Spade drill blade

The present invention relates generally to a spade drill blade and more particularly to a spade drill blade which is adapted to be clamped in the clamp slit of a holder and which has side surfaces that bear against the wall of the bore to be of a raised shape in such a way that during drilling it bears in extremely narrow lateral surface region in a diagonal plane against the inner surface of the bore, and the improvement, therefore, relates to the configuration of the blade with a very narrow contact area between the lateral surfaces of the cutting blade and the wall of the workpiece bore which reduces high surface friction and the risk of jamming or seizure and premature wear which prevents indexing to present the second cutting edge of the blade in the workpiece bore. The raised portion may be shaped in cross section, for example, as a ridge, trapezium, beading, lips or of other forms.

BACKGROUND OF THE INVENTION 
In general it is well known to provide spade drills in which a blade is 
clamped in a holder. Examples of this are shown, for example, in U.S. Pat. 
No. 3,076,357 and British Pat. No. 1,270,068. The blades that are used in 
this type of holder are generally provided with lateral edges that are 
cylindrically ground to provide a guide surface which engages the side of 
the hole being drilled and to prevent entire engagement these edges are 
sometimes relieved a certain amount as shown in U.S. Pat. No. 3,076,357. 
This, however, does not take care of all of the camming and heeling 
actions. 
SUMMARY OF THE INVENTION 
The present invention is directed to the basic concept that the lateral 
edges of a spade cutting blade be of a particular raised shape so that 
during drilling they bear against the bore of the hole being drilled in 
extremely narrow surface region. Further this narrow surface region should 
lie in a diagonal plane which extends approximately through the ends of 
the drilling edges. This reduces surface friction and thus reduces the 
risk of binding or jamming of the blade within the workpiece bore and 
premature wear which prevents indexing to present the second cutting edge 
of the blade in the workpiece bore. The lateral surfaces of the spade 
cutting blade may be of a number of shapes; for example, they can consist 
of a blade having a cylindrical curvature the radius of the curvature 
being less than the radius of the bit and the axis of the curvature must 
lie on a line parallel to the line joining the drilling edge ends of the 
bit. They may also be formed as a line contact with chamfered surfaces and 
a variety of other ways and in this fashion surface friction, a risk of 
jamming, binding, or seizing of the cutting blade in the workpiece is 
practically avoided, nevertheless the cutting blade is supported against 
the wall of the hole being drilled on a diagonal line to insure reliable 
guiding of the spade cutting blade in the hole.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings there is shown in FIGS. 1 and 6 a spade drill 
holder bar 10 which has at the end remote from the driving end a 
diametrical slot 21 (FIG. 6) cut therein. The bottom end of the slot 
defines a floor 22 and insert 30 is provided with a pair of protrusions 31 
which is adapted to be placed against the floor 22 and protrusions rest on 
either side of the ends of the floor so that lateral movement thereof in 
the slit is prevented. A double bladed spade drill blade 40 as shown in 
FIG. 2 is provided for association in the slot or slit 21 and this blade 
is formed with two opposite sets of cutting edges 41, 42, 43 and 44 
thereon. The clamping slit is provided with an extension slot 39 that 
extends from the bottom wall 22 to allow for greater bending moment 
between the legs and thus assure clamping of the drill bit blade within 
the slot. To retain the cutting blade in the slot 21, the head of clamp 
screw 45 in bore 24 bears against shelf 25 and threadingly engages bore 26 
to draw the two legs together. 
