Read this article, no more sleepless nights worrying about G76 Threading Cycle. Myth busting information that simplifies and demystified. Applies to Haas, Fanuc and Mazak ISO Be sure to read the end of this article to see a simple way to calculate the number of passes needed. I noticed quite a few people posting problems on Machining forums etc and as usual loads of misinformation. So here we are.

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Multiple repetitive cycles for CNC lathes have been an important part of control systems since the mid s. Still, to this day, they present the most innovative approach of material removal, particularly in the areas of turning, boring, and threading.

Over the plus years of their existence, multiple repetitive cycles have gone through only two major changes. Earlier controls require these cycles to be programmed in a single block; later controls require two blocks of program input.

At the beginning, lets look at the word convert. Changing from one format to another is not a true conversion or, at least, it is not a complete conversion. The reason is that a double-block format offers more features than a single-block format. Also, keep in mind that you have no choice here; the control system determines the programming method. What cycles are affected? All multiple repetitive cycles from G71 to G76 can be programmed in one or the other format, depending on the control.

The finishing cycle G70 always uses a single-block format. The single-block format is older of the two, and relies heavily on the settings of system parameters, generally inaccessible to the machine operator.

I will use the most commonly used G71 and G76 cycles as examples in this column, other cycles follow a similar pattern.

The single-block format of the roughing cycle G71 is: G71 P.. In this single-block format spindle speed is assumed to be in effect , P and Q addresses refer to the block numbers defining the finish contour. U and W are specifications of stock amount left over for finishing, D address is the depth of cut written without a decimal point , and the F address is the roughing feedrate.

In addition, some controls also accept I and K addresses that control the distance and direction of semi-finishing. For controls requiring a two-block format, the G71 must be written at the beginning of each consecutive block: G71 U..

G71 P.. The programmed data are similar, but a bit more flexible. In the first block, the U address is the cutting depth decimal point can be programmed , and the R address is the amount of retract from each cut. Apart from the more convenient way of programming the cutting depth, the addition of the R address represents the major change. In a single-block format, the retraction amount was controlled by a system parameter, in the double-block format, the programmer can specify such amount in the program directly.

Even more profound change can be found in the threading cycle G In its single-block format, the cycle uses the following data: G76 X.. In this case, the X specifies the final thread diameter, Z is the position of the thread end, I specifies the amount of taper if used , K is the thread depth, D is the depth of the first pass, A is the thread angle, and F is the thread lead feedrate.

The two-block version packs in a few more programmable features: G76 P.. G76 X.. The P address in the first block includes the number of finishing passes, length of lead for pullout at the thread end, and the thread angle all in one. The Q address specifies the minimum cutting depth, and R is the finish allowance.

In the second block, X and Z are the same as before, R specifies the amount of taper if used , P is the thread depth, Q is the depth of the first pass, and F is the thread lead feedrate. Neither P nor Q in the second block accept a decimal point. As you see from the two examples, the main difference between the two cycle formats is the additional programmable parameters which make the cycles much more flexible than using internal parameter settings.

In either double-block cycle, do not confuse the addresses in the first block with the same addresses in the second block. As you see, the conversion from a single-block format to a double-block adds certain features that are now programmable, offering more flexibility to the CNC programmer.


G76 Screw Thread Cycle

The end position is perhaps a bit easier, particularly in Z, as you generally know exactly the length you want threaded and where that thread starts. Incidentally, our G-Wizard Thread Calculator software has a nice database of common threads that calls out this sort of thing. There are many different thread standards such as the Unified Thread Standard , so make sure you have the correct data for your thread. The Start Position is a little more interesting. You need to leave some allowance in Z to give the CNC lathe time to synchronize the feedrate with the spindle rotational position. It turns out that cutting threads puts more stress on the cutter than a lot of other operations, so you want to turn the OD External threads or ID Internal threads to get close to the top of the thread to minimize the amount of cutting needed by the threading tool.


Fanuc G76 Thread Cycle for Dummies

G76 X The first defines the number of spring passes that the machine will take once the thread is cut to depth. This helps with surface finish and repeatability to help our threads remain in tolerance over many parts. The second value defines the angle of runout chamfer at the end of the thread while the third is the angle of the teeth of the thread i.

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