Machinery's Handbook, 31st Edition
1370 CNC FIXED CYCLE CODES takes place before tool retraction. Typically, the amount of Q__ is very small, usually in the range of 0.005-0.010 inches or 0.125-0.250 mm. In this internally complex cycle, the tool starts at the R-level and feeds to the Z-depth. When the depth is reached, the spindle stops and orients (so called OSS—oriented spindle stop). When oriented, the tool moves away by the Q__ amount, then retracts to the R-level. Once outside of the hole, it moves back by the Q__ amount and the spindle resumes normal rotation (see Fig. 23). When setting a boring bar used with G76 at the machine, the operator has to align the cutting tip in such a way that it is aimed in the opposite direction of the shift motion. This is done in oriented spindle mode, using the M19 command in MDI.
Q
Q
G98
G98 mode only
OSS
G99
Z0
Z0
OSS
Z-depth
Dwell OSS
R-level
Z-depth
Spindle start
Fig. 23. G76 Fixed Cycle. Fig. 24. G87 Fixed Cycle. Back Boring (G87): The G87 fixed cycle is used for back boring and has the following format: N__ G98 G87 X__ Y__ R__ Z__ Q__ F__ As the name suggests, this cycle is used to bore upwards, starting at the bottom of an existing hole. The R-level is below the part, and only G98 retract method can be used with this cycle. The Q__ amount is also the amount of shift away from the centerline. When the tool reaches X__Y__ hole location at the initial level, the spindle stops, tool orients and shifts away from hole centerline by the amount of Q__. Then, it rapids to the R-level, shifts back, and feeds to the Z-depth. When the depth is reached, the tool stops again, orients, and shifts by the Q__ amount. The following retract will be to the initial level, where the tool shifts back, and the spindle resumes normal rotation (see Fig. 24). This cycle can only be used if the boring bar can move clearly through the existing hole. The shift amount Q__ must be greater that one half of the difference between the back bore diameter and the existing hole diameter. Note that G99 retract mode cannot be used. Manual Boring (G88): The G88 fixed cycle is used for boring with manual intervention and has the following format: N__ G99 (G98) G88 X__ Y__ R__ Z__ P__ F__ The G88 fixed cycle is rather rare. Its use is limited to boring operations with special tools that require manual interference at the bottom of a hole. When such a operation is completed, the tool is moved out of the hole for safety reasons. This cycle may be used by some tool manufactures for certain special operations (see Fig. 25).
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