7 Multiple Repetitive Cycles on a CNC Lathe
Multiple repetitive cycles are the cycles that are used for roughing and finishing external and internal diameters.
By defining the profile of the part, the machine takes care of the tool paths for the material removal automatically.
Multiple repetitive cycles should not be confused with canned cycles that are used for operations such as drilling and tapping.
Here’s an overview of the main multiple repetitive cycles used on a CNC lathe:
1 . G70 – Finishing cycle
Once the roughing operation is complete, the G70 finishing cycle completes the profile to the finished size. The G70 can call upon the subroutine used during the roughing process or the tool paths can be programmed within the cycle.
2. G71 – OD/ID Roughing cycle
The G71 Roughing cycle removes material along the Z axis. A finishing allowance is added to the cycle that leaves material on for a finishing cut. The tool paths for the roughing cycle are generated automatically.
For more information about the programming of G71 roughing cycle, check out this article
3. G72 – Rough facing cycle
Where the G71 removes material along the Z-axis, the G72 facing cycle cuts along the X-axis while I feeding in along the Z axis. With the finished shape defined in the program, subprogram or subroutine, the workpiece is machined to this profile with an included finish allowance. The tool paths are automatically generated.
Click here for a more in-depth description on how to use the G72 facing cycle.
4. G73 – Pattern repeating cycle
Sometimes called the closed-loop cutting cycle, the G73 cycle machines the part to a defined shape by the tool moving in a fixed pattern for each pass. This is often used when machining castings as the shape of the part is already made so the machine follows this shape while removing material.
For more information about this cycle, read here.
5. G74 – Face cut off cycle
The G74 cycle can be used for face grooving. Infeed is made along the Z-axis intermittently. If the depth of cut along the X-Axis is omitted, it can be used as a deep hole drilling cycle.
6. G75 – Grooving cycle
Similar to the G74 cycle apart from the G75 cuts in the X-Axis. If the Z-axis value is missing, this cycle can be used to part off the component.
7. G76 – Screw thread cutting cycle
The thread pattern is repeated by feeding in the cutting tool in the X-axis to form a screw thread on a diameter. Depending on the software version installed in your machine, this may be done using different techniques. For a full description of each technique and how to program using them check out this article about G76 thread cycle programming.
The above cycles are used to speed up and simplify the programming of a part to rapidly remove material. This process can be sped up further by recalling the subroutine for the finishing cycle that was used during the roughing process.
Author: Marc Cronin, Senior CNC Machine Tools Engineer, and founder of GCodeTutor.com