Why 118? Why 135? (Drill angle)
Certain fundamental constants of nature and mathematics are set to mesh with reality: pi is the ration of circle circumference to diameter, and e works out to be geometrically set as well*.
Other numbers are set by man. An inch used to be a skosh over 2.54cm, now it's defined to be exactly 2.54cm.
Still other numbers are set to optimize the physics of a situation. 60degree centers give you optimal lateral stiffness, with less friction.
But because drills come in 118 degree, and 135 degree configurations commonly (and others, less commonly), I wonder who/what determined those angles. Also, why do the Starrett and General drill gauges only have the 59 degree half-angle?
Perhaps this is a no-brainer, but no obvious reason came to me.
*e is the unique real number such that the value of the derivative (slope of the tangent line) of the function f(x) = e^x at the point x = 0 is exactly 1.
Thank you. I did an (obviously lame) web search and didn't see that. Strangely, I even have a hardcopy of of Burghardt, Axelrod, and Anderson. Sometimes, those old things called books have useful information in them.
So 118 does the best job of centering without wedging. And the raison d' etre of 135 is...?
Here's one explanation... (funny that I just happened to be paging thru the old "Handbook for Drillers" today by the Cleveland Twist Drill co..)
OKAY! So there IS a reason. I asked the same thing here once upon a time, but didn't really get any explanation.
Still a little hazy on one aspect though-- why was 118° chosen to become so common? Honestly, 118° is a rather cumbersome number. It would have been easier to just round that off a bit to the more mathematically friendly 120°.
Just reading this info that John posted, it appears that the optimal angle was determined to be 118. Those old guys took that stuff seriously (the site that mentioned drilling 1.5 inch holes through steel by hand shows why!), and so I think they just said "If 118 is optimal, 118 it is". Then the sharpener/grinder folks made the jigs, and Starrett and B&S and Lufkin and others made the 59 degree bit gauges, and 118 was the standard.
Not a lot of experience here, but when I've ground stuff offhand 118 is just as easy to get as 120. Actually, if I target 118, then 120 IS easier to get. Or 114, or 122, or 119. Sheesh, come to think of it if I target 118, then ANY other number is easier to get. But that's just me.
The 135 degree point, though more efficient, does not work well without the additional facets of a modified split point. These additional facets substantially increase the cost of sharpening. The 118 degree point is still superior in some applications but is mostly used because it is cheaper to produce and takes less skill to resharpen.
Here is some more:
Drill Point Geometry
The drill point is the main factor to consider when trying to
optimize drilling efficiency. Good point geometry can accurately
locate a hole and allow productive feed rates without inducing
excessive cutting edge wear.
If a surveyors conical plumb bob, resting perfectly still, hanging
from its plumb line was suddenly released from the instrument, the
needle like point would penetrate the ground at precisely the place
over which it had been hanging. If the plumb bob was well balanced
and rotating on its axis at one thousand revolutions per minute the
point would appear the same as if it were standing still. It would
just be rotating. On release the point would strike same spot it
did before. The only difference being its rotation. This is true
because the sharp conical profile of the plumb bob appears the same
from all positions about its axis. It appears as a sharp point.
A jobbers length drill has a chisel point. At one position about
its axis the profile appears as a point, however at 90 degrees from
this position the point profile appears as a horizontal straight
line (just like a screwdriver). This spinning straight line causes
the drill point to "walk" and penetrate the work at unpredictable
So, desirable drill point geometry for accurate hole location should
include a profile which appears as a point from all angles about
the drill axis. In other words, when a drill is rotated between
the fingers the point profile should always show a sharp
intersection on the drill axis.
On close examination an airplane propeller has a varying twist
angle along each blade. Near the hub the twist angle is most
noticeable. As you proceed out toward the propeller tip the angle
becomes very slight. The reason for this design is during each
propeller revolution the airplane may travel four feet. For one
revolution a point near the propeller tip gets to go many feet
around the entire circumference of the circle to cut four feet of
air. Because the propeller tip travels far, slicing air, the angle
can be slight. A point near the hub only travels a relatively
short distance because its circle is much smaller. Since the hub
needs to do the same amount of work for its revolution it is given
a much greater angle or pitch so it can carve more air to make up
the four feet. This blade geometry is used to equal out the forces
along the blade and make the propeller more efficient.
A drill works in much the same way, for it cuts material at a
specific chip load. The proper relief or clearance angle at the
end of the drill is very important. Too little clearance prevents
the drill from penetrating the material for the selected chip load.
Too much clearance weakens the drills cutting edge and effects
premature edge wear.
To show this relationship we can look at the drill's helix angle
while it drills. The helix angle is part of a triangle. The two
essential legs of this triangle are the lead or penetration
distance per one revolution and the path around the circumference
of any particular point on the cutting edge as it makes that
Pi is approximately equal to 3.14. For the sake of discussion we
can round this figure to just 3 making easy some calculations about
various points along the cutting edge. The drill diameter
multiplied by the value of Pi equals the distance around the drill.
