Drill holes in aluminum alloys for dry drilling. However, the use of dry drilling holes in automotive production, the efficiency and cost is really more economic than using coolant processing?
In response to this problem, Daimler Chrysler, Ford Motor, and General Motors have formed a working group with a number of suppliers (such as machine tool supplier Gidding or Lewis, tool supplier Kennametal, etc.) to seek answers. The focus of their work is to develop high-speed drill holes for high-speed steel drills without the use of coolant in the A-319 aluminum alloy housing of automotive engines. The working group stipulated that the economical feasibility of dry drilling is to use a high-speed steel drill with a diameter of 6 mm to drill holes in this cast aluminum alloy material, using a rotation speed of 3000 r/min and a feed rate of at least 381 mm per minute. At least 10,000 20mm deep blind holes should be drilled. When drilling, it is required that the coolant be poured without supercharging coolant, to lubricate the tool and to dissipate heat. This work includes analysis, modeling, tools, materials, and tool coating.
The analysis was subdivided into three problems: the mathematical model for the establishment of deformation, the cost comparison between dry processing and wet processing, and the impact on human health and safety.
Because the dry machining will cause the temperature of the workpiece and the tool to increase compared to the wet machining, the heat affects the diameter of the drill bit and the position of the hole after cooling. Mathematical models can describe these effects and provide guidance on how to optimize the quality of the holes. To help the working group achieve this goal, the University of Michigan developed a model for evaluating selected experimental processes.
The quality of the processed hole was evaluated according to the diameter, cylindricity, and position accuracy. When measuring these parameters, it was found that the positional error was very sensitive to the order of the boreholes. The deformation of the part is largely influenced by the position of the clamping point and the friction between the tool and the chip.
Through analysis, it can be accurately known that dry drilling will produce an enlarged, bell-shaped hole, and the thermal characteristics of the drill bit have a significant effect on the hole diameter and hole shape. The effects of workpiece material, cutting speed and feed on the aperture and cylindricity are secondary.
The cylinder head part was modeled to calculate the hole position error caused by the heat generated by dry drilling. In addition, to determine the heat input to the workpiece, it is necessary to measure the drilling force and torque.
The location of 8 12mm and 16 6.4mm diameter holes is typically tracked, and the finite element analysis is used to quantify the change in temperature transfer. The thermal cycle is calculated from drilling until the workpiece cools to room temperature.
The working group determined that the ideal drilling process, the hole's maximum positioning error should be within ± 20μm. For the two worst-case scenarios: The workpiece was oscillated in the clamping device or an inappropriate drilling procedure was used. The maximum positioning error of the hole was calculated to be ±37 μm.
The mathematical model shows that the temperature-induced position error is small compared to the bit displacement and runout errors. If the drilling process is strictly controlled, the model can help to reduce the error caused by dry drilling.
For the cost comparison between dry and wet processing, Daimler Chrysler, Ford Motor, and General Motors provided data for machining typical engine aluminum alloy cylinder heads on the production line with and without coolant. This study shows that compared with wet drilling, dry drilling can reduce processing costs by 10% to 15%. In addition, if the concentration of metal particles produced by metal processing is reduced from 5.0 mg/m3 to 0.5 mg/m3, the savings in environmental, health and safety costs will increase by 20% to 30%.
The recovery of dry aluminum scrap is more profitable. For example, in 1997, the wet aluminum scrap price was US$0.07-0.23 per pound, and the dry aluminum scrap was US$1.05 per pound. The dry processing costs are also lower. According to the statistics of automobile manufacturers, the annual cost associated with the use of coolant is approximately US$ 3-5 million, while the dry processing system is US$ 100,000-300,000, and the personnel who operate and manage the coolant are omitted.
Machine tool manufacturers Horkos and Japan's Fukuyama also evaluated the relative costs of wet, dry and near-dry machining. From the assessment results, it can be seen that the related costs of dry processing are 32% less than that of near-dry processing and 69% less than wet processing, that is, dry processing is more cost-effective than wet processing and near-dry processing. From the point of view of cost structure, dry processing saves the cost of buying coolant, but it can also get some additional returns.
Basic life of drill bit and test process
The working group tested the effects of drilling tip offset, cold air injection and different drill manufacturing methods, different bit parameters, workpiece material and cutting speed, and feed rate on hole machining.
