The shape of an insert has a major influence on the impact resistance of the carbide. The weakest section of a milling insert is the corner, which is exposed to constant interrupted cuts and extreme heat changes. Since a round insert has no corners, it is more secure than any other shape (Figure 1).
Consider the tremendous forces that are placed on a milling insert in a typical cut. As the insert enters the cut, it is subjected to a large compressive load. The initial contact between the cutting edge and the workpiece may be very unfavorable depending on the position of the cutter in relation to the workpiece. As the insert moves through the cut, the chip thickness is constantly changing, varying the radial and axial cutting forces. Finally, as the insert leaves the workpiece, it is subjected to tensile stresses that break down the carbide. The chip bending away from the insert places large tensile or "ripping" stress on the face of the carbide.
In addition to normal milling operations, there are certain applications that are particularly damaging to carbide. Here are some of the most difficult jobs that demand a lot of strength from the shape of the insert:
- Milling of castings, where the axial depth often varies throughout the cut,
- Workpieces with hard spots or inclusions that cause unpredictable insert life,
- Raw materials that are flame or plasma cut, leaving very hard surfaces to machine,
- Forgings or castings with slag or scale on the surface that can result in the notching of the insert,
- Hardened steels, tool steels and cast irons, and
- Heat resistant materials like Inconel, titanium and Hastelloy.
Clearly the shape of the round insert is the major reason for its strength and reliability, but what else makes it such a popular choice?
One major reason for the success of the round insert in so many applications is that the axial and radial cutting forces can be managed by changing the axial depth of cut. A high radial cutting force can cause vibration and chatter. Conversely, a high axial force can cause the workpiece to move in its fixturing and cause poor tolerances. With the use of traditional milling cutters, the lead angle dictates the kind of cutting force produced.
As shown in Figure 2, when the lead angle (r) increases, the radial forces (fr) decrease and the axial forces (fa) increase, changing the resultant cutting force direction (R). At a 45-degree lead angle, the fr and fa are balanced. Which lead angle is best will depend on fixturing, part shape and size, workpiece material, machine power and type, and gage line or overhang.
However, the lead angle of a round insert cannot be truly defined. When the tool is used at different depths of cut (aa), the lead angle changes. The lead angle can be measured at the depth-of-cut line; however, the effective lead angle will change throughout the cutting depth. In Figures 3A, 3B and 3C, a 3/4-inch diameter (inscribed circle) insert is shown at a depth of cut of 0.375, 0.250 and 0.125 inch respectively. See first picture.
As the lead angle changes, the cutting forces are also changing. In Figure 3A where the insert is at the maximum depth of cut (one half the insert IC), the axial and radial forces are balanced. When the depth of cut is reduced, the radial forces decrease and the axial forces increase. At a very light depth of cut, the forces would be almost totally in the axial direction. Thus, depending on the setup and part parameters, maximum productivity can be gained with a round insert cutter by selecting the optimum depth of cut that best controls the cutting forces.
Another reason the round insert cutter is so widely used is the high table feeds that can be attained. This is due to the chip thinning effect of the round cutting edge. Chip thinning is what happens when you increase lead angle of the tool. The greater the lead angle, the thinner the chip since it is distributed over a greater length of the cutting edge. Therefore the insert load will be reduced allowing for higher feeds per tooth and superior metal removal rates. In Figure 4A, a tool with no lead is shown. Here, the chip thickness (hm) is equal to the feed per tooth.
Now look at Figure 4B. Note that the shape of the chip is longer and thinner. The area of the chip has not changed, but the shape has. So the average thickness of the chip will be less than that of the tool with no lead angle. This can be calculated by multiplying the cosine of the lead angle by the desired feed per tooth. Or, by dividing the desired chip thickness by the cosine of the lead angle, the needed feed per tooth can be calculated.
