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.

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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.

Transform production and profits dramatically, in just a few months.
With a CNC machine monitoring system.

Etc

A grave matter

I was in Aurangabad recently, the city named after Aurangazeb (1618-1707). Aurangazeb was quite a monster – had two of his brothers executed, and imprisoned his father Shah Jahan in Agra Fort till his death. After executing his eldest brother Dara Shikoh, he actually took Dara’s head to show his father (“Dad, Dara wanted to meet you one final time, so I got him to see you, heh, heh.”). The Taj Mahal is his mother Mumtaz Mahal’s tomb. His great-grandfather was Akbar.

I saw Aurangazeb’s tomb in Khuldabad, near Aurangabad. A remarkably simple structure compared to his wife’s and mother’s tombs. In the final years of his life, he is said to have knitted caps and sold them anonymously in the market, out of which he made around Rs. 14. He specified that his grave must be simple, funded solely from this money. Quite a contrast to his younger years spent bumping off various friends and relatives in his pursuit of power and wealth.

The tomb of his wife (Dilras Banu Begum), in Aurangabad, in contrast is a large monument that is a copy of the Taj Mahal. It’s called Bibi ka Maqbara, which means ‘Tomb of the Lady’ in Urdu. Its architect was the son of the architect of the Taj Mahal (small world, huh ?). The grave has a lot of notes and coins thrown in by visitors, a sign of their reverence for her.

It was initially designed to rival the Taj Mahal, but finally turned out to be an emaciated version of the original (actually looks like a Taj Mahal that went on a juice-only crash diet for 6 months – see the comparison picture). It was commissioned by Aurangazeb’s son, and the down scaling was caused by Aurangazeb cutting the budget (“Son, I think I’ve spent enough on your mom. I’ve moved on to another, hotter babe, and wooing her is costing me a bomb – restaurant bills and stuff, you know?”).

G76 Threading Cycle for CNC Lathes (Fanuc, Haas, Mach3, and LinuxCNC)

G76 Threading Cycle G-Code Basics

In this section, we walk through the different parameters to tell the G76 threading cycle how to cut the specific thread you want.  We’ll be covering a variety of dialects including Fanuc Controls, the Haas G76 Threading Cycle, Linux CNC/Path Pilot, and Mach 3.

Thread Start and End Positions

The first thing you’ll need to come up with is the thread’s start position and end position. The end point is perhaps a bit easier, particularly in Z, as you generally know exactly the length you want threaded and where that thread starts. The ending diameter is not too bad either as the thread’s specification will tell you that.

Incidentally, our G-Wizard Thread Calculator software has a nice database of common threads that calls out this sort of thing. Here’s a typical screen shot:

G-Wizard Thread Calculator has a database of common threads…

For more CNC Threading Machine Supplierinformation, please contact us. We will provide professional answers.

If you don’t have software like G-Wizard Calculator, you’ll have to leaf through your Machinery’s Handbook or something similar to find the information.  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’ll obviously want to start somewhere outside the threads. 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. This will determine the X coordinate you start from. I typically turn down to the top of the thread tooth and use a finish allowance so there’s a nice surface there.

Thread Height

This is another value that comes from a thread spec, so it’s easy to come by. In the screenshot above, use 0.0433″ for the 1/4-20 thread.  Note that this value is also referred to as thread depth.

Taper Amount

Pipe threads are often tapered to improve sealing. You can specify a taper amount on the G76 threading cycle when cutting such threads.

Thread Pitch or Lead

For most G76 threading cycles, this is specified as a Feedrate. It’s another value you can pull out of your thread database. For the 1/4-20 thread above, the pitch is 0.050″ (1/20 TPI) and the Lead is 20 TPI.

Thread Infeed Angle (also called Tool Nose Angle)

Back in the days of manual lathes, the machinist used to set the compound slide at an angle to the thread that’s called the thread infeed angle. Being able to set that angle is important to ensuring a constant amount of material is removed and that the cutting spreads the wear to both sides of the insert or cutter.

Illustration by Sandvik shows three infeed strategies…

The illustration above, courtesy of Sandvik, shows thread infeed strategies:

– Radial Infeed: Go straight in with a 0 degree angle. Note that the force increases steadily into the cut as more area is being cutter the deeper we go.

– Modified Flank Infeed: Go in at an angle, which helps spread the wear and keep the force constant.

– Incremental Infeed: Alternate side-to-side while feeding at an angle.

Radial Infeed is quick and dirty, but it produces a stiff V-shaped chip which may have difficulty forming. The insert tip can also reach very high temperatures. This method is suitable for fine pitches, but for coarse pitch threads it often results in too much vibration (chatter) and poor chip control.

The Modified Flank Infeed is the one most commonly programmed in CNC machines. By choosing the right angle, we can avoid having the insert rub. Chips are formed much more easily. The chips are thicker than for a Radial Infeed, but they are much better behaved. As a result, fewer passes are often required and less heat is generated.

When specifying the thread angle, most people use an A58 for a 60 degree thread tool. This will cause the tool to move in at a 29 degree ( 58 / 2 ) angle on each pass thus cutting mostly on one side and just a little on the other side. if you program an A60 the tool will feed in at a 30 degree angle just cutting on one side.

If you’ve got a really large thread to cut, the Incremental Infeed method is probably best. Unfortunately, Incremental Infeed is not available on most G76 threading cycles.

For more information, please visit 2-Axis CNC Lathe Machine.