The spade drill blade 40 is especially suitable for use in tools shown in 
FIG. 1 and basically comprises a reversible bit for use in connection with 
the boring bar made in accordance with the disclosure in co-pending 
application Ser. No. 735,143 filed Oct. 26, 1976 of Holloway and Grunsky 
now U.S. Pat. No. 4,060,335. On one bit surface which, as shown in FIG. 2 
faces to the left in the direction of the arrow P, there are two main 
cutting edge portions 41 and 42, while there is an opposite set of cutting 
edges 43 and 44. Each of these cutting edges are formed with top rake 
grooves which are illustrated as ground without any angle parallel to the 
upper surface of the blade and with a 12.degree. top rake angle, for 
example, and with a relief surface as shown, for example, at 15a. Various 
rake angles may be used consistent with boring technology. Also as shown 
the blade may be provided with chip divider grooves if these are found 
desirable or necessary. The sides of the blade 51 and 52 are those 
portions which will guide the spade bit and which will engage the wall of 
the hole being drilled. These sides or lateral surfaces can take a variety 
of shapes as seen in FIG. 5 and are all characterized by having contact 
portions that engage the wall of the hole extending along a line 23 (see 
FIG. 3) which joins the corner points of the cutting edges which are 
designated 17, 18, 19 and 20. In like fashion the line 26 diagrammatically 
represents the side of the blade 40 and the contact points for this 
lateral surface with the wall of the bore which joins the corner points 17 
and 18. Preferably lines 23 and 26 are between 27.5.degree. to 47.degree. 
to the plane of the blade 40. In the embodiment shown in FIG. 2 and FIG. 
5A, the lateral edge 52 or side of the blade 40 is a curved surface 52a 
and if one were to extend the radius of curvature, it would intersect the 
rotational axis 24 of the spade drill blade and will, of course, extend 
along the line 23. In some cases it may be preferable to form the contact 
with two chamfered surfaces 55 and 56 as seen in FIG. 5 so that the line 
or edge 52b where these surfaces join will extend along the diagrammatic 
line 23 as seen in FIG. 3. Additionally, as seen in FIG. 5B, a compound 
type of surface providing a small cylindrical radius such as 52c may be 
provided which again will extend along the line 23 as seen in FIG. 3. 
It should be noticed that the characterizing feature of this particular 
spade drill blade resides in the fact that there is virtually only a 
contact between the cutter blade 40 and the wall of the bore which contact 
extends along the lines denoted as 23 and 26 in FIG. 3. The angle of lines 
23, 26 may extend on an angle to the plane of the cutter preferably less 
than 30.degree. and ideally approximately 27.5.degree.. As seen in FIG. 4, 
contact with the bore of the workpiece is only near the cutting edges 17, 
18, 19 and 20. During drilling only a low friction will therefore exist 
between the lateral surfaces or sides of the blade 40 and the wall of the 
workpiece bore. In this way the risk of jamming or seizing or binding, 
shattering of the cutter blade in the workpiece bore, is virtually 
eliminated. Nevertheless, the spade drill blade, after it has drilled into 
the workpiece to the rear end 20 of its side surfaces, can now effectively 
bear along the diagonals 23, 26 against the workpiece bore, so that the 
risk of lateral deflection of the drill blade in the workpiece bore is 
also avoided. It will be understood that the outermost end points 17 and 
20 are situated upon the radially outermost periphery of the blade so that 
the drilling diameter is determined by the drilling edges. 
It will also be noted that the main cutting edge portions such as 41, 42, 
43 and 44 which are illustrated in FIG. 2, will normally extend parallel 
to the plane of the blade. They may extend oblique to this plane so that 
their distance from the central plane of the cutter blade becomes less 
toward the middle of the cutter blade. The parallel orientation of the 
cutting edges, however, possesses the advantage in that the rake surfaces 
such as 13a likewise extend parallel to the plane of the blade and are 
more easy to grind. To provide a web thinning area at the point of the 
blade, a grind such as 27 may be formed into the rake surface. 
It will be seen that what is provided here is something that is relatively 
easy to re-grind, since there are no unusual angles in connection with the 
rake surface and the blade therefor lends itself to re-grind and 
sharpening since nothing more than the original cutting edges need to be 
re-sharpened. From the foregoing it can be seen that the present invention 
provides a new form of a spade drill 40 having unique configuration of the 
outer edges 51 and 52 thereof in which essentially two point contact along 
a line is had with the workpiece bore eliminating such effects such as 
shattering and the like.