Indexable milling cutters are always accompanied by vibration when machining workpieces. In the workshop, you can feel the familiar sound and feeling caused by vibration. When the metal cutting rate is increased by increasing the cutting depth of the tool, the cutting edge of the tool is often damaged and the life of the machine tool is shortened.

In addition to improving machine tool performance, there is another way to improve processing conditions, and that is to use copy milling cutters. This kind of knife is also called a "button knife".

The copy milling cutter is just a simple change on the traditional end mill. Unlike the traditional diamond blade or square blade, the copy milling cutter uses a round blade. This change gives copy milling cutters a variety of advantages, most of which can be said to be a complement to today's development trends of small depth of cut, large feed, and high-speed machining. This article will introduce these advantages respectively:

Strong feed capability, enhanced spiral milling and bevel machining capabilities, higher cutting edge strength, more cutting edges participating in processing, and low power to obtain high cutting rate and rough machining close to finishing.

Feed the knife

Some copy milling cutters can feed directly into the workpiece like a drill. Such as: end mill type copy milling cutter, but can only cut in this direction if the manufacturer leaves sufficient clearance at the end of the tool. The sleeve copy milling cutter can also cut directly into the workpiece, but it consumes a lot of power during processing.

It is impossible for copying milling cutters to replace drill bits because the area involved in drilling is larger than the cutting depth that this type of tool can withstand. But the ability of copy milling cutters to cut directly into the workpiece solves a headache in machining: the need to pre-drill a starting hole before roughing.

Because ordinary indexable end mills cannot cut directly into the material along the Z-axis, a starting hole needs to be drilled in advance. Another method of entry is through an inclined plane, which usually requires the application of CAM software. With a copy milling cutter, this step can be omitted. The tool feed operation and cavity machining can be compiled into a complete program, without special consideration of the tool feed problem. This free-feed method is useful for roughing complex pockets and calling surface roughing routines.

Spiral interpolation

The combined use of copy milling cutters and helical interpolation can easily and quickly process large diameter holes. This technology is similar to thread milling, with three axes (X, Y, Z) moving simultaneously. But it is different from helical milling, because the copy milling cutter does not require a starting hole and can directly cut into the material. In addition, because the copy milling cutter has a large relief angle, the lifting angle during helical interpolation can be very large, and there is no need to worry about rubbing the bottom of the cutting edge. This simple and easy process offers the benefit of being able to machine holes of different sizes with the same diameter tool by simply programming the hole size changes.

A typical comparison illustrates the potential of this technology to improve efficiency.

Cutting edge strength

Because there are no sharp corners, round inserts have the highest cutting edge strength among indexable carbide inserts. Therefore it can be used for heavy cutting or roughing under unstable conditions. When long tools are required, round inserts can withstand greater tool deflection and vibration, allowing higher speeds and feeds during processing while reducing the risk of edge chipping.

When using round inserts, cutting forces can be effectively distributed. For a typical right-angle milling cutter, the tool pressure is mainly radial force, which results in large tool deformation and increases the possibility of vibration and tool breakage. The rounded cutting edge disperses cutting forces evenly, converting more cutting forces into axial force. This is what is desired when using extended tools, since reducing radial forces reduces tool deformation.

But be careful when using a horizontal machining center. Increased axial forces can cause a fixture mounted on a machine tool's corner plate to deflect because the structure is not as stable as it is on a vertical machining center. On a horizontal machining center, flexing creates vibrations that can cause slight chipping of the blade. The tool life will be shortened and the possibility of tool breakage will increase. In order to reduce or avoid this situation, a tool with a positive axial rake angle should be used, which can reduce the downward force on the workpiece.

Number of cutting edges

Another advantage of round inserts is that they have more available cutting edges than ordinary carbide inserts. Depending on the size of the insert and the depth of cut, a round insert can have 4 to 8 effective indexing times and remove at least twice as much material as ordinary diamond and square inserts. This advantage can reduce the number of times the operator goes to the tool room to replace new blades (ensuring the operator's effective working time), reduces the inventory of blades (lowers inventory costs), and reduces the cost of each cutting edge.

