Cutting tools, which act directly on the workpiece, come in a wide variety. Different tool geometries and materials offer distinct cutting performance characteristics, making them suitable for a diverse range of machining needs, such as face milling, contour milling, and cavity milling. Cutting parameters, on the other hand, act as the “rhythm regulator” of the machining process.
The proper setting of cutting speed, feed rate, and cutting depth not only ensures that the workpiece achieves the expected precision and surface quality but also maximizes the performance of the machine tool, prevents excessive tool wear or breakage, and reduces machining time and costs.
Therefore, careful consideration of the selection of CNC milling tools and cutting parameters is of paramount importance for promoting the optimized application of CNC milling technology.
Requirements for Cutting Tools in CNC Milling
Select Cutting Tools with Good Rigidity
In CNC milling, cutting tool rigidity is extremely critical.
Cutting tools with good rigidity can fully utilize the machine tool’s performance, enable deep-cut machining to improve production efficiency, and prevent vibrations in the machining system, thereby ensuring machining accuracy and surface quality.
When tool rigidity is insufficient, manual machining allows for flexible adjustments to layered cutting in the face of uneven material allowances.
However, CNC milling is constrained by the program: excessive material allowances can easily cause tool breakage, while using small allowances—though it prevents breakage—leads to problems such as excessive idle travel and low efficiency due to conservative cutting parameters.
However, when cutting tools have sufficient rigidity, there is no need to worry about uneven material allowances, allowing CNC milling to proceed more smoothly and efficiently, thereby significantly accelerating the machining process.
Selecting High-Durability Cutting Tools
When a cutting tool has low durability, its wear rate during machining increases significantly.
Changes in the tool’s condition directly affect the machining accuracy of the workpiece; tool wear leads to increased cutting forces, which in turn can cause parts to deform beyond tolerance limits.
Furthermore, during tool change operations, errors in tool setting can result in visible tool change marks on the surface, adversely affecting surface quality and machining accuracy.
In addition, an increase in the frequency of tool changes and tool setting reduces effective machining time, accelerates tool wear, impacts machine tool utilization, and correspondingly increases production costs.
Selecting High-Quality Cutting Tools
CNC machine tools are characterized by their ability to perform automatic, continuous machining and operate at high speeds.
However, without high-quality CNC cutting tools that are precisely matched to these machines, it is difficult to fully realize their potential for efficient machining.
Taking the high-speed machining centers—which are now widely used in the manufacturing industry—as an example, the linear speed of the cutting tool can reach extremely high levels during the machining process.
Under such conditions, if the cutting tool lacks sufficient red hardness and wear resistance, it will wear down extremely quickly, thereby failing to reliably meet the machining accuracy requirements for parts.
Conversely, if lower cutting speed parameters are used, it becomes difficult to effectively leverage the unique advantages of high-speed cutting technology.
Selection of CNC Milling Tools
Selection of Milling Tools
For machining large flat surfaces, indexable disc milling cutters are typically selected.
This is primarily due to their large tool diameter, a characteristic that effectively reduces the frequency of repeated passes, improves milling efficiency, prevents inter-pass marks, and optimizes surface finish.
For machining small flat surfaces, bosses, stepped surfaces, and side contours, cylindrical end mills are more suitable.
Keyway cutters, which are capable of over-center cutting, are better suited for machining closed grooves such as keyways and waist grooves.
Two-flute keyway cutters, with their relatively spacious chip clearance, are suitable for machining with large material removal rates and can ensure that the machined dimensions of the keyway meet precision requirements.
Ball-nose cutters are suitable for specific curved surface machining applications;
They effectively prevent interference and overcutting between the cutter tip and the workpiece surface, ensuring the precision and quality of curved surface machining.
Selection of Hole-Making Tools
In hole-making operations, the primary task is to precisely move the spindle or worktable for positioning to ensure accurate hole placement.
Twist drills have low rigidity because they are connected only by the drill shank; during drilling, coolant has difficulty reaching the cutting edge, high temperatures are generated, and chip evacuation is poor, which can easily cause the hole to become misaligned.
Therefore, the depth-to-diameter ratio of the drill bit must be kept within 5.
