Abstract: This article outlines solutions to some problems encountered during cam milling using milling and turning centers and cam grinding using CNC cam grinders.
I. Milling Cams in Turning and Milling Machining Centers
Cam machining is far more difficult than machining other parts such as shafts, discs, and housings, and its machining accuracy is also harder to guarantee. The Austrian WFL M40 CNC milling and turning center, with its five-axis linkage capability (see Figure 1), can complete the forming of cams, achieving a surface roughness of Ra = 1.6 μm. It allows for direct finish grinding without rough grinding, and its advanced control and programming systems can machine various types of camshafts. Because this high-precision CNC machining equipment has only been used in the domestic locomotive manufacturing industry for the past 10 years, many problems have been encountered during cam milling. Through continuous summarization and exploration of experience, some solutions to these problems are briefly introduced here.
1. Process design for milling cams
The WFL M40 CNC milling and turning machining center uses the Siemens 840D CNC system. The tool used for milling the cam is the T490 end mill (Figure 2 FLN D063-06-22-R-13). The final cam milling is achieved using the "MILLCAM" command developed by WFL based on the Siemens 840D CNC system and tailored to the machine tool's own characteristics.
Cam milling is the process of machining a rough-machined cylindrical cam blank into a cam shape with only grinding allowance (see Figure 3). Because the machining allowance in the base circle direction of the rough-machined cam blank is too large, cam milling requires two steps: rough milling and finish milling. Finish milling involves using the "MILLCAM" command to machine the cam profile to meet the milling process requirements. Rough milling, on the other hand, shapes the cam blank to ensure the final finish milling allowance is as uniform as possible, thus guaranteeing the quality of the finish milled cam. Therefore, how to perform rough milling is crucial in the cam milling process. There are generally three methods available on mill-turn machining centers for rough milling cams.
(1) Command method: The roughing of the cam is completed directly using the "MILLCAM" command, as shown in Figure 4. This method can ensure that the machining allowance of the final finish milled cam is uniform and the programming is simple. However, the "MILLCAM" command mills the cam in a complete 360° milling, and cannot mill the cam in segments. Therefore, a large part of the cam tip circle has no machining allowance during the rough milling process. That is, the machine tool is idle during this part of the cam machining, which greatly reduces the machining efficiency. Therefore, the method of rough milling the cam using the command is not suitable for batch machining of camshafts.
(2) Plane approximation method: The rough milling of the cam is completed by milling planes, as shown in Figure 5. The more planes milled on the circumference, the closer the actual profile of the cam is to the theoretical profile, and the more uniform the allowance of the finish milling cam. However, this method requires a high level of skill from the programmer. The starting coordinates of the tool and the rotation angle of the workpiece must be calculated in the drawing software for each plane milling, which is a huge workload.
(3) Combined Plane and Arc Surface Milling Method: This method combines milling of planes and arc surfaces to rough-mill the cam, as shown in Figure 6. It mainly consists of three steps (see Figure 7): After completing the first section of plane milling, the cutter remains stationary while the spindle rotates to mill the arc surface. Finally, the cutter moves forward to mill the second section of the plane. It is important to note that these two plane sections must be tangential to the arc surface; otherwise, the cutter will experience interference when milling the arc surface while the spindle rotates.
This phenomenon causes damage to the cutting tool and milling spindle. Considering factors such as machining efficiency, programming difficulty, and tool life, this method is currently used in the workshop for rough milling cams.
2. Selection of the tool's Y-value
As shown in Figure 7, during both rough and finish milling of cams, there is a section where the tool remains stationary while the spindle rotates to perform arc-shaped milling. This necessitates careful selection of the tool's Y-value, i.e., determining the tool position before the spindle begins rotary milling. Observation and testing of actual rough and finish milled cam workpieces reveal that a large Y-value results in a concave arc shape on the cam profile surface, lower in the middle and higher on both sides; a small Y-value results in a convex shape on the cam profile surface, higher in the middle and lower on both sides (see Figure 8). Furthermore, as shown in Figure 7...
If the Y value is too small, interference will occur when the tool rotates to mill the arc surface, causing damage to the tool and the milling spindle.
