MPost – Getting Correct Rotary Table Angles

Users have intermittantly complained about Mpost's output angles for 2-Axis Indexing vs. 4-Axis Contouring. This happens because of four facts of post operation:

1. For consistent angle calculation, the 'Set axis' data in SURFCAM must describe the same rotary table orientation as the POSTFORM.M file. (In many cases it isn't.) .
2. Unless a "Set Axis" record is included in the incoming INC data, the post uses the template's INDEX line to determine the rotary axis definition (direction).
3. SURFCAM's 2D operations do NOT include "Set axis" record in the INC file. (So when they are posted separately, the indexing could be different than they are when they are posted along with a tool path that DOES include "Set axis" data.).
4. MPost's "Pre-scan" picks up the "Set axis" data when the entire setup section is posted, but not when just a 2D operation is posted by itself. (Refer to #2 immediately above)

Below are the typical changes that need to be done to the post.

The original template typically has these defined incorrectly:

...
A ->4.>3 Mult -1
...
Index X
...

The template SHOULD be setup this way:

...
A ->4.>3 (Get rid of 'Mult -1')
...
Index X- ( Add '-' )
...

Below is a technical tip from PostHaste (the author of MPost) that addresses this issue.

There is a possibility of INCORRECT rotary angles being output with "CView Machining" (4 axis indexing" of 2D toolpaths. You can setup your posts correctly to avoid this problem.

Fact # 1:

The "Set axis" command in SURFCAM defines a vector that indicates which direction the indexer is "facing".

For example:

- (0,0,0) (1,0,0) indicates that the indexer is on the left end of the machine table (X-), facing towards the right (X+) end.

- (0,0,0) (-1,0,0) (or [1,0,0] [0,0,0]) indicates that the indexer is on the RIGHT end of the machine table (X+), facing towards the LEFT (X-) end. (Note - THE LATTER is the "default" setup that most vertical machines use.)

Fact # 2:

The INDEX statement in the post template (POSTFORM.M file) indicates WHICH END OF THE TABLE" the indexer is on (which is the OPPOSITE of the 'way it is facing').

SO... to setup your SURFCAM system AND the post for the most common 4-Axis setup (the latter of the above examples) you would use the following settings:

- In SURFCAM, the "Set Axis" should be (0,0,0) (-1,0,0) or any 'equivalent' such as (1,0,0) (0,0,0) which is typically "facing to the left" (towards X-) on a standard vertical mill.

- In your POSTFORM.M template, use "INDEX X" to indicate that the indexer is on the "X+" (right) end of the machine table (which means that it is 'facing left', matching the above 'set axis' data).

Fact # 3:

For consistent correct angle output, the 'Set axis' data in SURFCAM must describe the same rotary table orientation as the POSTFORM.M file.

In a nutshell, SURFCAM's vector indicates the direction the indexer is facing, but the INDEX line in the template tells the post "WHERE THE INDEXER IS" so you have to be careful that these "match"... which is a bit of a paradox, because they describe the same data in two different ways.

To avoid confusion, here's how to 'match' them:

- If the Set Axis vector is pointing towards X+, Use "Index X-".

- If the Set Axis vector is pointing towards X-, Use "Index X+".

(Of course, you must apply the same logic to support a "B-Axis" machine [typically horizontal mill] indexing - using Index Y or Y- accordingly.)

Finally, if you need to reverse the sign (+/-) on the rotary angle, use the MULT -1 modifier on the letter A (or B) - do NOT just add (or take away the '-' on the INDEX line in the template).
 

The bottom line..

Since it appears that the typical SURFCAM installation has the 'Set axis' data defined as (0,0,0 +1,0,0) in the configuration settings (in Tools, Options, NC Defaults, 4 axis, Rotary Options)...which is the opposite of the 'typical' rotary axis setup...NOT TO WORRY.

For the typical "rotary table on the right" setup we can simply compensate for this by setting the post template to Index X- (Again, you can use the "Mult -1" modifier if you need to reverse the sign on your rotary axis letter.)