#LyX 1.3 created this file. For more info see http://www.lyx.org/
\lyxformat 221
\textclass book
\language english
\inputencoding default
\fontscheme bookman
\graphics default
\float_placement !!h
\paperfontsize 10
\spacing single 
\papersize Default
\paperpackage a4wide
\use_geometry 0
\use_amsmath 0
\use_natbib 0
\use_numerical_citations 0
\paperorientation portrait
\secnumdepth 5
\tocdepth 5
\paragraph_separation skip
\defskip smallskip
\quotes_language english
\quotes_times 2
\papercolumns 1
\papersides 2
\paperpagestyle default

\layout Chapter

Coordinate System and G92 Offsets
\layout Section

Introduction
\layout Standard

You have seen how handy a tool length offset can be.
 Having this allows the programmer to ignore the actual tool length when
 writing a part program.
 In the same way, it is really nice to be able to find a prominent part
 of a casting or block of material and work a program from that point rather
 than having to take account of the location at which the casting or block
 will be held during the machining.
\layout Standard

This chapter introduces you to offsets as they are used by the EMC.
 These include;
\layout Itemize

machine coordinates (G53
\begin_inset LatexCommand \index{G53}

\end_inset 

) 
\layout Itemize

nine offsets (G54-G59.3
\begin_inset LatexCommand \index{G54}

\end_inset 


\begin_inset LatexCommand \index{G55}

\end_inset 


\begin_inset LatexCommand \index{G56}

\end_inset 


\begin_inset LatexCommand \index{G57}

\end_inset 


\begin_inset LatexCommand \index{G58}

\end_inset 


\begin_inset LatexCommand \index{G59}

\end_inset 


\begin_inset LatexCommand \index{G59.1}

\end_inset 


\begin_inset LatexCommand \index{G59.2}

\end_inset 


\begin_inset LatexCommand \index{G59.3}

\end_inset 

 ) 
\layout Itemize

a set of global offsets (G92
\begin_inset LatexCommand \index{G92}

\end_inset 

).
\layout Section

The Machine Position Command (G53)
\layout Standard

Regardless of any offsets that may be in effect, putting a G53
\begin_inset LatexCommand \index{G53}

\end_inset 

 in a block of code tells the interpreter to go to the real or absolute
 axis positions commanded in the block.
 For example
\layout Standard

g53 g0 x0 y0 z0 
\layout Standard

will get you to the actual position where these three axes are zero.
 You might use a command like this if you have a favorite position for tool
 changes or if your machine has an auto tool changer.
 You might also use this command to get the tool out of the way so that
 you can rotate or change a part in a vice.
 
\layout Standard

G53 is not a modal command.
 It must be used on each line where motion based upon absolute machine position
 is desired.
 
\layout Section

Fixture Offsets (G54-G59.3
\begin_inset LatexCommand \index{G54}

\end_inset 


\begin_inset LatexCommand \index{G55}

\end_inset 


\begin_inset LatexCommand \index{G56}

\end_inset 


\begin_inset LatexCommand \index{G57}

\end_inset 


\begin_inset LatexCommand \index{G58}

\end_inset 


\begin_inset LatexCommand \index{G59}

\end_inset 


\begin_inset LatexCommand \index{G59.1}

\end_inset 


\begin_inset LatexCommand \index{G59.2}

\end_inset 

 
\begin_inset LatexCommand \index{G59.3}

\end_inset 

)
\layout Standard


\begin_inset Float figure
wide false
collapsed false

\layout Standard
\align center 

\begin_inset Graphics
	filename offsets.png
	display color
	width 80text%

\end_inset 


\layout Caption


\begin_inset LatexCommand \label{fig:offsets}

\end_inset 

Work Offsets
\end_inset 


\layout Standard

Work or fixture offset are used to make a part home that is different from
 the absolute, machine coordinate system.
 This allows the part programmer to set up home positions for multiple parts.
 A typical operation that uses fixture offsets would be to mill multiple
 copies of parts on "islands" in a piece, similar to figure 
\begin_inset LatexCommand \ref{fig:offsets}

\end_inset 


\layout Standard

The values for offsets are stored in the VAR file that is requested by the
 INI file during the startup of an EMC.
 In our example below we'll use 
\begin_inset LatexCommand \index{G55}

\end_inset 

G55.
 The values for each axis for G55 are stored as variable numbers.
 