The correct Y value is related to the tool radius, cam width, depth of cut, insert thickness, and insert clearance angle. A general formula is: Y = (cam width / tool radius) × [(depth of cut + insert thickness) / insert clearance angle]. In actual machining, you can first select Y = tool radius / 2 - 3 for trial cutting, and then adjust the Y value according to the cam surface profile. A maximum of 3 trial cuts are needed to obtain the correct Y value. It is important to note that if the cam width is greater than 3/4 of the tool radius, a smooth cam surface cannot be obtained regardless of how the Y value is adjusted. In this case, it is necessary to consider changing the tool or performing multiple milling operations on the cam at that width.
3. Selection of "Mirror"
In the actual machining of camshafts, due to different clamping directions, the need for cam chamfering, or interference of the tool in the machine tool, the direction of the cam lift starting point will be different after the camshaft is clamped on the milling and turning machining center. This involves the selection of "mirror" in the "MILLCAM" command (see Figure 9).
4. Machining of the concave curve of the oil pump cam
To meet stringent emission requirements and improve diesel engine combustion quality, fuel injection pump cams are typically designed with a concave curve. However, the "MILLCAM" command on a milling machine cannot actually machine this concave curve. When the program reaches the concave curve, the spindle will rotate slightly in the opposite direction to the cutting direction, causing the tool's clearance angle to collide with the workpiece, resulting in damage to both the tool and the milling spindle. Therefore, when machining fuel pump cams with concave curves, it is necessary to use drawing software to first connect the two ends of this concave curve with straight lines. The cam lift for this section is then calculated using these straight lines. The calculated cam lift replaces the original section in the program. This allows the "MILLCAM" command to complete the machining of the fuel pump cam, replacing the original concave curve portion with a straight line. If the radius of curvature R of the concave curve replaced by a straight line is greater than 150 mm, the grinding allowance will not exceed 0.2 mm from the original, and no further machining is required; if R is less than 150 mm, the concave curve part needs to be milled multiple times in the cam width direction using a circular interpolation program.
II. Grinding cams on a CNC camshaft grinding machine
The KOPP SN320 CNC cam grinder (see Figure 10) uses the Siemens 840D CNC system. Grinding cams with this CNC camshaft grinder eliminates the need for template manufacturing. It's a process of digital and logical operations, allowing for the processing of various cam profile curves at any time, shortening the new product development and manufacturing process, and reducing new product development costs. Here, we briefly introduce some solutions to problems encountered during cam grinding.
1. Selection of grinding wheel diameter
Camshaft fuel injection pump cams are designed with a concave curve. Therefore, when selecting a grinding wheel for grinding the cam, the diameter of the grinding wheel must be smaller than the radius of curvature of the concave curve of the fuel pump cam in order to complete the grinding of the cam. Furthermore, the results of testing various cams show that the radius of the selected grinding wheel must be at least 15% smaller than the radius of curvature of the concave curve of the cam in order to ensure the accuracy of grinding the cam.
2. Elimination of dial error
CNC cam grinding machines use a "dial" to drive the camshaft to rotate for grinding (see Figure 11). Another important function of the "dial" is to transfer the reference zero point of the CNC cam grinding machine to the flange positioning hole of the camshaft, ultimately ensuring that the phase angle between the cam and the flange positioning hole meets the drawing requirements. However, this reference transfer always has errors. The method to eliminate this error is to inspect each new camshaft being ground, calculate the magnitude of the "dial" error based on the inspection results, and eliminate it in the program.
3. Selection of "Mirror"
Similar to milling and turning machining centers, the direction of the cam lift starting point will be different when the camshaft is clamped on a CNC cam grinder. This involves the selection of "mirror" in the programming of the CNC cam grinder (see Figure 12).
III. Conclusion
With the advancement of technology, railway locomotives are developing towards high speed, heavy load, economy, and low pollution, and the number of exported locomotives is constantly increasing. Cam curves are becoming more and more complex, and the curve update speed is also getting faster and faster. More and more advanced CNC equipment will be used to process camshafts. The processing process will certainly not be smooth sailing. As long as we work hard and study diligently, we will definitely be able to solve all the problems encountered in the processing.