Test the damage status of commercially available drills and find out the number of holes each drill can process without using coolant. Test bits were provided by Kennametal IPG and tested at the same time as Kennametal IPG and Ford. Ford's spindle speed is 15000r/min, and Kennametal IPG uses spindle speeds of 5000r/min and 3000r/min. The drill used for the test was a high-speed steel drill (uncoated) with a drill diameter of 6.75 mm and a total length of 100 mm. Based on the test results, drills with a 45° helix angle, a 150° point angle, and a standard land were selected for design refinement and evaluation tests.
In order to determine the process parameters that can increase the number of boreholes, the work items proposed by the working group include: (1) the performance of the workpiece material; (2) the manufacturing deviation of the drill bit; (3) the drill bit thinning and groove polishing; (4) Impact of higher speeds; (5) Drill tip offset; (6) Effect of cold air; (7) Effect of workshop air; (8) Equipment improvement; (9) Workpiece supplier; (10) Tool coating Floor.
The performance of the drill is determined by the number of boreholes drilled in the absence of built-up edge, chipping or excessive spindle power demand. The test conditions are shown in the table below.
Table Cutting conditions used by Kennametal IPG when seeking to improve drilling efficiency Test parameters - Test conditions Centering drill bit - drill diameter: 4mm, drill angle: 118°, hole depth: 127mm
Test bit - drill diameter: 6.75mm, drill angle: 145°, helix angle: 45°, polishing groove with 120# abrasive, standard edge depth -20.3mm (blind hole)
Speed ​​and feed rate -3000r/min, 381mm/min
Cooling - without coolant, or with Exair's cooling system Material - A-319 aluminum alloy manufactured by M&A Foundry Hardness - 48 ± 6HBN
Material composition - Al: 90.18%, (others: Si: 5.80%, Cu: 3.80%, Fe: 0.01%, Ti: 0.12%)
(1) The performance of the workpiece material It has been found through research that dry drilling is sensitive to changes in the performance of the workpiece. When the performance of the workpiece material is inconsistent, to accurately evaluate the quality of the drill bit, the test should be repeated to obtain statistical results. Changes in the hardness of the cast aluminum alloy A-319 workpiece, internal abrasive content and air bubbles, will affect the number of drill holes. Some of the drill bits in the test were 2 to 4 times as many as the other bits.
(2) Manufacturing error of the drill bit Eight randomly selected drill bits from the same batch were named J1 to J8. The drills were tested at cutting loads of 3500 rpm and 381 mm/min, and pilot holes were used to avoid deflection. Cold air guns were used to prevent buildup. In this way, the number of drilled holes per drill is in the range of 75-453. Remove J6 and J8 with excessive shape deviations and compress this range to 240 holes.
The drill bit metallurgical test made by Ionbond Company also can see the performance difference caused by the drill bit manufacturing: two bits with the same surface, one drilled 23 holes, and the other drilled 500 holes. Both trenches have built-up edges and small cracks near the drill tip. Both have the same microstructure and chemical composition. The key difference is the hardness of the drill bit at the two drill bits: one is less than 9% of the design value and the other is less than 19% of the design value. However, the hardness at the tip of the 100-μm cusp has reached the design requirements. Further analysis confirmed that improper tool sharpening softened the drill tip. This shows that the correct shape of the drill tip and the optimum physical properties of the tool material have a great influence on the cutting performance of the drill.
(3) Thinning and polishing of the drill with a horizontal edge A batch of drills undergoes an additional thinning process with a transverse edge, which increases the chip space and reduces the damage of the drill bit caused by chip jamming. Another batch with the same shape and grooved polishing has a low coefficient of friction between the chip and the drill bit. Kennametal IPG said these polished bits are "bright bits." Tests have shown that the thinning of the chisel edge significantly improves productivity compared to light grooves. The drill with a thin blade with a transverse edge has the best effect when the speed is 3500r/min and the feed rate is 381mm/min.
(4) Effect of higher speed When the feed rate is constant, the cutting load is lower when the speed is higher, and the drilling efficiency can be improved without using the coolant.
