Drill bit angles and accuracy.

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Jim
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Drill bit angles and accuracy.

Postby Jim » 26 Dec 2015 09:04

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.

Factor I

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
locations.

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.

Factor II

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
revolution.

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
Tangent function.

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
axis.

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