Profiling turning of complex parts

In mechanical processing, the processing technology of some parts with complex structure is very complicated, and sometimes the operator is required to process the workpiece in the shortest time, especially the workpiece with difficult-to-process materials, and the processing technology is more complicated. For this reason, manufacturers are constantly looking for more cost-effective methods to process complex parts, including turned parts. The advanced nature of CNC machine tools has allowed us to program almost imaginable tool paths. But when the tool moves along these trajectories, the relationship between the tool and the part (cutting angle, feed, cutting speed and depth) continues to change. Therefore, the key to solving the above problems is how to turn complex parts in the most effective and economical way.

Complicated turning processing may be processing in which the tool is fed simultaneously in the radial and axial directions to form different part profiles. In addition, other complex factors include the machinability of the workpiece material, the expected output and the capacity of the machine tool. Of course, there are also delivery dates and processing costs. It should be noted that a part considered complicated by one workshop may be regarded as a regular part by another workshop, that is, the complexity of the part is not always obvious.

Mr. John Campell of Voss Industries pointed out that a seemingly simple part may be more challenging than a complicated shape part. He compared the turning of 718 nickel alloy flanges with the machining of spiral pipe joints. Although spiral pipe joints require 10 installations and 24 procedures, once installed and processed, this process no longer requires adjustment. On the other hand, 718 alloy flanges require continuous adjustment during processing to compensate for material springback, shrinkage and tool wear. In addition, different materials and workpieces of the same shape respond differently to cutting forces. Taking flanges as an example, within the specified range of materials, changes in nickel and chromium may cause changes in the cutting conditions between one batch of materials and another batch of materials, and between parts.

Select blade geometry

Turning a complex part, the most basic requirement is that the cutting edge can enter the area where the part profile is located. This requires selecting the appropriate blade shape, leading angle, secondary angle, rake angle and back angle. When choosing the shape of the blade, the key is to consider the strength of the blade. Among them, the round blade has the highest strength. For non-circular blades, the larger the tip angle, the higher the strength. However, due to the clearance angle, profile turning usually uses 35° or 55° diamond inserts. The choice of the cutter bar is actually determined by the required cutting trajectory. If you need to perform complex profile turning, you can choose a J-shaped cutter bar with diamond blades, which can form a larger relief angle.

The nose angle and the entering angle of the blade together determine whether the tool can enter the contour of the workpiece; the gap between the workpiece and the main cutting edge of the blade, the secondary flank surface and the clearance angle of the lower half are very important. We often rely on estimation and experience to determine whether the tool can enter the workpiece and its related relief angle. This method is time-consuming. Now the CAD drawing and cutting simulation software can simulate cutting on the computer display screen and do not need to do it on the actual part.

Dale Hill of the American Green Leaf Company said that his company directly designs tools based on CAD drawings provided by customers. The designer can see whether the tool needs a shovel back or whether the tool can enter a deep groove area. For some parts with truly complex contours, it is usually not feasible to use standard tools, because they usually cannot enter the cavities and corners of complex parts. Computer simulation can speed up the design of special tools.

Blade relief angle

The main rake angle and the auxiliary rake angle of the blade will determine the clearance angle between the flank face and the workpiece. Different materials require different back angles. For example, when processing tough materials, especially nickel-based alloys, its resilience is very large. These alloys bulge in front of the cutting edge and spring back after the cutting edge passes. These rebounding workpieces will scratch the flank and generate a lot of cutting heat. In addition, the work hardening of nickel-based materials will also generate cutting heat, which will eventually lead to thermal failure of the tool. The failure mode may be chipping, but the thermal expansion of the cutting edge will cause the tool to break.

Titanium materials may rebound 0.05mm and 0.08mm. Therefore, when processing such materials, a clearance angle of 14° or 15° between the flank face and the workpiece is required to prevent thermal failure. However, titanium and plastic have similar resilience. When processing titanium, too small a clearance angle will cause thermal failure of the blade. When such a tool is processing plastic, the cutting force and cutting heat generated by springback will melt the plastic workpiece.

The clearance angle of the blade should not be too large, too large clearance angle will reduce the strength of the blade; the blade without clearance angle has enough strength, but it must be installed on the blade with negative rake angle to form sufficient clearance angle. The use of a cutting insert with a positive rake angle and no relief angle can ensure the required strength of the insert, and it can also form a positive rake angle for cutting.

Cutting force and chip control

Changes in the relationship between workpieces, tools, and other factors in the turning system will affect effective chip control. For example, in profiling turning, when the blade moves outward from the center of the workpiece, the chip thickness becomes thinner, the cutter depth increases, and chip control deteriorates. One solution is to divide one pass into two passes, and change the outward feed to the center feed to obtain the final profile.

Thin-walled and slender parts are difficult to clamp, and the cutting force may cause deformation of the workpiece and extremely poor surface roughness or even scrap the parts. A specially designed blade with chip control can reduce this deformation. Luye Company provides a finishing carbide insert called TurboForm, which has a large positive rake angle and a pressed chip breaker, so the cutting force generated is very low. At the same time, the periphery of the blade is precisely ground, so it has a very high finish. For example, when an aerospace manufacturer is processing thin-wall sealed titanium parts of a jet engine compressor, the blades are chipped and the surface finish is reduced due to tool vibration. After using TurboForm blades, the phenomenon of blade vibration and workpiece deformation is eliminated, and the life of the tool is prolonged.

If the machinability of the workpiece material causes the complexity of the turning process, a part composed of two materials may double this complexity. Therefore, when processing parts made of multiple materials, one way is to select blade grades that can process different materials. For example, when processing parts with 4340 steel on the inside and nickel-based alloy on the outside, the manufacturer must add a pause program during programming to change the blade. Use two different grades of blades for processing, and when the blade life is still very low, it is recommended to use the AC2000 CVD coated blades of Sumitomo Electric Co., Ltd. to process the above two materials by changing the feed rate and cutting speed, without stopping the replacement. Blade, significantly improves tool life.

Grind

The contours of some parts cannot be machined simply with ready-made inserts. When using a J-shaped toolholder with a negative rake angle for plunge cutting, the lower half of the secondary flank of the 35° insert may collide with the workpiece, causing the insert to break. One way to prevent this is to grind away the part that hinders cutting.

Mr. Jeff Carver, a manufacturer of steam turbine piping systems, said that they often use very sharp tools to machine parts, because there are often no standard tools, so they need to be sharpened frequently.

Although some manufacturers like to stock standard blades and grind them to meet specific processing requirements, these reground blades can also be purchased directly from tool manufacturers because they have prepared special blades for processing some special parts. When a sharpened tool cannot meet the requirements in one pass, the only option is to stop and use another blade to complete the cutting. The disadvantage is that it takes time and interrupts a pass, which will leave tool marks on the workpiece.

Machine tool

CNC machine tool manufacturers continue to introduce advanced technologies to simplify the turning of complex parts. A typical example is the multifunctional turning/milling machine from Mazak. These machines are like a machining center with a turning spindle at one end of the worktable. The B-axis of the machine tool can rotate 225° in the radial direction of the part during processing, which makes the tool nose radius always in the direction tangent to the cutting, and multiple processes can be completed with one tool.

In short, the progress of the metal cutting discipline will certainly make the turning of complex parts easier. To process some complex parts economically and effectively, the cooperation of technological factors is still needed.