(5) Offset of the drill tip A small defect in the drill tip or a runout of the spindle can cause a small displacement of the drill tip. If the offset is large enough before the drill tip is drilled into the workpiece surface, it will cause the hole position to shift. The hole offset increases the pressure on the bit and reduces the bit life. In order to avoid the hole displacement, it is usually pre-drilled with a diameter of 4mm drill hole 1.27mm. This will increase the number of boreholes from 36 to 52.
(6) Influence of cold air Two types of cold air transmission methods are compared, namely, the cooling air transmission inside the drill bit and direct cooling with an external nozzle near the drill tip. Both methods can reduce the generation of BUE. When the number of drill holes increased from 55 (cold air through the drill bit) to 162 (cool air sprayed directly to the drill tip), the result was still far from reaching the goal of drilling 10,000 holes. In addition, the cost of air-conditioning is still very high compared with the replacement of drills. Since the use of cold air did not significantly improve the bit life, it was decided after the comprehensive evaluation that it was no longer used.
(7) Effect of air in the workshop Air was filtered in a workshop with a pressure of 63 kg/cm2, and cooled by a main shaft and a drill. The test material was an A-319 aluminum alloy plate of Daimler Chrysler Automotive Company. The drill was a high-speed steel drill with a diameter of 6.75 mm. The spindle speeds were 3000 r/min and 1500 r/min, respectively, and the feed rates were 381 mm/min and 5715 mm/min. Without air cooling, the spindle speed was 3000 r/min and the feed rate was 381 mm/min. The drill bit was broken when the 38th hole was drilled. When cooled with air, the drill bit was broken when the 624th hole was drilled. When the spindle speed is 1500r/min and the feed rate is 5715mm/min, when the air is not used for cooling, the drill bit is overloaded when drilling the 23rd hole; when cooling with air, the torque is overloaded until the 34th hole is delayed. Under certain operating conditions, it is advantageous to cool the borehole with air through the drill bit. However, using a feed rate that is too low will still not achieve the drilling objectives for this project.
(8) Improvements in equipment Improvements in equipment include spindle speeds of up to 20000 r/min, ultrasonic vibration heads attached to drills, speed-increasing spindle heads, and drilling under vacuum conditions. However, none of these measures produced satisfactory results.
(9) Workpiece suppliers Repeated tests have shown that the same blanks provided by different casting suppliers have significantly different physical properties, which has a great impact on drilling productivity. Although some control measures are taken, the number of holes drilled by different material suppliers may still differ by 3 to 4 times. When the drill bit encounters holes, inclusions, and changes in hardness, additional stresses and deformations are generated, which is detrimental to the bit under working load. Therefore, ensuring the quality of the castings of the supplier and the material composition inside the aluminum alloy is the basis for optimizing drilling to achieve efficient drilling.
(10) Tool coating In the test procedure of the working group, the use of a coating in dry processing is the most effective means of achieving high productivity. In contrast, uncoated drills have a very short life span and rarely drill more than 25 holes before wear. The most promising coating is the physical coating. The diamond-like coating is used only for the drill tip and the front of the tool, not the entire groove surface. During the test, thinning of the transverse edge, normal cutting edge, drill with 45° helix angle, 150° drill tip angle, and PVD-coated drill bit on the tip were compared with 12 other structures on the market (without trimming the blade). Different drill bits are used for comparison. The cutting amount is: spindle speed 3500r/min, feed rate 381mm/min. The test results show that 4902 holes can be drilled with the drill bit with local PVD diamond coating, which has excellent results. The damage to the final drill bit is due to wear rather than clogging or sticking of the chips.
Based on the above test results, the drill bit of the aforesaid geometry (straight blade thinning, normal land, 45° helix angle, 150° drill angle, tip with PVD coating) was still used, and the working group decided to adjust the cutting amount. For: spindle speed 4000r/min, feed 381mm/min, and strive to achieve the ability to drill 8000 holes, the target is 10,000 holes. A large number of holes were machined during this project, but there was no upper limit target value. Test results with different rotational speeds and feed rates indicate that if the rotational speed is increased from 3000r/min to 4000r/min, the number of drilled holes may increase. If the blade is further processed, it is still possible to achieve this goal by 100%. It is clear that the use of a dry drilling process on the A-319 aluminum alloy plate is not mature enough to make it an economically viable method in automotive production. However, the extensive and rigorous tests conducted by the Working Group have clearly laid a good foundation for the adoption of this advanced technology and also clearly defined the direction for further improvement. Therefore, the economic feasibility of the process is not far off.