A one half inch drill then would have a one and one half inch
circumference. A point one thirty second of an inch from the drill
center would only travel three sixteenths of an inch around its
circumference. We get this by multiplying the radius by two to get
diameter and then again by Pi to equal circumference. This is one
eighth of the distance of a point on the circumference. If the
chip load per flute was eight thousandths of an inch per revolution
we could figure these helix angles as the drill "screwed" its way
into the work. Using some trigonometry and the arc tangent
function, the function which changes the tangent value back into
degrees, we can establish the two helix angles. At the drills
periphery the angle equals the arc tangent of sixteen thousandths
of penetration (two flutes multiplied by the chip load) divided by
one and one half inches. This angle would equal six tenths of one
degree. Another helix angle, near the drill center approaches
five degrees. The helix angle reaches NINETY degrees at the drill
center. On approaching the drill center the angle becomes very
severe very quickly. This condition is a unique property of the
To these helix angles we also must add proper relief for clearance.
Relief angles of from four to six degrees depending on the material
being cut added to the helix angle make up the total cutting lip
clearance. The angle near the drill center is handled one of two
ways. One, the drill may have a split point. The "split point
drill" if done correctly has a sharp point ground at precisely the
drills center for starting the hole on location and machining it on
size. It also has a varying clearance angle increasing near the
drill center for easy penetration. Or two, the drill may have a
chisel point as on the jobber length drill. This chisel point
mushes the material around until it is moved out from the drill
center and picked up by the flutes. It is then evacuated up and out
of the hole. The "chisel point drill" needs a center or spotting
pre-drill to direct its location.
On drill point geometry then, a conical point precision ground on
the drills axis allows the drill to machine a hole on size and with
a good finish. If the drill point is running concentric with the
spindle the hole will be created on location under the spindle
One of my favorite CNC drills for all materials is the Cleveland
"aircraft type C" drill list number 2130. I'm sure there are other
drills which would perform well but I stopped looking after I found
the one that worked.
While I am not one to believe the "Grandpaw did it this way so I will too" line of thought I do believe for most drilling in metal the angles on the drill now is the best for overall conditions. That's not to say for special applications a different angle is not better.
It's interesting to question established methods and I do this all the time. You have to remember that most of the old ways were established by "on the job" methods rather than scientific methods. the machines were simple and the methods to determine the best way was simple.
Now we have CNC machines that will scare the hell out of you when you hit the go button. I can just imagine the fear in the eyes of a machinist from 1900 if he walked in and saw what happens when you hit the button. The scream and the brown and yellow streaks on the floor away from the machine would be funny to most.
I guess I will stick with the angles of old and do the best I can with what I have.
There is some discussion of this in the older text "Tool Design" by Donaldson, Lecain and Goold. I have the 1973 edition.
The discussion is too lengthy to include in its entirety, but the salient point seems to be that increasing the included drill point angle increases the effective rake angle, increasing the cutting efficiency but at the same time increasing the amount of axial thrust required for proper feeding, and producing a greater tendency to "walk" when used without guide bushings.
The authors felt that in production drilling an increase in included point angle from 135 to 140 could produce "better results" but only with controlled speeds and feeds and guide bushings. They didn't define "better results' though.
Hence, the split point?
Originally Posted by Sea Farmer
Drill Point angles
I just want to cut to the chase. I don't know the reasons why 118 degrees was originally chosen for a standard drill tip back in 1920 or whatever but back then all the ever drilled through was steel and wood. With the development of new and exotic alloys the old standard just did not cut it pardon the pun. As a rule the harder the material the more the point angle. Stock drills for superalloys go up to 140 degrees and regarding the grind all the drill companies now have proprietary material specific tip grinds. Just grab a Precision Twist drill catalog or any drill manufacturer and look as the variety of tip grinds.
The factory drill points of 118 and 135 degrees shouldn't be directly compared. The 135 degree point comes on a cobalt drill which has a much thicker web. If you grind any point without web thinning on the cobalt drill, the drilling thrust forces are so high the drill may shatter.
Interesting - 118 sharpened normally has the best centering/ drill rate combination in normal materials drilling with a "normal" point that his easy to grind. Harder/tougher materials benefit from the higher angle, but require web-thinning, and benefit significantly from point splitting and even fancier grinding techniques.
Stan, I was intriqued by the drill type that is your favorite. Looked up the "aircraft type C" drill and found some offered by Besly. They remark:
"These drills are made to the A.I.A. NAS-907 Type C Standards for Aircraft Drills. They are designed for portable power tools or machines for drilling holes in hard and tough sheet metal alloys of the heat resistant, stainless and titanium types. Of sturdy construction, their split point allows ease of penetration and has made them popular in automobile body work and in the construction industry."
Cleveland reports (about it's "Aircraft Type C" drills): "AIA NAS 907 (Type C) approved for aircraft applications. Excellent for portable applications. Effective in alloyed steel, stainless steel, tool steel, and cast iron."
I needed a quick 7/16 hole in an aluminum block last night and a quick search located the right size drill, but it was dull. Had to grind it offhand. The General drill gauge I used (has a shape like a goose head) was 59 degrees, so 118 it was. I was surprised at how far "off" the previous grind was.
Thanks to all,
Originally Posted by bosleyjr
Thank you for your reply.
When I first used them I had a close size hole to drill in expensive steel parts. The drills put the holes on location and on size within .001 diameter. The point geometry had a varying relief as an airplane propeller and drilled without making a three lobed hole. *S
The drills under about .093 didn't have that geometry, so I turned to Tulon circuit board drills.
Those two drill brands were all I used unless something special came into the shop.
drill point geometry
I think the Drill point geometry article is the best one I've ever read.
I even split the points on 118 degree points. Before I sharpen them.
And I split points on bigger drills like 2 inch or more.
And I think parabolic drills are well worth using even in the easy to drill parts.