Modified tool holder to prevent insert slippage and fracture

A cutting tool holding system is disclosed. The system employs an improved chip breaker element having a lip mechanically gripping a radially extending and exposed edge of the cutting tool insert; the breaker element is held against relative sliding movement by a pin connection to a clamp which laterally urges the element and tool against the shank. Thus, the cutting insert is locked in place within the tool shank pocket avoiding premature failure from undue stress during prolonged tool cutting. The breaker element also has a chip breaker surface comprised of an arcuate segment spaced uniformly from the insert cutting point and a straight segment extending from the arcuate segment along the exposed edge of the insert.

BACKGROUND OF THE INVENTION 
Hot pressed silicon nitride containing at least a primary addition of 
Y.sub.2 O.sub.3, as a pressing aid has shown unusually superior results as 
a material useful in the cutting of cast iron. During the hot pressing, a 
crystalline second phase consisting of one or more of the three known 
types of yttrium silicon oxynitride is developed. 
When cutting cast iron, a relatively large mass removal rate is experienced 
during machine cutting. It has been typical for the materials used earlier 
by the prior art, such as tungsten carbide and aluminum oxide to fail by 
thermal cracking. However, with hot pressed silicon nitride materials, the 
cutting tool can be utilized for continuous or interrupted cutting for 
periods in excess of 1-15 minutes and longer, whereas it is typical for a 
prior art tool to be utilized for 1/10 of such time. Accordingly, this new 
material is experiencing dramatic increases in tool life. 
In spite of such increased longevity for the material from a metallurgical 
and wear resistant standpoint, the use of the tool has experienced one 
significant problem. This problem is connected with the manner in which 
the tool is mounted for cutting. It is typical for a metal shank to 
contain a pocket for receiving the cutting insert or tool. This insert is 
clamped in the pocket between the shank on one side and a chip breaker 
element on the other side. A chip breaker typically operates as a wedge in 
deflecting the cut chip so that they will break off from the workpiece and 
thereby be separated. The pocket in the tool holder is typically 
triangularly shaped, with the cutting insert projecting beyond the end of 
the shank, out of the pocket so that one point of the triangular insert is 
outside the sides of the shank to be free to cut; one end of the insert 
extends from such point in a direction generally aligned with the surface 
being cut. The cutting insert is held in place by frictional clamp; no 
holes are provided in the insert because of its ceramic nature which is 
notch sensitive. When the tool is withdrawn after each cycle of cutting, 
there may be slight frictional drag on the insert as it is pulled or 
dragged from the location of cutting for a repeat run. The drag tends to 
promote a pull on the insert causing it to pull slightly out of the 
pocket, allowing for debris and cut chips to become dislodged behind the 
insert; this eventually leads to undue forces on the insert as a result of 
an exaggerated overhang from the shank. The forces from the overhang 
position eventually result in premature fracture and failure of the tool 
insert with undesirable service life. This dilema of having a material 
which has an extremely long wear resistance and service life, but which 
can be mounted properly so that its service life is not prematurely cut 
short by unbalanced forces during machining, is a significant problem. 
SUMMARY OF THE INVENTION 
A primary object of this invention is to provide an improved tool holding 
system particularly suitable for use with increased-life type of ceramic 
cutting tools, which holding system prevents premature fracture failure in 
the cutting tool over extended use. 
Another object of this invention is to provide a tool holding system which 
promotes uniform seating forces on the tool cutting insert throughout its 
entire cutting life. 
Yet another object of this invention is to provide an improved method for 
cutting cast iron with silicon nitride, the method eliminating premature 
fracture failure resulting from unbalanced cutting forces and which method 
retains the integrity of the cutting tool even after long usage so that it 
may be ground for subsequent reuse. 
Features pursuant to the above objects comprise: 
(a) the definition of a chip breaker element so that it has a lip gripping 
mechanically the exposed edge of the cutting tool insert to provide a 
mechanical lock of the cutting tool insert within the holding pocket of 
the holder shank; 
(b) unique configuration of the chip breaking element so that it has a 
working surface comprised of an arcuate segment spaced uniformly from the 
cutting point, and a straight segment extending from the arcuate segment 
along the exposed surface of the cutting tool insert which is directed 
generally parallel to the surface being cut; 
(c) constituting the cutting tool insert of hot-pressed silicon nitride 
with pressing agents Y.sub.2 O.sub.3 and/or Al.sub.2 O.sub.3.

