Time to read: 7 min

This article is a companion toGetting Started with CNC，介绍不同类型的CNC铣床和用于设计其产生的部件的软件。在这里，我们将讨论CNC铣削术语，刀具路径和共同运营。

Chip Formation

You may have seen images like Figure 1, which show how a milling tool creates both compressive and shear stresses when it cuts a workpiece. Those stresses create a shear zone, and part of the workpiece shears away into what we call the chip, which tends to fly in the direction of the operator’s face — which is why machinists wear safety glasses!

This type of cut may appear simple and straightforward, but the mechanics behind chip formation are complex and have been studied for years.

An illustration of how chips form when a material is CNC milled.
图1：芯片形成

This cutting process generates quite a lot of heat, and the chips carry that heat when ejected from the workpiece. Good machinists pay attention to the color and shape of the chips to make sure the process is optimal — by observing the chips they know when to adjust coolant or cutting speed and feed rate.

将卷曲成形状的芯片通常为六或九个通常意味着该过程最佳地运行，但理想的芯片形状取决于材料的韧性。其他材料属性也影响芯片形状。例如，脆性材料通常在研磨时产生不连续的芯片。

攀登与传统铣削

There are two methods of milling material that depend upon whether the spindle rotation cuts from maximum to minimum thickness or vice versa. The first method, conventional milling, is more common with manual milling machines and cuts with a spindle rotation vector that’s the opposite of the cut direction. The tool rotates in a way that cuts the workpiece from smaller thickness to larger thickness (see Figure 2).

传统数控铣削方法的插图。
Figure 2: Conventional Milling

The second method, called climb milling, uses a spindle rotation vector that matches the cut direction. In this process, the tool rotates to cut the workpiece from larger thickness to smaller thickness (see Figure 3). Climb cutting with CNC milling is beneficial because it reduces cutting pressure and produces less heat, which results in a superior surface finish on the workpiece.

Illustration of the climb CNC milling method.
Figure 3: Climb Milling

切割速度和饲料

Tool rotation is defined as revolutions per minute (RPM), and the motion of the tool towards the workpiece is called the feed rate, defined as inches per minute (IPM). These are two of the main machining parameters that are critical to monitor and control.

In a manual process, machinists can tell if the cutting speed or tool rotation is too fast by feeling the pressure and vibration when cutting. When using automated CNC machines, operators rely on readings and chip color, but with practice and experience, you can get a feel for good starting speeds and feeds for different materials.

CAM模拟还可以给您概述初始参数，但它们不考虑切割工具的状况。购买新工具时，请参考制造商的目录是一个好主意，以熟悉您的工具的材料，耐用性和温度效应的具体考虑因素。

Typical formulas for determining appropriate speeds and feeds are shown below. Depending on the tool and your supplier, you may need additional equation parameters to account for different types of cutting diameters, cutting diameter at cutting depth, face effective cutting-edge count and others.

Equation for Rotational Speed

n =转速（RPM）

V =切割速度（SFPM）

D = Cutter Diameter Feed

Equation for Feed Rate

fr = Feed Rate [Distance/Min]

n =转速

nt = Number of Teeth on the Cutter

f = Feed (In/Tooth)

刀具路径

刀具路径are the trajectories the cutting tool follows during the machining process. CAD and CAM software simulate toolpaths by generating the coordinate locations that a cutting tool will follow in the machining process. This allows you to detect interferences between the tool holder and the workpiece, calculate speed, and know when you’ve programmed a cut outside of the workpiece by mistake.

2D刀具路径

Depending on the number of axes and how they move, CNC milling toolpaths can be classified as 2D, 3D, 4-axis, or 5-axis. Figure 4 shows an example of 2D toolpaths, which are operations defined on the X-Y plane.

例证例证2d CNC铣床工具路径。
Figure 4: 2D toolpath exampleImage Source: Autodesk Fusion

At this point, you may be wondering why we’re discussing 2D toolpaths. After all, don’t CNC machines work with three axes or more?

Well, the term 2D is a little misleading, because there’s a Z-axis value, but that value only positions the tool at a specific depth and is kept constant during the operation. So, if you look only at the cut operation, the 2D label makes sense since the cut happens only in the X-Y plane. In other words, the workpiece is cut normal to the tool.

Z-Levels and Machining Pockets

You may have heard 2D toolpaths referred to as 2 ½ D because some think it’s a better term for this type of toolpath. However, this terminology is problematic when machining pockets in parts. In Figure 4 above, the blue lines outside of the part are 2D toolpaths where the cutting motion happens in the X-Y plane.

