The reason for the decrease in boring machining performance!
When the machining performance of boring holes decreases, the reason may be caused by a specific factor, or it may be the result of multiple factors working together. These factors include workpiece stability, size of machining allowance, rigidity of tool system, blade grade and geometry, cutting speed and feed rate matching with tool performance. When encountering situations such as long processing cycle time, shortened tool life, or deterioration of part quality, these factors should be analyzed and identified.
In a specific boring process, the influence of certain factors may be more significant than other factors, but these factors may also be closely related to each other. Changing one factor may mean that in order to achieve the desired result, it is necessary to simultaneously change the other factor. However, when conducting cutting tests, do not change two or more factors at once.
1. Workpiece stability
Although machining centers and fixtures are usually not the first factors that machining workshops may consider, if the state of the workpiece is unstable during machining, the machine tool and fixture may also seriously affect the cutting performance of the tool.
If the clamping rigidity of the workpiece is guaranteed, the size and power of the machine tool will also affect the cutting parameters. Although machine tools with spindle taper holes of BT50, BT40, and BT30 can all use the same coarse boring head, not every machine tool can complete the same boring process. The same applies to the depth of the boring hole. On the BT50 machine tool, holes with a diameter of 75mm and a depth of 250-300mm can be bored. The BT40 machine tool can also complete the machining of this size range using an extended boring bar. However, any machine tool with a taper less than 40 does not support this type of machining.
Worn machine tool spindles and unstable fixtures are usually factors that cannot be changed but must be addressed. Sometimes, these factors may cause a machining task to fail completely, but generally speaking, changing the blade type or cutting parameters will provide a solution.
2. Processing allowance
Processing personnel are often unclear about how much margin should be reserved for boring machining. Users may be more familiar with the cutting speed/feed rate and required machining allowance in turning, but these experiences are not always applicable to boring. This is especially true in rough boring machining using boring cutters. It is not uncommon for the diameter of the drill bit to be very close to the final aperture of the workpiece (with only a machining allowance of 0.5-0.75mm left). Such a small material allowance is not enough to accommodate the two blade tips of the boring tool, which will lead to chattering and a decrease in cutting performance of the tool. If there is not enough machining allowance and loose diameter tolerance (plus or minus one thousandth), it is better to use a boring cutter (or a boring cutter with one of the blade clamps removed) for better machining results.
On the other hand, for parts with core holes, if the position of the core hole is incorrect, there may be too much workpiece material that needs to be cut off. Even if the diameter of the core hole is within the typical range of rough boring allowance standards, deviation from the core may result in the boring tool taking more on one side of the hole than the blade can withstand the chip load.
3. Rigidity of tool assembly
When selecting a boring tool for a machining task, it is usually based on the required boring diameter and nominal depth, with little consideration given to the actual boring depth and the required additional overhang (if needed). For example, in a certain boring process, the depth of the boring hole is only 50mm, but the tool may require a hanging length of 200mm to reach the boring hole through the workpiece and/or fixture. This is completely different from the required boring depth of 250mm.
In order to maximize tool rigidity and usage range, modular boring system can provide an unlimited number of module combinations. In cases where the required tool length is longer, it is important to first choose a larger basic diameter of the boring bar, and then reduce the diameter of the boring bar as needed, rather than using the same diameter size throughout the entire length of the boring bar.
For long overhanging boring with limited space, it is possible to consider using integral hard alloy boring rods (instead of using multiple extended rods). This configuration can provide higher rigidity and better control, but is usually limited to boring holes with smaller diameters.
For long overhang boring, compared to tool configuration schemes that only consider nominal boring hole length and aperture, modular boring systems that use larger overhang connection sizes and only reduce tool diameter when necessary have better rigidity.
4. Blade brand and geometric shape
The blade is the key contact point between the workpiece and the cutting tool. If the blade does not match the boring process, even if the boring system has excellent rigidity and the boring head is precisely balanced, it may still be difficult to achieve ideal machining performance.
If the geometric shape of the blade cannot guarantee cutting stability, using the best blade grade is of no use. Boring blades with suppressed geometric shapes usually use a relatively conservative chip breaking table, which can maintain a longer service life under stable processing conditions, but their radial cutting depth should be at least half of the tool tip arc radius. In some harsh boring processes (such as deep hole or long overhang boring, long chip material boring, or unstable workpiece clamping caused by machine tools and/or fixtures), grinding geometric shaped boring blades can cut more freely.
For specific boring processes, the blade grades and coatings used are constantly being upgraded and replaced. When boring steel workpieces, the most commonly used grades are metal ceramics and three-layer coated hard alloy. Coated hard alloy grades can also be used for boring of cast iron. If the processing conditions are stable, silicon nitride ceramic blades and certain cubic boron nitride (CBN) grades can also be used for boring of cast iron. Aluminum and other non-ferrous metal materials can be bored using uncoated hard alloy blades, which typically have large grinding chips at regular angles to prevent the generation of elongated chips. For high-speed precision boring of these materials, blades with polycrystalline diamond (PCD) tips or coatings may also be a good choice. It must be remembered that cutting stability is the first requirement for extending the life of the blade.
5. Cutting speed and feed rate
After considering all other factors, it is also necessary to determine whether the cutting speed and feed rate are appropriate. These cutting parameters are crucial for obtaining the optimal free cutting conditions. The ideal boring state is to use a high cutting speed and a moderate feed rate, but this may also be limited by various conditions mentioned above.
A common mistake in rough boring machining using boring cutters is simply multiplying the feed rate of single point boring by 2. This calculation method is usually not correct: for boring machining with the same aperture, the feed rate of the coarse boring cutter can reach 4 times that of the fine boring cutter, because the coarse boring cutter can use a larger tool tip arc radius. For example, if the tip radius of a precision boring cutter is 0.2mm or 0.4mm, a coarse boring blade can use a tip radius of 0.8mm. By doubling the radius of the tool tip arc and using two blades, the feed rate can reach 4 times that of a precision boring tool.
Generally speaking, rough boring does not require a very fine surface finish, so a more rigid boring cutter can be used to process at a higher cutting speed. If the feed rate of the boring cutter is too small, it will cause chatter due to inappropriate machining allowance. Rough boring cutters are used for boring machining with high loads, requiring the removal of more workpiece materials and the use of higher feed rates.
Processing personnel sometimes find it difficult to determine the appropriate surface cutting speed for precision boring. Optimizing cutting speed is crucial for extending blade life. If heavy load boring is carried out at a very high cutting speed, it will generate a large amount of cutting heat and shorten the life of the blade. Reducing the chip load can lower the cutting temperature, allowing the boring blade to process at a higher surface feed rate.
