At present, various difficult-to-process materials such as hardened steel, super-hard sintered metal, heat-resistant super alloys, bimetallic materials, etc. have been increasingly widely used in the manufacture of industrial parts. Although parts made of such materials can achieve excellent performance, they also bring a problem: how to achieve the final forming of parts at a reasonable cost per piece.
Fortunately, CNC cutting tool suppliers have successfully developed various new cutting inserts for milling and turning difficult-to-process materials. Such as coated carbide inserts, metal ceramic inserts, CBN inserts, PCD inserts, etc. These new material inserts use special geometries and surface coatings, have excellent wear resistance, and can withstand mechanical and thermal shocks during processing. However, how to use these cutting inserts reasonably and effectively in production requires close cooperation with tool suppliers who have professional knowledge.
Since the cost of cutting inserts is relatively low (generally, the cost of carbide inserts accounts for only 3% of the total processing cost, and CBN inserts account for 5% to 6% of the total processing cost), it may not be cost-effective to blindly choose cheaper inserts to save processing costs. Although new material inserts are more expensive, they can shorten processing time, extend tool life, and improve product quality, so they may have better economic efficiency.
On the other hand, blindly choosing new material inserts without considering actual processing needs may also increase processing costs (the price of CBN inserts can be 8 to 10 times that of carbide inserts). In addition, when using new material inserts, if incorrect cutting speeds and feed rates are used, the workpiece processing quality and tool life will also be affected. Therefore, when selecting cutting inserts for difficult-to-process materials, it is necessary to correctly evaluate the economic efficiency of processing and comprehensively consider the entire processing process.
What Factors Should be Considered When Choosing Cutting Inserts
When selecting cutting inserts, the entire machining task needs to be evaluated. Under the premise of meeting the workpiece dimensional accuracy and surface finish requirements, and taking into account the machining time and insert replacement, the relatively low-priced carbide inserts can achieve better machining economy. By accurately understanding and comprehensively weighing the production batch, machining time and insert performance, the cutting inserts can be reasonably selected to achieve the machining effect of improving productivity.
Taking the milling of gas turbine blades made of sintered titanium carbide as an example, when the batch size of workpieces is small, the use of coated carbide inserts can also achieve better processing results. At a cutting speed of 35m/min, the cutting edge life of carbide inserts is only 5 to 10 minutes, while the reasonable insert life for large-scale processing of difficult-to-process workpieces is generally required to reach 15 to 30 minutes. In small-batch processing, the impact of shorter insert life and more frequent replacement of inserts on productivity is not obvious; but in large-scale full-load processing, longer insert life is of vital importance to reducing tool change auxiliary time, reducing labor intensity, and improving machine tool utilization and production capacity. Therefore, when the batch size of turbine blades is large, it may be more reasonable to use CBN inserts with higher hardness and higher price.
In order to fully utilize the cutting performance of advanced material inserts, it is also necessary to select the correct feed rate and cutting speed. Taking CBN inserts as an example, the cutting edges of these inserts have been strengthened and passivated, which can effectively avoid chipping when cutting workpiece materials with a hardness of >50HRC. Although CBN inserts have excellent toughness, the selection of cutting parameters is still very strict. If the selected cutting speed is 10% higher or lower than the ideal value, the cutting performance of the inserts may be greatly reduced.
In order to implement the cutting of difficult-to-machine materials, you can consider seeking technical support from professional tool suppliers, who can provide reasonable solutions based on other similar processing examples. When cutting tests are required, trial-and-error methods can usually be used, that is, first cutting with carbide inserts, and then switching to new material inserts for comparative cutting to compare the processing effects of different inserts. Advanced insert shapes, high-rigidity toolholders, and optimized processing procedures can usually make lower-priced carbide inserts suitable for cutting difficult-to-machine materials. Whether it is necessary to replace inserts with new materials should be determined according to the specific processing tasks and processing conditions. For the same category of difficult-to-process materials, there are usually certain commonalities in the selection of cutting inserts.
