so (try to keep this time the same and use the same slow jog every time)
thanks Friedrich

On my machine, that would result in a lot of sheared-off dowel pins, or worse.....

Regards,
Ray L.

It was actually a suggestion i found somewere on the net related to Sherline and Tiag micromills, with small motors that you can actually stop by hand without ripping all the skin off , it would be dangerous on big strong systems that can break things.
i should have said it was aimed at MICROMILLS

Well, thanks for all of the suggestions.
I gotta say, though, I know how Machine coordinates work in the real world, most of my problem has been caused my my not reading Mach's instruction manual.
Clearly, 15 years worth of 'I know what I'm doing' isn't helping.

The other day I hit the 'GOTO Z' button accidentally, and the mill ran back to the home coodinates I'd set at the same time I'd set G54. Unfamiliar with Mach's single-DRO offset page, I've just been homing the machine AND setting a work offset at the same time.
Anyway, as the machine moved, I noticed that it positioned X and Y first, and then Z, which, for a homing function, would be backwards, and potentially disasterous.
Since then, I've learned that the GOTO Z is just heading to Work zero, and the positioning of Z last makes complete sense in THAT case.
Still, it's a button I'll remove when I start experimenting with VB.
Again, thanks to everyone for your help.

GoToZ will first move Z to the "safe" height, then move Y to zero, then move X to zero, then finally move Z to zero.

Regards,
Ray L.

Ray, it will only move the Z first if a safe Z is set up then on mine it moves X and Y together, does yours move seperately?
Hood