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Issue cutting threads CNC lathe (Fanuc 21i-T)

maguilera

Plastic
Joined
Jan 4, 2022
Yesterday I was running a program that has been used many times before, but this time the internal threads were coming out really bad, like spoiled. I replaced the insert, checked that the tool is on center, made sure X and Z offsets were good and gave it another go but had no luck again, threads came out awful.

I then ran the threading section of the program without the work piece to take a look, and the linear velocity of the tool doesn’t look constant, seems like it is breaking and accelerating really quickly a bunch of times during the threading, in some weird way looks like those chip breaking cycles in Z direction. I’ve tried decreasing the spindle speed by half and still would behave the same way, but slower.

The only thing that got rid of this issue was adding a dwell of 2 seconds before the G76 cycle.

The part is 1020 steel with external diameter of 125mm and 40mm exposed, clamped using soft jaws; the tool holder body is 20mm with about 70mm of overhang, so rigidity shouldn’t be an issue.

As can be seen in the program below and the drawing attached, enough run-in and run-out were provided.

Has anyone experienced something like this?

The lathe is an INDEX MC-170 with Fanuc 21i-T controller.

T1010
G97 S1000 M3
M8 M20
G0 G54 X53 Z6
G4 P2000
G76 P010060 Q65 R0.065
G76 X60.3 Z-60 P1300 Q395 F2

Cheers
 

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Philabuster

Diamond
Joined
Jul 12, 2006
Location
Tempe, AZ
Yesterday I was running a program that has been used many times before, but this time the internal threads were coming out really bad, like spoiled. I replaced the insert, checked that the tool is on center, made sure X and Z offsets were good and gave it another go but had no luck again, threads came out awful.

I then ran the threading section of the program without the work piece to take a look, and the linear velocity of the tool doesn’t look constant, seems like it is breaking and accelerating really quickly a bunch of times during the threading, in some weird way looks like those chip breaking cycles in Z direction. I’ve tried decreasing the spindle speed by half and still would behave the same way, but slower.
Sounds like the spindle encoder in the machine is going out. The program is not the issue.
 

Philabuster

Diamond
Joined
Jul 12, 2006
Location
Tempe, AZ
Replace the spindle encoder. You will not be able to cut threads until then. The spindle encoder is what keeps track of the spindle orientation when threading.
 








 
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