In this brief how-to, we walk you through the steps for programming thread milling operations in Fusion 360. We often get asked if the Bantam Tools Desktop CNC Milling Machine supports rigid tapping, and while it doesn’t, there are other strategies you can use to machine threads into your part, such as thread milling.
When programming a thread milling operation in Fusion 360, there are two important numbers you need: the thread pitch and the pitch diameter offset. Finding the thread pitch for a screw is straightforward—you can take it directly from the fastener that you’re using (see common metric thread pitches and imperial threads per inch). However, calculating the pitch diameter offset is a little more involved. Every machine screw has a major diameter (D) and a minor diameter (d), marking the respective crest and root of the threads. In other words, the pitch diameter offset is the difference between the major and minor diameters.
The pitch diameter offset is crucial because the tool you’re using may not always come to a fine point—like the Harvey Tool 60º double-angle shank cutter we’re using for this threading operation. To ensure precision, you don’t want to randomly input values for the pitch diameter offset, but rather calculate it.
John Saunders from NYC CNC has created a calculator that takes the geometry of your tool into account and generates the pitch diameter offset for you—saving you time and ensuring accuracy. John gives an in-depth explanation in his thread milling video.
Once you’ve downloaded and opened the calculator, click the “Internal - Simple” tab. It’ll look like this:
Enter the values for the major and minor diameter of your screw, the cutting diameter of the tool you’re using (you can easily find these on the manufacturer’s website), and the flat (or crest) into the spreadsheet. And just like that, you have your pitch diameter offset value!
Now that you have the values for the thread pitch and pitch diameter offset, head over to Fusion 360. If you haven’t already, create your design and use the Thread tool to model your threads. In the pop-up window, enter the thread type, size, designation, class, and direction of your screw.
Note: If you’re still getting comfortable with using Fusion 360, refer to our Fusion 360 Workflows: Programming CAM support guide. We also recommend checking out video tutorials from NYC CNC and Lars Christensen.
Next, head over to the Manufacturing workspace in Fusion 360. Go to 2D toolpaths and select Thread. In the pop-up window, select your tooling and enter the appropriate speeds and feeds. Then, in the Passes tab, enter your values for the Thread Pitch and Pitch Offset Diameter.
Note: If the thread pitch is too tight (i.e., it’s hard to thread the screw), you can increase that offset number a thousandths of an inch (0.001”) at a time until your threads fit perfectly.
When you’re finished programming your CAM, post-process your G-code files and load them into the Bantam Tools software to set up your job!
Happy thread milling and be sure to check out John Saunders’ video for a more in-depth explanation.