Limits, Fits and Tolerances

Interchangeability An interchangeable part is one which can be substituted for

similar part manufactured to the same drawing.

 When one component assembles properly (and which satisfies the functionality aspect of the assembly) with any mating component, both chosen at random, then it is known as interchangeability.

Or The parts manufactured under similar conditions by any company

or industry at any corner of the world can be interchangeable

Before the 18th century production used to be confined to small number of units and the same operator could adjust the mating components to obtain desired fit.

Devices such as guns were made one at a time by gunsmith. If single component of a firearm needed a replacement, the entire firearm either had to be sent to an expert gunsmith for custom repairs, or discarded and replaced by another firearm.

Historical Background

Examples Keys Couplings Pin Joints Screwed Fasteners Gears Clutches

The advantages of interchangeability1. The assembly of mating parts is easier. Since any

component picked up from its lot will assemble with any other mating part from another lot without additional fitting and machining.

2. It enhances the production rate.3. It brings down the assembling cost drastically.4. Repairing of existing machines or products is

simplified because component parts can be easily replaced.5. Replacement of worn out parts is easy.6. Without interchangeability mass production is not

possible.

Selective assembly The discussion so far has been in connection with full

interchangeability or random assembly in which any component assembles with any other component.

Often special cases of accuracy and uniformity arises which might not be satisfied by certain of the fits given under a fully interchangeable system.

For example if a part at its low limit is assembled with the mating part a high limit, the fit so obtained may not fully satisfy the functional requirements of the assembly.

Also machine capabilities are sometimes not compatible with the requirements of interchangeable assembly.

For selective assembly, components are measured and sorted into groups by dimension, prior to the assembly process. This is done for both mating parts.

Consider a bearing assembly Hole with , Shaft Clearance should be 0.14mm Randomly if we take and clearance will be 0.08mm Hole and Shaft pairing respctively which gives 0.14mm

clearance 24.97 and 24.83, 25.0 and 24.86, 25.02 and 24.88

1. Shaft: All outer elements of a part including those which are not cylindrical.

2. Hole: All inner elements regardless of their shape.

3. Basic Size: It is the standard size for the part & is same for both the hole & Shaft. This is obtained by calculation for strength.

4. Actual Size: It is the dimension which is measured on the actual manufactured part. Actual & Basic Size will never be equal.

5. Limits of Size: It is the maximum & minimum Permissible Size of the part.

System’s Terminologies

6. Maximum Limit: The maximum Size Permitted for the part.

7. Minimum Limit: The Minimum size permitted for the part.

8. Zero Line: A straight line to which deviations are referred. It is a line of zero deviation.

9. Deviation: It is the algebraic Difference b/w a size (actual, limit of a size etc.) & the Corresponding basic Size.

10. Upper Deviation: It is the algebraic Difference b/w maximum limit of size & the Corresponding basic Size.

11. Lower Deviation: It is the algebraic Difference b/w minimum limit of size & the Corresponding basic Size.

12. Fundamental Deviation: The deviation which is the nearest to the

zero line.

13. Actual Deviation: It is the algebraic difference b/w an actual size & corresponding basic size.

14. Mean Deviation: It is the arithmetical mean b/w the upper & lower deviation.

15. Tolerance: The difference b/w upper & lower limit of a dimension.

16. Tolerance Zone: It is defined by its magnitude & by its position in relation to the zero line.

17. Tolerance Grade: It is the degree of accuracy manufacturing.

18. Tolerance Class: This term is used for a combination of fundamental deviation & tolerance grade.

19. Allowance: It is an intentional difference b/w the maximum material limits for the mating parts.

20. Fits: The relationship existing b/w two parts which are to be assembled w.r.t the difference in their sizes is called fit.

There are two extreme possible sizes of a component.

The largest permissible size for a component is called upper limit and smallest size is called lower limit.

Limits

A tolerance is the total permissible variation from the specified basic size of the part.

There are two types of tolerance:

Tolerance

Minimum & Maximum Metal Limits

The degree of tightness or looseness between two mating parts is called a fit.

Fits

1. Clearance Fit: It is a Fit that always enables a clearance b/w the hole & the shaft. There may be: Slide Clearance Fit (SC) Running Clearance Fit (RC)

Types of Fits

2. Transition Fit: It is a Fit where both clearance & interference may occur in the coupling. Tolerance zones of the hole & shaft partly or completely interfere. It may be: Push Fit Wringing Fit

3. Interference Fit: It is a fit that always ensure some interference b/w the whole & shaft in the coupling. It may be Longitudinal Press (force fit) Transverse Press (shrink fit)

System of Fit

Limit gauges are made to the limits of the dimensions of the part to be tested. There are two limit of dimensions, so we need two limit gauge.

Go gauge should pass through or over a part while Not Go gauge should not pass through or over the part.

Limit Gauges

On the basis of, TYPE Standard Gauges Limit Gauges

On the basis of, PURPOSE Workshop Gauges Inspection Gauges Reference/Master Gauges

On the basis of, GEOMETRY Plug Gauges Snap/Ring Gauges

On the basis of, DESIGN Single/Double Limit Gauges Fixed/Adjustable Gauges Solid/Hollow Gauges

Types of Gauges

They are used for checking Holes that whether the hole dimensions are with in specified tolerance.

Go-plug gauge check the lower limit of the hole. No-Go gauge check the upper limit of hole.

Plug Gauges

They are external or internal diameter measuring gauges.

Ring Gauges

The most satisfactory method of testing a taper is to use taper gauges.

They are also used to gauge the diameter of the taper at some point.

Taper gauges are made in both the plug and ring styles and, in general, follow the same standard construction as plug and ring gauges.

Taper Gauges

Thread gauges are used to check the pitch diameter of the thread.

For checking internal threads (nut, bushes, etc.), plug thread gauges are used, while for checking external threads (screws, bolts, etc.), ring thread gauges are used.

Single-piece thread gauges serve for measuring small diameters.

For large diameters the gauges are made with removable plugs machined with a tang.

Thread Gauges

RADIUS GAUGE The function of these gauges is to check the

radius of curvature of convex and concave surfaces over a range from 1 to 25 mm.

The gauges are made in sets of thin plates curved to different radius at the ends.

Each set consists of 16 convex and 16 concave blades.

FEELER GAUGE Feeler gauges are used for checking

clearances between mating surfaces. They are made in form of a set of steel,

precision machined blade 0.03 to 1.0 mm thick and 100 mm long.

Each blade has an indication of its thickness. To find the size of the clearance, one or two

blades are inserted and tried for a fit between the contacting surfaces until blades of suitable thickness are found.

◦ Design of Limit Gauges1. Guidelines for gauge design2. Material Consideration for Gauges3. Gauge Tolerance4. Wear Allowance5. Gauging Force

Home Assignment

Angular Metrology

Circles are divided into 360 equal parts, each being a degree.

Each of these degrees can be evenly divided into 60 equal parts. These parts are called minutes.

These minutes can be evenly divided into 60 equal parts. These parts are called seconds.

Another common measurement unit for angle is Radian.

Introduction

Angle Measuring Devices

Protractors Protractor

Machinists Protractor

Arm Protractor

Squares

Angle gauges

Bevel Protractor

Used when accuracy of angle must be checked to less than 5 minutes.Consists of steel bar with two cylinders of equal diameter fastened near ends.

Centers of cylinders exactly 90º to edgeDistance between centers usually 5 or 10 inches and 100 or 200 millimeters.

Made of stabilized tool hardened steel.

Sine Bar

Sine Bar

Autocollimator