In high-speed milling applications, axial chip thinning is often used to increase the feed rate when machining with small end mills. The machine shop uses the same effective machining method on one of the medium speed machines to increase the metal cutting speed of standard size end mills.
From the company name itself, it is clear that Robertson EDM is not a specialized processing shop that started as a machining center. The shop is located in Edgerton, Ohio, with four EDM wire-cutting machines and one EDM tooling machine, but currently there is only one CNC machining center – this is a Mazak VTC-16B vertical machining purchased at the shop. Center, performance is very reliable.
However, as the workshop continues to grow and the volume of business continues to increase, the scope of its work and the opportunities for work continue to expand. For example, in order to undertake the turning operation of large-diameter workpieces, the workshop recently added an investment to purchase a Johnford ST-40A slant bed turning machining center from Absolute Machine Tool Company. The turning diameter of the bed is 31in (1in=25.4mm). ). At present, the workload of the machining center is constantly increasing. In order to meet new challenges, the processing shop has implemented a high-speed machining method in a vertical machining center such as VMC.
The term “high-speed machiningâ€, especially in steel processing applications, is often aimed at high-speed feed small tools that run at high speeds on the spindle and have a shallow depth of cut. Despite its shallow depth of cut, the cutting speed is fast and a very high metal removal rate can be achieved with this machining method. If a ball end mill is used, the cutting speed can be further improved. Although the ball end mill has a shallow depth of cut, it has the advantage of making the axial chips thin. Because of this effect, the chip thickness is smaller than the advancement amount per tooth. This means that the advance speed of each tooth can be further increased, and the linear feed rate of a few inches per minute can be increased accordingly, even beyond the range allowed by the high speed spindle.
This is high speed machining. On the contrary, Robertson EDM's machine tools have no particularly high spindle speeds, and no complicated 3D milling work can benefit from small diameter ball-end tools. For this shop, in high-speed machining, the practice of thinning the chips to increase the feed rate is still a really valuable idea. In order to increase its own metal cutting speed, the shop uses a tool designed by Iscar to achieve the purpose of thinning the chips. This insert is called a “feed milling cutter†and has a curved surface that makes it possible to thin the chips. In other words, chip thinning does not necessarily require ball end mills or small diameter tools. At least this aspect of high-speed machining can be applied to medium-speed machine tools for more standard roughing purposes.
Practical application of the chip thinning process Jeffrey Robertson is one of the main partners of the workshop. Iscar's representative for local machine tool sales and applications is Mr. Greg Mallett. Mr. Mallet and Mr. Robertson also saw the opportunity to apply axial cutting and thinning inserts and jointly planned the tools used in the new turning center. Robertson EDM has roughed a fixture part from cold-rolled steel, and the shop's method of machining this part provides a good basis for comparison. Since the part is too large and the machining center has insufficient stroke, the shop has to use a partial machining method to machine the part.
After the first half of the part was machined with a 0.75in radius insert tool, the shop agreed to machine the other half of the part with a 0.75in radius insert that cuts the chips. A tool with the latter insert uses one tooth instead of two teeth for cutting, but still uses a shallow depth of cut. However, since the chip is thinned, a high feed rate is obtained, so productivity is improved. The service life of the tool has also increased.
The surface of the insert starts at a radius of 0.106 in. and then transitions to a radius of 0.5 in. The shallower depth of cut formed by this curved surface allows the advancement speed of each tooth to be increased because the chip thickness is less than the advancement of each tooth, not equal.
Mr. Mallett said that the tool life extension is also due to the curved surface of the tool. He said that Robertson EDM's ability to improve tool life is mainly due to its stable machining. In the case of a ball end mill, the material is only in contact with the tool surface at a point below the full radius of the tool. The cutting force produced is not exactly the lateral thrust but an upward thrust towards the center point of the ball end. In other words, only a portion of the cutting force is moving in the direction of the X and Y axes. The other cutting forces move in the direction of the Z axis, that is, in the direction of the main shaft. Among the three axes, the rigidity of the main shaft is the best. Since the force received along one side of the tool is relatively small, there is much less chance of the tool being biased, so the cutting is more stable.
In fact, the production capacity of the processing center in the workshop has been greatly improved. Nowadays, the tool shop often uses this tool as a roughing process for steel, for example, it is used to machine the gearbox cover, which is another example of the tool that is suitable for machining a part. With the continuous development of the processing workshop, there is no doubt that the number of machining centers will continue to increase, and perhaps a high-speed machining machine will be added in the future. However, in the current situation, the excess throughput provided by the faster roughing feed rate is sufficient to meet current production needs.
What is the axial chip thinning process?
It is machined with a ball end mill and its cutting state is different from that of a standard milling cutter. Its cutting contact surface with the tool is smaller than the full radius of its curved surface. The chip thickness and the amount of advancement per tooth will also vary - less cutting thickness. Due to the thinner chip size, the feed per tooth can be increased, resulting in a higher linear feed rate for higher metal cutting speeds. For applications where a relatively shallow depth of cut has been used, this is used. The process is more suitable.
This effect is usually closely related to ball end mills in high speed machining. However, it is also advantageous for milling cutters that use circular inserts and tools that use curved surfaces to achieve the goal of thinning chips (such as the "feed milling cutter" mentioned here).
The tool force is not lateral, but extends from the material to the center of the curve along the diagonal direction indicated by the dashed line. In other words, some forces do not move completely along the X and Y axes, but move along the Z axis. This is the most rigid spindle in the machine tool, which makes the cutting more stable. status.
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