CNC FIXED CYCLE CODES Machinery's Handbook, 31st Edition
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Spindle start - G98
G98
Spindle start - G99
G99
Z0
Z0
Dwell
Dwell
Z-depth
Z-depth
Spindle stop
Fig. 25. G88 Fixed Cycle. Fig. 26. G89 Fixed Cycle. Boring with Dwell (G89): The G89 fixed cycle is used for boring with dwell and has the following format: N__ G99 (G98) G89 X__ Y__ R__ Z__ P__ F__ From the R-level, the boring bar will feed to the Z-depth, dwells for the number of milli seconds specified by P__, than feeds back to R-level, both at the programmed feed rate F__. G85, a similar cycle, does not have a dwell. Either one of these cycles is commonly used for reaming (see Fig. 26). Fixed Cycle Cancellation (G80) : The G80 fixed cycle cancel is used to cancel any fixed cycle and has the following format: N__ G80 G80 is the recommended method for fixed cycle cancellation, although programming any motion command (G00, G01, G02, G03) will also cancel any active cycle. General Notes on Fixed Cycles: Except G87, all fixed cycles can retract to either the R-level (using G99) or the initial level (using G98). The R-level is often called the feed plane and is the Z-position from where the feed rate mode begins. In the cycle, the Z-address is used for depth and the R-address is used for the feed plane to avoid a conflict of two identical addresses. The initial level is the last Z-address programmed before a fixed cycle is called. Although the initial level can be equivalent of the R-level, it usually is higher. Its purpose is to provide means for special clearance setting to bypass obstacles between holes. Fixed cycles for boring can be applied to either single point or double point boring bars, with the exception of G76 and G87 cycles, which require a shift. In all cases, boring bars must be preset to the diameter of the hole machined. Contouring.— Contouring is the most common semi-finishing and finishing toolpath for both machining centers and lathes. The programmer has to calculate every single point of the part in the order of machining. Using a cutter radius offset, the toolpath is based on the drawing dimensions. Not every point is identified in the drawing and its coordinates have to be calculated manually. For vertical and horizontal lines the calculations are quite simple, but for angular lines and partial arcs trigonometric calculations are necessary. Fig. 27 shows typical calculations for start and end points of an arc tangent to two lines, a common situation in programming. As in any calculations, certain features must be known. For the calculations shown, point P, radius R, and angle A (as shown) must be known.
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