Pre-drilling a center hole with a center drill before drilling the main hole can meet the positioning accuracy requirements. Before fine reaming, chamfering the hole opening facilitates reamer guidance.
To eliminate positional deviations during reaming, a floating reamer should be selected.
When boring, multi-fluted boring tools can balance boring forces and suppress vibration; it is recommended to choose a short, thick shank.
In case of interference, only remove material at the point of interference to enhance rigidity, ensuring the hole’s dimensions and surface quality.
Principles for Hole-Machining Methods
The selection of hole-machining methods is equally critical and must follow a series of principles.
First, consolidate tool types to reduce the total number. Once a tool is selected for machining, strive to complete all machining tasks with that tool in a single operation.
Distinguish between roughing and finishing tools; although this may increase the number of tools, it reduces wear on finishing tools and ensures part accuracy.
Machine holes on a surface only after the surface has been milled to prevent drilling slippage.
When machining curved surfaces and side contours, it is advisable to finish the curved surfaces before the contours, using tools with excellent durability, hardness, and wear resistance as the preferred choice.
Although this approach may appear to increase costs at first glance, the longer cutting time and better precision retention improve machining quality and efficiency while reducing interruptions.
When production costs are calculated comprehensively, they are actually significantly reduced.
Importance of Proper Hole-Making Tool Selection
The proper selection of hole-machining tools and methods is a core element in achieving high-efficiency, high-precision hole machining.
It plays an irreplaceable role in optimizing the overall machining process and enhancing product quality, providing manufacturing enterprises with strong technical support and cost-control assurance in the face of intense market competition.
Selection of Other Cutting Tools
Other cutting tools used in CNC milling play a critical role in expanding the versatility of milling machines and enhancing their adaptability to different workpieces.
When selecting these tools, the following aspects should be given priority: First, ensuring the precision of the cutting tools is of the utmost importance.
Second, priority should be given to tools with good durability. During prolonged machining, both the tool’s surface and internal structure will sustain a certain degree of damage.
When the tool’s outer surface is damaged, its machining performance will decline significantly, manifesting as reduced machining accuracy and slower machining speeds;
Conversely, damage to the internal structure can easily lead to tool breakage, which will adversely affect both the workpiece and the milling machine.
However, when cutting tools are selected appropriately, the probability of internal structural damage leading to breakage is relatively low.
Finally, cutting tools should be inspected regularly and replaced promptly; after replacement, be sure to perform the necessary adjustments to ensure that the cutting process proceeds smoothly and efficiently.
Determining Cutting Parameters for CNC Milling
Determining Cutting Speed
Cutting speed is inversely proportional to tool life T, back depth of cut ap, side depth of cut ae, the number of teeth Z on the milling cutter, and feed per tooth fz, while it is directly proportional to the diameter d of the milling cutter.
When the depth of cut (ap), side cut (ae), number of teeth (Z), and feed per tooth (fz) increase, the number of teeth engaged in the cutting process increases simultaneously, placing a greater load on the cutting edges.
This results in more cutting heat being generated in the cutting zone, which not only raises the temperature but also accelerates tool wear.
Therefore, when determining the cutting speed, it is necessary to consider the tool’s durability.
Large-diameter milling cutters provide better heat dissipation, so consider using higher cutting speeds in such cases.
Table 1 lists reference cutting speeds for milling steel and cast iron under different hardness conditions.
| Workpiece Material | Hardness (HBS) | Carbide Milling Cutter (m/min) | High-Speed Steel (HSS) Milling Cutter (m/min) |
|---|---|---|---|
| Cast Iron | <190 | 66–150 | 21–36 |
| Cast Iron | 190–260 | 45–90 | 9–18 |
| Cast Iron | 260–320 | 21–30 | 4.5–10 |
| Steel | <225 | 66–150 | 18–42 |
| Steel | 225–325 | 54–120 | 12–36 |
| Steel | 325–425 | 36–75 | 6–21 |
Table 1. Reference Cutting Speeds for Milling
Determining Cutting Depth
In the field of CNC milling, differences in machine tool types and machined materials can affect the cutting depth, so it is crucial to match the cutting depth to the specific machine tool and material.
