12 Important Things to Know About CNC Machining: What Is It?
Computer Numerical Control (CNC) machining is the process of making things with machine tools that are controlled by computers. CNC machining is controlled by computer programs after an initial setup. This means that it has many benefits, such as stable processing quality, high machining precision, excellent repeatability, the ability to machine complex shapes, and high production efficiency. But in the real world of machining, human factors and experience on the job are very important in deciding how good the finished product will be. Let's look at twelve very useful pieces of advice from a CNC machining veteran with ten years of experience...
I. How Should CNC Machining Processes Be Split Up?
There are a few ways to divide CNC machining processes:
1. **Process Division by Tool Concentration:** This method sorts operations by the type of cutting tool used. Before moving on to the next tool, one tool is used to finish all of the machining features on the part. After that, the next tools (the second, third, and so on) are used to finish the rest of the features that they can handle. This method cuts down on tool changes, cuts down on "idle" time when the machine isn't cutting, and cuts down on positioning mistakes that aren't needed.
2. **Process Division by Machining Feature:** For parts that need a lot of machining, the work can be split up based on structural features like internal features, external contours, curved surfaces, or flat surfaces. In general, flat surfaces and datum surfaces (reference planes) are machined first, then holes; simple geometric shapes are machined before more complicated ones; and features that don't need as much accuracy are machined before those that do.
3. **Process Division by Rough vs. Finish Machining:** Rough machining can cause parts to change shape, which means that shape correction is needed later. So, in general, operations that include both roughing and finishing passes should be split into two separate processes. In short, when splitting up processes, you need to be flexible and use your judgment based on the part's structure and how easy it is to make, the machine tool's capabilities, the scope of the machining tasks, the number of setups needed, and the specific production organization in the facility. Also, the choice between "concentrated processes" and "dispersed processes" should be based on the specific situation, with the goal always being to find a logical and efficient way to do things.
2. What rules should be followed when deciding the order of CNC machining operations?
When planning the order of machining operations, you should carefully think about the part's structure, the condition of the raw workpiece (blank), and the needs for positioning and clamping. The main goal is to make sure that the workpiece's structural rigidity stays the same during the machining process. In general, the order of operations should follow these rules:
1. The CNC machining done in the previous step must not get in the way of the positioning and clamping needed for the next step. If there are regular machine tool operations mixed in with CNC operations, they must also be fully taken into account.
2. First, the internal features and cavities should be machined, and then the external contours should be machined.
3. To cut down on the number of times you have to move, change tools, and adjust clamps, you should group operations that use the same positioning and clamping method or the same cutting tool.
4. When doing more than one operation on a single workpiece, the ones that have the least negative effect on the workpiece's structural rigidity should be done first.
III. What things should you think about when choosing a way to clamp the workpiece?
When setting up positioning datums and coming up with a clamping plan, you should think about these three things:
1. Make sure that the datums used for design, manufacturing, and programming calculations are all the same.
2. Try to keep the number of setups to a minimum. Ideally, you should be able to finish all of the surfaces that need machining in one positioning operation.
3. Don't use clamping methods that need to be adjusted by hand while the workpiece is on the machine tool, since this takes up machine time.
4. The clamping setup should be open and free of obstructions. Its positioning and clamping mechanisms should not get in the way of the cutting tool's path during CNC machining (for example, by causing collisions). If this kind of interference can't be avoided, other ways to clamp things down, like using a machine vise or a baseplate with screw-down fasteners, should be used.
IV. How can you figure out a reasonable tool-setting point? How do the programming coordinate system and the workpiece coordinate system work together?
1. You can set the tool-setting point directly on the workpiece, but it's important that this point matches a positioning datum or a surface that has already been finished machining. Sometimes, the original tool-setting point may be destroyed (machined away) during the first operation, making it impossible to find for the second and later operations. So, during the first step of setting up the tool, it's best to set up a *relative* tool-setting position at a place that keeps a fixed size relationship with the positioning datum. This makes it possible to accurately reset the original tool-setting point based on the known relative position of the two points in space. Most of the time, this relative tool-setting position is set up on the fixture itself or on the machine tool's worktable. The following are the requirements for choosing this position:
1) It makes it easy to set up and align.
