Broach and process of manufacturing a broach

A broach has replaceable cutting inserts. The inserts are supported to have equal hook and rake angles of about 11.degree. to 14.degree.. A method of producing the broach in a simplified milling setup is described.

FIELD OF THE INVENTION 
The invention relates to a broach which is designed for roughing blade root 
forms in turbine discs and similar types of broach slotting operations. 
BACKGROUND OF THE INVENTION 
High speed steel broaches are a common cutting tool in many types of metal 
working applications. 
The application of pre-sharpened disposable cutting inserts to broaches in 
general has been very limited. In the case of roughing the blade root 
forms in turbine wheel discs, it has been virtually non-existent. This 
invention is intended to address the application of pre-sharpened 
disposable cutting insert broaches to turbine wheel discs and other uses. 
It is further the intent of the invention to accomplish this in a most 
economical way, through initial low cost of the broach and its improved 
operating efficiency, by virtue of much lower machining times and lowered 
tool maintenance costs. 
Standard positive rake indexable inserts usually allow only 
5.degree.-8.degree. rakes. However, somewhat higher positive rakes are 
much more suited to the special metals used in turbine discs. 
Few broaching operations require the deep slotting associated with turbine 
discs. This deep slotting necessitates a series of many broach holders, 
making the total broach assembly extremely long with lengths occasionally 
reaching 100 feet or more. This extreme length does not allow the luxury 
of providing one series of inserts for cutting the slot bottom and a 
separate series for the slot walls, as is the case with crank shaft pin 
diameter broaching. (Reference U.S. Pat. No. 4,740,115) 
BRIEF DESCRIPTION OF THE INVENTION 
The above problems are solved in the present invention by using inserts 
with staggered tooth configuration using about 11.degree.-14.degree. 
positive side rakes, and about 11.degree.-14.degree. hook angles. It is 
also preferred that the inserts have approximately 2.degree. to 
approximately 31/2.degree. inward taper, with each pair of right-hand and 
left-hand pre-sharpened disposable inserts cutting both the slot bottom 
and side wall of the part with a very free cutting action. 
A particular benefit of the staggered side rake configuration is that the 
length of the chip is distinctly shorter than the width of the slot being 
broached. This makes the chip more manageable. The higher positive side 
rake also tends to steer this shorter chip toward the center of the broach 
and away from the sidewall of the cut. 
The very free cutting action consequent to the higher positive rakes and 
inward taper result in a very prolonged edge life for still further 
operating economy. 
Extraordinary economy and accuracy in the manufacture of the broach holder 
is achieved by virtue of the insert's side taper. 
The usual inserts have a width that differs from their height.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1, 2 and 3, a broach assembly comprises one or more 
holders 1 with a cut direction of arrow E. Each holder 1 has a series of 
staggered right-hand and left-hand stations 2 and 3 respectively, cutting 
both the bottom 4 and side walls 5 and 6 of the work piece 7 (see FIG. 3). 
Each right-hand station 2 contains a disposable pre-sharpened insert 8, a 
wedge 9 and a holding screw 10. Each left-hand station 3 contains a 
similar disposable pre-sharpened insert 11, a wedge 9, and a holding screw 
10. The stations typically are uniformly staggered on a pitch 12, between 
5/8 inch and 2 inches. A right-hand side rake 13 is between 
11.degree.-14.degree. and a left-hand side rake 14, is preferably at the 
same angle as its right-hand counterpart. 
Referring to FIG. 2, a positive hook angle 15 is shown as between 
11.degree.-14.degree.. 
Referring now to FIG. 4, the holder 1, during machining, is preferably 
tilted by the same amount as is the hook angle 15 so that the hook plane 
18 is then oriented in the vertical plane. This provides for simplified 
milling and permits all inserts to be milled in a single set up with a 
milling cutter having a milling face parallel to the milling cutter axis. 
As can be seen by reference to a station side view in FIG. 4 and to a 
station plan view in FIG. 7 it is relatively simple to mill all the 
right-hand and all the left-hand stations efficiently and with enhanced 
accuracy, in just one fixturing. 
Another feature of the invention permits simplified station machining for 
the centerline 19 of the threaded hole 20 which is vertical in FIG. 4. 
When changing insert 8, a gap must be achieved between the face 18 of the 
insert 8 and the face 21 (see FIGS. 4 and 12) of the wedge 9. This is 
accomplished by virtue of the 0.005 inch approximate difference 22 between 
offset 24 which defines the centerline of the screw hole in wedge 9 and 
offset which defines the center position of the centerline 19 of threaded 
hole 20. 