DETAILED DESCRIPTION OF THE DRAWINGS 
High metal removal rates and increased productivity can be achieved with 
relatively new ceramic tool materials based in silicon nitride. In order 
to fully utilize the benefits of this tool material, it is necessary to 
modify the tool holder assembly currently used in production. Instead of 
drilling openings through the cutting tool insert itself so that the 
insert may be clamped and bolted in place, the silicon nitride based 
materials must be clamped in place by frictional force; any type of 
interruption of the hot-pressed material by machining of internal openings 
would lead to premature fracture and weakening of the tool insert. Silicon 
nitride ceramics have a relatively low transverse rupture strength usually 
in the range of 70-110 ksi, and have other physical characteristics such 
as low resistance to crack propogation and notch sensitivity which could 
result in premature brittle fracture and hence internal machining of such 
inserts should not be carried out. In addition, such internal machining is 
very expensive and also induces micro-cracks which promote fracture. 
Hot pressed silicon nitride material typically is prepared by mixing a 
first powder consisting substantially of alpha base silicon nitride having 
a cation impurity content of no greater than 1%, excluding free silicon. 
The first powder is well milled to a particle size of 1.5 microns. A 
second powder is prepared consisting of Y.sub.2 O.sub.3 and a third powder 
consisting of aluminum oxide. Approximately 7-8 weight percent of the 
second powder and about 2.5 weight percent of the third powder is mixed 
with the first powder, and then placed in a compacting die where it is 
heated to approximately 1700.degree. C. with about a 5000 psi load 
applied. The load is applied at room temperature before the die is brought 
up to temperature and the load is continued for 2 hours thereafter. The 
resulting material provides exceptional tool life when cutting cast iron; 
x-ray analysis shows the presence of crystallized yttrium silicon 
oxynitride compounds. The resulting hot-pressed product of course is 
shaped as a cutting tool (according to conventional cutting tool 
configurations) by a diamond cutting or other equivalent means. The 
product will have a density of 95% or more theoretical, a thermal-shock 
parameter at 1200.degree. C. of at least (110.times.10.sup.9 
BTU-lbs.)/Hour(in..sup.3), a hardness value of at least 87 Rockwell 45-N, 
and a bend strength of at least 70,000 psi at 1200.degree. C. 
Turning now to FIG. 1, a typical prior art modification to hold the new 
material in place, is shown. A cylindrical work piece 10 is shown as being 
machined by a cutting tool insert 13 held in a tool holder 11. The insert 
13 is a flat piece, triangularly shaped, adapted to fit within a truncated 
triangular recess 12 or pocket within the holder 11. The recess is 
arranged so that it intersects with one side and the end of the holder 11. 
The insert is held in place by clamp 15 which has one end 15a bolted to 
the holder 11 by conventional bolting assembly 16. The other end 15b of 
the clamp presses inwardly to impart a clamping force to the insert 10 via 
a chip breaker element 14. The chip breaker element 14 is interposed 
between the clamp and the insert and has direct frictional engagement with 
the cutting tool. Thus the insert is frictionally gripped by the side of 
pocket 12 and element 14. 
Two kinds of machines exist for moving the tool holder, one in which a 
slide positions the tool holder to the corrent dimension and feeds it 
vertically downward producing the required surface. The tool holder is 
withdrawn away from the surface, before lifting it upward to the initial 
position. This machine is undesirable because of cost and complexity. 
In a second kind of machine the tool holder remains in one position 
corresponding to the size of the part and is fed vertically downward for 
cutting and is withdrawn vertically upward to its original position. This 
kind of machine is easier to build since it has a less number of movements 
for the tool holder and for the positioning slide. However, it has one 
significant disadvantage. While cutting, the tool tip or insert is under 
considerable pressure. The cutting forces deflect the tool holder assembly 
away from the workpiece. At the end of a cutting cycle the tool holder 
assembly returns to its original position. In the withdrawn position, it 
produces a helical groove on the workpiece. In addition, this drag causes 
the cutting tool insert to be pushed out of the triangular shaped pocket, 
as shown in FIG. 3, leaving a space 17. This sequence, occurring 
repeatedly over a number of cycles, will result in considerable overhang 
18 (increased over the original overhang 19) of the tool out of the tool 
holder and result in unbalanced forces producing fracture and catastrophic 
failure. The extent of the drag force will depend on the initial 
deflection of the holder assembly, which in turn depends upon the cutting 
forces. Ceramic cutting tool inserts in general use a chamfer 20 along the 
point 21, which results in increased cutting forces rather than a 
nominally sharp edge. These forces also increase with the wear of the 
insert. 
Unlike other cutting tools, the silicon nitride based ceramic materials 
perform successfully over a considerably longer period of time even when 
wear begins to take place in the insert. Thus, higher cutting forces can 
be expected during the useful life of such ceramic inserts. The "drag" and 
pull-out of the insert is more likely to occur with these ceramic inserts. 
This circumstance can be a serious matter in that it dissipates the 
longevity characteristics of such ceramic insert and reduces them to 
regular premature failure. 
In order to prevent such premature mechanical failure, the present 
invention employs a modified chip breaker element 22 (see FIGS. 4-5) which 
is placed along side of the tool insert; it is arranged to provide uniform 
clamping pressure of the insert 13. 
The chip breaker element has a lip 23 which extends around and grips the 
lower edge 24 of the triangular shaped insert. In this manner, the chip 
breaker element mechanically and positively holds the cutting tool insert 
in place during the return stroke, retaining it tightly in the recess 
pocket against walls 25 and 26 during the entire life of the insert. At no 
time is the insert allowed to move away from the internal sides 25-26 of 
the pocket as a result of drag forces or unbalanced cutting forces. The 
element 22 is fixed against relative sliding movement with the clamp 15 by 
a pin 30 fitting tightly in aligned openings in said clamp and element. 
The modified chip breaker element also has a chip breaking surface 27 which 
is comprised of an arcuate segment 27a spaced a uniform distance 28 from 
the cutting point 21. A straight segment 27b extends from the arcuate 
segment 27a along the exposed edge 29 of the cutting tool insert (and 
extends generally parallel to the workpiece surface being cut.) 
The improved tool holder system of the present invention provides: 
(a) securement for the cutting tool insert to eliminate pull-out resulting 
from drag of the insert on the workpiece, and does not require holes in 
the cutting tool insert itself for securement, 
(b) improved productivity and reduced down time resulting from the use of 
such modified holding system, 
(c) catastrophic failure of the ceramic insert is eliminated and therefore 
the insert after being worn down an incremental amount during its normal 
service life can be fround or reused through salvage operations, 
(d) the modified chip breaker element will result in better chip breaking 
and the elimination of overhang by the insert.