At present, many alloy steel workpieces have higher and higher requirements for hardness. In the past, the application hardness of tool steel was usually 45HRC, but now the tool steel used in the mold industry is generally required to be hardened to 63HRC. In order to avoid heat treatment deformation, some molds that could only be cut before heat treatment in the past need to be precision milled in a fully hardened state. When milling fully hardened steel, the cutting heat and cutting pressure generated may cause plastic deformation of the cutting inserts and cause the inserts to fail quickly. For example, when milling hardened steel with a hardness of 60HRC (the hardness of the carbide particles in the material can reach 90HRC), ordinary coated carbide inserts will experience rapid wear of the back face
Although hardened steel is difficult to cut, fully hardened steel workpieces can be machined economically using carbide inserts. Taking the processing of aerospace parts as an example, after replacing the original cermet inserts with carbide inserts, the secondary hole processing of large-size forgings made of hardened 3000M steel (4340 modified) was successfully completed. Most of the machining allowance of the processed hole has been removed before heat treatment (material hardness 30 ~ 32HRC), but in order to correct the heat treatment deformation, precision holes on such large-sized workpieces must be completely hardened after the workpiece (hardness reaches 54 ~ 55HRC) ) for secondary cutting. Since the hole to be processed is located deep in the workpiece, the special workpiece topography makes processing quite difficult, so it takes three cutting passes to achieve the required dimensional accuracy and surface finish.
The high hardness of the material coupled with the intermittent cutting method causes the cutting edge of the original cermet inserts to collapse and become ineffective before completing a single cutting pass. The collapsed inserts may cause the risk of scrapping the workpiece. After switching to PVD-coated fine-grained carbide inserts, the toughness and sharpness of the tool are significantly improved, and cutting can be successfully completed in 6 to 9 passes. After switching to carbide inserts, the tool supplier recommended reducing the cutting speed from the original 90m/min to 53m/min, but the cutting depth remained unchanged. After the cutting speed is reduced, it takes about 20 minutes to complete the three cutting passes of the hole with carbide inserts, while it originally took more than an hour to process with cermet tools. More importantly, it enhances the safety of the carbide inserts cutting edge, greatly reducing the risk of expensive workpieces being scrapped due to tool chipping.
In order to obtain reasonable cutting parameters for carbide inserts milling hardened steel, tool cutting tests can be carried out. During the trial cutting, the cutting speed can usually be selected from 30m/min to 45-55m/min; the feed rate is usually 0.075-0.1mm/tooth. Generally speaking, inserts with zero rake angle or small negative rake angle are stronger than those with positive rake angle. When machining hardened steel, it is also more advantageous to use round carbide inserts because round inserts have higher strength and the blunt cutting edge is not easy to break.
When selecting carbide insert grades, consider using high-toughness grades. The cutting edge safety of such inserts is better and can withstand the large radial cutting force and severe cutting-in and cutting-out impact when cutting hardened steel. In addition, specially designed high-temperature carbide grades can withstand the large amount of cutting heat generated when cutting hardened steel (HRC60). Impact-resistant carbide inserts with an aluminum oxide coating can also withstand the high temperatures generated when milling hardened steel.
Insert Processing Powder Alloy
With the continuous development of powder metallurgy technology, various superhard sintered metal (powder alloy) materials used in different fields are emerging in an endless stream. For example, a manufacturer has developed a composite powder nickel alloy containing tungsten carbide (WC) or titanium carbide (TiC) particles, with a hardness of 53-60HRC, and the hardness of the carbide particles in the nickel alloy matrix can reach 90HRC. When milling this material, the coated carbide inserts will soon wear the back face, and the main cutting edge will be worn flat; the superhard particles in the microstructure of the material will cause “micro-vibration”, resulting in accelerated wear of the inserts; the shear stress generated when cutting the workpiece may also cause the carbide inserts to break.
The use of CBN inserts can better solve the cutting problem of hard powder alloy materials containing tungsten carbide and titanium carbide particles. The improved insert geometry can effectively overcome the “micro-vibration” phenomenon. When a user milled a composite powder alloy workpiece, he found that the processing life of the new CBN inserts was more than 2,000 times longer than that of the best carbide inserts. Cutting tests have shown that the machining efficiency of hard powder alloy materials (cutting speed 60m/min, feed rate 0.18mm/edge) can be increased by 75% compared with electrical discharge machining (wire cutting) by using a face milling cutter equipped with 5 CBN inserts.
In order to fully utilize the best performance of CBN inserts, the cutting parameters must be strictly controlled within a reasonable range. Although the cutting speed of about 50m/min and the feed rate of 0.1-0.15mm/tooth are not high, they can achieve high productivity when processing powder alloy materials. The optimal cutting parameters can be accurately determined through a 30-60 second trial cut. During the trial cut, you can start with a low speed and gradually increase the cutting speed until the insert cutting edge is excessively worn.
When processing difficult-to-process materials, dry cutting should generally be used to keep the insert cutting edge temperature constant. In most cases, a circular tool with a double negative angle geometry has the best processing effect, and the cutting depth should usually be controlled at 1-2mm.
