Fanuc G71 Roughing Works With a Small-to-Large Contour

The submitted lathe program runs the contour left to right, puts the groove inside the roughing cycle, and uses cycle parameters that do not match the contour it is meant to cut. Three changes fix it: flip the contour so it runs small diameter to large diameter, take the groove out of the roughing contour, and make the cycle end reference point at the last contour block. The reference block below is written in Fanuc G71 format. The reviewed program may have been written for a Sinumerik-style control, so every cycle parameter must be checked against your own control's manual.

Three ways to program this part, compared

The review produced three candidate approaches. Only one fits a chuck-held part on a conventional lathe.

Recommendation: use approach B on any lathe that holds the part in a chuck. Use approach C only after confirming the machine is a Swiss-type. The rest of this reference follows B.

Deciding the Z direction for your machine

Z- toward the workpiece is normal on chuck lathes, and the review reports the same on Sinumerik 840D lathes and turning centers. A two-spindle machine with both spindles on the same Z axis cuts the left part in Z- and the right part in Z+. A Swiss-type behaves differently: the bar stock moves in +Z through the tool zone, the turning tools move only in X, and the parting cut is made at Z+. If the programmer says the machine "makes a piece along the Z axis", the machine type has not been established yet.

1. Read the machine nameplate or manual and record whether it is a chuck lathe, a two-spindle machine, or a Swiss-type. Gate: the type is written down before any coordinates are chosen.
2. Jog Z in the minus direction with the tool near the part in MPG or handwheel mode. Gate: the tool moves toward the chuck (chuck lathe) or the axis direction matches the manual for your machine type.
3. Set the Z sign for the whole program from that reading. Do not mix signs inside one contour.

Reference roughing block with the contour running small to large

The block below is the corrected Fanuc-format reference. The paste it came from had lost its line breaks, so the line layout here is reconstructed. The coordinates belong to an example part, so replace them with drawing values.

G71 U1.0 R0.5
G71 P100 Q200 U2.0 W0.1 F0.25
N100 G0 G42 X22.0
G1 Z2.0
X30.0 Z-2.0
Z-60.0
X40.0
X60.0 Z-100.0
Z-105.0
G2 X70.0 Z-110.0 R5.0
G1 X80.0
G3 X100.0 Z-120.0 R10.0
G1 Z-200.0
N200 G1 X110.0
G0 G40 Z5.0

Read the block for these features:

• X grows and Z decreases monotonically from the small end at Z2 to the chuck end at Z-200. Roughing cycles of this type need a monotonic contour. This one has no undercut and no groove.
• The first contour block, N100, carries only an X move. The Z approach follows in the next block.
• Both arcs carry an end diameter, an end length, and a radius. The G2 arc goes from X60 Z-105 to X70 Z-110 with R5. The G3 arc goes from X80 Z-110 to X100 Z-120 with R10. The original program's R10 segment lacked its end diameter and length.
• P100 and Q200 bracket the contour exactly. Q names the last contour block, which is why the N numbers must stay in the program even though other line numbers are optional.
• U1.0 in the first line is the depth of cut, 1 mm here. Confirm on your control whether that value is radial or diametral.
• The roughing cycle leaves U2.0 W0.1 of stock. A finish pass over the same P/Q range (G70 P100 Q200) belongs after the cycle. The reference block does not include it, so add it and confirm the finishing tool number and offsets.

Corrections mapped to the symptoms they fix

Grooving tool, holder clearance, and tooling knowledge

The insert sits in a holder, and the holder's back side is wider than the cutting edge. On a big-to-small cut the shoulder that was cut first sits behind the insert, and the holder collides with it. The same collision happens if the groove is programmed into the G71 contour, because the cycle plunges into the groove and retracts along the wall. Cut the roughing contour with a turning tool that clears the shoulders, and cut the groove afterward with a grooving tool that has its own tool number, offset, and block.

Work out the tooling before writing coordinates. Confirm insert geometry, holder clearance angle, and grooving insert width against the drawing. G-code cannot correct a tool that does not fit the profile. An approach that also works is to start with the small side of the part and leave the groove out of the first program. Prove the outer contour, then add the groove as its own operation.

Line numbers and cycle references

Advice to remove all line numbers is only partly right. Numbers on ordinary blocks are optional. The N numbers named by P and Q are not: the cycle finds the contour by those sequence numbers. Keep N100 and N200 in the reference block, and drop the others if they add clutter. If the program is edited later, renumber only after confirming that P and Q still point at the first and last contour blocks.

Commissioning procedure for the corrected program

1. Set the work offset and tool offsets for the turning tool and the grooving tool. Gate: touching off on the stock diameter and face reproduces the X and Z values displayed on the offset page.
2. Load the program and run graphic simulation. Gate: the roughing paths run parallel to Z, step by the depth of cut, and stop at the finish allowance U2.0 W0.1.
3. Check the contour against the drawing. Gate: every arc has an end diameter, an end length, and a radius, and the largest diameter is at the chuck end.
4. Check P and Q. Gate: P equals the number of the first contour block and Q equals the number of the last contour block.
5. Check the depth-of-cut value at the cycle call. Gate: it reads about 1 mm, not 10 mm.
6. Run the first pass in single block with feed override reduced and the distance-to-go display visible. Gate: the entry move lands at the correct diameter with the length offset (for example X100 Z0.5) before any cut begins.
7. Add the finish pass, then add the groove as its own operation with the second tool. Gate: the holder does not touch any shoulder in simulation or in single-block motion.

FAQ