Tolerancing System

Tolerancing System

In an assembly process the degree of "clearance" or "tightness" desired between mating parts is important.

In a manufacture of a machine, quality is a primary consideration. Manufacturing precision taken into the product determines its quality, its cost and selling price.

Parts of a machine are designed in order to make a function. The working parts have a definite relationship with each other: free rotation, free longitudinal movement, clamping action, permanent fixed position. Precision is the degree of accuracy necessary to ensure the functioning of a part as intended.

There are two types of surfaces:
1) Nonmating parts - left in their original rough-cast form
2) Mating parts - are machined to proper smoothness and must be at correct distance from each other. So greater manufacturing precision is required for these surfaces. However because of impossiblity to make a distance to an absolute size some variation must be allowed.

TOLERANCE is the allowable variation for any given size in order to achive a proper function.

Basic size : 1.500 in
Tolerance on the shaft : 0.002 in
Tolerance on the slot : 0.001 in
Allowance : 0.004 (tightest possible fit - maximum material limit)

Cumulative tolerance effect : Dimensions will vary in position by the sum of the tolerances. This difficulty can be eliminated by dimensioning from one position (base-line dimensioning)

Typical tolerances available with specific machining operations

A clearance fit (the shaft is always smaller than the hole)
Tolerance on shaft : 0.002
Tolerance on hole : 0.002
minimum clearance : 1.500 - 1.495 = 0.005 in
maximum clearance : 1.502 - 1.493 = 0.009 in
clearance 0.005 in for the tightest possible fit

An interference (force) fit (when the shaft is always larger in diameter than the hole - parts must be assembled by pressure or heat expansion)
Tolerance on shaft : 0.001
Tolerance on hole : 0.001
minimum clearance : 2.000 - 2.003 = -0.003 in (the tightest fit 0.003 in interference)
maximum clearance : 2.001 - 2.002 = -0.001 in (the loosest fit 0.001 in interference)

Maximum clearance=Minimum interference

Minimum clearance=Maximum interference

A transition fit exist when the maximum clearance is positive and the minimum clearance is negative
Tolerance on shaft : 0.005
Tolerance on hole : 0.005
minimum clearance : 1.000 - 1.007 = -0.007 in (the tightest fit 0.007 in interference)
maximum clearance : 1.005 - 1.002 = 0.003 in (the loosest fit 0.003 in clearance)

Transition fits are used only for locating a shaft relative to a hole, where accuracy is important but either a clearance or an interference is permitted.

Standardized Tolerances

We use standardized tolerances for fits of parts.

Cylindrical Fits:

Decimal inch system, based on ANSI Standard (B4.1 - 1967(1979))
Millimeter system (ISO), based on ANSI Standard (B4.2 - 1978)
ANSI - American National Standards Institute
ISO - International Organization for Standardization

Basic - Hole System : (Nominal) size is given to Hole
Basic - Shaft System: (Nominal) size is given to shaft

Basic(or Normal) Hole System : Hole dia. constant, shaft dia. changing either smaller or larger to satisfy the required fit (min hole dia. always nominal size)

Basic (Normal) Shaft System : Shaft dia. constant, hole dia. can be smaller or larger to satisfy the required fit (max shaft always nominal size)

Normal - Hole System is prefered because of availability of drills and reamers in machine shop. But for a transmission system bearings must fit the shaft so shaft system is used.

Decimal inch system, based on ANSI Standard (B4.1 - 1967(1979))

The letter symbols are used in ANSI Standard

RC, running or sliding fit
LC, locational clearance fit
LT, locationa transition fit
LN, locational interference fit
FN, force or shrink fit

RC# (The letter symbols are used in conjunction with numbers)
the lower RC numbers are the tighter fits
the higher numbers are the looser fits

FN4 - Represents a complete fit (class 4 force fit)

Appendix 8A : Running and Sliding Fits

Appendix 8B : Locational Clearance Fits

Appendix 8C : Locational Transitional Fits

Appendix 8D : Locational Interference Fits

Appendix 8E : Force Fits

EXAMPLE

RC6 fit

Nominal size : 1 in.

Nominal Size Range	Limits of Clearance	Standard Limits
		Hole H9	Shaft e8
0.71 - 1.19 (Nominal Size : 1 in)	1.6	+2.0	-1.6
	4.8	0	-2.8

1.0000 - 0.9984 = 0.0016 (clearance)
1.0020 - 0.9972 = 0.0048 (clearance)

Appendix 8A , page 661

FN4 fit

Nominal size : 2 in.

Nominal Size Range	Limits of Clearance	Standard Limits
		Hole H9	Shaft e8
(Nominal Size : 2in)	2.3	+1.2	+4.2
	4.2	0	+3.5

2.0000 - 2.0042 = -0.0042 (interferance)
2.0012 - 2.0035 = -0.0023 (interferance)

Appendix 8E , page 666

ANSI Standart (b4.2 - 1978) related to ISO standarts

In ISO System letters are used to show different types of fits according to deviation from 0 line.

Hole tolerances are shown with capital letter A-Z

Shaft tolerances are shown with small letters a-z

Quality of tolerances is given with a # (H7, g6, m6, k8)

Quality 1-5 (measuring instruments)
Quality 5-11 (mechanical parts)
Quality 11-18 (coarse work)

Appendix 9A : Basic Hole Clearance Fits

Appendix 9B : Basic Hole Transition and Interference Fits

Appendix 9C : Basic Shaft Clearance Fits

Appendix 9D : Basic Shaft Transition and Interference Fits

EXAMPLE

H7s6 fit

Basic dia.	Tolerance pos & #
f60	H7	60.030
		60.000
	s6	60.072
		60.053

Interference fit
60.030 - 60.053 = -0.023 (max fit)
60.000 - 60.072 = -0.072 (min fit)

Appendix 9B , page 671

h9D9 fit

Basic dia.	Tolerance pos & #
f25	h9	25.000
		24.948
	D9	25.117
		25.065

Clearance fit
25.065 - 25.000 = 0.065 (max fit)
25.117 - 24.948 = 0.169 (min fit)

Appendix 9C , page 672