While thread milling cutters can produce threads with relative ease, there are many factors that a machinist must consider before starting a job to obtain consistent results. To conceptualize these characteristics and select the correct tool, the machinist must first understand the basic thread milling application. The article will detail what factors need to be considered in the selection of a thread milling tool.
Mechanical part complexity, material, and machine setup all affect the selection of tools for CNC machine tools. Thread milling is ideal for machining asymmetrical or hard materials and when chip breakage problems are encountered. Machinists have many options when machining threaded parts, including thread milling cutters with interchangeable heads, indexable tools, and solid tools.
There are many variables involved in the machining of threaded parts, and each type of tool has its advantages for specific applications and metal removal rates. And, while the selection is highly dependent on the workpiece itself, the most important variables are the thread size and thread depth, the volume of the part for a given thread size, the material and hardness of the workpiece, and the machine tool being used. Among them is the selection of the right type of milling cutter to remove a given volume of metal in the shortest possible time. Improper selection of a thread milling cutter will result in lost production, short insert life, and increased production costs.
Reference factors for the selection of thread milling cutters
1. Integral carbide option
Due to the nature of thread milling cutters, we often think that fully milled cross-section solid carbide tools are the fastest way to produce large thread sizes. While this is true, this type of tool is limited to a specific pitch and a maximum depth of cut of 2xD. When threading tough materials such as high-temperature alloys and die steels, multiple radial passes are required to bring the threads up to measurement, increasing the cycle time per hole and the overall cycle time per part.
2. Indexable options
For shops producing 7/8" and larger threads, where part size is small and cycle time is not an issue, consider the benefits of the versatility and simplicity of a single-plane indexable tool. With the ability to index tools, these tools can be used for a wide range of thread sizes, diameters, and pitch combinations (in some cases up to 3xD thread depth and up to 4x longer tool life).
These tools are designed with precision ground recesses for each insert, making them extremely stable and the system of choice for shops machining large threads in tough materials. They are also used in setups where existing machines cannot handle the torque required for rotary taps. They are also a good choice for small production runs where odd-pitch combination threads are required.
3. Interchangeable Head Options
Interchangeable heads or modular systems are designed to be easy to set up and achieve very high metal removal rates. The use of high-edge count inserts allows for higher feed rates in most materials, resulting in significantly shorter cycle times. Modular systems are a good choice when producing large thread sizes at 2xD thread depth ratios when a combination of tools is required to increase machine tool magazine space and reduce cycle times. A single-edge insert can produce multiple pitch and diameter combinations, and the tool body/holder design allows the insert to have more cutting edges on the same circumference than in an indexable system.
Production variables, thread size, and thread depth are decisive factors in determining whether a multi-purpose tool is beneficial. Set-up time and cycle time are not critical factors (depending on the application, type of shop, and the number of parts). A single-plane indexable thread mill is not the fastest method, but it is an economical way to minimize costs and obtain good threads.
