How to Threadmill!

Threadmilling is a great solution for cutting clean threads at any size – but it’s difficult to get a great fit on the first try.

The days of chasing your thread when you can’t get the right fit are over! We’ve found the perfect formula to cut the perfect thread with any tool every time, and it makes learning how to thread mill easy!

Why Threadmill?

  • You can adjust the tolerance of the thread to your desired fit
  • It’s better for hard materials, it allows you to cut material in progressive passes
  • Unlike taps, thread mills won’t get stuck in parts if they break, and often won’t even damage the part
  • You can cut countless thread sizes with a single tool
  • Thread milling allows cutting odd sized threads
  • You can also use a thread mill to back chamfer the hole
  • Thread milling requires much less horsepower from the machine than tapping
  • Threadmills are readily available in carbide, as opposed to most taps being HSS or powdered metal

Stepover and Multiple Passes

When it comes to choosing a radial DOC and number of passes for your thread, there are a few common schools of thought. In the end, nothing is better than testing what works best for you.

A few processes are:

  • Taking multiple shallow, fast cuts
    • Taking shallow, quick passes is relatively low-strain on your tool and can cut down on tool deflection. However, more passes can dull the teeth of the threadmill more quickly over time.
  • Taking a “60/40” slower cut
    • Splitting the cut into two passes can be beneficial as well. Since the profile of the thread is triangular, each pass is removing more material than the last so staggering passes is critical. Cutting 60% of your thread on the first pass (30% of your PDO) and 40% on your final pass (20% of your PDO) eliminates the need for many quick, light passes. However, you may need to utilize repeat passes or adjust your PDO out to account for tool deflection and ensure a proper fit.
  • A combination of the two
    • I find that a happy medium works best for me. I like to set my WOC to about 5% of the threadmill’s cutting diameter (e.g. .019″ for a .388″ threadmill)
    • Number of passes should equal ((PDO/2) / (Cutting Diameter * .05)) rounded up to the next whole number.


AB Tools TM1/4

Lakeshore Carbide 5/16-SPTRMLB

Lakeshore Carbide 1/2-SPTRMLB

Tormach 34693


Thread Mill Calculator

Standard Threads Sample

NPT Sample

Parametric Form Mill

Micro Threadmilling

Threadmill Calculator Changelog

Revision 5:

  • Corrected formulas for Common Size Calculations sheet
  • Added tool checker to eliminate cutting too deep of threads with a tool, causing it to break

Revision 6:

  • Added a custom hole size/thread series sheet for custom, non-standardized threads

Revision 7:

  • Made adjustments to NPT sheets for a better fit

Revision 8:

  • Adjusted NPT sheets to match the updated, simpler modeling method

Revision 9:

  • Added effective feedrate/chipload calculation.

Related Videos & Resources:

Back Chamfer Tools and Info!
Threadmilling on a Tormach with Fusion 360
HSMWorks: Thread Milling Tutorial!