The Appeal of Trochoidal and Peel Milling

The use of high-speed circular machining techniques with low radial engagements has risen more than ever. Trochoidal and peel milling thrive in most manufacturing industries due to their compatibility with various materials and machining situations. To a large extent, CAM and control developers now make them more accessible to different sizes of machine shops to add these machining operations to their capabilities. However, figuring out all aspects of these complex high-speed machining operations can be somewhat challenging. This article unfolds the trochoidal and peel milling principle and its suitable applications.

CNC Milling Equipment

The Principle of Trochoidal and Peel Milling

Peel milling utilizes high feed rates and low radial depth of cut combined with a significant axial depth of cut up to the cutter maximum. Deduced from the X-Y-Z plane, peel milling entails using a small stepover, typically around 10% of the cutting tool’s diameter, but with a maximum Z depth of cut between transitions. So, before applying this machining process, you should consult experts to recommend workable cutting parameters.

Peel milling makes the most of the chip thinning principle to lengthen tool life, especially along the entire flute length. For instance, suppose you have a 12.7mm endmill, you’ll get a depth cut of 25.4mm on the workpiece but with a minor stepover of 1.27mm. When you compare this to the standard milling process carried out at a depth of 6.35mm and a stepover of 70%, you’ll find that peel milling has a higher engagement and removal rate than the conventional milling approach.

Besides, the chip sizes you get with low radial engagement come out super thin, which means you can crank up the feed rate while maintaining the standard chip thickness. Based on the example above, peel milling enables a 70% feed rate higher than the traditional model and still gives an exact chip thickness.

Trochoidal Milling:

Trochoidal milling is a common variety of peel milling. It leverages high speeds with overlapping circular cutting paths in the X-Y plane. Generally, peel milling applies to clearing easily accessed workpieces, but trochoidal milling is an excellent method for carving out deep slots and other narrow features at full depth.

Even though slotting is a roughing operation, trochoidal milling often produces an improved finish compared to the conventional slotting toolpath. This is due to its decreased radial depth of cut and cutting forces. However, it may require semi-finishing operations along the walls of the workpiece to eliminate any cusps left from the winding motion of the cutter.

Typical Trochoidal Milling Toolpath:

Trochoidal milling uses a specialized cutter to fabricate slots of varying sizes wider than the cutting diameter. This is possible because trochoidal milling cutters have smaller diameters that move at high speed, with each trochoid penetrating deeper into the slot. Thus, the basic principle is that each cut forms a circular arc around the slot instead of a straight line. Moreover, specific trochoidal paths include a “D” shaped toolpath faster than completing full circles.

When milling using conventional techniques, you’ll need a wrap angle of 180º for the maximum cut. But the machining situation is different with trochoidal milling as the wrap angle is determined as a function of the cutting depth, often limited to a maximum of 70º. While some miniature tools benefit from trochoidal milling, you risk encountering tool breakage. As such, trochoidal milling remains prohibited for tools with cutting diameters less than 0.031-inch. Regardless, it still proves difficult with cutting diameters below 0.062-inch.

5-Axis For CNC Milling

Trochoidal Milling Feeds and Speeds:

Trochoidal milling helps keep a steady load on the cutter, allowing for reduced tool contact so you can operate at higher speeds and feeds. Machinists use low feed rates and cutting depth for conventional milling processes to avoid chipping problems, usually at a maximum milling depth of 1.5xD. However, with trochoidal milling, experts achieve cutting depths of up to 2xD even without specially developed cutters.

Standard Tools Required for Trochoidal and Peel Milling

Trochoidal and peel milling operations give desired results when you use the right tools and equipment. Check them below:

1. Tool Holder:

Like most CNC machining processes, peel milling is a high-performance machining procedure that generates a lot of vibration and heat. Thus, it would be best to use a compact and solid tool holder to resist the pressure on the tool during the milling operation. Some of the best tool holders you can opt for include quality hydraulic tool holders and shrink-fit systems. These options provide great versatility and vibration-dampening properties. Avoid using ER collet end mill holders or Weldon shank, inch cheater bar. They do not give good results because of their inability to hold the tool firmly enough and reduce vibration.

2. Specialized Cutters:

Suitable cutter selection helps take advantage of chip thinning in high-performance peel milling operations. The conventional milling approach uses cutting tools with large flute spaces for chip clearance. In contrast, peel milling needs less chip clearance, bringing about two possibilities.

  • Select flutes not deep on a cutter so the core diameter becomes substantially larger. With this, you can opt for a much stronger tool and push it harder.
  • Use more flutes. Many cutters designed for peel milling have 6 or 8 flutes, unlike the 4-flute end mill for traditional milling.

3. High Speed Mills:

Trochoidal and peel milling require fast and highly capable mills that can handle hard acceleration, deceleration, and maximum feed rate. The high-speed mill must be able to make small and fast cuts accurately. For most machines, experts input codes to change operating modes from high speed to stop mode and other processes. More so, the machine must conform to the programmed toolpath during high-speed mode without overshooting.

4. CAM System:

To carry out trochoidal and peel milling, you need software capable of executing it. There are different CAM systems in which some allow tighter control than others. So before selecting a CAM package for your machining project or shop, examine the quality of the CAM toolpath and post-processed results. For instance, you wouldn’t want to end up using G1 line motions only to keep getting errors from the machine controller showing that there’s an issue with your G2 and G3 values.

Trochoidal and Peel Milling Applications

While peel milling provides great machining solutions, it does not apply to every manufacturing project. Below are some typical applications of peel milling:

1. Slotting:

Trochoidal milling is well-suited for slotting, but it depends on the part’s geometrical features and finishes requirements. With trochoidal milling, engineers make the best use of an endmill between 50 to 75% of the slot width. Thus, a trochoidal toolpath comprising a 0.5-inch endmill is an excellent choice for a slot measuring 0.75-inch wide and 1-inch deep.

2. Pocketing:

Trochoidal milling is very often the best choice for deep pockets. The material removal rate is very high if you employ the entire flute length. So, when applying this machining method, the thin chips get cleared from the pocket of the workpiece rapidly.

3. Hard Materials:

Peel milling is an excellent choice for abrasive and hard materials since it distributes and maximizes tool wear along the entire flute length. Experts often apply peel milling in machining operations where notch wear or chipping is needed. In addition, you can best use peel milling as a roughing process for hard materials such as titanium, hard tool steels, cobalt-chrome, and Inconel.

CNC Milling Material


Trochoidal and peel milling operations have proven valuable for high-performance machining in many industries. Their high speed and feed rate features save time and lengthen tool life in machining operations. However, it is vital to have the required tools and software to handle high speed, feed rate, and pressure to avoid tool failure. Even though trochoidal and peel milling operations improve machining efficiency in various applications, always consult the tool manufacturer beforehand.