12 CNC Machining Lessons from Experience: A Must-Read for Machinists!
Due to the complexity of CNC machining (such as different machine tools, different materials, different tools, different cutting methods, different parameter settings, etc.), reaching a certain level in CNC machining (whether machining or programming) requires a relatively long period of time. This manual is a summary of experience accumulated by engineers over long-term actual production, covering CNC machining processes, operations, selection of commonly used tool parameters, monitoring during machining, and other aspects, for your reference.
I. Q: How should machining operations be divided?
A: The division of CNC machining operations can generally be carried out by the following methods:
(1) Tool-concentrated sequencing method: Operations are divided according to the tools used, completing all possible features on the part with the same tool, then using the second and third tools to complete the remaining features they can handle. This reduces the number of tool changes, shortens air-cutting time, and reduces unnecessary positioning errors.
(2) Machining-feature sequencing method: For parts with many machining features, the machining portions can be divided into several sections according to their structural characteristics, such as internal shapes, external shapes, curved surfaces, or flat surfaces. Generally, machine flat surfaces and locating surfaces first, then holes; machine simple geometric shapes first, then complex geometric shapes; machine low-precision areas first, then high-precision areas.
(3) Rough and finish machining sequencing method: For parts prone to machining deformation, since deformation may occur after rough machining and requires correction, generally all operations requiring both rough and finish machining should be separated.
In summary, when dividing operations, it is essential to consider the part structure and manufacturability, machine tool functions, the amount of CNC machining content, the number of setups, and the production organization conditions of the unit. It is also recommended to determine whether to adopt the principle of operation concentration or operation dispersion based on actual conditions, but always strive for reasonableness.
II. Q: What principles should be followed in arranging the machining sequence?
A: The machining sequence should be arranged based on the part structure and blank condition, as well as the need for locating and clamping, with emphasis on not compromising the rigidity of the workpiece. The sequence should generally follow these principles:
(1) The previous operation must not affect the locating and clamping of the subsequent operation; where general-purpose machine tool operations are interspersed, they must also be comprehensively considered.
(2) Machine internal shapes and cavities first, then external shapes.
(3) Operations using the same locating and clamping method or the same tool should preferably be performed consecutively to reduce repeated positioning, tool changes, and clamp plate adjustments.
(4) For multiple operations performed in the same setup, arrange the operations that cause less rigidity damage to the workpiece first.
III. Q: What aspects should be noted when determining the workpiece clamping method?
A: When determining the locating datum and clamping scheme, the following three points should be noted:
(1) Strive to unify the design, process, and programming calculation datums.
(2) Minimize the number of setups, ideally completing all surfaces to be machined in a single locating.
(3) Avoid manual adjustment schemes that occupy machine time.
(4) The fixture should be open; its locating and clamping mechanisms must not interfere with tool travel during machining (e.g., causing collisions). In such cases, clamping methods such as a vise or adding a base plate with screws can be used.
IV. Q: How to determine a reasonably positioned tool setting point? What is the relationship between the workpiece coordinate system and the programming coordinate system?
1. The tool setting point can be set on the workpiece being machined, but note that the tool setting point must be a datum position or an already finished surface. Sometimes after the first operation, the tool setting point is destroyed by machining, making it impossible to find the tool setting point for the second and subsequent operations. Therefore, during tool setting in the first operation, establish a relative tool setting position at a location with a fixed dimensional relationship to the locating datum, so that the original tool setting point can be recovered based on their relative positional relationship. This relative tool setting position is usually set on the machine table or fixture. Its selection principles are as follows:
1) Easy to align. 2) Convenient for programming. 3) Small tool setting error. 4) Easy to check during machining.
2. The origin position of the workpiece coordinate system is set by the operator. After the workpiece is clamped, it is determined through tool setting, reflecting the distance relationship between the workpiece and the machine zero point. Once the workpiece coordinate system is fixed, it is generally not changed. The workpiece coordinate system and the programming coordinate system must be unified, that is, during machining, the workpiece coordinate system and the programming coordinate system are consistent.
V. Q: How to select the tool path?
The tool path refers to the movement trajectory and direction of the tool relative to the workpiece during CNC machining. The reasonable selection of the machining path is very important because it is closely related to the machining accuracy and surface quality of the part. When determining the tool path, the following points should be mainly considered:
1) Ensure the machining accuracy requirements of the part. 2) Facilitate numerical calculation and reduce programming workload. 3) Seek the shortest machining path to reduce air-cutting time and improve machining efficiency. 4) Minimize the number of program blocks. 5) Ensure the roughness requirements of the machined contour surface; the final contour should be continuously machined in the last pass. 6) The tool entry and exit (cut-in and cut-out) paths should also be carefully considered to minimize tool marks left by stopping at the contour (elastic deformation caused by sudden changes in cutting force), and to avoid scratching the workpiece by plunging vertically onto the contour surface.
VI. Q: How to monitor and adjust during machining?
After the workpiece is aligned and the program is debugged, the automatic machining stage can begin. During automatic machining, the operator must monitor the cutting process to prevent abnormal cutting from causing workpiece quality problems and other accidents.
