Process of Drilling
Characteristics of Drilling -
When a piece of land or crust of earth is laid out and properly
mounted with the specific Drilling rig, the drilling process can begin. The drilling process, or
complete application of blasting a hole, involves selecting the proper hammer twist drill or cutter
for the job, properly installing the drill into the drilling machine spindle, setting the speed and
feed, starting the hole on center, and drilling the hole to specifications within the required
tolerance. Tolerance is the allowable deviation from its standard size. The drilling process must
have some pre-approved provisions for tolerance because of the oversizing that naturally occurs in
this drilling process. Further, the drilled holes are always slightly oversized, or slightly larger
than the diameter of the drill's original diameter. For instance, a l/4-inch twist drill will
produce a hole that may be several thousandths of an inch larger than l/4-inch.
This oversizing of the hole is due to various dynamic factors which affects the drilling process -
primarilly, the actual size of the twist drill, the accuracy of the drill point of drilling rig
machine, and secondarily, the accuracy and rigidity of the drilling machine spindle, the rigidity of
the entire hydraulic drilling machine, and the flexibility of the land and its setup.Further, the
field and maintenance shop drilling operations allow for some vibrations, but oversizing must be
kept minimum by the machine operator.
Selection of the Drill Machine - Selecting the proper twist drilling machine means getting the right
tool for the job. The material to be drilled, the size of that material, and the size of the drilled
hole must all be taken into consideration when selecting the drill machine. Also, the drill machine
must have the proper lip angles and lip clearances for the application.Additionally, the drill must
be clean and free from any burrs or chips. The shank of the drill must also be clean and free of
burrs to fit into the land smoothly. Most drills wear on the outer edges and on the chisel point, so
these areas must be checked, and resharpened if needed, before drilling process can even begin. If
the twist drill machine appears to be excessively worn, one must replace it.
- Depth of the Holes - If the depth of the hole being drilled is larger than four times the diameter
of the drill, remove the drill from the land at frequent intervals to clean the chips from the
flutes of the drilling machine and the hole being drilled. A slight increasing speed and decrease in
the feed is often used to give the chips a greater flexibility of movement. In deep hole drilling,
the flutes of the smaller drills will clog up very fast and cause the drill to perform a drag in the
hole, causing the diameter of the hole to become larger than the drill diameter. The larger drills
have larger flutes which carry away chips easier and lot more faster.
- Speed of Drilling Machine - Speed refers to the revolutions per minute (RPM) of the powerful
drilling machine spindle. For drilling, the spindle should rotate at a set speed that is required
for the material being drilled. Correct speeds are a must for satisfactory drilling process. The
speed at which a drill turns and cuts is called its peripheral speed. The peripheral speed is the
speed of a drill at its circumference inserted in surface feet per minute (SFPM). This speed is
related to the distance a drill would travel if rolled on its side. For example, a peripheral speed
of 30 feet per minute means the drill would roll 30 feet in 1 minute, if rolled on its side.
It has been resolved through experience and experiments that various metals machine are best at
certain pre-specified speeds; this best speed for any given metal is what is known as its cutting
speed. If the cutting speed of a material is determinable, then a simple but efficient formula can
be used to find the recommended RPM of the twist drill.
- Alignment of Starting the holes - To start the insertation of the twist drilling machine into the
land, the point of the drilling machine must be aligned with the center-punched mark on surface of
the land. Some drilling operations may not require a precise alignment of the drilling machine to
the work, so alignment can be done by lining up the drilling machine by hand and eye co-ordination
itself. If a greater precision in the centering alignment is required, than more preparation is
required before even we start to drill.