The G32 thread cycle allows us to have full control over each point of the positioning, although we tend to call it a cycle, it is actually modal in that it locks the rotation of the spindle to the feed rate, much like a manual lathe does. This gives us the ability to screw cut while programming point to point.

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The above video lesson is covered in more depth during the CNC Lathe programming course available here

G32 Screw cutting mode

G00 X8.0 Z5.0;
X4.9;
G32 Z-15.0 F0.8;
G00 X8.0;
Z5.0 X6.0;
X4.8;
G32 Z-15.0 F0.8;
G00 X8.0;
Z5.0 X6.0;
X4.75;
G32 Z-15.0 F0.8;
Etc.

Before we look at cutting tapered threads, let's take a look at how the G32 works with a standard threading sequence. Here is a section of the program that is using the G32 G-Code.

Feel free to download the infographics on this page as a reference.

Let's look at each block to see what is happening.

G00 X8.0 Z5.0;
Rapid to a safe start position, assuming the datum is at the front face of the part.

X4.9;
Still in rapid mode with G00 active, we go down to the depth of the first cut, taking 0.1mm deep.

G32 Z-15.0 F0.8;
Here, we tell the machine to lock the spindle rotations to the feed rate so we can cut our screw thread in the same place each time. From now on, every time we call G32 the machine will cut a thread following the same path as the previous one. The Z distance is the end of the thread and F is the pitch. We are cutting an M5 thread so the pitch is 0.8mm.

G00 X8.0;
After our first pass, we program the retract points. Coming up to X8.0mm gives us some clearance for when we move back to the start of the thread.

Z5.0 X6.0;
We can move in more than one axis to speed things up if we wish. Moving to 5mm to the right of the front face will give us a lead into the thread, to eliminate any backlash that may be present.

4.8;
Now we are ready to take the next cut, I've come down to take another 0.1mm deep pass. We can control the depth of each pass and decrease it as we get nearer the final depth.

G32 Z-15.0 F0.8;
G32 will lock our turret and spindle in the same place as before, so our next thread pass cuts over the top of the last. Then we just repeat this as we come down in X until our final depth of the thread.

G32 Taper Thread

G00 X7.5 Z3.5;
G32 Z-73.5 X62.5 F1.0;
G00 X70.0;
Z3.5;
X7.4;
G32 Z-73.5 X62.4 F1.0;
G00 X70.0;
Z3.5;
X7.3;
G32 Z-73.5 X62.3 F1.0;
Etc.

To use G32 to produce a thread on a taper, its a similar process. The differences are we have to calculate the start and end positions and give an X dimension during the G32 block.

G00 X7.5 Z3.5;
When we calculate our start position, we have to take into account the angle of the taper. This is where our Trigonometry classes come into use. If you need to brush up on your trig, I have a course here.

G32 Z-73.5 X62.5 F1.0;
When we get to the G32 block, it is exactly the same but we also give an X dimension for the end position of our thread. I have gone past the endpoint by 3.5mm and calculated the X position to maintain the correct angle of the taper to give a run out cutting fresh air

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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.

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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…

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.

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