\layout Quote

5241\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5242\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5243\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5244\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5245\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5246\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Standard

In the VAR file scheme, the first variable number stores the X offset, the
 second the Y offset and so on for all six axes.
 There are numbered sets like this for each of the fixture offsets.
 
\layout Standard

Each of the graphical interfaces has a way to set values for these offsets.
 You can also set these values by editing the VAR file itself and then issuing
 a [reset] so that the EMC reads the new values.
 For our example let's directly edit the file so that G55 takes on the following
 values.
\layout Quote

5241\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 2.000000
\layout Quote

5242\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 1.000000
\layout Quote

5243\SpecialChar ~
\SpecialChar ~
 -2.000000
\layout Quote

5244\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5245\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5246\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Standard

You should read this as moving the zero positions of G55 to X = 2 units,
 Y= 1 unit, and Z = -2 units away from the absolute zero position.
\layout Standard

Once there are values assigned, a call to G55 in a program block would shift
 the zero reference by the values stored.
 The following line would then move each axis to the new zero position.
 Unlike G53, G54 through G59.3 are modal commands.
 They will act on all blocks of code after one of them has been set.
 The program that might be run using figure 
\begin_inset LatexCommand \ref{fig:offsets}

\end_inset 

 would require only a single coordinate reference for each of the locations
 and all of the work to be done there.
 The following code is offered as an example of making a square using the
 G55 offsets that we set above.
\layout Quote

G55 G0 x0 y0 z0
\layout Quote

g1 f2 z-0.2000
\layout Quote

x1
\layout Quote

y1
\layout Quote

x0
\layout Quote

y0
\layout Quote

g0 z0
\layout Quote

g54 x0 y0 z0
\layout Quote

m2
\layout Standard


\begin_inset Quotes eld
\end_inset 

But,
\begin_inset Quotes erd
\end_inset 

 you say, 
\begin_inset Quotes eld
\end_inset 

why is there a G54 in there near the end.
\begin_inset Quotes erd
\end_inset 

 Many programmers leave the G54 coordinate system with all zero values so
 that there is a modal code for the absolute machine based axis positions.
 This program assumes that we have done that and use the ending command
 as a command to machine zero.
 It would have been possible to use g53 and arrive at the same place but
 that command would not have been modal and any commands issued after it
 would have returned to using the G55 offsets because that coordinate system
 would still be in effect.
\layout Quote


\begin_inset LatexCommand \index{G54}

\end_inset 

G54\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 1
\layout Quote


\begin_inset LatexCommand \index{G55}

\end_inset 

G55\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 2
\layout Quote


\begin_inset LatexCommand \index{G56}

\end_inset 

G56\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 3
\layout Quote


\begin_inset LatexCommand \index{G57}

\end_inset 

G57\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 4
\layout Quote


\begin_inset LatexCommand \index{G58}

\end_inset 

G58\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 5
\layout Quote


\begin_inset LatexCommand \index{G59}

\end_inset 

G59\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 6
\layout Quote

G59.1
\begin_inset LatexCommand \index{G59.1}

\end_inset 

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 7
\layout Quote

G59.2
\begin_inset LatexCommand \index{G59.2}

\end_inset 

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 8
\layout Quote

G59.3
\begin_inset LatexCommand \index{G59.3}

\end_inset 

\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
use preset work coordinate system 9
\layout Subsection

Default coordinate system
\layout Standard

One other variable in the VAR file becomes important when we think about
 offset systems.
 This variable is named 5220.
 In the default files its value is set to 1.00000.
 This means that when the EMC starts up it should use the first coordinate
 system as its default.
 If you set this to 9.00000 it would use the nineth offset system as its
 default for startup and reset.
 Any value other than an interger (decimal really) between 1 and 9 will
 cause the EMC to fault on startup.
 
\layout Subsection

Setting coordinate system values within G-code.
\layout Standard

In the general programming chapter we listed a G10 command word.
 This command can be used to change the values of the offsets in a coordinate
 system.
 (add here)
\layout Section

G92
\begin_inset LatexCommand \index{G92}

\end_inset 

 Offsets
\layout Standard

G92 is the most misunderstood and maligned part of EMC programming.
 The way that it works has changed just a bit from the early days to the
 current releases.
 This change has confused many users.
 It should be thought of as a temporary offset that is applied to all other
 offsets.
 
