Time to read: 7 min

This article is a companion toGetting Started with CNC, an introduction to the different types of CNC milling machines and the software used to design the parts they produce. Here, we’re going to discuss CNC milling terminology, toolpaths, and common operations.

Chip Formation

您可能已经看到如图1所示的图像，它在切割工件时，铣削工具如何创​​造压缩和剪切应力。这些压力造成剪切区，部分工件剪切到我们所谓的芯片中，这往往会在操作员的脸部的方向上飞行 - 这就是机械师穿安全眼镜的原因！

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.
Figure 1: Chip formation

切削过程产生相当多的热量, 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.

Chips that curl into shapes like the numbers six or nine typically mean the process is operating optimally, but the ideal chip shape depends upon the toughness of the material. Other material properties affect the chip shape as well. For example, brittle materials typically produce discontinuous chips when milled.

Climb vs. Conventional Milling

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).

Illustration of the conventional CNC milling method.
图2：常规铣削

第二种方法称为爬升研磨，使用与切割方向匹配的主轴旋转矢量。在该过程中，该工具旋转以将工件从较大厚度切割成更小的厚度（参见图3）。使用CNC铣削攀爬切割是有益的，因为它降低了切削压力并产生较少的热量，从而导致工件上的优异表面光洁度。

攀登数控铣削方法的例证。
图3：爬研磨

Cutting Speed and Feed

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.

在手动过程中，机械师可以通过切割时感受压力和振动来判断切割速度或刀具旋转太快。当使用自动数控机器时，操作员依赖读数和芯片颜色，但具有实践和经验，您可以为不同材料提供良好的起始速度和饲料的感觉。

CAM simulations can also give you general initial parameters, but they don’t account for the condition of the cutting tool. When buying new tools, it’s a good idea to reference the manufacturer’s catalog to familiarize yourself with the specific considerations for materials, wearability, and temperature effects of your tools.

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.

转速方程

N = Rotational Speed (RPM’s)

v = Cutting Speed (SFPM)

D = Cutter Diameter Feed

Equation for Feed Rate

fr = Feed Rate [Distance/Min]

N = Rotational Speed

nt =刀具上的牙齿数量

f =饲料（in /牙齿）

Toolpaths

Toolpaths 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 Toolpaths

根据轴的数量以及它们的移动方式，CNC铣削刀具路径可以分为2D，3D，4轴或5轴。图4显示了2D刀具路径的示例，其是在x-y平面上定义的操作。

Illustration of an example 2D CNC milling toolpath.
Figure 4: 2D toolpath example图像来源：Autodesk Fusion

此时，您可能会想知道我们为什么要讨论2D刀具路径。毕竟，NOT CNC机器用三个轴或更多工作？

嗯，第2D术语是有点误导，因为存在z轴值，但该值仅在特定深度处定位工具，并且在操作期间保持恒定。因此，如果只关注剪切操作，则2D标签是有道理的，因为切割仅在X-y平面上发生。换句话说，工件被切割到工具上。

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.

Common 2D operations

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 operations图像来源：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.
  • 零用：顾名思义，这种类型的操作消除了生产口袋的材料。
  • Slot Milling: Similar to pocketing, the tool works in Z-levels but produces specialized slot shapes.
  • 倒角铣削：该刀具路径在比角度产生倒角边角。
  • Radius (Corner-Round) Milling: Similar to chamfer, but the produced edge corner is round.

3D Toolpaths

2D刀具路径在x-y平面上定义，但是当操作也包括z轴运动时，它成为一个3D刀具路径。这些刀具路径用于加工更复杂的形状，如模具，模具工具，有机拓扑和具有复合轮的几何形状 - 通常，3D刀具路径用于雕刻不使用2D刀具路径的形状。

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 example资料来源：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:

Clearing Operations

清除操作是粗加工操作，或者换句话说，它们主要用于去除散装材料。

  • 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.
Illustration of common 3D CNC milling operations.
Figure 7:3D operations图像来源：Autodesk Fusion

Finishing Operations

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

  • Parallelfinishing 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.
  • 轮廓finishing is similar to the 2D contour operation, but it consists of multiple contours at different Z elevations.
  • Ramp整理与轮廓整理类似，但由于名称表示，斜坡壁角而不是用恒定的Z高度加工。
  • Flatfinishing is similar to the 2D facing operation.
  • Scallopfinishing consists of passes at a constant distance from one another with inwards offset along the surface.
  • Spiralfinishing is a spiral projected down on the machined surface.
  • 径向finishing 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.

  • Horizontal clearing与袋清洁相同的方式机器的平面区域，适用于粗加工和整理操作。
  • 铅笔operations create 3D toolpaths along with internal corners and fillets with radii smaller than or equal to the tool corner radius.
  • Morphed spiral操作类似于螺旋操作，除了从所选​​边界产生螺旋，而不是将产生的通道修剪到加工边界。
  • Morphtoolpaths involve machining between 2 curves, following their shapes.

Conclusion

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 centerfor more articles, webinars, and guides that will help you fully leverage the power of CNC machines to realize your next product design.

And remember, if you need help with your next project, Fictiv has a team of experts waiting to help you build better CNC parts, faster.Create an accountand upload your design today!