1. ADVANCED MECHANICAL DRAFTING
LECTURE #3

2. Common Hole Types DRILLED REAMED BORED COUNTERBORED SPOTFACED
COUNTERSUNK

3. DRILLING Drill size chart Drill names
Specifying drilled holes on drawings
Ø.250 (1/4) THRU or
Ø.250 (1/4)  .50

4. 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

5. USE “FREE FIT” CLEARANCE HOLES UNLESS OTHERWISE SPECIFIED
EXAMPLE:
Specify clearance hole for 3/8” screw on the drawing.
Ø.397 (X) THRU

6. ¼” SOCKET HEAD CAP SCREW & ¼” MEDIUM LOCK WASHER
GSSP FASTENERS
¼” SOCKET HEAD CAP SCREW & ¼” MEDIUM LOCK WASHER

7. SOCKET HEAD CAP SCREW

8. LOCK WASHER

9. COUNTERBORE LOCATIONS 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
FIRST NOTE LINE
SECOND
NOTE LINE

11. 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 > 5 UP 
Round digits < 5 DOWN 
Round 5 UP if previous digit is ODD
Round 5 DOWN if previous digit is EVEN
Examples:
.625 (5/8)  (7/8)  .88
.3125 (5/16)  (3/8)  .38
.125 (1/8)  (3/16)  .188