\layout Standard

In response to criticism of it, Ray Henry studied it by comparing the way
 the interpreter authors expected it to work and the way that it worked
 on his Grizzly minimill.
 The following quoted paragraphs are extracted from his paper which is available
 in several text formats in the dropbox at 
\begin_inset LatexCommand \url{http://www.linuxcnc.org}

\end_inset 

.
\layout Subsection

The G92 commands
\layout Standard

This set of commands include;
\layout Description

G92 This command, when used with axis names, sets values to offset variables.
\layout Description

G92.1
\begin_inset LatexCommand \index{G92.1}

\end_inset 

 This command sets zero values to the g92 variables.
\layout Description

G92.2
\begin_inset LatexCommand \index{G92.2}

\end_inset 

 This command suspends but does not zero out the g92 variables.
\layout Description

G92.3
\begin_inset LatexCommand \index{G92.3}

\end_inset 

 This command applies offset values that have been suspended.
\layout Standard

When the commands are used as described above, they will work pretty much
 as you would expect.
\layout Quote

A user must understand the correct ways that the g92 values work.
 They are set based upon the location of each axis when the g92 command
 is invoked.
 The NIST document is clear that, 
\begin_inset Quotes eld
\end_inset 

To make the
\noun on 
 
\noun default 
current point have the coordinates
\begin_inset Quotes erd
\end_inset 

 x0, y0, and z0 you would use g92 x0 y0 z0.
 G92 
\emph on 
does not work from absolute machine coordinates
\emph default 
.
 It works from 
\emph on 
current location
\emph default 
.
 
\layout Quote

G92 also works from current location as modified by any other offsets that
 are in effect when the g92 command is invoked.
 While testing for differences between work offsets and actual offsets it
 was found that a g54 offset could cancel out a g92 and thus give the appearance
 that no offsets were in effect.
 However, the g92 was still in effect for all coordinates and did produce
 expected work offsets for the other coordinate systems.
 
\layout Quote

It is likely that the absence of home switches and proper home procedures
 will result in very large errors in the application of g92 values if they
 exist in the var file.
 Many EMC users do not have home switches in place on their machines.
 For them home should be found by moving each axis to a location and issuing
 the home command.
 When each axis is in a known location, the home command will recalculate
 how the g92 values are applied and will produce consistent results.
 Without a home sequence, the values are applied to the position of the
 machine when the EMC begins to run.
\layout Subsection

Setting G92 values
\layout Standard

There are at least two ways to set G92 values.
\layout Itemize

right mouse click on position displays of tkemc will popup a window into
 which you can type a value.
 
\layout Itemize

the g92 command
\layout Standard

Both of these work from the current location of the axis to which the offset
 is to be applied.
 
\layout Quote

Issuing g92 x y z a b c does in fact set values to the g92 variables such
 that each axis takes on the value associated with its name.
 These values are assigned to the current position of the machine axis.
 These results satisfy paragraphs one and two of the NIST document.
\layout Quote

G92 commands work from current axis location and add and subtract correctly
 to give the current axis position the value assigned by the g92 command.
 The effects work even though previous offsets are in.
\layout Standard

So if the X axis is currently showing 2.0000 as its position a G92 x0 will
 set an offset of -2.0000 so that the current location of X becomes zero.
 A G92 X2 will set an offset of 0.0000 and the displayed position will not
 change.
 A G92 X5.0000 will set an offset of 3.0000 so that the current displayed
 position becomes 5.0000.
 
