In CNC machining, tool life refers to the time from when the tool tip cuts the workpiece to when the tool tip is scrapped or the actual length of the workpiece surface. Tool nose machining time is the main evaluation index for tool companies to calculate tool life.
The service life of the general tool is 15-20 minutes of continuous processing of each blade. Tool life is measured by companies under relatively ideal conditions in the laboratory. According to the different cutting depths and feeds of different workpiece materials, each blade is continuously processed for 15-20 minutes, and the relationship between the corresponding linear speed and feed is calculated to form the corresponding cutting parameter table. Therefore, the cutting data table of each company is also different.
1. Can The Service Life Of The Tool Be Improved?
Tool life is only 15-20 minutes, can tool life be further improved? Obviously, tool life can easily be improved, but only at the expense of line speed. The lower the line speed, the more obvious the increase in tool life (but the lower the line speed, the vibration during machining will reduce the tool life).
2. Is There Any Practical Significance For Improving Tool Life?
In the processing cost of the workpiece, the proportion of tool cost is very small. If the line speed decreases, even if the tool life increases, the number of workpieces processed by the tool will not necessarily increase, but the machining cost of the workpiece will increase.
It needs to be properly understood that it makes sense to increase the number of workpieces as much as possible while ensuring the tool life as much as possible.
3. Factors Affecting Tool Life
1. Line Speed
Line speed has a great impact on tool life. If the line speed is higher than 20% of the specified line speed of the sample, the tool life will be reduced to 1/2 of the original. If it is increased to 50%, the tool life will be only 1/5 of the original. In order to improve the service life of the tool, it is necessary to know the material, state and linear speed range of the tool for each workpiece to be machined. Each company’s knives have different line speeds. We can conduct a preliminary search from the relevant samples provided by the company, and then adjust it according to the specific situation during the processing, so that a relatively ideal effect can be achieved. The line speed data for roughing and finishing are inconsistent. Roughing is mainly to remove the allowance, and the line speed is low. Finishing is mainly to ensure dimensional accuracy and roughness, and the line speed is high.
2. Cutting Depth
Depth of cut has less effect on tool life than line speed. Each groove has a relatively large range of cutting depths. When roughing, the depth of cut should be increased as much as possible to ensure a large stock removal rate. When finishing, the depth of cut should be as small as possible to ensure the dimensional accuracy and surface quality of the workpiece. However, the cutting depth cannot exceed the cutting range of the groove. If the cutting depth is too large, the tool cannot withstand the cutting force, resulting in tool chipping. If the cutting depth is too small, the tool will only scratch and squeeze on the surface of the workpiece, causing serious wear on the flank, thereby reducing tool life.
Compared with line speed and depth of cut, feed has little effect on tool life, but has a large effect on workpiece surface quality. When roughing, increasing the feed can improve the removal rate of the stock. When finishing, reducing the feed can improve the surface roughness of the workpiece. If the roughness allows, the feed rate can be increased as much as possible to improve the machining efficiency.
In addition to the three cutting factors, vibration has a large impact on tool life. There are many reasons for vibration, including machine rigidity, tooling rigidity, workpiece rigidity, cutting parameters, tool groove, tool nose arc radius, blade clearance angle, tool shank extension length, etc., but the main reason is that the system rigidity is not enough to resist the processing time. The cutting force causes the tool to vibrate continuously on the surface of the workpiece during machining. To remove or reduce vibration, a comprehensive consideration must be given. The vibration of the workpiece surface can be understood as the constant knocking between the tool and the workpiece, rather than normal cutting, which will cause some tiny cracks and chipping at the tip of the tool, and these cracks and chipping will increase the cutting force and further aggravate Vibration, which further increases the degree of cracks and chipping, greatly reducing the life of the tool.
5. Blade Material
When machining workpieces, we mainly consider workpiece materials, heat treatment requirements, and whether intermittent machining is required. For example, inserts for machining steel and inserts for machining cast iron are not necessarily the same as those for machining hardness HB215 and HRC62, and inserts for intermittent machining and continuous machining are not the same. Steel parts are used for machining steel parts, casting inserts are used for machining castings, CBN inserts are used for machining hardened hard steel and so on. For the same workpiece material, if it is continuous processing, it is necessary to use a higher hardness blade, which can improve the cutting speed of the workpiece, reduce the wear of the tool tip, and reduce the processing time. If it is intermittent processing, it needs to use a better toughness blade. Effectively reduce abnormal wear such as chipping and prolong the service life of the tool.
6. Number Of Blades Used
The tool will generate a lot of heat during use, which will greatly increase the temperature of the blade, but when the tool is not processed or cooled with cooling water, the temperature of the blade will decrease, so the blade is always in the high temperature range, causing the blade to be continuously thermally expanded and shrinkage, resulting in tiny cracks in the blade. When machining the insert with the first edge, the tool life is normal, but as the usage of the insert increases, the crack will spread to other cutting edges, resulting in a reduction in the life of other cutting edges.
4. Can The Number Of Workpieces Processed Within The Tool Life Range Be Increased?
Since the tool life is only 15-20 minutes, the effective cutting length of the tool determines the number of workpieces actually machined within the tool life range. Increasing the throughput of workpieces means reducing the machining time and cutting length of individual workpieces. By properly adjusting the cutting parameters of the tool, the machining time of the workpiece can be reduced:
- Within a reasonable range of line speed, properly increasing the line speed cannot reduce the processing length of the workpiece, but can shorten the processing time of the workpiece.
- Increasing the depth of cut can reduce the number of rough machining and the effective cutting length of the workpiece, thereby shortening the processing time.
- Appropriately increasing the feed rate can also effectively reduce the cutting length and shorten the processing time.