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.

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.