Monitoring the cutting process mainly considers the following aspects:
1. Machining process monitoring: Rough machining mainly considers the rapid removal of excess material from the workpiece surface. During automatic machining, the tool automatically cuts along the predetermined cutting trajectory according to the set cutting parameters. At this time, the operator should observe the changes in cutting load during automatic machining through the cutting load meter, and adjust the cutting parameters according to the tool's load capacity to maximize machine efficiency.
2. Cutting sound monitoring during cutting: During automatic cutting, generally at the start of cutting, the sound of the tool cutting the workpiece is stable, continuous, and light, and the machine movement is smooth. As cutting progresses, when there are hard spots on the workpiece, tool wear, or tool clamping issues, the cutting process becomes unstable. The manifestation of instability is a change in cutting sound, impact noise between the tool and workpiece, and machine vibration. At this time, cutting parameters and conditions should be adjusted promptly. If the adjustment effect is not obvious, the machine should be paused to check the tool and workpiece condition.
3. Finish machining process monitoring: Finish machining mainly ensures the dimensional accuracy and surface quality of the workpiece, with higher cutting speed and larger feed rate. At this time, attention should be focused on the influence of built-up edge on the machined surface. For cavity machining, attention should also be paid to overcutting and tool deflection at corners. To solve these problems: first, adjust the coolant spray position so that the machined surface is always under optimal cooling conditions; second, observe the quality of the machined surface of the workpiece and adjust cutting parameters to minimize quality variation. If adjustment still has no obvious effect, stop the machine and check whether the original program is reasonable.
Special note: when pausing for inspection or stopping the machine for inspection, pay attention to the tool position. If the machine is stopped during cutting, the sudden spindle stop will leave tool marks on the workpiece surface. Generally, consider stopping the machine when the tool is out of the cutting state.
4. Tool monitoring: Tool quality largely determines the machining quality of the workpiece. During automatic cutting, judge the normal wear condition and abnormal breakage condition of the tool through sound monitoring, cutting time control, pause inspection during cutting, and workpiece surface analysis. Handle tools promptly according to machining requirements to prevent machining quality problems caused by failure to handle tools in time.
VII. Q: How to reasonably select machining tools? What are the major elements of cutting parameters? What types of tool materials are there? How to determine tool speed, cutting speed, and cutting width?
1. For face milling, use indexable carbide face milling cutters or end mills. For general milling, try to use two-pass machining; the first pass should preferably use a face milling cutter for rough milling, with continuous travel along the workpiece surface. The recommended width of cut per pass is 60%–75% of the tool diameter.
2. End mills and carbide-insert face milling cutters are mainly used for machining bosses, grooves, and box opening surfaces.
3. Ball nose cutters and round nose cutters (also called bull nose cutters) are often used for machining curved surfaces and variable-angle contour profiles. Ball nose cutters are mostly used for semi-finish and finish machining. Carbide-insert round nose cutters are mostly used for roughing.
VIII. Q: What is the purpose of a machining program sheet? What content should be included in a machining program sheet?
A: (1) The machining program sheet is one of the contents of CNC machining process design and is also a procedure that operators need to follow and execute. It is a specific explanation of the machining program, with the purpose of enabling operators to clearly understand the program content, clamping and locating methods, tools selected for each machining program, and issues requiring attention.
(2) The machining program sheet should include: drawing and programming file names, workpiece name, clamping sketch, program name, tools used for each program, maximum cutting depth, machining nature (such as rough machining or finish machining), theoretical machining time, etc.
IX. Q: What preparations should be made before CNC programming?
A: After determining the machining process, before programming, understand: 1. The workpiece clamping method; 2. The size of the workpiece blank — to determine the machining range or whether multiple setups are needed; 3. The workpiece material — to select which tools to use for machining; 4. What tools are in stock — to avoid needing to modify the program during machining due to the absence of a tool; if a particular tool must be used, it can be prepared in advance.
X. Q: What principles apply to setting the safety height in programming?
A: The principle for setting the safety height: generally higher than the highest surface of the island. Alternatively, set the programming zero point at the highest surface, which can also minimize the risk of tool collision.
XI. Q: After the tool path is programmed, why is post-processing still required?
A: Because different machine tools recognize different address codes and NC program formats, the correct post-processing format must be selected for the machine tool in use to ensure that the generated program can run.
XII. Q: What is DNC communication?
A: Program transfer methods can be divided into CNC and DNC. CNC means the program is transferred via media (such as floppy disks, tape readers, communication cables, etc.) to the machine tool's memory for storage, and during machining the program is called from memory for machining. Since memory capacity is limited, DNC machining can be used when the program is large. In DNC machining, the machine tool reads the program directly from the control computer (i.e., drip-feeding), so it is not limited by memory capacity.
Cutting parameters have three major elements: cutting depth, spindle speed, and feed rate.
The overall principle for selecting cutting parameters is: Light cut, fast feed (i.e., small cutting depth, fast feed rate).
Classified by material, tools are generally divided into ordinary high-speed steel tools (material: high-speed steel), coated tools (such as titanium-coated, etc.), and alloy tools (such as tungsten carbide, boron nitride tools, etc.).