For example, a regular diamond-shaped insert costs about $8 a piece, has two usable cutting edges, and costs $4 per cutting edge. The cost of a square insert is $10, so that averages out to $2.50 per cutting edge.

Compare the cost of these blades to round blades, which cost $11 per blade (less in most cases).

Assuming that under the worst conditions - heavy cutting - the insert can only be indexed 4 times, the cost per cutting edge is $2.75. In more cases, it can be indexed 8 times, so the cost per blade is $1.38. Through practical applications and cost comparisons, circular inserts not only have a higher metal cutting rate than other types of inserts, but their economics are also quite attractive.

High cutting rate, low energy consumption

If used properly, round inserts can achieve high metal removal rates without requiring high machine tool power. Because of the high strength of round inserts, workpieces can be machined at feed rates not possible with right-angle milling cutters, even allowing for heavy-duty roughing on light machine tools. The key thing to understand is that the greater the depth of cut, the thicker the chips, which will increase power consumption. With shallow cuts—a depth of cut of 0.025 to 0.050 inches—a common round-insert cutter can machine steel at a feed-per-tooth rate of 0.040 inches, and in some cases, as much as 0.06 ipt. The maximum value of most diamond blades and square blades is only 0.010-0.012ipt.

It is worth noting that some users of button cutters will encounter such a problem. During the machining process, or after the blade is worn, the blade will move within the tool mounting seat. In both cases, the tool is subjected to excessive pressure that exceeds the clamping force of the insert clamping element. Therefore, possible situations need to be considered when designing the tool. For example, some knives use screws to fix the blade and at the same time there is a small clamp on it. This double clamping method ensures the safety of the blade.

Another important issue is the positive locking feature of the blade mount. Many button cutter users use cheap die-cast blades with smooth round surfaces that don’t lock radially. Cutting forces acting tangentially to the insert cause the screw on the insert to lose torque. Stiffer copy milling cutters solve this problem by adding locking surfaces on the sides of the insert - the locking surfaces mate closely with the locking surfaces of the cutter body, minimizing the possibility of relative movement between the two.

When cutting at high feed rates, the tool needs to be able to provide maximum support for the cutting edge. Using a copy milling cutter with a negative axial rake angle (tilting the blade downward, toward the workpiece) and conservative machining parameters can achieve good machining results, but it will fail when the feed rate is very high. This design is inherently flawed and lacks support in the main areas where the cutting edge is stressed. Using a copy milling cutter with a positive axial rake angle can provide good support for the cutting edge because the carbide behind the cutting edge is close to parallel to the cutting force. Using carbide in this way allows users to feel the ability of carbide to absorb compressive stress.

But in this case, the stiffness of the tool clamping is important. Therefore, it is highly recommended to use a short end mill chuck or a sleeve mill chuck.

With the right machining, even a machine with only 10 or 15 horsepower can achieve competitive cuts, with fewer setup steps and more flexible factory production schedules.

Rough machining close to finishing

Using round inserts for roughing provides better "preparation" for semi-finishing or finishing. Roughing with a right-angle cutter leaves a step as you cut downwards. The greater the cutting depth in each pass, the more obvious this step effect will be. This uneven workpiece surface will cause uneven force on the tool during semi-finishing, causing impact to the tool and deformation of the tool, making it impossible to directly transition from roughing to finishing. Not only semi-finishing is required, but multiple finishing operations are also required.

The use of round blades greatly reduces the occurrence of the above situation. Instead of leaving a step like a right-angle cutter, there are only some very small "wrinkles" that are very low and can be machined easily. When the cutting depth is small, round inserts are also the best choice. The height of the "wrinkles" becomes smaller. The surface of the workpiece after rough machining is relatively flat and can be easily semi-finished. In some cases, finishing can even be done directly.