Milling is an interrupted cutting process. During machining, the continuous impact of workpiece materials with a hardness of 60HRC or higher on the tool will cause huge machining stress. Therefore, in order to provide sufficiently high impact resistance in milling, the machining machine and tool system must have the highest rigidity, the smallest overhang length and the greatest strength.
Inserts Machining Heat-resistant Super Alloys
Heat-resistant super alloys (HRSAs) developed for the aerospace industry are now increasingly used in the automotive, medical, semiconductor, power generation equipment and other industries. In addition to common heat-resistant super alloy grades (such as Inconel 718/625, Waspalloy, 6A14V titanium alloy, etc.), a variety of new titanium-based alloys and aluminum- and magnesium-based alloy grades have been developed. All heat-resistant super alloys belong to the category of difficult-to-machine materials.
Superalloys have high hardness, and the processing hardness of some titanium alloy grades reaches 330HB. For ordinary alloys, when the temperature in the cutting zone is higher than 1100℃, the molecular bonding chains in the material will soften, and a flow zone that is conducive to chip formation will appear. On the contrary, the excellent high temperature resistance of heat-resistant superalloys enables them to maintain high hardness throughout the cutting process.
Heat-resistant superalloys also have a tendency to cold harden when being cut, which can easily cause premature chipping and failure of cutting inserts. During cutting, a wear-resistant cold hardened scale layer will be generated on the cut surface of the workpiece, causing the insert cutting edge to wear quickly.
Given the difficult-to-machine characteristics of superalloys, lower cutting speeds are usually used during machining. For example, the cutting speed for milling superalloy Inconel 718 brake keys with carbide inserts is 60m/min; the cutting speed for external cylindrical/end turning of Inconel 718 with CBN inserts is 80m/min. In contrast, the cutting speed for cutting tool steel with uncoated carbide inserts can generally reach 120-240m/min. The feed rate when cutting superalloys is usually equivalent to the feed rate for cutting tool steel.
When machining superalloys, the choice of cutting inserts mainly depends on the material being machined and the type of workpiece. In order to improve machining efficiency, carbide inserts with positive rake cutting edges can be used when machining thin-walled workpieces, while ceramic inserts with negative rake cutting edges are required when machining thick-walled workpieces to enhance the “ploughing” effect of inserts during cutting. For most difficult-to-machine materials, dry cutting should be preferred to keep the insert cutting edge temperature constant. But when machining titanium alloys, coolant must be used even at low cutting speeds.
Since heat-resistant superalloys maintain high hardness during cutting, the wear of the chamfered end of the cutting inserts is accelerated. Using round inserts with blunt cutting edges can greatly improve the strength of the cutting edge, but the cold work hardening tendency of superalloys can lead to more severe insert chipping. By changing the cutting depth during multiple consecutive passes, the inserts can avoid the cold work hardening layer formed on the surface of the workpiece, thereby reducing insert chipping and extending the working life of the cutting edge. The cutting depth can vary by 7.6mm for one pass and 3.2mm and 2.5mm for subsequent cuts.
Inserts for Bimetallic Materials
Bimetallic materials are composed of harder materials placed in selected wear-prone areas and then surrounded (or mixed) with other softer alloy materials. Bimetallic materials are increasingly used in the automotive industry and other industries, but they also bring special processing challenges. CBN inserts can efficiently cut hard alloys with a hardness greater than 50HRC, but may break when cutting soft alloys in bimetallic materials. PCD inserts can cut wear-resistant aluminum alloys, but are prone to excessive wear when cutting ferrous metals.
To achieve efficient machining of bimetallic materials, users, tool suppliers and machine tool manufacturers need to jointly develop precise cutting programs. For example, a certain bimetallic material is made by embedding a high-hardness composite powder alloy into an inexpensive 316 stainless steel matrix through a hot isostatic pressing process. During machining, the spiral interpolation tool path program needs to be compiled and input into the machine tool control system to machine the powder alloy material part first and then the matrix part at an optimized feed rate and cutting speed.
To efficiently machine a bimetallic cylinder block composed of an aluminum alloy and a cast iron cylinder head gasket, automakers must overcome both the wear resistance of the aluminum alloy and the high hardness of the cast iron. Because the harder cast iron cylinder head gasket (a wear-prone part) is difficult to isolate from the softer aluminum alloy cylinder block, it is not appropriate to use a separate machining method. However, by rationally programming the machine tool, using very low cutting speeds and very small cutting depths, wear-resistant PCD inserts can be used to process both aluminum alloy and cast iron, thus avoiding frequent tool changes during the machining process.
