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## MECHANICAL DRAWING Chapter 10: TOLERANCES AND FITS

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Chapter Objectives

- Describe the nominal size, tolerance, limits allowance …
- Identify a clearance fit, interference fit, and transition fit.
- Describe the basic hole and basic shaft systems.
- Describe the classes of fit.
- Draw geometric tolerancing symbols.
- Specify geometric tolerances.

INTRODUCTION

Objectives of specifying tolerances:

- Makes it possible that any two mating parts will fit together.
- Increased precession makes the part more expensive to manufacture.
- The degree of accuracy is according to functional requirements.

Tolerance:

- Is the difference between the maximum and minimum permitted size of a part.
- Is the total amount a specific dimension is permitted to vary.

TOLERANCES

Example:

Maximum size

Maximum Limit

Shaft

Hole

Minimum size

Minimum Limit

Tolerance = 31.70 – 31.67 = 0.03

Tolerance = total amount of variation

31.78 – 31.75 = 0.03

Dimension could be 31.78 or 31.75 or any value between them

It is a decision of the designer to choose a suitable dimension with lower cost.

Note that the previous parts are interchangeable (mating parts).

TOLERANCES

Example:

DEFINITIONS

- Nominal size: the dimension to which a part to be made if it were possible to manufacture to exact size.
- Tolerances: the total amount by which a given dimension may vary or the difference between limits.
- Limits: the max. and min. sizes indicated by a tolerance dimensions.

For the previous ex.: limits for the hole = 1.251 and 1.250

limits for the shaft = 1.247 and 1.248

- Allowance: minimum clearance space.

clearance fit = (+ve) allowance interference fit = (-ve) allowance

DEFINITIONS

- Manufacturing costs increases considerably as tolerances zones decrease. Hence, general tolerances are often adopted, which allow reasonable limits for every day applications. They may vary depending on the type of product and degree of precision required.
- General tolerances:

Manufacturing processGeneral tolerances (mm)

Drilling + 0.125

Turning – diameters ± 0.15

Grinding – surfaces ± 0.025

Relating limits to fits

- Manufacturing limits are specified relative to the basic size and expressed unilaterally or bilaterally according to the system of limits adopted.
- Basic size/limits: the nominal size to which tolerances must be added or subtracted, and is equal for both mating parts.
- Unilateral limits: limits whose difference are from one side only of the basic size.
- Bilateral limits: limits whose difference result in a tolerance zone on both sides of the basic size.

Max. limit

Max. limit

Min. limit

Min. limit

Bilateral

Unilateral

Hole

Hole

Basic size (could be any one)

Basic size

Basic hole and shaft systems

Basic hole system: the minimum hole is taken as the basic size. (Normally standard drilling machines are used to generate the hole and the shaft is easier to be machined to the desired diameter).

Basic shaft system: the maximum shaft is taken as the basic size (shaft cannot be machined to size easily).

Basic hole and shaft systems

Example:

Given basic size of 15mm, allowance of 0.02mm, hole machining has a precision of ±0.01mm and shaft machining of ± 0.02mm. Determine the hole and shaft limits for a basic hole fit.

Solution:

Basic hole fit min. hole size = 15mm

Hole tolerance 0.02

Allowance = 0.02

Shaft tolerance 0.04

Min

hole

Dia.

Max hole

Dia. = 15.02

Max

shaft

Dia.

15mm

Min shaft

Dia. = 14.94

14.98

FITS

Fits: relation between mating pairs before assembly:

Pairs: shaft/hole.

Fits purpose: assemble/disassemble of parts.

sliding a part relative to the other.

Clearance

- Types of fit:
- Clearance fit: always there is a clearance between the max. limit of the shaft and the min. limit of the hole.
- Interference fit: min. limit of the shaft is always greater than the max. limit of the hole.
- Transition fit: the fit could be either clearance or interference. (small interference and clearance)

Interference

Transition

METRIC SYSTEM OF TOLERANCES AND FITS (ISO)

International tolerance grade: IT, uniform degree of accuracy provided to the basic size

Fine tolerance,

Precise work

Coarse tolerance

SPECIFYING TOLERANCES

- General Tolerances Notes

"ALL TOLERANCES ±1 MM

UNLESS OTHERWISE NOTED.

ANGLES ±1 DEGREE."

- Limit Tolerances/Dimensions

The high limit is placed above the low limit.

In single-line note form, the low limit precedes the high limit separated by a dash

METRIC SYSTEM OF TOLERANCES AND FITS

Terms Related to Metric Limits and Fits

METRIC SYSTEM OF TOLERANCES AND FITS (ISO SYMBOLS)

- Hole base system:

Hole: H (uppercase)

Shaft: (c, d, f, g, h), (k, n), (p, s, u) (lowercase)

- Shaft base system:

Hole: h (lowercase)

Shaft: (C, D, F, G, H), (K, N), (P, S, U) (uppercase)

Each letter has a meaning in accordance with the class of fit. As will be explained in the next slides/tables

CLASSES OF FITS (ISO SYMBOLS)

- Clearance fits:
- Loose running fit: H11/c11

Wide commercial tolerances. Precision is not important.

- Free running: H9/d9

Accuracy is not important, or large temp. variation, or both conditions.

- Close running: H8/f7

Running on accurate machines, accurate locations at moderate speeds and journal pressure. Accurate location and minimum play are desired.

- Sliding fit: H7/g6

For easy move and turn (no free motion), and accurate location.

- Locational fit (precision clearance fit): H7/h6

Location of stationary parts (static applications).

- Transition fits:
- Interference fits:

CLASSES OF FITS (Clearance fits)

Example:

For free running clearance fit, find hole and shaft limits for a base size of 12mm.

Also provide the fit symbol in the preferred form.

Solution:

From the previous table for free running fit,

Hole limits: 12.000-12.043

Shaft limits: 11.907-11.950

Fit symbol: 12H9/d9

CLASSES OF FITS (ISO SYMBOLS)

- Clearance fits:
- Transition fits:

Compromise between clearance and interference fits. For application where accuracy of location is important (small clearance or interference is permissible). Ex., gears and keys on shafts.

- Location transition: H7/k6
- Location transition: H7/n6 (more accurate location)
- Interference fits:
- Location interference: H7/p6

Slight interference (press fit). Parts to be assembled/disassembled with accuracy without bore pressure. Ex: couplings, brass collars…

- Medium drive: H7/s6

Ordinary steel parts, shrink fit on light sections, tightest fit for cast iron.

- Force fit: H7/u6

Parts to be joined with high stresses, shrink fit on medium sections.

CLASSES OF FITS (Transition & Interference Fits)

Example:

For a basic hole diameter of 40mm, H7p6, determine the class of the fit, the limits for the hole and the shaft.

Solution:

From the previous table for free running fit,

Type of fit: locational interference

Hole limits: 40.000 – 40.025

Shaft limits: 40.026 - 40.042

40H7/p6

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