However, the pockets in the part are tricky: if the tool is already at the pocket-specific height, and cutting only occurs on the X-Y plane, how can the tool carve the pocket? Doesn’t it drill in the Z direction to get the tool inside of the workpiece at that specific height and turn it into a 3D toolpath?

The short answer is no. To mill the pockets, the machine utilizes Z-Levels, where the tool is positioned at one elevation in Z and the cutting operation is performed at the X-Y plane at that Z elevation. Then the machine moves to the next Z-level and so on until the pocket is completely milled out — which gives the impression of cutting in the Z-axis when in reality you’re not.

共同的2D操作

2D toolpaths and 2D operations are defined on the X-Y plane, and Figure 5 below illustrates some of the most common 2D operations:

Illustration of common 2D CNC milling operations.
Figure 5: 2D operationsImage Source: Autodesk Fusion
  • Facing: This toolpath is used as a starting point to produce flat faces and square edges.
  • 2D Contour operations: These are used to rough outline and finish outside part walls, like the blue lines in Figure 4.
  • Pocketing: As the name implies, this type of operation removes material to produce pockets.
  • 插槽铣削：类似于袋装，该工具在Z级别工作，但生产专业的槽形状。
  • Chamfer Milling: This toolpath produces chamfer edge corners at specific angles.
  • Radius (Corner-Round) Milling: Similar to chamfer, but the produced edge corner is round.

3D Toolpaths

2D刀具路径are defined on the X-Y plane, but when the operation also includes Z-axis motion, it becomes a 3D toolpath. These toolpaths are used to machine more complex shapes, like molds, die-tools, organic topologies, and geometries with compound rounds — generally, 3D toolpaths are used to carve shapes not typically possible with 2D toolpaths.

If you’ve used a 3D scanner, you know that instead of generating a typical CAD geometry, scanned geometries are created as a mesh or array of triangles in the model. Similarly, CAM software creates a triangular mesh representing the geometry to be machined, and this mesh is used to calculate the 3D toolpath. This process is complex and the simulation takes more time since the software must calculate and constantly monitor the toolpath to ensure the tool doesn’t interfere with the workpiece.

Illustration of an example 3D CNC milling toolpath.
Figure 6: 3D toolpath exampleSource: Autodesk Fusion

Common 3D Operations

Now that you know how 3D toolpaths and 3D operations are defined, let’s take a look at what the different 3D operations do. Figure 7 illustrates some of the most common 3D operations, and is followed by a breakdown of each:

清除ing Operations

清除ing operations are roughing operations, or in other words, they’re primarily used to remove the bulk material.

  • Adaptive Clearingis used for removing large areas of material. This strategy divides the workpiece into a series of Z-levels (different elevations) and then machines them starting from the bottom.
  • Pocket Clearingis similar to Adaptive Clearing in that the CAM software divides the workpiece into Z-levels. But, with pocket clearing, the machining goes in stages, from the middle to the edge of the horizontal area along offset passes.
共同的3D CNC铣削操作的例证。
图7：3D operationsImage Source: Autodesk Fusion

完成操作

Finishing operations improve surface finish, dimensional accuracy, and tolerances. In other words, these milling operations generate the designedsurface finish。

  • 平行线finishing is used for geometries that can be divided into planes with similar contours. The passes are parallel in the X-Y plane and follow the surface in the Z-direction.
  • Contour精加工类似于2D轮廓操作，但它由不同Z高度的多个轮廓组成。
  • Rampfinishing is similar to contour finishing, but as the name indicates, ramps down walls rather than machining with a constant Z elevation.
  • 平坦的精加工与2D面向操作相似。
  • 扇贝整理包括在彼此的恒定距离处与沿着表面上向内偏移的通过。
  • Spiralfinishing is a spiral projected down on the machined surface.
  • Radialfinishing consists of passes along the radii of an arc, then projected down on the machined surface.

Other Operations

Other operations can be defined as clearing or finishing depending on the parameters of the CAM software.

  • 水平清除machines flat areas of the part in the same way as pocket clearing, and is applicable for both roughing and finishing operations.
  • Pencil操作创建3D刀具路径以及带有小于或等于刀具半径的半角的内部角落和圆角。
  • Morphed spiraloperations are similar to the spiral operation, except the spiral is generated from the selected boundary instead of trimming the generated passes to the machining boundary.
  • Morphtoolpaths involve machining between 2 curves, following their shapes.

结论

Hopefully, this article has given you a better understanding of CNC Milling cuts and toolpaths. If you want to learn more about CNC machining, check out ourresource center对于更多的文章，网络研讨会和指南，可以帮助您充分利用数控机器的电力来实现您的下一个产品设计。

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