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Spotting & Step Drills, Countersinks &
Center Reamers Technical Information

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Basics of...Spotting & Step Drills,
Countersinks & Center Reamers
Spotting Drill Countersink (Single Flute) Combined Drill & Countersink (Plain Type)

 

Holes are drilled in a workpiece for many different reasons. Often, the process begins with a spotting operation and ends with a countersinking operation.

Spotting/Center Drills

Starting a hole with a spot drill provides a true center for more accurate drilling. Always choose a spot drill with an equal or greater angle than the drill that will
be used to make the finished hole.

ANGLES

90°

  • Used to create a chamfer for the hole that is being produced
  • Always select a size larger than the drill to be used to create the hole

120°

  • Used for preparing a hole for a 118° or 120° drill bit

140°

  • For preparing a hole for a 130° or 140° high performance drill bit

Countersinks & Center Reamers

Countersinks are versatile tools that are typically used after a hole is drilled.
They can chamfer and deburr, but their primary function is to make an angled
cut at the top of a hole to form a seat for a fastener.
Center reamers are designed for countersinking holes for centers, flathead
screws and rivets. They can also be used to clean and enlarge existing holes.

TYPES

Single Flute

  • Provides the best centering of all countersinks and a smoother surface finish
  • Suitable for hand-held operations and production set-ups

3 & 4 Flute

  • Provide the best chip clearance and are suitable when machining plastics and non-ferrous metals such as aluminum, copper, brass and bronze

6 Flute

  • Allow the highest feed rates of all countersinks
  • Provide a longer life, as cutting load is distributed across more edges

Drill Point

  • Allows user to spot drill, countersink and edge chamfer with the same tool

CNC

  • Provide the tightest tolerances

Chatterless

  • Reduce vibration and noise

Jobber Length Step Drills

  • Step drills can produce holes with two or more diameters during one set-up.
  • The step drill is a modified standard drill and functions as a combination tool
    because of the point which is ground to a desired diameter. This implies it has
    the same flute for the different diameters.
  • Jobber length step drills, sometimes known as subland drills, can be used for
    drilling and countersinking flat head machine screws, drilling chamfered holes
    prior to tapping and drilling and counterboring cap screw clearance holes.

Combined Drills & Countersinks

As the name implies, these tools are designed to improve productivity by
performing the drill and countersink operation in one pass.

TYPES

Plain

  • Most common style
  • Standard flute clearance

Radius

  • Have increased flute clearance
  • Provide better productivity and more accurate holes than all other countersinks

Bell or Bugle

  • Bevels the outer edge of the hole, preparing the workpiece for additional
    machining processes

ANGLES

60°

  • Specifically for creating holes that will be used to machine between centers

82°

  • Designed to create holes that match the angle of the most popular flat and oval head screws

90°

  • Use to create holes that match the angle of flat and oval head screws, and to enlarge existing holes

MATERIALS

High Speed Steel (M1, M2, M7, M50)

  • Combines good tool life and productivity with minimal cost
  • Works well in free cutting and carbon steels as well as soft, non-ferrous
    materials like aluminum, brass, bronze and copper

Cobalt (M35, M42)

  • Better wear resistance, higher hardness and toughness than HSS
  • Very little chipping or micro chipping under severe cutting conditions, allowing
    the tool to run 10% faster than HSS
  • With the right point angle and helix, cobalt is the most cost-effective for
    machining cast iron, heat-treated steels and titanium alloys

Solid Carbide

  • For high-performance applications
  • Can run faster and withstand higher temperatures, while providing good wear
    resistance
  • Carbide is brittle and tends to chip when conditions are not ideal; heavy feed
    rates are more suitable for HSS and Cobalt tools
  • Used in abrasive and tough-to-machine materials: cast iron, non-ferrous
    alloys, glass, plastics and composites
  • Not recommended for hand held operations
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