2) It makes programming easier.
3) It cuts down on mistakes when setting up tools.
4) It makes it easy to check on the machining process and verify that it is going well. 2. The operator sets the position of the workpiece coordinate system's origin. After the workpiece has been securely fixed, it is set up through tool setting procedures. It shows the distance between the workpiece and the machine tool's zero point, which is the position of the workpiece. Once the workpiece coordinate system is set up, it usually stays that way. The workpiece coordinate system and the programming coordinate system must be the same. This means that these two coordinate systems must be the same during the actual machining process.
V. How to Choose the Tool Path?
The tool path is the path and direction that the cutting tool moves in relation to the workpiece during CNC machining. Choosing a machining path wisely is very important because it has a direct effect on both the dimensional accuracy and the surface quality of the finished part. When figuring out the tool path, you should mainly think about the following important points:
1. Make sure that the part meets the machining accuracy requirements.
2. Make it easier to do math and cut down on the work that comes with programming.
3. Find the shortest machining path to cut down on "air-cutting" time (travel time without cutting) and improve the overall efficiency of CNC machining.
4. Minimize the total number of program blocks (segments) whenever possible.
5. After machining, make sure that the surface roughness requirements for the workpiece's contour are met. In particular, the final contour should be made in a single, continuous finishing pass.
6. You need to carefully plan how the tool will enter and exit the material (how it cuts into and out of it). This is to reduce the number of times the tool stops directly on the contour, which can cause elastic deformation because of sudden changes in cutting forces and leave visible tool marks. It also helps keep the workpiece surface from getting scratched by avoiding direct, perpendicular plunging cuts onto the contour.
VI. How to Keep an Eye on and Change Things During CNC Machining?
After the workpiece has been aligned and the program has been debugged, the process can move on to the automatic machining stage. During automatic machining, the operator must continuously monitor the cutting process to prevent abnormal cutting conditions that could lead to workpiece quality defects or other operational accidents.
When you watch the cutting process, you mostly pay attention to the following:
1. During rough machining, the main goal is to quickly remove excess material (machining allowance) from the surface of the workpiece. When the machine tool is in automatic mode and the cutting parameters are set, the tool makes cuts along the path it was set to follow. During this step, the operator should keep a close eye on the cutting load meter to see how the cutting load changes. The operator can then change the cutting parameters to make sure the machine tool works at its best based on how much weight it can handle and how much stress it is under. 2. Monitoring Cutting Sounds During Machining: When a machine is cutting a workpiece automatically, the sound of the tool cutting the workpiece is usually stable, continuous, and clear at the start of the process. At this point, the machine tool is working well. As the cutting process goes on, it may become unstable because of things like hard spots in the workpiece, wear on the tool, or slippage of the tool. This instability shows up as changes in the sounds of cutting, impact noises between the tool and the workpiece, and vibration of the machine tool. When this happens, cutting conditions and parameters should be changed right away. If these changes don't make a big difference, the machine tool should be stopped so that both the tool and the workpiece can be checked.
3. Monitoring the Finishing Process: The main goal of finishing is to make sure that the workpiece is the right size and has a good surface finish. This stage usually has faster cutting speeds and bigger feed rates. When finishing, you need to pay special attention to how built-up edges might affect the machined surface. When machining cavities, it's also important to avoid problems like overcutting or tool deflection (chatter) at the corners. To fix these problems, first make sure that the cutting fluid nozzle is in the right place so that the machined surface stays cool all the time. Second, keep a close eye on the quality of the finished surface on the workpiece and change the cutting parameters as needed to keep the quality from getting worse. If making changes still doesn't help, stop the machine to check if the original machining program was written correctly and logically. When you stop or pause the machine to look at it, you need to pay close attention to where the tool is. If the spindle suddenly stops while the tool is cutting, it could leave unwanted marks on the surface of the workpiece. In general, the machine should only be turned off after the tool has stopped cutting.
