Machining of hard alloy parts
Unlike traditional concepts, it is not difficult to machine a quench-hardened workpiece to 5 μm with the right machine tool and the right tool. Currently, tool suppliers have developed new tools that can withstand such machining conditions. Machine tool builders have also produced CNC lathes that meet the stiffness required for these new tools. At the same time, hard material turning can promote production from the following aspects.
I. Hard material turning saves time compared to grinding
The final size specification is less time consuming because the metal cutting rate for turning is higher than for grinding. Moreover, after the workpiece is clamped once, multiple processes can be processed. The operator can complete all the machining in one loading and unloading, thus achieving just-in-time production.
Reducing costs and saving time is saving money. Furthermore, the use of a lathe instead of a grinding machine means a lower total investment in the machine tool.
Turning of better quality hard materials is much better than grinding in terms of dimensional accuracy and surface roughness. You can turn most hard materials to a total tolerance of 5μm with a surface roughness Ra of 0.1μm. Of course, such precise surface quality requires special measures, but Ra 0.15μm-Ra0.2μm is a breeze.
Less process hard material turning eliminates many forms of grinding, as well as polishing and other finishing operations. Not only does it save processing costs, but moreover, it does not require part outsourcing to assist in processing.
Wasting less hard materials, there is very little waste in turning. Because the coolant used is the same as normal turning, there is no need to add additional coolant, resulting in a small waste. What's more, dry turning can be done without coolant to avoid waste of coolant and waste workpieces. Hard material turning can also avoid the problem of grinding waste residue - that is, the laborious and costly waste residue recovery and disposal problem.
II. Power and stiffness requirements
In order to achieve the best results in turning hard materials, the correct combination of machine tools and tools is important. For example, in a rough turning process, when using a ceramic knife and using a cutting amount of 0.28 mm/min and a cutting depth of 1.9 mm, it is necessary to output a load of 65% for a 15 kW motor. The lathe turns one end of the quenched 50mm diameter steel piece to about 25mm, while also turning out the taper surface, arc radius and other complex shapes.
Although high power is required for large depth of cut ap, many users find that this is not the case in most cases. Because the processing of most hard materials is finishing. Generally, it can be rough-turned on a common screw lathe, leaving a margin of 0.25-0.38mm, and then finishing with a hard material turning technology after heat treatment. Another requirement for hard material turning is stiffness, which can cause tremors in any part of the machine tool, tool and workpiece, and machine tool if it lacks stiffness. In order to be able to achieve precision and dimensional tolerances compared to grinding, the machine must have a particularly high stiffness from the bed. Traditional cast iron beds are not suitable for hard material turning compared to granite polymers. The granite polymer bed is 20 times more resistant to vibration than cast iron. The vibration on the main shaft is only 1/3 or less of the main shaft mounted on the cast iron bed.
Reducing the amount of overhang is another way to reduce vibration. In theory, the workpiece holding device should keep the workpiece as close as possible to the spindle bearing, and the tool should be as close as possible to the turret body without the cantilever state. For this purpose, a lathe in which the collet chuck can be directly clamped in the spindle. The collet holder keeps the workpiece away from the spindle bearing, which not only has a small clamping force, but also increases the chance of vibration.
III. New variety of tools
It is good to machine hard steel with ceramic knives and cubic boron nitride (CBN) tools. These tools have developed rapidly in a short period of time, and some tool manufacturers claim that: As long as the machine is used correctly, their tool can reduce the time by 300% than grinding and improve the cutting accuracy.
The ceramic knives recommended by these suppliers have a negative rake angle and a large T-chamfer which adds pressure only at the tool-workpiece interface. In contrast, the CBN according to the specific circumstances of the workpiece, there may be no such adverse T- chamfer. For example, when CBN is used for interrupted cutting, a large T-chamfer is required to withstand vibration.
The use of ceramic tools is becoming more common than CBN due to its lower cost and because of their larger cutting edges. However, they are extremely susceptible to heat cracking. In addition, if the blade is not properly ground, it is not advisable to perform interrupted cutting. Although CBN is much more expensive than ceramics, it has much better cutting performance, higher processing precision and longer service life.
Choosing between these two tool materials is how to adapt the workpiece material, geometry, and cutting speed and feed rate. Some users usually do not stick to the comparison one by one, but the cost of the same processing for the tool, and the principle of how much benefit can be generated. These users use a cheap ceramic knife to rough-cut the scales and scales left in the heat treatment. Then, finish with a better CBN tool. In general, if the hardening depth is greater than the depth of the material to be cared, it is best to use CBN.
IV. Choose the right processing method
From a practical standpoint, hard material turning is a process that is closely related to the characteristics of the workpiece. It is especially suitable for cutting workpieces with complex geometries such as intricate arcs, angles, and radii. You can use a single-tip programming turning method to complete the machining quickly and economically without having to buy a forming wheel for this purpose. As a rule of thumb, most hard material turning is performed at a cutting speed of 91-150 m/min and a feed rate of 0.05-0.13 mm/min. The depth of the cutting knife is generally 0.08-0.5mm. Therefore, how to control the purity of the material, as well as the heat treatment, roughing and forming processes before turning, is the determining factor of the precision that can be achieved by this processing. If the workpiece hardness fluctuates only three Rockwell hardness values (for example), the cutting pressure of the tool will change enough to ensure that you can't guarantee the dimensional accuracy of 5μm. Unstable stiffness and uneven depth of cut can damage this process. Therefore, qualified materials should be used to maintain a constant metal removal rate and to ensure that the heat treatment hardness is within ±1 or ±2 HRC.
The length to diameter ratio of the workpiece also affects the machine's ability to maintain tolerance accuracy. A typical horizontal lathe generally achieves rather stringent precision when the tailstock is used for a soft workpiece with a length to diameter ratio of 5:1 to 6:1. Due to the hardness of the workpiece, the negative rake angle of the tool and the T-chamfering necessary to cut the hard material, the pressure on the tool is quite large. Therefore, when the workpiece is supported without a tailstock, the length to diameter ratio of the workpiece must be limited to 3:1-4:1.
Of course, this limitation will vary with the process of the process - that is, with the material, hardness, and roughness and accuracy requirements of the workpiece. We have not used the tailstock support before, and turning the workpiece with a diameter of 13mm and a length of 64mm is very poor precision.
The tailstock can play a role, but it is a panacea for all problems. For example, the tip cannot prevent chattering of a workpiece having a diameter of 25 mm and a length of 300 mm. For this type of workpiece, you can only use grinding. To ensure tighter tolerances than 0.005mm, it is better to use the grinding process. Although hard material turning cannot completely replace grinding. But it can replace a considerable part. In particular, workpieces with a small length and diameter ratio and a complicated shape.
