ADVANCED MECHANICAL DRAFTING

LECTURE #11

POSITIONControls locations of mating features

IDEAL CONDITION

ACCEPTABLE CONDITION

UNACCEPTABLE CONDITION

POSITION ZONE TYPES• Zones are located from datums

with basic dimensions

• Zone sizes are specified in feature control frames

• Zone is round (Ø) when it locates an axis (hole, post,)

• Zone is flat (no Ø) when it locates a center plane (slot, tab)

• Zone size may be modified (,) when dependent on feature size

AXIS

CENTER PLANE

POSITION TOLERANCE ZONE FOR HOLES

HOLE ATTRUE

POSITION

ACCEPTABLE LOCATIONAL VARIATION

ACCEPTABLE ANGULAR VARIATION

IDEAL HOLE LOCATION = CENTER OF ZONE

ACTUAL HOLE AXIS MUST LIE INSIDE ZONE

FIXED and FLOATING fastenersONE CLEARANCE HOLE TWO CLEARANCE HOLES

FIXEDFLOATING

CALCULATING POSITION TOLERANCE WHEN USING FASTENERS

FLOATING FASTENER CONDITION

TOLERANCE = MMC CLR HOLE – MMC FASTENER

MMC = largest screw & smallest clearance hole

FIXED FASTENER CONDITION

TOLERANCE = MMC CLR HOLE – MMC FASTENER

2

DETERMINING MMC SIZES OF HOLES & FASTENERS FOR CALCULATIONS

RULE #1The MMC of a fastener is its nominal size

Fact #1 - Fastener companies manufacture screws so that their NOMINAL SIZES are at MMC (worst case is nominal size)

RULE #2The MMC of a drilled hole is its nominal size

Fact #2 – Drill bits create holes that are always larger than their NOMINAL SIZES (worst case is nominal size for calculations)

The MMC modifier The MMC modifier provides bonus tolerance whenever

the tolerance zone size is based on the feature size

Allowable movement of hole with hole at MMC

Enlarged hole provides for more movement of hole

ZONE SIZE CAN INCREASE

NO CLEARANCEFASTENER

CLEARANCE HOLE ADDITIONAL CLEARANCE

MMC question

If feature size deviates from MMC, can the feature move

farther out of position and still function correctly?

If answer is “yes”, belongs in feature control frame

Example – call out hole for 3/8” screw in fixed condition

Nominal size of 3/8” screw is 3/8” (Ø.375)

Clearance hole for 3/8” screw is made with drill bit “X” (Ø.3970)

TOLERANCE ZONE DIAMETER = (.397 - .375) ÷ 2 = .011

Ø.3970 (X) THRU

1. Establish datums

2. Locate tolerance zone

3. Define hole and zone

ON THE DRAWING

Calculating Actual Zone Size in Inspection

.505

.4972XØ THRU

MMC of hole = .497

HOLE SIZE ZONE SIZE

.497 .013

.498 .014

.499 .015

.500 .016

.501 .017

.502 .018

.503 .019

.504 .020

.505 .021

.506 N/A

Actual Zone Size = MMC zone size +

feature size deviation from MMC

The MMC modifier is a powerful tool for allowing bonus

tolerance in the location of a feature without decreasing its

function or quality

Bonus tolerance= faster manufacturing time= less manufacturing cost= MORE MONEY FOR YOU

YOKE FORGING

Phantom lines indicate areas

where extra material will be machined off

Dimensions reflect finished

features

Yoke Holes