In response to this problem, Daimler Chrysler, Ford Motor, and General Motors have formed a working group with a number of suppliers (such as machine tool supplier Gidding or Lewis, tool supplier Kennametal, etc.) to seek answers. The focus of their work is to develop high-speed drill holes for high-speed steel drills without the use of coolant in the A-319 aluminum alloy housing of automotive engines. The working group stipulated that the economical feasibility of dry drilling is to use a high-speed steel drill with a diameter of 6 mm to drill holes in this cast aluminum alloy material, using a rotation speed of 3000 r/min and a feed rate of at least 381 mm per minute. At least 10,000 20mm deep blind holes should be drilled. When drilling, it is required that the coolant be poured without supercharging coolant, to lubricate the tool and to dissipate heat. This work includes analysis, modeling, tools, materials, and tool coating.
The analysis was subdivided into three problems: the mathematical model for the establishment of deformation, the cost comparison between dry processing and wet processing, and the impact on human health and safety.
Because the dry machining will cause the temperature of the workpiece and the tool to increase compared to the wet machining, the heat affects the diameter of the drill bit and the position of the hole after cooling. Mathematical models can describe these effects and provide guidance on how to optimize the quality of the holes. To help the working group achieve this goal, the University of Michigan developed a model for evaluating selected experimental processes.
The quality of the processed hole was evaluated according to the diameter, cylindricity, and position accuracy. When measuring these parameters, it was found that the positional error was very sensitive to the order of the boreholes. The deformation of the part is largely influenced by the position of the clamping point and the friction between the tool and the chip.
Through analysis, it can be accurately known that dry drilling will produce an enlarged, bell-shaped hole, and the thermal characteristics of the drill bit have a significant effect on the hole diameter and hole shape. The effects of workpiece material, cutting speed and feed on the aperture and cylindricity are secondary.
The cylinder head part was modeled to calculate the hole position error caused by the heat generated by dry drilling. In addition, to determine the heat input to the workpiece, it is necessary to measure the drilling force and torque.
The location of 8 12mm and 16 6.4mm diameter holes is typically tracked, and the finite element analysis is used to quantify the change in temperature transfer. The thermal cycle is calculated from drilling until the workpiece cools to room temperature.
The working group determined that the ideal drilling process, the hole's maximum positioning error should be within ± 20μm. For the two worst-case scenarios: The workpiece was oscillated in the clamping device or an inappropriate drilling procedure was used. The maximum positioning error of the hole was calculated to be ±37 μm.
The mathematical model shows that the temperature-induced position error is small compared to the bit displacement and runout errors. If the drilling process is strictly controlled, the model can help to reduce the error caused by dry drilling.
For the cost comparison between dry and wet processing, Daimler Chrysler, Ford Motor, and General Motors provided data for machining typical engine aluminum alloy cylinder heads on the production line with and without coolant. This study shows that compared with wet drilling, dry drilling can reduce processing costs by 10% to 15%. In addition, if the concentration of metal particles produced by metal processing is reduced from 5.0 mg/m3 to 0.5 mg/m3, the savings in environmental, health and safety costs will increase by 20% to 30%.
The recovery of dry aluminum scrap is more profitable. For example, in 1997, the wet aluminum scrap price was US$0.07-0.23 per pound, and the dry aluminum scrap was US$1.05 per pound. The dry processing costs are also lower. According to the statistics of automobile manufacturers, the annual cost associated with the use of coolant is approximately US$ 3-5 million, while the dry processing system is US$ 100,000-300,000, and the personnel who operate and manage the coolant are omitted.
Machine tool manufacturers Horkos and Japan's Fukuyama also evaluated the relative costs of wet, dry and near-dry machining. From the assessment results, it can be seen that the related costs of dry processing are 32% less than that of near-dry processing and 69% less than wet processing, that is, dry processing is more cost-effective than wet processing and near-dry processing. From the point of view of cost structure, dry processing saves the cost of buying coolant, but it can also get some additional returns.
Basic life of drill bit and test process
The working group tested the effects of drilling tip offset, cold air injection and different drill manufacturing methods, different bit parameters, workpiece material and cutting speed, and feed rate on hole machining.