\layout Subsection

G92 Cautions
\layout Standard

Sometimes the values of a G92 offset get stuck in the VAR file.
 When this happens reset or a startup will cause them to become active again.
 The variables are named 
\layout Quote

5211\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5212\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5213\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5214\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5215\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Quote

5216\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
 0.000000
\layout Standard

where 5211 is the X axis offset and so on.
 If you are seeing unexpected positions as the result of a commanded move,
 or even unexpected numbers in the position displays when you start up,
 look at these variables in the VAR file and see if they contain values.
 If they do, set them to zeros and the problems should go away.
\layout Quote

With these tests we can see that reset returns g92 to the condition that
 it had when the interpreter started up.
 The reader should note that we have established ...
 that no write of these values occurs during a normal run so if no g92 was
 set at the startup, none will be read in during a reset.
\layout Quote

It may be that this is the heart of the problem that some have experienced
 with differences between the old and the new interpreter.
 It may well be, but I leave it to others to test, that the old interpreter
 and task programs immediately wrote values to the var file and then found
 those values during a reset.
 
\layout Standard

On the other hand, if G92 values existed in the VAR file when the EMC started
 up
\layout Quote

...
 starting the EMC with g92 values in the var file is that it will apply
 the values to current location of each axis.
 If this is home position and home position is set as machine zero everything
 will be correct.
 Once home has been established using real machine switches or moving each
 axis to a known home position and issuing an axis home command, g92 commands
 and values work as advertised.
\layout Quote

These tests did not study the effect of re-reading the var file while they
 contain numbers.
 This could cause problems if g92 offsets had been removed with g92.1 but
 the var file still contained the previous numbers.
 
\layout Standard

It is this complexity that causes us to say that G92 values must be treated
 as temporary.
 They should be used to set global short term offsets.
 The G54-59.3 coordinate systems should be used whenever long lasting and
 predictable offsets are needed.
\layout Section

Sample Program Using Offsets
\layout Standard

This sample engraving project mills a set of four .1 radius circles in roughly
 a star shape around a center circle.
 We can setup the individual circle pattern like this.
\layout Quote
\noindent 
G10 L2 P1 x0 y0 z0 (ensure that g54 is set to machine zero) 
\layout Quote
\noindent 
g0 x-.1 y0 z0
\layout Quote
\noindent 
g1 f1 z-.25
\layout Quote
\noindent 
g3 x-.1 y0 i.1 j0
\layout Quote
\noindent 
g0 z0
\layout Quote

m2
\layout Standard

We can issue a set of commands to create offsets for the four other circles
 like this.
\layout Quote

G10 L2 P2 x0.5 (offsets g55 x value by 0.5 inch) 
\layout Quote

G10 L2 P3 x-0.5 (offsets g56 x value by -0.5 inch) 
\layout Quote

G10 L2 P4 y0.5 (offsets g57 y value by 0.5 inch) 
\layout Quote

G10 L2 P5 y-0.5 (offsets g58 y value by -0.5 inch) 
\layout Standard

We put these together in the following program.
\layout Quote

(a program for milling five small circles in a diamond shape)
\layout Quote

G10 L2 P1 x0 y0 z0 (ensure that g54 is machine zero)
\layout Quote

G10 L2 P2 x0.5 (offsets g55 x value by 0.5 inch) 
\layout Quote

G10 L2 P3 x-0.5 (offsets g56 x value by -0.5 inch) 
\layout Quote

G10 L2 P4 y0.5 (offsets g57 y value by 0.5 inch) 
\layout Quote

G10 L2 P5 y-0.5 (offsets g58 y value by -0.5 inch)
\layout Quote

g54 g0 x-.1 y0 z0 (center circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g55 g0 x-.1 y0 z0 (first offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g56 g0 x-.1 y0 z0 (second offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g57 g0 x-.1 y0 z0 (third offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g0 z0
\layout Quote

g58 g0 x-.1 y0 z0 (fourth offset circle)
\layout Quote

g1 f1 z-.25
\layout Quote

g3 x-.1 y0 i.1 j0
\layout Quote

g54 g0 x0 y0 z0
\layout Quote

m2
\layout Standard

Now comes the time when we might apply a set of G92 offsets to this program.
 You'll see that it is running in each case at z0.
 If the mill were at the zero position, a g92 z1.0000 issued at the head
 of the program would shift everything down an inch.
 You might also shift the whole pattern around in the XY plane by adding
 some x and y offsets with g92.
 If you do this you should add a G92.1 command just before the m2 that ends
 the program.
 If you do not, other programs that you might run after this one will also
 use that g92 offset.
 Furthermore it would save the g92 values when you shut down the EMC and
 they will be recalled when you start up again.
\the_end