Brazed rotary cutting tools include, but are not limited to: endmills, router bits, shapers, drills, reamers, counterbores, countersinks, and taps. In designing and using such cutting tools, significant attention is paid to both the wear resistance and to the dimensional precision of a tool. As a result, significant efforts are made to improve those characteristics, while constraining any cost increases such improvements might have upon the construction price of the cutting tools.
Wear resistance is typically affected by the material that forms the cutting surface of the tool, the geometry of the cutter, and the material being cut. Cutter wear is typically measured by monitoring specific changes in the cutting edge of the tool. As the tool experiences wear, the cutting edge will begin to break down and exhibit a loss of cutting edge material. Such loss may be exhibited by the formation of a wear radius on the cutting edge and/or by the loss of larger chunks of cutting edge material through chipping or other destructive means. Over time, material suppliers have developed new or improved materials which reduce the rate of cutter wear. Manufacturers have dramatically reduced wear in many applications by moving from high-carbon steel, to high-speed steel, to carbide, and finally to diamond products such as polycrystalline diamond (“PCD”). However, this increased wear resistance comes at the price of increased material costs and manufacturing difficulties.
Diamond has proven to be an extremely wear resistant tool material, especially for machining advanced, but abrasive, composite materials such as carbon fiber and aramids. However, unlike steel and carbide, the high cost and difficult working characteristics of diamond prevent its use as a both a structural body material and a cutting edge material. Instead, diamond tools are typically of a two piece design in which one material acts as the tool body and a segment of PCD is situated only at the cutting edge. While this is the common structure of PCD cutting tools, manufacturers still find it difficult to attach diamond directly to the tool body material and to thereafter shape it to the desired cutting edge profile.
To solve the attachment problem, manufacturers have resorted to a number of construction methods. In one exemplary method, manufacturers fix thin slabs of diamond to thicker carbide disks, where the carbide serves as a backer material supporting the brittle diamond structure. Controlled atmosphere brazing is one common method of fixing the diamond to the carbide slug. Another common method is the pressing and then sintering of a carbide and diamond composite compact.
In FIG. 1, a commercially available PCD and carbide disk 100 is shown. The disk consists of a wear layer 101 of PCD typically ranging in thickness from 0.005″ to 0.125″, and most typically from 0.010″ to 0.040″. It is known by those skilled in the art that the wear layer 101 may instead consist of a layer of polycrystalline cubic boron nitride (“PCBN”). The PCD layer 101 is permanently bonded to a carbide layer 102, where the carbide consists primarily of tungsten carbide and various other compounds which are used as binders or to impart specific properties such as toughness or hardness to the carbide layer.
Using the PCD and carbide disk 100 as raw material stock, an insert manufacturer will typically cut or grind specifically shaped cutting tool inserts from the disk 101. Most often, the insert manufacturer will use electrical discharge machining (“EDM”) to cut the insert from the disk 100. For purposes of illustration, a representation of a portion of an EDM wire 103 is shown passing through the disk 100 at an angle 104.
FIG. 2 is a top view of the PCD and carbide disk 100 showing EDM cut paths for an exemplary insert shape. The EDM wire 103 may be used to form a flat faced insert illustrated by the EDM cut pattern 201. One skilled in the art will understand that other methods, such as grinding, may also be used to form an insert from the disk 100.
FIG. 3 shows a perspective view of an exemplary flat face insert 300 formed from the PCD/carbide disk 100. The insert 300 is cut in such a manner that a straight cutting edge 301 is formed at the interface of the flat rake face 303 and the flat relief surface 304. The EDM cut path results in a straight bottom edge 302 at the base of the rake face 303. Typically, a portion of the insert 305 may be removed from the back bottom corner of the insert. This portion 305 is removed by any practical material removal process, such as grinding or EDM, and the resulting angle 306 is typically formed at substantially 90 degrees in order to facilitate later mounting of the insert 300 in a fluted tool body.
Angle 307 of the insert is known as the included angle and is defined herein as the angle formed substantially at the cutting edge 301 and between the rake face 303 and the relief surface 304. The included angle 307 may typically be less than 90 degrees. The benefit of having an included angle 307 of less than 90 degrees is that the insert may be mounted to a tool body and no further material must be removed to create an appropriately shaped relief surface 304 behind the cutting edge. FIGS. 7 and 8, discussed later in relation to aspects of the invention, illustrate the location of the relief surface in more detail.
Typically, the cut pattern 201 in the disk 100 is oriented such that wear layer 101 forms the relief surface 304. This is done so that a wear resistant cutting edge 301 is produced with minimal use of the expensive wear layer material. In an alternative configuration, where the wear layer 101 is oriented to form the rake face (configuration not shown), the cutting edge may still be wear layer material; however, more wear layer material is required to cover the large rake face and, consequently, the insert is more expensive to produce. In either configuration, a significant portion of either or both the rake face and the relief surface consist of wear layer material.
