Early carbide cutting tools were typically carbide blanks brazed to steel holders. The blank would be resharpened by grinding. Clearance angles, cutting point radii, and other features could also be ground into the tools to suit a particular cutting operation. Although these early carbide cutting tools provide significant productivity increases over previous tools, certain disadvantages became apparent. Regrinding to sharpen the dulled tool would change the size and shape of the cutting insert. This would require an adjustment of the relationship between the cutting tool and the workpiece each time the tool is sharpened to compensate for the smaller sized tool.
Additionally, the brazed connection between the carbide insert and the holder could only withstand a limited range of operating temperature, thereby reducing the number of potential applications for the carbide tool. Additionally, carbide inserts which with coated surfaces could not be resharpened and had to be replaced.
Presently, replaceable, indexable inserts are more widely used. Indexable inserts resemble brazed tools except that the carbide insert is secured in the holder by clamping rather than brazing. When a cutting edge dulls, the insert may be removed, rotated, and replaced to provide a new sharp edge for further cutting. This eliminated the need for readjustment of the cutting operation between changes. Typically, an indexable cutting insert is held in place by a screw with a tapered head that fits a conical hole in the insert and thus holds the insert securely in the tool holder. Clamps on top of the insert may also be used in conjunction with the screw to further secure the cutting insert to the tool holder.
Consistency and ease of replacement are the main advantages of indexable inserts. Consistency of positioning of the cutting edge from index to index simplifies machine tool setup and helps ensure a more uniform product.
There may be a number of problems, however, in using a round shape inserts in a tool holder for metal machining, in particular in milling operations. Typical problems for machining with round inserts include inconsistent insert life in machining operations due to an imprecise indexing mechanism compared to square or diamond shape inserts, insecure clamping and positioning of a round inserts since no straight edge is available for indexing, cutting edge chipping may also result from rotation of a round shape insert in the pocket of the cutting tool holder, as well as other reasons.
Several tool holders and round cutting inserts have incorporated antirotation mechanisms in an attempt to overcome these problems in machining metals with round shape inserts in tool holders. For example, U.S. Pat. No. 5,658,100, describes a mechanism to secure the round shape insert in a pocket of a milling cutter wherein a plurality of depressions are built on the top surface of the round shape insert. The insert is then secured in the tool holder by a clamping claw supported on the milling cutter body, to prevent the rotation of a round shape insert in the pocket of a milling cutter. However, since the clamping claw described in U.S. Pat. No. 5,658,100 engages the top face of the cutting insert, the chip control geometry must perform the duel function of preventing rotation and to controlling the chip formation of the insert in operation.
In U.S. Pat. No. 6,053,671, a round shape insert is manufactured in a way that the edge surface of the insert consists of two portions separated by a step. The upper portion is a conical frustum which functions as a normal round cutting surface and the lower portion is in the shape of a polygon which creates an interface contact with the lower wall of the pocket, thus preventing the rotation of a round shape insert against the pocket.
In U.S. Pat. No. 5,346,336, another cutting insert comprising a polygonal pattern of facets is described. The polygonal pattern is located around the circular side surface of a round shape insert and then a complementary shaped polygonal pattern of the pocket is provided. The polygonal facet on a round shape insert interfaces with the complementary polygonal wall on the corresponding pocket of a milling cutter body, thus preventing the rotation of a round shape insert in the pocket.
U.S. Pat. Nos. 6,053,671 and 5,346,336 are basically similar in design, that is, the polygonal-shaped facets on a round shape insert interacts with the complementarily-shaped polygonal wall in the receiving pocket on a milling cutter body. The edges of the full radius formed by the polygonal pattern on the hard carbide material insert may slowly cut the relatively soft steel material of the pocket pin during the machining operations. In addition, the limited contact provided by the polygonal pattern-based design of these inserts may result in one-point contact between the edges of the insert facets and the flat wall of the pocket, which can result in localized chipping or cracking of the insert over time.
In U.S. Pat. No. 6,238,133, a insert and tool holder are described wherein a plurality of curved and obliquely 3-dimensional surfaces are built around the side wall of a round shape insert which interfaces with a substantially complementary curved surface, also 3-dimensional, in the insert-receiving pocket of a milling cutter body, therefore providing an anti-rotation mechanism for a round shape insert in the pocket on a milling cutter.
U.S. Pat. No. 6,238,133 provides a mechanism that uses curved stop surfaces around the side wall of the round shape insert and at least one complementary anti-rotation curved surface in the pocket of a cutter body. Both curved surfaces are defined by partial radius curves such that the interface can be in a form of either a broad/line-type contact or double convex, lenticular, contact. This is largely because there are no sharp edges on the insert to weaken the contact interface between the hard carbide insert and the relatively soft pocket on a steel cutter body.
There are several disadvantages and limitations in the mechanism described in U.S. Pat. No. 6,238,133, these disadvantages are primarily due to the complexity of the curved and oblique 3-dimensional surfaces in the pocket. Specifically, first, the complementarily-shaped oblique geometry can require complex machining procedures in the manufacturing of the insert and the corresponding pocket to ensure the necessary geometric and dimensional accuracy required to keep the desired functionality at the contact interface between the insert and the pocket. Second, the curved and oblique 3-dimensional surface limits the manufacturing methods for the pocket to an axial machining method along the pocket axis. Third, as a result of the manufacturing limitations of the complex geometry of the tool holder, it further limits number of pockets, thus the number of teeth, in a milling cutter.
The number of teeth on a milling cutter is an important factor affecting the cutting efficiency of the milling operation. A milling cutter should have enough teeth to ensure uninterrupted contact with the work piece. If the milling cutter has too few teeth, one tooth will lose contact with the workpiece prior to the next tooth is engaging the workplace. This will lead to vibration and chatter resulting in poor finishing, dimensional inaccuracies, and excessive tool wear. The quality of the surface finish is typically better using milling cutters with more teeth.
Although all the above-mentioned mechanisms provide feasible means of preventing undesired rotation of a round shape insert in the corresponding pocket on a milling cutter body, the mechanisms described all have some shortcomings.
Therefore, there is a need for an indexable cutting insert and tool holder that effectively functions to prevent rotation of the round shape insert in an insert pocket, but also are easy to manufacture and provides the necessary accuracy for stable and secure positioning. There is also a need for an indexable cutting insert, for example, a cutting insert made from tungsten based carbide or cermet and tool holder combination that allows the possibility to increase number of teeth on a cutter body.