CNC Tips and Techniques. Peter Smid

even X.1 — all versions have the same meaning. Formats such as X1.0000 can be safely shortened to X1.0 or even X1. — all versions have the same meaning as well.

       Combine Single-Axis Motions

      Before combining two or more single-axis motions into one, always consider how such changes affect the machining safety. For example, it is quite common to program a tool approach along XY axes first than in the Z-axis, whereas tool return will be the opposite — the Z-axis first, followed by XY axes motion. Combining such motions will save only a few characters, but also may endanger machining safety.

      Some additional methods may also be used. Shortening the length of CNC programs should always be considered a special situation, never the standard method of programming. There is nothing wrong with including convenience features in the program; such programs are easier to read, easier to interpret, and much easier to change. If the methods described are still not enough, remember, there is always the DNC method waiting in the wings.

Conversion of Lathe Cycles June 2005, updated February 2013

      Most CNC lathe programmers would agree that the most useful features of a CNC lathe control system are the multiple repetitive cycles. Multiple repetitive cycles for CNC lathes have been an important part of control systems since the mid 1980s. Still, to this day, they present the most innovative approach of material removal, particularly in the areas of turning, boring, and threading. Over the thirty years of their existence, multiple repetitive cycles have gone through only two major changes. Earlier controls require these cycles to be programmed in a single block, later controls require two blocks of program input. This difference in programming method often presents a situation when one type has to be converted to another type — usually from a single block format to the double block format.

      Converting Formats

      To start, let’s look at the word convert. Changing from one format to another is not a true conversion or — at least, it is not a complete conversion. The reason is that a double block format offers more features than a single block format. Also, keep in mind that you have no choice here; the control system determines the programming method. Typically, Fanuc control models 10/11/15 use a single block format, other control models (0/16/18/20/21...) use the double block format. What cycles are affected? All multiple repetitive cycles from G71 to G76 can be programmed in one or the other format, depending on the control. The finishing cycle G70 always uses a single block format.

       Single Block Format

      The single block format is the older of the two, and relies heavily on the settings of system parameters, generally inaccessible to the machine operator. I will use the most commonly used G71 and G76 cycles as examples in this column; other cycles follow a similar pattern. The single block format of the roughing cycle G71 is:

       G71 P.. Q.. U.. W.. D.. F..

      In this single block format (spindle speed is assumed to be in effect), P and Q addresses refer to the block numbers defining the finish contour. U and W are specifications of stock amount left over for finishing, D is the depth of cut (written without a decimal point), and F is the roughing feedrate. In addition, some controls also accept I and K addresses that control the distance and direction of semi-finishing.

       Double-Block Format

      For controls requiring a two-block format, the G71 must be written at the beginning of each consecutive block:

       G71 U.. R..

       G71 P.. Q.. U.. W.. F..

      The programmed data are similar but a bit more flexible. In the first block, the U address is the cutting depth (decimal point can be programmed), and the R address is the amount of retract from each cut. The second block has the same meaning as before — finish contour block number range P and Q, stock allowances U and W, and feedrate F. Apart from the more convenient way of programming the cutting depth, the addition of the R address represents the major change. In a single block format, the retraction amount was controlled by a system parameter; in the double block format, the programmer can specify such amount in the program directly.

      G76 Threading Cycle