Round insert cutters, like all lead angle cutters, have a change in chip thickness when feed per tooth is changed. But unlike other lead angle cutters, the chip thickness for a round insert is also affected by a change in depth of cut. Determining the average chip thickness is important to consider to evaluate the load on the cutting edge and to achieve maximum table feeds. In Figure 5, a 3/4-inch diameter insert (Di) is shown taking a feed per tooth (Sz) of 0.008 inch, at a depth of cut (aa) of 0.375, 0.250 and 0.125 inch respectively. Note that each has a significantly different average chip thickness.
See Picture Two and Three.
To illustrate how much difference this chip thinning can make, let's look at an example comparing a round insert cutter to a traditional 45-degree lead cutter that utilizes a square insert. In this example, we will base our calculations on a 0.125-inch axial depth of cut.
|Avg. Chip Thick||0.008||0.008||0.008|
|Feed per tooth||0.011||0.016||0.019|
|# of inserts||6||6||6|
Notice that the round insert cutter requires more table feed to maintain the same 0.008-inch chip thickness as the 45-degree lead cutter. The round insert is more productive, yet not working any harder than the square! This is the true benefit of the round insert. Also notice how the table feed must increase when the round insert IC is 3/4 inch. This is because the chip is being spread (thinned) over a larger radius than with a 1/2-inch IC.
Multiple Cutting Edges
A milling insert is limited to the number of indexes by its number of corners. A round insert, on the other hand, is able to provide many cutting edges. It all depends on how deep the cut is. The fewest edges available would be where the insert is "buried" at one-half the IC of the insert -- this would provide four edges. The maximum amount of edges can be as many as eight or more for very light cuts. Thus, the per-index cost of a round insert is less than an insert with three or four indexes.
In the past, it was difficult for an operator to index to a new edge with a round insert. After loosening the insert screw, he would have to "eyeball" to find the new cutting edge. Recent insert pressing advancements have allowed some manufacturers to press-in location facets into the back of the insert (Figure 6). With these facets, the operator gets a true, distinct index every time he goes for a new edge. This way the guess work is taken out, and all of the insert may be utilized. The facets also help to hold the insert in the pocket, keeping it from rotating under heavy loads.
Positive Top Rakes
The oldest milling cutter designs are of double negative geometry. Inserts were totally flat, so the cutting rake or "top rake" was also negative. Thanks to the advancements in design and pressing of carbide inserts, another geometric dimension has been added to milling operations through the insert geometry itself. These insert geometries have further enhanced the milling cutter performance.
While it is known that negative inclinations of the insert are the strongest way to present the insert to the workpiece, it is also true that positive top rakes make for a freer, quieter, lower horsepower cut. Designers of round insert cutters have been able to maintain the strength of negative inclination, while not compromising on a high positive top rake. This is due to the pressed-in geometries of the inserts (Figure 7). By putting positive angles and optimized edge preps on the round inserts, security is maintained and a free cutting action is produced.
Perhaps the most interesting thing about round insert cutters is the remarkable amount of ways that they can be applied. Since the cutter body and insert seat areas are rounded to accommodate the insert, these tools have tremendous amounts of clearance in almost every direction. This allows them to be used in applications that are not practical for traditional lead angle tools. For instance, up ramping is an operation most any milling cutter can perform.
However, down ramping requires bottom clearance that most cutters do not have. The round insert cutter is capable of down ramping at extreme angles because the tool is actually cutting on the outside and inside of the insert (Figure 8). This same bottom clearance allows the tool to be directly plunged into the workpiece material, which is ideal when pocketing in restricted areas. These clearances are why the round insert is a standard in mold and die shops throughout the world.
Another unique capability of these cutters is that they can produce a hole from a solid using helical interpolation (Figure 9). This can significantly reduce cycle times and the amount of tools in the setup when producing rough holes.
Many machine tools are available today with multi-axis capability with the goal of producing parts with as few setups as possible. This makes the round insert cutter ideal for producing parts that are concave or convex, where the cutter must be able to machine contoured surfaces. Also they are well suited for machining impellers, turbine blades and propellers.
With all of this application potential, and with the strength, reliability and productivity of the round insert cutter, it is no wonder that it is the first choice milling tool for many shops today.