Hi All,

Certainly a novice at Gcode on my end. Using R in G2 or G3 or for that matter I,J,K seems straight

forward. Who's advice do you want to follow? ( BTW, not mine )

NITS came out with a standard on programing a long time ago. Different machining centers and manufactures, programs, post processors, etc........ seems like over time it's been implemented differently in use. Then you have folks out there that have written articles to clarify ARC coding and books by different authors. Many users, myself included, will just click the reverse arc's box, the arc problem ( crop circles ) goes away, SOMETIMES, and life is good. Just look at postings from different programs and what happens when "interpreted" by MACH. The Smid book dosn't even talk about the "R" that i could find.

So you may ask " should i do something and it will always turn out correct". I don't know, because it's not as simple as one may think based on the post processor, the programs interpretation of the code, a math error, dia / rad mode, incremental / absolute, the plane your working in, how you inialize Mach, etc.........

May I remark, that sometimes a simple question or problem is "loaded" and if a little code was provided it sure can make it easier in trying to help. The R stuff i posted was from something saved some time ago and don't even remember where i got it. Here is some more for reading pleasure......

RICH

THIS FOLLOWING IS A LINK TO THE NIST RS274/NGC LANGUAGE

http://www.linuxcnc.org/handbook/RS274NGC_3/RS274NGC_33a.htmlThe following info was extracted from the handbook.

3.5.3.1 Radius Format Arc

In the radius format, the coordinates of the end point of the arc in the selected plane are specified along with the radius of the arc. Program G2 X- Y- Z- A- B- C- R- (or use G3 instead of G2). R is the radius. The axis words are all optional except that at least one of the two words for the axes in the selected plane must be used. The R number is the radius. A positive radius indicates that the arc turns through 180 degrees or less, while a negative radius indicates a turn of 180 degrees to 359.999 degrees. If the arc is helical, the value of the end point of the arc on the coordinate axis parallel to the axis of the helix is also specified.

It is an error if:

· both of the axis words for the axes of the selected plane are omitted,

· the end point of the arc is the same as the current point.

It is not good practice to program radius format arcs that are nearly full circles or are semicircles (or nearly semicircles) because a small change in the location of the end point will produce a much larger change in the location of the center of the circle (and, hence, the middle of the arc). The magnification effect is large enough that rounding error in a number can produce out-of-tolerance cuts. Nearly full circles are outrageously bad, semicircles (and nearly so) are only very bad. Other size arcs (in the range tiny to 165 degrees or 195 to 345 degrees) are OK.

Here is an example of a radius format command to mill an arc: G17 G2 x 10 y 15 r 20 z 5.

That means to make a clockwise (as viewed from the positive Z-axis) circular or helical arc whose axis is parallel to the Z-axis, ending where X=10, Y=15, and Z=5, with a radius of 20. If the starting value of Z is 5, this is an arc of a circle parallel to the XY-plane; otherwise it is a helical arc.

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3.5.3.2 Center Format Arc

In the center format, the coordinates of the end point of the arc in the selected plane are specified along with the offsets of the center of the arc from the current location. In this format, it is OK if the end point of the arc is the same as the current point. It is an error if:

· when the arc is projected on the selected plane, the distance from the current point to the center differs from the distance from the end point to the center by more than 0.0002 inch (if inches are being used) or 0.002 millimeter (if millimeters are being used).

When the XY-plane is selected, program G2 X- Y- Z- A- B- C- I- J- (or use G3 instead of G2). The axis words are all optional except that at least one of X and Y must be used. I and J are the offsets from the current location (in the X and Y directions, respectively) of the center of the circle. I and J are optional except that at least one of the two must be used. It is an error if:

· X and Y are both omitted,

· I and J are both omitted.

When the XZ-plane is selected, program G2 X- Y- Z- A- B- C- I- K- (or use G3 instead of G2). The axis words are all optional except that at least one of X and Z must be used. I and K are the offsets from the current location (in the X and Z directions, respectively) of the center of the circle. I and K are optional except that at least one of the two must be used. It is an error if:

· X and Z are both omitted,

· I and K are both omitted.

When the YZ-plane is selected, program G2 X- Y- Z- A- B- C- J- K- (or use G3 instead of G2). The axis words are all optional except that at least one of Y and Z must be used. J and K are the offsets from the current location (in the Y and Z directions, respectively) of the center of the circle. J and K are optional except that at least one of the two must be used. It is an error if:

· Y and Z are both omitted,

· J and K are both omitted.

Here is an example of a center format command to mill an arc: G17 G2 x 10 y 16 i 3 j 4 z 9.

That means to make a clockwise (as viewed from the positive z-axis) circular or helical arc whose axis is parallel to the Z-axis, ending where X=10, Y=16, and Z=9, with its center offset in the X direction by 3 units from the current X location and offset in the Y direction by 4 units from the current Y location. If the current location has X=7, Y=7 at the outset, the center will be at X=10, Y=11. If the starting value of Z is 9, this is a circular arc; otherwise it is a helical arc. The radius of this arc would be 5.

In the center format, the radius of the arc is not specified, but it may be found easily as the distance from the center of the circle to either the current point or the end poi