CNC threading: G76 Fanuc threading cycle – first depth of cut calculation
CNC threading often fails because of improper depths of cut. Too high depth of cut causes excessive load on the tool and part, high tool wear, and poor part quality. Too low depth of cut causes high cycle time and work hardening. In the G76 Fanuc threading cycle you have to specify the first depth of cut. This is also common to other controllers like Haas and Mitsubishi.
Calculating the first depth of cut involves a simple formula, but is not done on 90 % of shop floors. Most programmers have a thumb rule that has no scientific basis.
Here’s the explanation
The cutting load is proportional to the cross sectional area of material being removed. If the depths of cut are constant, the load increases with each cut. The load in cut 2 is twice that in cut 1, in cut 3 it is 4 times more than in cut 1, in cut 4 it is 6 times more, etc. Disastrous for the tool and the part.
To prevent this, controllers have a constant area cutting logic in the threading cycle. The depth of each successive cut is reduced to keep the cutting area and hence cutting load constant. Unfortunately for the programmer, the G76 Fanuc threading cycle (as well as on Haas and Mitsubishi) requires that you specify the first depth of cut in the threading cycle command. They calculate the remaining depths of cut from this. This involves a small calculation, and most programmers do not do this, ending up getting the thread right after some trial and error that involves rejecting a few parts.
This is how you calculate the first depth of cut – just 3 steps:
1. Determine the number of cuts based on the workpiece material, type of thread (Metric, UNI, etc.) and the pitch, from the tool manufacturer’s catalog.
2. Use this formula to determine the first depth of cut.
E.g., if the thread depth is 1.28 and the number of cuts is 8, the depth of the first cut is 0.45.
3. Use this value in the threading cycle. E.g. on a Fanuc controller you would program this as Q450 in the second line of the G76 cycle (the value is programmed in microns in Fanuc).
Action point
Ensure that this simple method is used for calculating the first thread depth to program threading.
OR
Get a software like CADEM CAPSturn CNC lathe programming software that automatically determines the number of cuts based on the type and size of thread, and outputs the correct value in the threading cycle in the program.
