So, how to choose? Let's look at various factors involved in the process, and the strengths and weaknesses of each tool design.
Square-shoulder tools are capable of heavier depths of cut and are, therefore, more apt to consume available horsepower quickly than are button mills. The exception here would be in the case of extremely high feed rates, of which some button mills are capable, while square-shoulder mills are not. In some situations, button mills can run feeds per tooth of over 0.050" (1.27 mm), at depths of cut around 0.050" or possibly more, depending on the I.C. (inscribed circle) of the insert. In these situations, the metal removal rate can far exceed the capabilities of most square-shoulder tools, but horsepower consumption will go up proportionately.
In cases where horsepower is more limited, as in many newer high-speed machining centers, button cutters have an advantage due to the previously mentioned capacity for high feed rates. In cases like these, button cutters can be run at light depths of cut, and a heavy feed per tooth. This yields an acceptable metal removal rate, even for a light-duty machine. A square-shoulder tool, however, will be limited to whatever depth of cut the machine tool will allow, limiting metal removal.
Rigidity is a factor in both the machine tool and the workpiece setup. A machine tool with box ways will allow heavier cutting than a machine with linear ways. A workpiece setup with solid, well-distributed clamping will allow more tool pressure than a part that has areas left unsupported (for example, if the part is longer than the vise is wide). These issues are relevant to cutting tool selection.
As far as machine rigidity is concerned, a good rule of thumb is that a square-shoulder tool will thrive on a rigid machine, and a button cutter will perform more satisfactorily on a less-rigid machine. The reasons for this lie mainly in tool geometry. The square-shoulder tool, having a 90º cutting edge, will generate primarily radial cutting forces and offer potentially heavier depths of cut. Heavier cutting demands good machine spindle and way rigidity, or vibration will almost certainly occur.
With the sharper edge (corner) on a square-shoulder tool, excessive vibration creates the potential for edge chipping, which could in turn lead to catastrophic failure. The button cutter offers good metal removal on the less-rigid machine for two primary reasons. First, the round cutting edge is much stronger and more durable than the sharper, square-shoulder cutting edge. This greater strength allows the insert to better absorb shock and vibration during the cutting process, and should also be taken into account for some interrupted cutting. Second, the round cutting edge generates variable tool pressure, meaning that the forces are much more evenly split between axial and radial (up and to the side) than is the case with a square-shoulder tool. This split in the tool forces puts much of the tool pressure back into the workpiece, helping to lower the demand on the machine spindle and ways. With a button cutter, cutting at aggressive feed rates is possible on machines not previously adept at such things, but lighter depths of cut (usually <0.060" or 1.5-mm) will be necessary.
Workpiece rigidity is a different issue altogether. Here, the focused tool pressure of the square-shoulder cutting tool can frequently be an advantage. Take, for example, a workpiece that is poorly supported underneath the part. In this case, any cutting pressure generated in the axial (down) direction will tend to cause chatter due to the lack of a stable base for the pressure, and a square-shoulder tool may be better-suited to the milling. A button cutter will be more likely to produce vibration of the workpiece in this situation, causing poor surface finish, decreased tool life, and increased noise levels.
Nowhere is this phenomenon more evident than when machining through-pockets. As the cutter gets closer to breaking through the bottom, a button cutter will produce significant floor vibration that compromises the remaining material in the pocket. In cases like these, it is not uncommon to see the button cutter's inserts break before ever breaking through the bottom. Square-shoulder cutters are best suited for applications such as these, because pressure will be directed into the sidewalls of the part, not the floor being machined.
The final shape of the workpiece plays a basic, yet important, role in the selection of the roughing tool. Selecting the wrong cutting tool for roughing can create extra steps in the process, reducing profitability and negatively affecting delivery.
For slotting, step milling, face milling up to a shoulder, and most 2-D profile milling, the square-shoulder tool makes the most sense. Using a button cutter in these cases will make it necessary to undertake the extra step of machining away the radius created by the round insert, adding an additional tool to the program and setup. A round insert will also leave a scalloped sidewall finish in these cases, creating the need for a finishing tool to clean up the lines.
For 3-D profiling, cavity/core roughing, surfacing, or face milling of an open face, a button-style cutting tool is the logical choice. In most of these cases, the final surface has anything but a straight wall. The use of a round insert (especially at lighter depths of cut) creates a smooth, flowing surface that is easier to cut during a semifinishing or finishing routine. Square-shoulder tools leave steps on surfaces like these, creating uneven tool pressure and (typically) poor surface tolerance. Parts roughed with square-shoulder tooling frequently need multiple semifinish or finish cuts to achieve the desired profile tolerance. Additionally, the stepped surfaces are harder on the cutting tool, with excessive variation in tool pressure typically reducing tool life and surface-finish quality.
Hole interpolation is done in many different ways, most of them slow. Depending on the type of machine tool, cycle times can be improved dramatically, especially in 2" (51-mm) diam or larger holes. Square-shoulder milling tools are better-suited for circular interpolation than button cutters. This process involves pre-drilling a start hole somewhat bigger than the cutting tool to be used. The square-shoulder tool is then plunged into the drilled hole, at which point the circle is milled to size at a specified depth of cut per pass, continuing in this approach to the finished depth. If the machine tool has sufficient horsepower to allow the tool to operate at depths of cut greater than 1/4" (6.4 mm) per pass, this process can be done economically. If not, holemaking done in this manner will most likely be slow and inefficient.
Button cutters offer the most aggressive option for hole creation, and also allow elimination of the pre-drilling operation. How? By helical interpolation--the combined movement of three (X, Y and Z) axes while interpolating the hole. This operation is performed by positioning the OD of the cutting tool at the inside of the finished diameter, rather than positioning the cutter at the hole center. The program then instructs the tool to begin circular interpolation of the hole, but with downward Z-axis movement through each 360º rotation in the hole. Tool movement creates a ramping effect, allowing the tool gradual entry into the workpiece and rapid metal removal, all with a smooth-sounding cut that's easy on the cutting tool and the machine itself. A typical operation would be:
- Hole size: 4" (203-mm) diam
- Hole depth: 2" (51-mm) depth
- Tool used: 2" (51-mm) diam button mill with four inserts
Approach: Position the tool at the three o'clock position in the hole, approximately 0.100" (0.03 mm) above Z-zero (workpiece top). Execute an arc command that brings the tool back around to the start position and also moves the tool down in Z by an incremental amount, often 0.050 - 0.100" (1.25 - 2.5 mm). (There are many different programming methods available for the helix command, but they won't be covered here.) Continue this motion until the insert centerline breaks through the part bottom. At this point, bring the tool back to centerline and retract.
While square-shoulder tools are capable of this motion, button tools allow much more aggressive ramp angles and provide better protection to the cutting edge for re-cutting of chips, the most challenging aspect of helical interpolation. Strong air blast is highly recommended for this process.
There are big differences in cost per edge between button cutters and square-shoulder cutters. Most parallelogram inserts carry an end-user price of approximately $9. With two cutting edges, this translates to a cost per edge of $4.50--quite expensive. Button cutters, however, offer between four and eight cutting edges. With a typical end-user price approximating $10, that translates into a cost per edge of $1.25 - 2.50.
Price per edge, however, is not the only important economic aspect of cutting tool selection. Total value must be determined, based on all of the previously mentioned aspects, including tool life and cycle time.
Dramatically different results can be attained, depending on the situation. Knowing how to intelligently evaluate each one can be the difference between profiting or losing on a job.