This provides simplicity and economy in the holder's manufacturing process, 
because of the unique relationship wherein the centerline 19 of the screw 
in the present invention is parallel with the clamped surface 18 of the 
insert. In the prior art, (see FIG. 6) this centerline is canted relative 
to the clamped surface of the insert, the canting usually being the same 
as the wedge taper angle. This feature of the present invention becomes 
practical (as shown in FIG. 4) by virtue of the threaded hole 20 in the 
holder being on a slightly greater offset 26 relative to the insert face, 
than is the offset 24 to the threaded hole in the wedge 9. This difference 
in offset predisposes the wedge face 9A to back away from the insert face 
21 when the wedge is loosened, allowing the insert 8 to be readily 
replaced. By using slightly larger than normal thread diameters in both 
the wedge and holder, the screw is allowed to assume a modest 
catercornered position, thus maintaining proper function, when tightened 
in place. 
For maximum economy in the broach holder manufacture, the insert pocket is 
oriented around the hook angle. This orientation, coupled with the side 
rake, would project the insert's trailing end 27 above the cut line of the 
broach (see FIG. 9). By adding a correcting side taper 28 on the insert, 
this projection ("walkout") is eliminated thus allowing the broach to 
produce a straight cut. Further by adding this same angle to the adjacent 
side, an inward taper 36 is created up the side of the broach (see FIG. 
10). 
Referring to FIG. 8 the holder 1 is single plane tilted by the same amount 
as the hook angle 15 so that the hook plane 18 is then oriented in the 
vertical plane for simplified machining. When the station is machined in 
this plane, a conventional square or parallelogram insert 8 will 
"walk-out" from the cut plane 4 as depicted by line 27 in both FIG. 8 and 
FIG. 9. When the holder is moved during cutting in a direction parallel to 
that shown in E (FIG. 1). This "walk-out" angle 28, FIG. 9, ranges between 
approximately 2.degree. for an 11.degree. hook and an 11.degree. side rake 
to approximate 31/2.degree. for a 14.degree. hook and 14.degree. side 
rake. The methods required to correct for this "walk-out" in the holder 
can be very complex, time consuming and costly. Referring to FIG. 12 in 
this invention a correcting side taper angle 29 equal to this "walk-out" 
angle is added to the insert 8, cutting edges 30 and 31. Thus the cutting 
edge 30 stays in the straight cutting plane 4 and retains a flat bottom in 
the slot being broached With this method of fixturing combined with the 
"walk-out" correcting angle 29 and vertically threaded hole 20 FIG. 4, it 
then becomes possible to machine all the stations in a holder in one 
fixturing thus achieving both very low machine times and very close 
tolerances. 
To further enhance chip control, in this invention, the left-hand station 
is at the same height as its right-hand station mate, in lieu of the 
conventional practice of the rise per tooth being evenly divided between 
teeth. This is of particular value when the insert width is substantially 
greater than one half of the broach cut width. A uniform rise per tooth 
would, in the overlap area cause the chip to be one-half the thickness of 
the chip in the non-overlap area. This differential would length, 
resulting in a loss of directional control over the chip. 
Referring to FIGS. 10, 15, and 18, the side taper 29 FIGS. 15 and 18 is 
incorporated on side 31 of FIGS. 15 and 18 as well as side 30 so as to 
provide an inward taper 36 FIG. 10 from 2.degree.-31/2 from the side walls 
5 and 6 FIG. 3 of the cut. 
Referring to FIGS. 2 and 10, the right-hand tooth 8 and the left-hand tooth 
11 of a pair cutting the same height 16R and 16L, where the rise 17 occurs 
between each pair of teeth and not from tooth to tooth as is the 
conventional fashion as shown in FIG. 11. In the referenced conventional 
arrangement in FIG. 11, the rise 32 is split in half 33 by locating the 
left-hand tooth midway between the two adjacent right-hand teeth. Thus the 
chip thickness through the overlap area 34 is half the thickness of the 
non-overlap areas making the chip less manageable. In this invention as 
shown in FIG. 10 where each pair of teeth are at the same height 16R and 
16L, the lead tooth 8 cuts an essentially uniform chip its full width with 
the following tooth 11 cutting the remainder of the cut width. Hence, this 
invention cuts just two manageable chips in the cut width 35 with 
essentially uniform chip thickness. 
Referring to FIG. 12, the effective cut length 37 of the disposable insert 
in this invention is between 50% and 90% of the broach cut width. 
Reference FIG. 15 cut length 38, FIG. 18 cut length 39 and 40 FIG. 10. 
Further, referring to FIG. 10 the chip formed in length 40 is not more than 
90% of the broach cut width. 
In this invention, where the width of cut allows it, the pre-sharpened 
disposable insert height and width are preferably equal, thus allowing the 
insert to be uni-handed and usable in both right-hand and left-hand 
stations. For cut widths too narrow or too wide to allow use of the 
uni-handed insert, both right-hand and left-hand inserts are required. 
In a most preferred form of the invention, the hook angle and rake angle 
are both 12.degree.. The inserts are preferably tungsten carbide.