Technical Reference
220
Tables for Determining Metric Tolerances
Table 1: Description of Preferred Fits Table 1 is pulled from the ANSI metric tolerances section of the machinists’ handbook, which conforms to the ISO and DIN tolerances for hole basis and shaft basis fits. Table 1 provides an overview of the hole basis and shaft basis fits that make up clearance fit, transition fit and interference fit combinations.
ISO SYMBOL HOLE BASIS SHAFT BASIS
FOR WIDE COMMERCIAL TOLERANCES OR ALLOWANCES ON EXTERNAL MEMBERS. NOT FOR USE WHERE ACCURACY IS ESSENTIAL, BUT GOOD FOR LARGE TEMPERATURE VARIATIONS, HIGH RUNNING SPEEDS, OR HEAVY JOURNAL PRESSURES. FOR RUNNING ON ACCURATE MACHINES AND FOR ACCURATE LOCATION AT MODERATE SPEEDS AND JOURNAL PRESSURES. NOT INTENDED TO RUN FREELY, BUT TO MOVE AND TURN FREELY AND LOCATE ACCURATELY. PROVIDES SNUG FIT FOR LOCATING STATIONARY PARTS; BUT CAN BE FREELY ASSEMBLED AND DISASSEMBLED. FOR ACCURATE LOCATION, A COMPROMISE BETWEEN CLEAR- ANCE AND INTERFERENCE. FOR MORE ACCURATE LOCATION WHERE GREATER INTERFER- ENCE IS PERMISSIBLE. FOR PARTS REQUIRING RIGIDITY AND ALIGNMENT WITH PRIME ACCURACY OF LOCATION BUT WITHOUT SPECIAL BORE PRES- SURE REQUIREMENTS. FOR ORDINARY STEEL PARTS OR SHRINK FITS ON LIGHT SEC- TIONS, THE TIGHTEST FIT USABLE WITH CAST IRON. SUITABLE FOR PARTS WHICH CAN BE HIGHLY STRESSED OR FOR SHRINK FITS WHERE THE HEAVY PRESSING FORCES REQUIRED ARE IMPRACTICAL.
H11/c11
C11/h11
LOOSE RUNNING FIT
H9/d9
D9/h9
FREE RUNNING FIT
CLEARANCE FITS
H8/f7
F8/h7
CLOSE RUNNING FIT
H7/g6
G7/h6
SLIDING FIT
H7/h6
H7/h6
LOCATIONAL CLEARANCE FIT
H7/k6
K7/h6
LOCATIONAL TRANSITION FIT
TRANSITION FITS
H7/n6
N7/h6
LOCATIONAL TRANSITION FIT
H7/p6*
P7/h6
LOCATIONAL INTERFERENCE FIT
INTERFERENCE FITS
H7/s6
S7/h6
MEDIUM DRIVE FIT
H7/u6
U7/h6
FORCE FIT
Excerpt from Machinists’ Handbook , pg. 661, 25 Ed., Industrial Press. *The H7/p6 Hole Basis fit is a transition fit for basic sizes in ranges from 0 through 3mm.
Table 2: Commonly Used Hole-Basis System of Fits Table 2 shows closer detail on the hole-basis system of fits than Table 1. When using the hole-basis system of fits, the smallest diameter in the hole tolerance range is fixed to the zero line (basic nominal hole size or diameter), and the clearance between the shaft and hole extends below the zero line, or negative relative to the basic nominal hole size.
LETTER SYMBOLS AND GRADE NUMBERS OF SHAFTS
CLEARANCE FITS
TRANSITION FITS
INTERFERENCE FITS
BASIC HOLES
b
c
d
e
f
g 4 5 6 6 7
h 4 5 6 6 7 7 8
js
k 4 5 6 6 7
m n
p
r
s
t
u
x
4 5 6 6 7
4 5 6 6 7
H5
H6
6 6 7 7 8
6*
6* 6* 7*
6 7
6 7
6* 7*
6 7
6 7
6 7
6 7
H7
8 9 8 9
H8
9 8 9
8 9
H9
9 9
H10 9 *Exceptions occur in some steps of dimensions. 9
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