Once the insert 300 is produced, one or more of the inserts 300 are mounted to a cutting tool body in order to create a semi-finished tool. One carbide surface of the insert 300 is typically oriented towards the mounting interface with the tool body, as shown with reference to FIGS. 4A and 4B. Brazing is the common method of fixing the insert to the tool body. The semi-finished tool is then subject to some form of material removal process, such as grinding or electrical discharge machining (“EDM”), where the cutting edge is brought to the desired condition, shape, and dimensions.
A problem with this method for producing PCD (or PCBN) edged cutting tools is that carbide and diamond inserts warp during the brazing process that fixes them to the tool body. PCD and carbide typically exhibit different coefficients of thermal expansion. During the brazing process, the high temperature used to melt the silver solder or other brazing material causes the PCD and carbide sections of the insert to expand at different rates. As a result, the insert warps in one or more dimensions. This warpage causes numerous problems. First, the warped surfaces will change the dimensional characteristics of the assembled tool, thereby necessitating allowances for additional grinding stock to compensate for high and low spots on the rake face and relief surface. Second, warpage can result in the insert rocking against the bottom mounting surface of a fluted tool body during or before brazing, resulting in difficulty maintaining alignment of the insert to the tool. Third, PCD is extremely difficult to machine. Significant cost is attributable to the removal of excess grinding stock from the assembled tool by traditional EDM or tool grinding methods.
To illustrate the problems cause by braze-induced warping of the insert, FIG. 4A shows an endmill type rotary cutting tool 400 with exemplary flat faced insert 300 mounted in a cutting tool body 406. Normally, the insert 300 is seated such that its flat bottom edge 302 is in contact with the bottom of the flute, thus helping to align the cutting edge 301 to within some desired level of concentricity, taper, and/or runout to the cutting tool 400 and/or to other cutting edges present on the cutting tool.
As previously stated, the common means for attaching insert 300 to the tool body 406 is by brazing. During brazing, flux and silver solder or other appropriate materials are placed between the insert 300 and the tool body 406. The assembly is then heated to a high temperature using a torch or induction heating methods. The assembly is heated to the point that the silver solder melts and bonds the insert 300 to the tool body 406. A known problem is that the heating of the insert 300 can cause it to permanently warp due to the different coefficients of thermal expansion between the insert's 300 wear layer and carbide layer.
Two specific problems of insert warping are illustrated in FIG. 4B. First, during the heating and cooling associated with brazing the insert 300 to the tool body 406, the PCD and carbide expand at different rates and the insert 300 becomes deformed, as shown by the post-braze insert 409. As a result of the insert warpage, the cutting edge 401 may exhibit a curved shape. The cutting edge corners, such as corner 411, may circumscribe a larger cutting edge diameter than the diameter circumscribed by the middle of the cutting edge 412.
The amount of warp, approximated as dimension 404 in this view, is measured as the diametric radial distance between the high corner 411 and the low center point 412. Diametric radial distance is defined herein as the radial distance between the circles circumscribed by two points, where the circles are approximately concentric to the axis of rotation of the tool. Though the amount of cutting edge warp 404 depends on numerous factors, including, but not limited to, the composition of the PCD layer, the composition of the carbide layer, the heating method, and the length of the insert, the amount of warp 404 is generally predictable, repeatable, and within a narrow band of variance for most commercially available carbide and PCD disk 100 compositions and insert sizes.
Braze induced insert warpage of a flat face insert creates a cutting edge profile that may exhibit substantial problems with concentricity, taper, and/or runout due to the movement of various portions of the post-braze cutting edge 401 from the straightness of the pre-braze cutting edge 301. Similar problems exist for shape face inserts. As a result, the cutting edge tolerances of the finished tool must either be loosened to accommodate these warp effects or, more commonly, additional material removal operations must be performed on the cutting edge 401 after the inserts are brazed to the tool. These additional finishing operations are undesirable because current material removal processes are not well suited for the work. Grinding operations using diamond superabrasive wheels are slow when used for grinding hard polycrystalline diamond cutting edges and EDM machines are also slow when performing finishing work on rotary cutting tools.
A second problem associated with braze-induced insert warpage is illustrated by the insert's warped bottom edge 402. This edge, along with the entire bottom insert surface, warps similarly to the cutting edge, though the amount of bottom edge warp 405 may be greater or less than the amount of cutting edge warp 404. This bottom warp can result in a single point of contact 408 at the interface between the bottom of the flute 407 and the bottom edge of the insert 402. Compared to a normally flat interface, such as between the pre-braze bottom edge 302 and the bottom of flute 407 in FIG. 4A, this single point of contact 408 allows the insert 409 to shift or rock back and forth within the flute 407 during brazing. Consequently, it is difficult to maintain accurate positioning of the insert 409 relative to the tool body 406. This further contributes to problems of concentricity, taper, and/or runout and again requires looser tolerances or additional material removal operations after brazing.