R....
a lot of folks say to stay away from that..........
So i think not to confuse myself i'll do the I J thing...LOL
When using I/J/K format, you describe to the control, the position of the arc centre relative to the start coordinate of the arc, which you have also specified in the program. The main reason for treating the "R" format with caution, is that when using I/J/K format, a check is done by the control to ensure the radius of the arc from the arc centre to the end point of the arc specified in the program is the same, within a tlerance set in parameters, as the radius from arc centre to the start point specified in the program. If its not, the program is halted and an alarm is raised.
When using the "R" format, within the program, you specify the start and end coordinates, plus the radius of the arc. From that, the control calculates where the arc centre is using the algorithm for an arc through 2 points with a given radius. If a mistake is made with regards to specifying the start, or end point coordinates of the arc, the control simply calculates the arc centre based on the wrong coordinates and shifts the arc centre from where it would be for the correct start/end coordinates. This, of course, will affect the accuracy of the profile and usually doesn't become known until the finished machined part goes to inspection. With I/J/K format, the control alerts to the error before the arc feature is cut and remedial steps can be taken. Start/end point coordinate errors are far more likely with a manually calculated, Finger Cam program than one produced using CAM software.
A second reason I/J/K format is more desirable than R format, is that with some (many) Start/End Coordinate and Radius specified arcs, the result is not as accurate as when using I/J/K format, particularly when programming a full circle. When programming using the I/J/K format, whether using a calculator, or CAM software, the centre of the arc is calculated using double precision numbers, or can be in the case when using a calculator (its up to the operator as to the accuracy of the numbers used). All motion control software calculates the trajectory of an arc based on the Centre Coordinate of the arc, the Start Coordinate and the number of degrees the End Coordinate is from the Start Coordinate of the arc. Accordingly, the accuracy of the arc centre is very important.
As mentioned, when using I/J/K format, the arc centre is pre-calculated for the control using high precision numbers. When using R Format, the numbers specifying the Start/End and Radius of the arc have already been rounded to the least programmable increment of the control and therefore, at best, the control can only use numbers that are accurate to three or four decimal places when calculating the centre coordinates of the arc. In the majority of cases, in practical terms, this inaccuracy won't be detectable in the work-piece, but it can be measurable when the feature being machined is a full circle.
When programming a full circle using R Format, most will opt to program 2 x 180 degree arcs and this is how the Post Processor of probably all CAM software will be set up. In this case, the control will have to calculate two arc centre coordinate sets in the execution of the full circle; one set for each 180deg arc. If one is hell bent on using the "R" format and where full circles are involved, a more accurate result will be obtained by programming, say, 359degs of the arc for the first part and the final 1deg for the last part of the circle. Two arc centres will still be calculated by the control, but for practical purposes, extremely hard to detect an error. When programming an arc through more than 180degs using "R" Format, the "R" address is specified as a minus value.
Regards,
Bill
(way more than 1 ....
)