Consider the actual machining environment when determining the cutting depth.
During rough machining, increase the cutting depth appropriately when the tool has sufficient rigidity to remove more material in a single pass and improve machining efficiency.
However, if the tool lacks sufficient rigidity or the material is thick, increasing the cutting depth can lead to problems such as abnormal cutting or even failure to cut through the material, thereby hindering the machining process.
Orient the cutting edge inward during cutting to prevent it from contacting and damaging the machine tool’s metal components.
When the tool has low rigidity and the material is thick, high impact loads can occur. Reduce the cutting depth in such cases to improve cut quality and minimize the risk of tool damage.
Consider the machining allowance when determining the cutting depth. Precisely control the allowance and reduce the depth appropriately to minimize resource waste while improving machining quality and efficiency.
Under normal conditions, use a cutting depth of 0.05 mm to 1.0 mm. For semi-finishing, maintain a depth of 1 mm to 3 mm.
Adjust the cutting depth within the appropriate range according to the machining requirements.
Select a suitable depth to achieve optimal cutting results and maintain smooth CNC milling operations.
Apply appropriate cutting-depth settings to meet different machining requirements, achieve the required machining accuracy, and optimize the overall process.
Determining Feed Rate
Feed rate F is a key factor affecting the efficiency of CNC machining.
Determine the value by considering factors such as the cutting tool material, the machinability of the workpiece material, and the condition of the cutting allowance.
When machining part contours, cutting forces can suddenly change at specific locations, potentially damaging the cutting tool.
When the tool approaches a corner or encounters an abrupt change in cutting allowance, reduce the feed rate appropriately to prevent over-cutting or under-cutting at the part corners.
The following principles apply to feed rate settings:
First, while ensuring the machining quality of the workpiece, select the highest possible feed rate to improve efficiency; for conventional cutting, this typically ranges from 100 mm/min to 200 mm/min.
Second, for parts cut with a saw-tooth cutter or during deep-hole machining, select a lower feed rate of 20 mm/min to 50 mm/min to accommodate the specific process requirements.
Third, for parts requiring high machining accuracy and strict surface quality, select a relatively low feed rate of 20 mm/min to 50 mm/min.
Finally, when the tool is in an idle feed state, the feed rate setting must be based on the maximum idle feed rate parameter preset in the machine tool’s CNC system.
This ensures the precision and coordination of tool movement throughout the entire machining process, meets the specific speed control requirements of different machining stages, and thereby guarantees high-quality part machining.
These principles, when applied in conjunction, help determine appropriate feed rates for various machining scenarios, ensuring the smooth execution of CNC milling operations and the achievement of high machining quality.
Determining the Back-Cutting Depth
Provided that the surface roughness of the machined part meets the relevant requirements, the selection of the back-cutting depth should comprehensively consider the part’s process rigidity, the machine tool, the fixture, and the cutting tool.
When the process system has sufficient rigidity, use a larger back-cutting depth to reduce the number of passes and improve machining efficiency.
For a required workpiece surface roughness of approximately Ra 3.2 μm to 12.5 μm, perform machining in two stages:
rough milling and semi-finish milling, with a remaining allowance of 0.5 mm to 1.0 mm after rough milling;
When the required surface roughness is Ra 0.8 μm to 3.2 μm, use three machining stages: rough milling, semi-finish milling, and finish milling.
For these three machining processes, the respective back cutting depths should be as follows:
During semi-finishing, set the back cutting depth and side cutting depth for end milling to 1.5 mm to 2 mm.
For finishing, set the back cutting depth to 0.5 mm to 1 mm. Set the side cutting depth for peripheral milling to 0.3 mm to 0.5 mm.
Conclusion
The proper selection of CNC milling tools and cutting parameters is key to achieving high-quality, high-efficiency CNC milling.
Correctly selecting tools and cutting parameters not only effectively improves part machining accuracy, reduces surface roughness, and ensures that products meet strict quality standards, but also maximizes machine tool performance, reduces tool wear, saves costs, and enhances a company’s competitiveness.