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Handbook on Twist Drills and Drill Point Geometry by Jeff Toycen

  • Toycen Book on Drills

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Handbook on Twist Drills written by Jeff Toycen read more
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Table of Contents
Chapter One History of the Twist Drill …………………………………. 1
Chapter Two Types of Drills ………………………………………………… 3
Twist Drills …………………………………………………. 4
Brad Point …………………………………………………… 5
Brazed Carbide…………………………………………….. 6
Solid Carbide Drills ………………………………………. 7
Chapter Three Coolant Through Drills ……………………………………. 9
Chapter Four Glossary of Terms …………………………………………. 11
Chapter Five Drill Anatomy ……………………………………………….. 17
Chapter Six Common Drill Tip Geometry ………………………….. 21
Chapter Seven About Twist Drills …………………………………………. 23
Twist Drills ……………………………………………….. 25
Multi-Facet ……………………………………………….. 27
Split Point …………………………………………………. 28
S-Shaped Chisel Edge …………………………………. 30
Dubbed Drills …………………………………………….. 30
Avoid Unequal Lands ………………………………….. 32
Lip Cutting Edge Clearance ………………………….. 34
Productivity ………………………………………………. 35
Chapter Eight Tips and Chips ………………………………………………. 37
Basic Chip Profiles from Common Drill Tips …… 38
Chapter Nine Drill Sharpening ……………………………………………. 41
Worn Portions ……………………………………………. 42
Regrinding the Point ……………………………………. 43
Web Thinning ……………………………………………. 44
More on S-shaped chisel edge……………………….. 50
Chapter Ten Useful Tables and Charts ……………………………….. 51
Chapter Eleven Cutting Tap Drill Chart …………………………………. 53
Chapter Twelve Cold Form Tap Drill Charts …………………………… 55
Chapter Thirteen Decimal Drill Conversion Chart …………………….. 59
Chapter Fourteen Other Useful Conversion Formulas …………………. 61
Chapter Fifteen Tool Steel Materials ……………………………………….. 63
Chapter Sixteen Notes ……………………………………………………………. 65
About the Author …………………………………………. 74

Preface
This is the second in our series of shop reference books about
cutting tools. This handbook provides general information
about two flute drills. Most commonly associated with
producing machined holes, drilling process is one of the fastest
material removal opportunities in the machine shop.
Many other processes contribute to the production of holes,
including boring, reaming, broaching, and internal grinding,
however drilling accounts for the majority of holes produced in
most shops. This is because drilling is the most simple, quick,
and economical method of hole production. The other methods
are used principally for more accurate, smoother, larger holes.
They are often used after a drill has already made the pilot
hole. Drilling is one of the most complex machining processes.
The chief characteristic that distinguishes it from other
machining operations is the combined cutting and extrusion of
metal at the chisel edge in the center of the drill. The high
thrust force caused by the feeding motion first extrudes metal
under the chisel edge with a conventional helical tip profile.
Then the Material tends to shear under the action of a negative
rake angle tool. Center Cutting tip profiles discussed later in
the book greatly increase the efficiency of the drilling
operation by shearing the material at the center of the tool.
The cutting action along the lips of the drill is not unlike that in
other machining processes. Due to variable rake angle and
inclination, however, there are differences in the cutting action
at various radii on the cutting edges. This is complicated by the
constraint of the whole chip on the chip flow at any single
point along the lip. Still, the metal removing action is true
cutting, and the problems of variable geometry and constraint
are present, but because it is such a small portion of the total
drilling operation, it is not a distinguishing characteristic of the
process.

Drill wear starts as soon as cutting begins and instead of
progressing at a constant rate, the wear accelerates
continuously. Wear starts at the sharp corners of the cutting
edges and, at the same time, works its way along the cutting
edges to the chisel edge and up the drill margins. As wear
progresses, clearance is reduced. The resulting rubbing causes
more heat, which in turn causes faster wear.
Wear lands behind the cutting edges are not the best indicators
of wear, since they depend on the lip relief angle. The wear on
the drill margins actually determines the degree of wear and is
not nearly as obvious as wear lands. When the corners of the
drill are rounded off, the drill has been damaged more than is
readily apparent. Quite possibly the drill appeared to be
working properly even while it was wearing. The margins
could be worn in a taper as far back as an inch from the point.
To restore the tool to new condition, the worn area must be
removed. Because of the accelerating nature of wear, the
number of holes per inch of drill can sometimes be doubled by
reducing, by 25 percent number of holes drilled per grind.
Unless buying a drill designed for a specific purpose, the
average drill as it comes from the manufacturer has a “best
general purpose’ tip on it when it comes out of the package.
What this means to the shop owner is that there is good chance
the tip on the drill will be wrong for the intended application.
It has been estimated that about 90 percent of drilling troubles
are due to improper grinding of the drill point. Therefore, it is
important that care be taken when re sharpening drills.
A good drill point will have: both lips at the same angle to the
axis of the drill; both lips the same length; correct clearance
angle; and correct thickness of web.
In North America we have had easy access to inexpensive tools
a few cost effective options in drill point grinding equipment.
To a great extent we have become accustom to using drills

“right out of the bag”. This approach is flawed and we may run
the risk of losing our competitive edge to those who can run
faster cycle times, less expensive set ups, and reduced down
time because all of their shop staff can drill point grind. This is
the reason for our focus on getting good, easy to use tool
grinders onto the shop floor.
Drilling today’s wide variety of materials and at speeds not
even considered possible when these tools were invented
requires a great variety of drill tip geometries, new uses of
materials and coatings and various types of types of web
thinning techniques. The Basic geometric principles can be
applied to almost any drill style and will help determine the
following:
 Control of the formation of the chip
 Control of the size and shape of the ship
 Control of the chip flow along the flutes
 Determining the strength of the cutting lip
 Managing lip wear
 Minimize torque required
 Control of hole size, quality and straightness
 Control the amount of burr on both ends of the drilled hole
 Create greater options in speeds and feeds
 Reduce heat generated

It is necessary to understand these basic principles in order to
effectively drill point to suit specific applications. The use of
controlled drill pointing and web thinning can result in
substantial savings in drilling costs. On spec drill point
grinding includes producing the proper included point angle,
centering considerations, cutting lip relief, lip clearance , edge
reinforcing, and edge prep (in the case of carbide). According
to one of the best drill point grinding houses in the US, there
are over 140 common drill tip geometries in use today,
however the lion’s share of the work is still being done by
basic two flute helical, split points or basic four faceted tips.
The Cuttermaster Professional Tool Room Series Drill point
grinders produced by GSC are designed to be used on the shop
floor, can be operated with minimal training, and produce NAS
907b tip profiles in less time than it takes to load a new drill.
We hope this book finds it way into the hands of those who
make their living as trades people. And we are thankful for
those who have kindly volunteered material for this book,
amongst them Joe Mazof, Dormer Tools, Greenfield and David
Oates for his patient proof reading and technical contributions.
Thank you. From the Cuttermaster Professional Team

 

Shipping Dimensions

Weight
1 kg
Shipping Dimensions
24 × 24 × 5 cm

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