advanced mechanical drafting

ADVANCED MECHANICAL DRAFTING

LECTURE #6

MMC & LMC• MMC = Maximum Material Condition

– Largest shaft diameter– Smallest hole diameter

Worst case in most assembly situations

• LMC = Least Material Condition– Smallest shaft diameter– Largest hole diameter

Certain uncommon situations only

FITSFITS

Ø1.4941.490

1.5051.500

Ø

SHAFTHOLE

CLEARANCE AT MMC = 1.500 – 1.494 = .006

CLEARANCE FIT

CLEARANCE AT LMC = 1.505 – 1.490 = .015

Ø2.7562.752

FITSFITS

2.7472.741

Ø

SHAFTHOLE

INTERFERENCE AT MMC = 2.756 – 2.741 = .015

INFERFERENCE FIT

INTERFERENCE AT LMC = 2.752 – 2.747 = .005

Ø3.3213.315

FITSFITS

3.3283.319

Ø

SHAFTHOLE

INTERFERENCE AT MMC = 3.321 – 3.319 = .002

TRANSITION FIT

CLEARANCE AT LMC = 3.328 – 3.315 = .013

GENERAL FIT CONCEPT

MMC=MAXIMUM INTERFERENCE or

MINIMUM CLEARANCE

LMC=MAXIMUM CLEARANCE or MINIMUM INTERFERENCE

FITS IN THE MHB• Five Major Fit Classes

– RC – running or sliding clearance fits– LC – locational clearance fits– LT – locational transition fits– LN – locational interference fits– FN – force or shrink interference fits

• Each fit class contains several subclasses• All values in fit tables must be multiplied by

1000 before using them in calculations

EXAMPLECalculate Shaft and Hole diameters and clearances or interferences for a Ø1.50”

basic mating shaft and hole in the following fit conditions:

RC-7

FN-4

LT-3

SHAFT1.50 - .0030 = 1.49701.50 - .0046 = 1.4954

HOLE1.50 + .0025 = 1.50251.50 + 0 = 1.5000

CLEARANCE.003O @ MMC.0071 @ LMC

SHAFT1.50 + .0031 = 1.50311.50 + .0025 = 1.5025

HOLE1.50 + .001 = 1.5011.50 + 0 = 1.500

INTERFERENCE.0031 @ MMC.0015 @ LMC

SHAFT1.50 + .0007 = 1.50071.50 + .0001 = 1.5001

HOLE1.50 + .001 = 1.5011.50 + 0 = 1.500

CLR or INT.0007 INT @ MMC.0009 CLR @ LMC

GSH END DESIGN

CHANGING DIMENSION PROPERTIES

• Left click on dimension to show grips

• Right click to display change menu

• Select PROPERTIES

• Change or add prefix, suffix, precision, tolerances etc.

ADD PREFIX OR SUFFIX

• Prefixes:– 2X, Ø (%%C), R, SØ, SR

• Suffixes:– TYP, MAX, MIN, etc.

CHANGE TO LIMIT

.877

.872Ø TYP

EXAMPLE:

Dimension .875

Lower limit .003

Upper limit .002

Precision 0.000

MOVE DIMENSION OUTSIDE OF EXTENSION

LINES

1.5051.497