Drill Bits and Sizes

Drill Bits and Sizes

• Drill bits are cutting tools used to create holes of circular cross-section in various materials.
• They come in many sizes and shapes, and can be attached to a drill to cut through a workpiece by rotation.
• Standard sizes are available, and a comprehensive drill bit and tap size chart lists metric and imperial-sized drill bits alongside required screw tap sizes.

Metric Sizing

• Drill bits in metric sizes are most commonly used in nearly all countries except the United States.
• Drill bits are available in increments of 1 mm, 0.5 mm, and 0.1 mm.
• Sets of bits in 1-mm increments can be found in a market stall, in 0.5-mm increments at a hardware store, and in 0.1-mm increments at an engineer's store.

Centre Drill Bit Sizes

• Centre drills are available with two different included angles: 60° (standard for drilling centre holes) and 90° (used for locating holes prior to drilling with twist drills).
• Centre drills are made specifically for drilling lathe centres, but are often incorrectly used as spotting drills.

Bolt Tolerance

• Close-tolerance bolts can be placed in a freezer prior to fitment to eliminate friction.
• Standard-tolerance bolts are manufactured to +0.000 to -0.0055 in. of the diameter's nominal size.
• Close-tolerance bolts are manufactured to +0.000 to -0.0005 in. of the diameter's nominal size.

Bolt Holes

• Bolt holes of primary connecting elements have close tolerances.
• For general application, it is generally permissible to use the first lettered drill size larger than the nominal bolt diameter.

Decimal Fraction Equivalents

• Decimal places for 0.25, 0.5, and 0.75 are shown to thousandths (0.250, 0.500, 0.750).
• Machinists usually think about decimal fractions in terms of 'two-fifty', 'five hundred', and 'seven-fifty'.

Jobber-Length Drill

• Jobber-length drills are the most common type of drill.
• The length of the flutes is between 9 and 14 times the diameter of the drill, depending on the drill size.

Aircraft Length Drill

• Aircraft length drills are commonly used for manufacturing riveted aluminum aircraft.
• They are available in fixed lengths, such as 6, 8, 12, or 18 inches, rather than the progressive lengths of jobber drills.

Unilateral and Bilateral Tolerances

• Unilateral tolerance means variation is permitted only in one direction from the specified dimension.
• Bilateral tolerance means variation is permitted in both directions from the specified dimension.

Limits of Size and Fit

• Limits of size are the extremes of size allowed for a dimension.
• Basic size is the size about which the limits of a particular fit are fixed.
• Upper limit of size (ULS) is the largest size permitted by a toleranced dimension.
• Lower limit of size (LLS) is the smallest size permitted by a toleranced dimension.
• Tolerance is the total amount of variation permitted by the limits of the dimension.

Calculating Clearances

• The amount of clearance is found by calculating the upper and lower limits of both the male and female components.
• The smallest shaft is subtracted from the largest hole to calculate the slackest fit.
• The largest shaft is subtracted from the smallest hole to calculate the tightest fit.
• The tolerances of the individual items will produce a minimum clearance and a maximum clearance.