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ADVANCED MECHANICAL DRAFTING. LECTURE #3. DRILLED. REAMED. BORED. Common Hole Types. COUNTERBORED. SPOTFACED. COUNTERSUNK. Ø.250 (1/4) THRU or Ø.250 (1/4)  .50. DRILLING. Drill size chart. Specifying drilled holes on drawings. Drill names. CLEARANCE HOLES.

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common hole types

DRILLED

REAMED

BORED

Common Hole Types

COUNTERBORED

SPOTFACED

COUNTERSUNK

drilling

Ø.250 (1/4) THRU

or

Ø.250 (1/4)  .50

DRILLING
  • Drill size chart
  • Specifying drilled holes on drawings
  • Drill names
clearance holes
CLEARANCE HOLES
  • Used for holes that accommodate fasteners (usually screws)
  • Must be larger diameter than the screw for a clearance fit
  • Must be THRU holes
  • Tapping section of MHB contains necessary data
clearance holes1

USE “FREE FIT” CLEARANCE HOLES UNLESS OTHERWISE SPECIFIED

Ø.397 (X) THRU

CLEARANCE HOLES

EXAMPLE:

Specify clearance hole for 3/8” screw on the drawing.

gssp fasteners
GSSP FASTENERS

¼” SOCKET HEAD CAP SCREW & ¼” MEDIUM LOCK WASHER

counterbore locations and sizes

FIRST NOTE LINE

SECOND

NOTE LINE

COUNTERBORELOCATIONS and SIZES
  • Required location data
    • Number of hole locations
    • Diameter of bolt circle
    • Angle between holes
  • Required size data
    • Number of holes
    • Drill size and depth
    • Counterbore diameter and depth
rounding decimal dimensions
ROUNDING DECIMAL DIMENSIONS

Dimensions are normally rounded to two (2) places (hundredths), but may be rounded to three or four places if tighter tolerances are required or if dimension corresponds to a tool size

  • Rounding Rules
    • Round digits > 5UP 
    • Round digits < 5DOWN 
    • Round 5UP if previous digit is ODD
    • Round 5DOWN if previous digit is EVEN
  • Examples:
    • .625 (5/8)  .62 .875 (7/8)  .88
    • .3125 (5/16)  .312 .375 (3/8)  .38
    • .125 (1/8)  .12 .1875 (3/16)  .188
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