When Type 2 is specified by the programmer the Z Axis PQ path is allowed to vary (for example, the Z-axis tool path can reverse direction).
The Z Axis PQ path must not exceed the original starting location. The only exception is on the Q block.
Type 2 roughing when Setting 33 is set to YASNAC, must include R1 (with no decimal) on the G71 command block.
Type 2, when Setting 33 is set to FANUC, must have a reference move, in both the X and Z Axis, in the block specified by P.
Roughing is similar to Type 1 except after each pass, along the X Axis, the tool will follow the path defined by PQ. The tool will then retract parallel to the Z Axis by the distance defined in Setting 73 (Can Cycle Retraction). The Type 2 roughing method does not leave steps in the part prior to finish cutting and typically results in a better finish.
A side effect of using a X finish or roughing allowance is the limit between the two cuts on one side of a trough and the corresponding point on the other side of the trough. This distance must be greater than double the sum of the roughing and finish allowances.
For example, if G72 Type 2 path contains the following:
... ; X-5. Z-5. ; X-5.1 Z-5.1 ; X-8.1 Z-3.1 ; ... ;
The greatest allowance that can be specified is 0.999, since the horizontal distance from the start of cut 2 to the start point on cut 3 is 0.2. If a larger allowance is specified, overcutting occurs.
Cutter compensation is approximated by adjusting the roughing allowance according to the radius and tip type of the tool. Thus, the limitations that apply to the allowance also apply to the sum of the allowance and the tool radius.
caution: If the last cut in the P-Q path is a non-monotonic curve using a finish allowance, add a short retraction cut (do not use U).
Monotonic curves are curves that tend to move in only one direction as x increases. A monotonic increasing curve always increases as x increases, i.e. f(a)>f(b) for all a>b. A monotonic decreasing curve always decreases as x increases, i.e. f(a)b. The same sort of restrictions are also made for the monotonic non-decreasing and monotonic non-increasing curves. As shown in the following Figure, as X increases, Z decreases, then increases, then decreases, and finally increases. This X-Z curve is definitely non-monotonic. Thus, the need for a short retraction cut.
% O60723(G72 END FACE REMOVAL) ; (G54 X0 is at the center of rotation) ; (Z0 is on the face of the part) ; (T1 is an end face grooving tool) ; (BEGIN PREPARATION BLOCKS) ; T101 (Select tool and offset 1) ; G00 G18 G20 G40 G80 G99 (Safe startup) ; G50 S1000 (Limit spindle to 1000 RPM) ; G97 S500 M03 (CSS off, spindle on CW) ; G00 G54 X2.1 Z0.1 (Rapid to clear position) ; M08 (Coolant on) ; (BEGIN CUTTING BLOCKS) ; G72 P1 Q2 D0.06 I0.02 K0.01 U0.0 W0.01 F0.015 (Begin G72) ; N1 G01 Z-0.46 X2.1 F0.005 (P1 - Begin toolpath) ; X2. (1st position); G03 X1.9 Z-0.45 R0.2 (Tool path) ; G01 X1.75 Z-0.4 (Linear feed) ; G02 X1.65 Z-.4 R0.06 (Feed CW) ; G01 X1.5 Z-0.45 (Linear feed) ; G03 X1.3 Z-0.45 R0.12 (Feed CCW) ; G01 X1.17 Z-0.41 (Linear feed) ; G02 X1.03 Z-0.41 R0.1 (Feed CW) ; G01 X0.9 Z-0.45 (Linear feed) ; G03 X0.42 Z-0.45 R0.19 (Feed CCW) ; G03 X0.2 Z-0.3 R0.38 (Feed CCW) ; N2 G01 X0.01 Z0 (Q2 - End of tool path) ; G70 P1 Q2 (Finish Pass) ; (BEGIN COMPLETION BLOCKS) ; G00 G53 X0 M09 (X home, coolant off) ; G53 Z0 M05 (Z home, spindle off) ; M30 (End program) ; %