Test the damage status of commercially available drills and find out the number of holes each drill can process without using coolant. Test bits were provided by Kennametal IPG and tested at the same time as Kennametal IPG and Ford. Ford's spindle speed is 15000r/min, and Kennametal IPG uses spindle speeds of 5000r/min and 3000r/min. The drill used for the test was a high-speed steel drill (uncoated) with a drill diameter of 6.75 mm and a total length of 100 mm. Based on the test results, drills with a 45° helix angle, a 150° point angle, and a standard land were selected for design refinement and evaluation tests.
In order to determine the process parameters that can increase the number of boreholes, the work items proposed by the working group include: (1) the performance of the workpiece material; (2) the manufacturing deviation of the drill bit; (3) the drill bit thinning and groove polishing; (4) Impact of higher speeds; (5) Drill tip offset; (6) Effect of cold air; (7) Effect of workshop air; (8) Equipment improvement; (9) Workpiece supplier; (10) Tool coating Floor.
The performance of the drill is determined by the number of boreholes drilled in the absence of built-up edge, chipping or excessive spindle power demand. The test conditions are shown in the table below.
Table Cutting conditions used by Kennametal IPG when seeking to improve drilling efficiency Test parameters - Test conditions Centering drill bit - drill diameter: 4mm, drill angle: 118°, hole depth: 127mm
Test bit - drill diameter: 6.75mm, drill angle: 145°, helix angle: 45°, polishing groove with 120# abrasive, standard edge depth -20.3mm (blind hole)
Speed ​​and feed rate -3000r/min, 381mm/min
Cooling - without coolant, or with Exair's cooling system Material - A-319 aluminum alloy manufactured by M&A Foundry Hardness - 48 ± 6HBN
Material composition - Al: 90.18%, (others: Si: 5.80%, Cu: 3.80%, Fe: 0.01%, Ti: 0.12%)
(1) The performance of the workpiece material It has been found through research that dry drilling is sensitive to changes in the performance of the workpiece. When the performance of the workpiece material is inconsistent, to accurately evaluate the quality of the drill bit, the test should be repeated to obtain statistical results. Changes in the hardness of the cast aluminum alloy A-319 workpiece, internal abrasive content and air bubbles, will affect the number of drill holes. Some of the drill bits in the test were 2 to 4 times as many as the other bits.
(2) Manufacturing error of the drill bit Eight randomly selected drill bits from the same batch were named J1 to J8. The drills were tested at cutting loads of 3500 rpm and 381 mm/min, and pilot holes were used to avoid deflection. Cold air guns were used to prevent buildup. In this way, the number of drilled holes per drill is in the range of 75-453. Remove J6 and J8 with excessive shape deviations and compress this range to 240 holes.
The drill bit metallurgical test made by Ionbond Company also can see the performance difference caused by the drill bit manufacturing: two bits with the same surface, one drilled 23 holes, and the other drilled 500 holes. Both trenches have built-up edges and small cracks near the drill tip. Both have the same microstructure and chemical composition. The key difference is the hardness of the drill bit at the two drill bits: one is less than 9% of the design value and the other is less than 19% of the design value. However, the hardness at the tip of the 100-μm cusp has reached the design requirements. Further analysis confirmed that improper tool sharpening softened the drill tip. This shows that the correct shape of the drill tip and the optimum physical properties of the tool material have a great influence on the cutting performance of the drill.
(3) Thinning and polishing of the drill with a horizontal edge A batch of drills undergoes an additional thinning process with a transverse edge, which increases the chip space and reduces the damage of the drill bit caused by chip jamming. Another batch with the same shape and grooved polishing has a low coefficient of friction between the chip and the drill bit. Kennametal IPG said these polished bits are "bright bits." Tests have shown that the thinning of the chisel edge significantly improves productivity compared to light grooves. The drill with a thin blade with a transverse edge has the best effect when the speed is 3500r/min and the feed rate is 381mm/min.
(4) Effect of higher speed When the feed rate is constant, the cutting load is lower when the speed is higher, and the drilling efficiency can be improved without using the coolant.
(5) Offset of the drill tip A small defect in the drill tip or a runout of the spindle can cause a small displacement of the drill tip. If the offset is large enough before the drill tip is drilled into the workpiece surface, it will cause the hole position to shift. The hole offset increases the pressure on the bit and reduces the bit life. In order to avoid the hole displacement, it is usually pre-drilled with a diameter of 4mm drill hole 1.27mm. This will increase the number of boreholes from 36 to 52.
