In the metalworking industry, ensuring a high-grade surface finish has always been one of the main aims in cutting tool development. This is also true for indexable face milling cutters.
Despite significant improvements in the high surface quality of milled surfaces, tool designers still believe that available resources have not yet been exhausted and that intelligent applications of the last generation of advanced milling cutters can substantially improve surface texture further. When considering an indexable milling cutter, the key factors that determine the quality of a generated surface are the cutter’s geometry and accuracy. Both characteristics are mainly related to inserts carried by the cutter.
Powder metallurgy advancements have enabled the production of carbide inserts with complex shapes, ensuring optimal cutting geometry while maintaining substantially increased accuracy of inserts as sintered inserts. Not surprisingly, such advancements in technology have significantly improved the surface finish in face milling. However, when indexable milling cutters were successfully applied to machining high-strength materials and manufacturers started to notice hard milling as an alternative to grinding, the metalworking industry started demanding milling grades capable of higher surface finishes.
An indexable face milling cutter is a multi-tooth tool and the more teeth, the higher the productivity. This is an undeniable advantage of the face mill cutter. But, in terms of surface texture, a large number of teeth may be a problem. A fine distinction in teeth protrusion leads to irregular feed for the teeth and it contributes to chatter, which will ultimately affect the surface finish.
Insert accuracy can considerably be increased by grinding. Moreover, grinding provides a sharp cutting edge that is very important in maintaining cutting action and preventing plastic deformation of metal in fine milling, which features shallow depths of cut. Ensuring a highly accurate cutting edge requires grinding both the top and side surfaces of an insert. This may cancel the advantages of powder metallurgy in generating complicated surfaces to provide required rake and clearance angles along a cutting edge. To avoid such an adverse impact, cutting tool engineers should be very resourceful when designing the inserts intended for fine grinding.
An important factor regarding the loss of tooth accuracy is the insert pocket in a cutter, as it has its own dimensional and form tolerances. As a result, even for very precise inserts, tooth protrusion will vary. Whilst the protrusion may be within acceptable limits, it cannot ensure an extra fine surface finish when compared to grinding. A way to overcome this problem is by using a fly cutter that carries only one insert. The fly cutter is successfully used in various milling applications, and it facilitates a smooth and clean-cut, providing excellent surface texture parameters. But then again, productivity, in this case, is far below multi-tooth indexable face mills.
How do you solve a difficult situation and find an acceptable balance between surface quality and productivity? The cutting tool manufacturers have developed different answers to the quandary.
An integrated wiper flat with a specially shaped minor cutting edge is a classical element of various milling inserts. Its width should be greater than the feed per revolution. Despite being called flat, the minor edge sometimes has a complex geometry to compensate for a negative effect of wear development. When an insert is mounted on a cutter, the wiper flat sits parallel to the machined surface. Hence, the surface will be formed by the insert protruding most from the cutter. Introducing a wiper flat in an insert design is an effective way to improve the surface finish. Even today, rough milling inserts may have an integrated wiper (Figure 1).
ISCAR’s DOVEIQMILL family of face mills carry double-sided inserts with a wide wiper flat. These tools are intended for rough and semi-rough milling with a surface roughness that usually features semi-finish to finish pass quality. The DOVEIQMILL cutters have successfully delivered the hopes of their designers – and the adoption of the cutters in various processes resulting in the cancellation of finish milling operations in many instances. The cutters provide roughness values of up to Ra0.4 when milling steel and cast iron.
Increasing the number of teeth in large-diameter face mills and fine pitch cutters can determine the appropriate growth of the integrated wiper width, which is naturally bound by the design and dimensional limitations. In such cases, a high surface finish can be achieved with the use of a specially designed wiper insert (or two inserts for large diameter tools), whereby the wiper flat is significantly wider than the standard one. This insert is mounted in the same pocket but protrudes several tenths of a millimetre axially, relative to the standard insert (Figure 2).
Very good results can be reached by applying adjustable milling cutters that utilise different mechanisms to adjust the position of an insert cutting edge within very strict limits of only several microns. But the beneficial adjustability of the cutting tools also has a flip side. It is additional work that takes time. A desirable solution looks like a tool that after mounting an insert has no adjusting requirements needed to achieve a high surface quality grade. That is why improving accuracy and advanced geometries remains the mainstream in updating indexable cutters for finish face milling.
Simultaneously, cutting tool manufacturers offer tailored solutions that attract attention with their originality. An example is ISCAR’s TANGFIN family of milling cutters with tangentially clamped inserts with wide integrated wiper flats (Figure 3). The inserts are positioned in a TANGFIN cutter with a gradual displacement in both radial and axial directions. This means that each insert cuts a small section of the machined material providing an extra fine surface finish with a roughness of Ra0.1.
To ensure a high surface quality in milling relatively small faces, mainly bounded by shoulders, ISCAR has developed dedicated exchangeable solid carbide heads in a diameter range from 12 to 50mm for its MULTI-MASTER and T-FACE families (Figure 4). The heads are fully ground and provide high precision that facilitates a sharp cutting edge and enables a greater number of teeth when compared to mills with indexable inserts of the same diameter. In combination, these features guarantee high-performance finish milling.
A tendency to decrease machining allowance due to the active introduction of technologies for precise workpiece production and 3D printing makes the issue of obtaining a high surface finish by face milling particularly relevant. Can toolmakers find a simple and effective answer to the new needs of manufacturing? The near future will tell.
