ADVANCED MECHANICAL DRAFTING

LECTURE #3

Common Hole Types

DRILLED

REAMED

BORED

COUNTERBORED

COUNTERSUNK

SPOTFACED

DRILLING

• Specifying drilled holes on drawings

Ø.250 (1/4) THRU

or

Ø.250 (1/4) .50

• Drill names

• Drill size chart

CLEARANCE HOLES

• Used for holes that accommodate fasteners (usually screws)

• Must be larger diameter than the screw for a clearance fit

• Tapping section of MHB contains necessary data

• Must be THRU holes

CLEARANCE HOLES

EXAMPLE:

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

Ø.397 (X) THRU

USE “FREE FIT” CLEARANCE HOLES UNLESS OTHERWISE

SPECIFIED

GSSP FASTENERS

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

SOCKET HEAD CAP

SCREW

LOCK WASHER

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

FIRST NOTE LINE

SECONDNOTE LINE

ROUNDING DECIMAL DIMENSIONS

• 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) .62 .875 (7/8) .88– .3125 (5/16) .312 .375 (3/8) .38– .125 (1/8) .12 .1875 (3/16) .188

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