(6) Influence of cold air Two types of cold air transmission methods are compared, namely, the cooling air transmission inside the drill bit and direct cooling with an external nozzle near the drill tip. Both methods can reduce the generation of BUE. When the number of drill holes increased from 55 (cold air through the drill bit) to 162 (cool air sprayed directly to the drill tip), the result was still far from reaching the goal of drilling 10,000 holes. In addition, the cost of air-conditioning is still very high compared with the replacement of drills. Since the use of cold air did not significantly improve the bit life, it was decided after the comprehensive evaluation that it was no longer used.
(7) Effect of air in the workshop Air was filtered in a workshop with a pressure of 63 kg/cm2, and cooled by a main shaft and a drill. The test material was an A-319 aluminum alloy plate of Daimler Chrysler Automotive Company. The drill was a high-speed steel drill with a diameter of 6.75 mm. The spindle speeds were 3000 r/min and 1500 r/min, respectively, and the feed rates were 381 mm/min and 5715 mm/min. Without air cooling, the spindle speed was 3000 r/min and the feed rate was 381 mm/min. The drill bit was broken when the 38th hole was drilled. When cooled with air, the drill bit was broken when the 624th hole was drilled. When the spindle speed is 1500r/min and the feed rate is 5715mm/min, when the air is not used for cooling, the drill bit is overloaded when drilling the 23rd hole; when cooling with air, the torque is overloaded until the 34th hole is delayed. Under certain operating conditions, it is advantageous to cool the borehole with air through the drill bit. However, using a feed rate that is too low will still not achieve the drilling objectives for this project.
(8) Improvements in equipment Improvements in equipment include spindle speeds of up to 20000 r/min, ultrasonic vibration heads attached to drills, speed-increasing spindle heads, and drilling under vacuum conditions. However, none of these measures produced satisfactory results.
(9) Workpiece suppliers Repeated tests have shown that the same blanks provided by different casting suppliers have significantly different physical properties, which has a great impact on drilling productivity. Although some control measures are taken, the number of holes drilled by different material suppliers may still differ by 3 to 4 times. When the drill bit encounters holes, inclusions, and changes in hardness, additional stresses and deformations are generated, which is detrimental to the bit under working load. Therefore, ensuring the quality of the castings of the supplier and the material composition inside the aluminum alloy is the basis for optimizing drilling to achieve efficient drilling.
(10) Tool coating In the test procedure of the working group, the use of a coating in dry processing is the most effective means of achieving high productivity. In contrast, uncoated drills have a very short life span and rarely drill more than 25 holes before wear. The most promising coating is the physical coating. The diamond-like coating is used only for the drill tip and the front of the tool, not the entire groove surface. During the test, thinning of the transverse edge, normal cutting edge, drill with 45° helix angle, 150° drill tip angle, and PVD-coated drill bit on the tip were compared with 12 other structures on the market (without trimming the blade). Different drill bits are used for comparison. The cutting amount is: spindle speed 3500r/min, feed rate 381mm/min. The test results show that 4902 holes can be drilled with the drill bit with local PVD diamond coating, which has excellent results. The damage to the final drill bit is due to wear rather than clogging or sticking of the chips.
Based on the above test results, the drill bit of the aforesaid geometry (straight blade thinning, normal land, 45° helix angle, 150° drill angle, tip with PVD coating) was still used, and the working group decided to adjust the cutting amount. For: spindle speed 4000r/min, feed 381mm/min, and strive to achieve the ability to drill 8000 holes, the target is 10,000 holes. A large number of holes were machined during this project, but there was no upper limit target value. Test results with different rotational speeds and feed rates indicate that if the rotational speed is increased from 3000r/min to 4000r/min, the number of drilled holes may increase. If the blade is further processed, it is still possible to achieve this goal by 100%. It is clear that the use of a dry drilling process on the A-319 aluminum alloy plate is not mature enough to make it an economically viable method in automotive production. However, the extensive and rigorous tests conducted by the Working Group have clearly laid a good foundation for the adoption of this advanced technology and also clearly defined the direction for further improvement. Therefore, the economic feasibility of the process is not far off.
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