Re: Re: G53-57 was Re: Need Zero Help!!

Hi Alan

Comments mixed in

>    From: Alan Marconett KM6VV <KM6VV@...>
> Subject: Re: G53-57 was Re: Need Zero Help!!
>
> Thanks Ray,
>
> I'd like to understand the use of G53-57 (fixture offsets) a little
> better.  The handbook has a little data.  I got that G53 is machine
> coordinates, and G54-57 are work coordinates (and that they alias to
> T0-4?).

I'm lost by the alias stuff but g53 is supposed to always be close to the
zero position where EMC sets itself when you home your machine.

This zero need not be where the switches are located but where you tell
the program you want zero to be using the variables

HOME = 0 0 0
HOME_OFFSET = 0.0

in each axis of the ini file.

> so are they just saving AXYZ offsets (and tool comps?), which are then
> added to the machine coordinates?  Or is there more to it then that?

That's exactly it. And these offsets are saved to the var file when you
shut down so they are there when you start up again.

For a bit of extra insurance, I wrote the Set_Coordinates.tcl script so
that it writes them as you press <write> to set them. And after watching
David Munro work with his bridgeport, I decided that we needed separate
teach buttons for each axis. This way you can use one of those little
ball nosed edge finders on each side of a corner of stock or the corner
of your vice and capture each axis when you're ready. (I do need to add
an automatic size offset for this.)

> If I do G92 in "G53", this sets the machine coordinates; then if I
> reposition to a new fixture, do I issue G54 and then G92 again?

Quick answer is no because the g92 is still active. To deactivate it you
would have to set zero position back to g92 0 or issue a g92.1 or g92.2.

Let me take a minute and sort out two things. Setting the value of g92
by itself and using it with other offsets. And then setting g92 with
other offsets already active. For all of these examples I will use

g55 offsets are x1 y1 z-2
g56 offsets are x3 y1 z-3

Actual distance to first side is -3.106

I flip this part when complete into the second holder.

Actual distance to second side is -4.106

You need to work here with a great deal of caution because G92 will add
or subtract an amount from all of the existing coordinate systems. I'll
use BDI-TNG because I can sit here and surf, write this and run EMC all
at the same time. (shameless plug for a product I sell)

G92 is not real intuitive. For example if I drop z to where I'm about
ready for the first surface cut (-3.1060) and right click the z display
it will popup a widget that contains the value 0.0. If I press enter or
ok it sets that axis position to zero by creating a work offset of z
-3.106.

As an aside, I could have put a value (2.0) in that popup rather than
accepting the 0.0 and it would have computed the real g92 offset from the
difference between the value I typed in and the current position of z.
In this case using the previous distance of -3.106 the value 2 gave me a
z -5.106 offset. In effect I told it that I wanted this place, -3.106,
to be +2 and it did exactly that. This can be a killer!

Whenever you set a g92 it adds itself to whatever offsets are already in
g54-g59.3 when you activate them. If I'm using the g55 and g56 values
above, and without thinking, ran the tool down and set g92. If my
program called for a g55 at the start, where am I going to find the tool
working? Well its down there among the ballscrews and gears again. The
g55 will put it at -5.106 while the g56 will put it at -6.106.

Now if I happen to have g55 g0 x0 y0 z0 already active when I position my
endmill and right click to set a g92, I will get a different offset.
From here (z-2 but zero on the display) I again drop the tool in z until
it is about ready to cut, (-1.106) click the display and set zero. The
work offsets say that I've got -3.106 set for z. Now 2" was already
apportioned to the existing g55 and g92 is set to -1.106. Now g56's work
offset is -4.106. They are right where I wanted them.

> After that, I should be able to switch "fixtures" by a G54-57 ?

Yep! Only you can use all ten of them.

Ray