|
Processes
- Drilling
Drilling machines
are available in several formats; from the small manually operated
"hand drills" which are powered by the user’s operation
of a rotating handle which drives the drill bit, to large electrically
powered industrial units for cutting large, deep holes in thick
metals.
The chuck which holds the drill bits is held in place usually
by a threaded section that engages with a similar thread on
the smaller machines, or by a "Morse taper" section
on larger machines and lathes. The Morse taper system works
by having a tapered section on the chuck shank, which mates
with a corresponding tapered hole in the machine. This are is
covered further in the lathe section.
A common variety of drilling machine is the pillar drill. This
consists of the main assembly (motor, chuck, drive belts etc)
being mounted at the top of a pillar – putting them at around
the same height as the user’s head/chest. A flat table is attached
to the pillar further down and can be moved vertically to allow
for a variety of sizes of work and drill bit length. |
The
picture in the gallery shows a pillar drill, its basic controls
and features. Many of these are common to most drills.
The
Basic Drill Bit
Perhaps the most important component
of a drilling system is the drill bit. This is a shaft that
has cutting edges at one end to remove the material being drilled.
Typically, the cutting ends of drill bits are made of hardened
alloy steel, whereas the shafts are left soft to reduce the
possibility of the bit shattering.
The most common drill bits are two fluted, that is they have
twin flutes running around half the length of the bit in a helical
pattern from the cutting end. These flutes end at the tip, which
is usually angled at around 35 degrees to facilitate alignment
and cutting.
The flutes of the drill bit aid the removal of material from
the area being drilled – if they were not present waste material
or swarf would collect around the cutting surfaces of the bit,
creating an oversized hole at best, or a seized and possible
broken bit at worst.
If the materials being drilled are particularly hard or resistant
to drilling, it is important to use some form of coolant during
the drilling process – as the heat generated will expand the
bit and the work, possibly causing it to seize. Also, if excess
heat is allowed to build up the drilled hole will become oversized,
and the drill may become annealed and lose its hardness and
hence cutting ability.
Obviously these problems are usually encountered on harder metal
alloys, and rarely on wood and plastics.
Specialist
Drill Bits
There are some situations where
the standard type of drill bit mentioned above are not suitable.
For example, if a hole is required with a flat base (as opposed
to the usual concave base left by standard drill bits) a slot
drill is used. This is similar to a standard bit, apart from
its cutting edges, which are perpendicular to the length of
the bit as opposed to being angled away from the length.
Drilling
Speeds
The speed at which a drill is
operated depends on several factors; these are typically the
properties of the material being drilled and the size of the
hole required.
As a general rule, softer materials can be drilled at higher
speeds than harder materials, as they are easier to cut and
hence material is removed faster. Drilling too fast will result
in an overheated drill bit, an oversized and badly finished
hole, and a possibility of the bit breaking. Drilling too slowly
may splinter woods, and is obviously more time consuming.
When a large diameter hole is required in a material, the drill
must be operated at a slower speed than for a smaller diameter
hole. This is because for a standard speed, the perimeter edge
of a large drill bit will be travelling much faster than that
of a small one. For example the circumference of 6mm drill bit
is around 18mm, so at a speed of 200 RPM any point at the edge
of the bit will be travelling at a velocity of 0.6 metres per
second. However, a 12mm bit has a circumference of around 38mm,
so at the same chuck speed of 200 RPM the outside edges of the
bit will be travelling at a velocity of 1.3m/s – twice as fast
as the 6mm bit.
Work
Holding
Drilling takes place in countless
situations, in some, such as using a battery powered hand drill
to cut a hole in a car body panel the work is large enough and
has enough mass to stay still while being worked on.
If a smaller component is to be drilled however, it must be
held securely – both to assure accurate placement of the hole,
and to prevent it binding to the drill bit and spinning around
with the rotation of the drill. This can obviously be a safety
hazard, especially if the work is a piece of sheet metal with
sharp edges.
There are several methods of work holding available, from clamping
the work in a vice to be drilled with a hand drill, to fixing
it in a drilling vice attached to the bed of a pillar drill
or clamping it to the bed with G-cramps if the work is larger.
Either way, when being drilled the work must always be held
securely.
|
