Proper application of speed and feed for the material hardness are critical factors for good tool life when hard milling with ceramic inserts. Unfortunately, depth- and width-of-cut, as well as cutter lead angle, are commonly overlooked. A basic understanding of how these factors influence tool life can make a huge difference in metal removal rates.

If a tool is running with the incorrect chip load or at inefficient speeds and feeds, it not only sacrifices tool life, but productivity as well. When considering all of the factors, productivity increases and decreases can be astounding with small changes in cutting parameters.

Speed
The cutting speed necessary for successful hard milling with ceramics is based on the actual material hardness, usually in the 45–65 Rockwell C hardness range. At elevated temperatures the metal being machined becomes plasticized, or softened, which lessens the cutting forces, and aids in chip separation. To take advantage of the strength and hardness of a whisker-reinforced ceramic insert, it is necessary to run at a surface speed (SFM) high enough to create sufficient heat in the cutting zone, which may be as much as five times the cutting speed of conventional carbide.

This is not to suggest that hard milling with ceramic inserts requires a new high-speed machining center with 50 horsepower and the latest advanced processor. The speed necessary for milling hardened materials with ceramics is within the range of many of the machines in a modern mold shop. The horsepower consumption is actually quite low due to the low cutting forces being generated. The ability to hard mill with ceramics on most machines already available in your shop is another benefit to the tooling system.

Feed
Before programming a hardened part it is important to consider the factors that influence the difference between the programmed feedrate per tooth and the actual thickness of the chip being formed. The feed per tooth specified in the program can be dramatically reduced by the cutting conditions. The actual thickness of a chip being formed is affected by the depth- and width-of-cut as well as the insert radius or the lead angle of the tool.

Actual chip thickness is a crucial factor for heat dispersion (i.e., the chip must have enough mass to carry away a majority of the heat). When the depth- and/or width-of-cut is below an acceptable level, the chip generated will not be thick enough to carry away all of the heat being produced by the cut. The heat that is not absorbed by a thin chip has to go somewhere. It will be pushed into the part, the inserts, the cutter and the fixture. It is much more efficient to send the largest portion of the heat away with the chips.

Feedrate and the effect on the actual chip thickness also are directly related to the insert’s cutting edge. Most ceramic inserts require some edge protection such as a negative land commonly know as a T land in addition to a light hone (.0005”-.001”). The inherent edge strength of whisker-reinforced ceramics allows the insert to mill difficult materials with only a small negative land (.002”-.004”).

This land helps strengthen the insert by redirecting the cutting forces into the body of the insert, which puts the cutting edge into a stronger, more compressive condition. The edge condition must be taken into account when hard milling with ceramics to ensure that the actual feed is at or above the negative land on the insert. A chip that is too thin works on the very edge of the insert, where it is the weakest, causing premature insert failure.

Lead Angle Effect on Chip Thickness
In many cases ceramic milling cutters are specified with a lead angle, or more commonly, with round inserts. The lead angle of a cutting tool reduces the programmed chip thickness by an easily calculated percentage.

For instance, on a milling cutter using square inserts on a 45-degree lead angle the actual chip thickness will only be about 70 percent of the programmed feed per tooth. This would allow you to increase the programmed feedrate by up to 40 percent to achieve an equal ratio of programmed feed to actual chip thickness. The same principle applies to cutters using round inserts.

The lead angle on the radius of a round insert is infinitely variable based on the depth-of-cut. This makes it more difficult to calculate the percentage of chip thinning with a round insert. However, this is a common issue that must be addressed when using ball nose and bull end mills, and many machinists and programmers in the moldmaking industry are familiar with the chip thinning compensation formulas.

If you consider a ceramic milling cutter using round inserts like a bull nose mill, and apply chip thinning compensation for the radius of the insert you will assure proper chip formation. In this way you also can see increased feedrates of 20 to 40 percent over the initial programmed feed, and metal removal rates many times that of carbide end mills.

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