4. Tool Monitoring: The quality of the cutting tool has a big impact on the final quality of the machined workpiece. During automated machining operations, various methods-such as sound monitoring, cutting time control, mid-process inspection pauses, and surface analysis of the workpiece-should be employed to assess whether the tool is exhibiting normal wear or has sustained abnormal damage. To avoid quality problems caused by tools that weren't taken care of quickly, tools must be taken care of (for example, replaced or resharpened) on time based on the specific machining needs.
7. How to Choose the Right Cutting Tools? What Are the Most Important Parts of Cutting Parameters? What kinds of materials are used to make tools? How do you figure out the tool's cutting speed, cutting width, and rotational speed? 1. When face milling, you should use carbide face mills or end mills that can't be regrinded. If you're doing general milling, it's best to use a two-pass strategy. The first pass should use a face mill to rough out the workpiece surface in a continuous pass. For each pass, the best width of cut (stepover) is between 60% and 75% of the tool diameter.
2. End mills and face mills with carbide inserts are mostly used to cut bosses, grooves, and the faces of box-like structures.
3. Ball-nose mills and radius mills (also called bull-nose mills) are often used to shape curved surfaces and contours with different angles. Ball-nose mills are mostly used for semi-finishing and finishing work. Radius mills with carbide inserts are more often used for roughing.
8. What does a machining process sheet do? What should be in it?
1. The machining process sheet is an important part of the design for NC machining. It is a set of rules that the machine operator must follow exactly. It gives a full description of the machining program so that the operator knows exactly what it is, how to clamp and position the workpiece, which tools to use for each step of the machining process, and any important points that need to be paid attention to.
2. The machining process sheet should have the names of the drawing and programming files, the name of the workpiece, a sketch of the clamping setup, the name of the program, the specific tools used for each program segment, the maximum cutting depth, the type of machining operation (e.g., roughing or finishing), the estimated theoretical machining time, and any other important information.
9. What should you do to get ready before you start NC programming?
Before programming, you should find out the following information once the machining process strategy has been set:
1. How to clamp and hold the workpiece in place;
2. The size of the workpiece blank (raw material) is important because it helps you figure out how big the machining job will be and whether you will need more than one clamping setup.
3. The material of the workpiece, which makes it easier to choose the right cutting tools for the machining process;
4. The list of cutting tools that are available so that the program doesn't have to be changed during machining because a certain tool isn't available. If a certain tool is absolutely necessary, it can be prepared ahead of time.
10. What are the main ideas for setting the "safety height" during programming?
The safety height should be set above the highest point of any "islands" (raised features) on the workpiece surface. Another option is to set the programming zero point at the highest point on the workpiece. This also helps reduce the chance of tool collision.
11. What is the purpose of "post-processing" after the toolpaths have been made? It is important to choose the right post-processing format for the machine tool being used because different machine tools understand different address codes and NC program formats. This will make sure that the program can be run correctly.
XII. What does DNC Communication mean?
There are two main types of ways to move programs: CNC and DNC. CNC is the process of moving a program to the machine tool's internal memory using different types of media, like floppy disks, tape readers, communication cables, and so on. The program is then stored there and used during machining operations. But since the machine's internal memory isn't very big, the DNC method is used for big programs. In DNC machining, the machine tool reads the program directly from a control computer, which means that it runs the program in real time as it is being sent. This gets around the limits of the machine's internal memory.
1. There are three main parts to cutting parameters: cutting depth, spindle speed, and feed rate. The basic rule for choosing cutting parameters is "shallow cut, fast feed," which means using a small cutting depth and a high feed rate.
2. There are three main types of cutting tools based on the materials they are made of: standard high-speed steel (HSS) tools, coated tools (like titanium-coated tools), and alloy tools (like tungsten carbide tools, cubic boron nitride tools, and so on).
