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Thread milling, thread mill and how to thread mill

What is Thread Milling and How to Thread Mill

By STG Machine

Thread milling is a special thread cutting process done on a CNC machine to create internal or external threads. Thread milling has a few advantages over traditional cut or roll taps but it can oftentimes take longer and requires a bit of finesse.

Background and Set-Up

above all, understanding the tools and the math behind threading is critical for thread milling. Thread mills are typically available in 2 forms: single cutter and multiple-cutter method. 

Single Cutters:

Due to their versatility, single cutters are the most popular option. They can be used to manufacture any type of thread as long as the included angle is the same as the cutter. These tools allow the user to control the thread pitch and the diameter of the threading tool on each helical movement.

Multiple-Cutter Method:

An alternative is the multi cutter thread mill. They possess the ability to make a thread in just 1 or 2 passes. They achieve this by having threads throughout the cutter. However, they are limited in their pitch range due to the threads being pre-made. These tools are a nice option if you are in need of a tool with increased rigidity.

Subsequently, once your tool has been selected the approach mimics any ordinary threading operation: set-up, spot and drill. The thread size and class will determine what drill should be used for the thread. For information on how to calculate the proper drill size please refer to our article here or take a look at our drill chart here.

After the preparation operations have been completed the following information will be needed:

  1. Thread Mill diameter
  2. Crest on the tool (if any)
  3. Major diameter of thread
  4. Root of thread

Carefully drop the thread-mill down into the bottom of the hole and program it to move up in a helical fashion. It should be running at:

(Major Diameter) + (Root of Thread) – (Crest of the tool)

This calculation should account for the proper amount of over-travel to ensure that the thread is cut correctly. We recommend checking the thread while it is still in the original set up.  Additional compensation may be required which can be done at the controller level.

Beware that the material has a large impact on the outcome of thread milling. It is usually best to practice on a scrap piece of metal before attempting this on your work piece.


Programming a thread mill can be difficult without a CAD/CAM software. We use HSMWorks and CAMWorks for our programming and it helps out tremendously. The software allows you to simulate your cuts with a plethora of options to help dial in the thread. Being able to see the cuts happen under the machine simulation tool helps ensure that you will not break an expensive tap.

Despite having a CAM system it is always best to go with the recommendations from the manufacturer. They will usually advise on speeds, feeds and depth based on internal or external threads.

If you don’t have a CAD/CAM system we recommend checking out Scientific Tool. They have a very easy to use and intuitive g-code generator for all of your thread milling needs here. The calculator can account for multiple threads, multiple passes and many other options.


A thread milled tap is still a tap, so inspection procedures should not change. For internal threads you will want to use a go/no-go thread gauge. For external threads you will want to use a ring gage. The most common mistake when thread milling is related to the lead in and lead out. It is important to ensure that the thread milling tool goes past your desired thread depth. Without compensating for some additional travel the thread may not be cut deep enough. When inspecting a thread always make sure to check for the desired depth with the appropriate gauge.

In conclusion, it is important to take into consideration every aspect of the thread. The thread milling tool and its geometry is the  most critical aspect of the thread milling process. With a solid CAM software the process can become much easier but always make sure to inspect the treadmill after machining.


Find thread milling tools at:

  1. Harvey
  2. Abetools
  3. Lakeshore tools

Find inspection tools at:

  1. Amazon
  2. Mcmaster-Carr
  3. Vermont gage
  4. Deltronic gage

Find thread information and details at:

  1. Machinist handbook
  2. Thread check
  3. G-wizard

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