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Nomenclature and Basic Parts Of Drilling tool (Twist bit)

Drill bit Nomenclature:

Drilling is the process of cutting or originating a round hole from the solid material. There are many ways of classifying drills. The tool(drill) and not the work piece is revolved and is fed into the material along its axis.
For example , according to material, number and types of flutes, drill size , type of shank(straight or taper) and cutting point geometry etc. However the most common type of drill is the fluted drill shown in figure.

drill bit nomenclature

Drill Construction: Drill Construction:

A twist drill is made up of three components: A twist drill is made up of three components:
– Shank
It is the portion of the drill by which it is clamped in the spindle. The shank may be either straight or tapered. Straight shank drills are used with a chuck. Tapered shank drills have self-holding tapes that fit directly into the drill press spindle. On the taper shank is the another term is used which is called tang. This fits into a slot in the spindles sleeve.

– Body
It is the portion between the point and the shank.

– Drill point

Drilling tool Nomenclature :
  • Cutting Diameter: 
“Largest diameter measured across the top of the lands behind the point”
  • Back Taper –
 The diameter reduces slightly toward the shank end of the drill, this is known as “back taper”. Back taper provides clearance between the drill and workpiece preventing friction and heat
  • Flute Length: Flute Length:
“The length of flute measured from the drill point to the end of the flute runout” 
  • Land:
 “The part of the drill body “The part of the drill body between the flutes” between the flutes”
– The lands provide the drill with much of its torsional strength
– Reducing the land width increases chip space, but reduces strength.
  • Webs
“The thickness “The thickness measured measured across the across the base of the base of the flutes” flutes”
– Contributes to the torsional strength of the drill.
  • Helix Angle: Helix Angle:
Angle formed between a line drawn parallel to the axis of the drawn parallel to the axis of the drill and the edge of the land.
Helix angles generally fall into three categories: Helix angles generally fall into three categories:
– Slow Spiral– 12° to 22° helix angle
– Regular Spiral– 28° to 32° helix angle
– Fast Spiral– 34° to 38° helix angle
  • Margins:
“The cylindrical portion of “The cylindrical portion of the land that is the land that is not cut not cut away to provide clearance” to provide clearance”
– The balance of the land is reduced in diameter, known as “cleared diameter” or “body clearance”.
  • Cutting Lips Cutting Lips
“Cutting edges that extend from “Cutting edges that extend from the center of the drill to the the center of the drill to the outer diameter” outer diameter”
– On most standard drills, the cutting edge should form a straight line.
– Some specialty and high performance drills have curved cutting lips.
  • Chisel Edge Chisel Edge
“The edge at the end of the web, that “The edge at the end of the web, that connects the cutting lips” connects the cutting lips”
– The chisel edge does not cut does not cut- it penetrates displacing the workpiece material.
  • Chisel Edge Angle Chisel Edge Angle
“The angle formed between the “The angle formed between the chisel edge and the cutting lips” chisel edge and the cutting lips
– The chisel edge angle is generally 125° to 135°
  • Point:
The point is the cone shaped end and it does the cutting. It consists of the following:
(A)   dead center: It is the sharp edge at the extreme tip of the drill. This should always be the exact center of the drill.
(B)   Lips: these are the cutting edges of the drill.
(C)   Heel : It is the portion of the point back from the cutting edge.

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