OutlineIntroduction

Limits, fits and tolerances

Alignment tests

Types of measurements and instruments

Introduction to engineering metrologyMetrology is the science of measurement (Ostwald and Munoz

1997) Importance terms in metrology:Precision: the repeatability of measurementAccuracy: a measurement proximity to the true value

Measurement of dimensions such as length, thickness, diameter, angle, flatness, profile and others.

Dimensional tolerance permissible variation in the dimension of the part

Rule of thumb the smaller the tolerance, the higher the production cost

Limits and fits

It is impossible to make a part to a exact size and if, by chance, an exact size is achieved it is impossible to measure it accurately enough to prove it.

Since a part can not be made to an exact size it is necessary to specify the amount by which the size may deviate from the ideal size. The successful functioning of most manufactured items depends not only upon the individual sizes of the parts but also upon the relationships of those parts in an assembly:

Limits and fits

A perfect part can not be produced.

Therefore it is necessary to specify the amount by which a size may deviate from its ideal size and still fulfill its required functions.

Therefore it is necessary to specify the amount by which a size may deviate from its ideal size and still fulfill its required functions.

Metric nomenclature

Limits and fits

Basic Size- size to which limits or deviations are assignedDeviation- algebraic difference between size and corresponding basic size

Upper deviation- indicates maximum differenceLower deviation- indicates minimum differenceFundamental deviation- which of the above is closer to the basic size

Limits and fits

Tolerance- difference between max. and min. size limits

International tolerance grade numbers (IT)-designated groups of tolerances that vary depending on basic size

Hole basis- system of fits corresponding to hole sizes (H is the fundamental deviation)

Shaft basis- system of fits corresponding to shaft size (h is the fundamental deviation)

Tolerance control

Tolerances are added to:

Control the products that are produced

Dimensions can not be reproduced exactly

Control the accuracy of the process and to reduce functional or assembly failures.

Create more careful production procedures and more rigorous inspection

Tolerance control

There are two different types of conventional tolerances:

Unilateral: specify dimensional variation from the basic size in one direction.

Bilateral: specified dimensional variation from the basic size in both directions.

Tighter tolerances improve the quality of the product but generally increase the manufacturing cost.

Tolerance control

Type of fits and their descriptionType of fit Description Symbol

Clearance Loose running fit: for wide commercial tolerances or allowances on external members H11/c11

Free running fit: not for use where accuracy is essential, but good for large temperature variations, high running speeds, or heavy journal pressures

H9/d9

Close running fit: for running on accurate machines and for accurate location at moderate speeds and journal pressures

H8/f7

Sliding fit: where parts are not intended to run freely, but must have and run freely and locate accurately

H7/g6

Locational clearance fit: Provides snug fit for location of stationary parts, but can be freely assembled and disassembled

H7/h6

Transition Locational transitional: fit for accurate location, a compromise between clearance and interference

H7/k6

Locational transitional fit for more accurate location where greater interference is permissible H7/n6

Interference Locational interference fit: for parts requiring rigidity and alignment with prime accuracy of location but without special bore pressure requirements

H7/p6

Medium drive fit: for ordinary steel parts or shrink fits on light sections, the tightest fit useable with cast iron

Force fit: Suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces are impractical

H7/s6

H7/u6

Limits and fits

Type of fitsClearance fit: The largest permitted shaft diameter is smaller

than the diameter of the smallest hole. LMC of the hole LMC of the shaft = Clearance

Interference fit: The minimum permitted diameter of the shaft is larger than the maximum permitted diameter of the hole.

Least amount of Interference is:LMC Shaft = 1.2513

- LMC Hole = 1.2506Min Interference = .0007

Greatest amount of Interference:MMC Shaft = 1.2519

- MMC Hole = 1.2500Max Interference = .0019

Type of fits

Transitional fit: The diameter of the largest permitted hole is greater than that of the smallest permitted shaft and the smallest permitted hole is smaller than the largest permitted shaft.

LMC Hole = 1.2506- LMC Shaft = 1.2503- Positive Clearance = .0003

MMC Shaft = 1.2509- MMC Hole = 1.2500Negative Allowance (Interference) = .0003

Type of fits systemsThese different types of fits are used in conjunction with two

distinct bases: 1. Hole basis system: The desired clearances and

interferences in the fit are achieved by combinations of various shaft tolerance zone with the hole tolerance zone H. In this system of tolerance and fits, the lower deviation of the hole is always equal to zero.

2. Shaft basis system: The desired clearances and interferences in the fit are achieved in the combination of various hole tolerance zone with the shaft tolerance zone h. In this system of tolerance and fits, the upper deviation of the hole is always equal to zero

Limits exampleJournal bearings are designed to operate at high rotational speeds.If the clearance between inner and the outer diameter is too small the bearing will sieze.If the clearance is too big the shaft will vibrate.Limits on the size of the shaft and hole provide correct operation.

Close running fit H8 f7H8 hole= 20,000 to 20.033f7 shaft= 19,980 to 19,959 clearance= 20 to 74 micron

Nominal diameter 20 mm.

Limits exampleSpool valve has a shaft that translates.This time the clearance should be a sliding fit.Nominal diameter 20 mm.Sliding fit H7/g6.g6 shaft= 19,993 to 19,980H7 hole = 20,000 to 20, 021Clearance= 7 to 28 microns

Limits exampleA 20 mm nominal diameter journal/shaft is to have a clearance, but close accurate running fit. Within what size tolerances should the parts be manufactured? Use the basic hole system.

Solution: A H8/f7 fit is suitable. From the BS chart, for a 20 mm diameter nominal size the H8 limits are + 0.033 and 0.000 and the f7 limits are-0.020 and -0.041 mm. Hence the hole diameter should be between 20.000 and 20.003 mm and the shaft diameter should be between 19.959 and 19.980 mm.

Example

A fit is designated as diameter 130 H7 p61. State the classification of fit produced2. Determine the limits of size both the shaft and the hole.3. State the extremes of fit i.e. the maximum or minimum

interference or clearance.4. Determine the fundamental deviations on both the hole

and the shaft.5. State the tolerance grades for both the hole and shaft.

Solution1. Classification of fit: Interference2. Hole: + 0.040 Shaft: + 0.068, + 0.043Hole: 130.040 shaft: 130.068, 130.0433. Maximum interference occurs between the

smallest hole and the largest shaft; i.e. 130.000 130.068 = - 0.068 mmMinimum interference occurs between the largest

hole and the smallest shaft; i.e. 130.040 130.043 = - 0.003 mm

Solution4. Fundamental deviation for Hole = + 0.040 and +

0.000, Fundamental deviation for shaft = + 0.068, + 0.0435. Tolerance grade for Hole is IT7 = 0.040Tolerance grade for shaft is IT6= 0.025

Question: loose running fitDetermine the loose running fit tolerances for a shaft and hole that have a basic diameter of 32 mm. 32H11/32c11Tolerance Grade Hole Shaft

Upper deviationLower deviationMax DiameterMin Diameter

Average Diameter

0.160 mm0.000 mm32.160 mm32.000

32.080 mm

-0.120 mm-0.280 mm31.880 mm31.720 mm31.800 mm

Max ClearanceMin ClearanceDimensions tolerance in drawing

Dmax- dmin=0.44 mmDmin-dmax=0.12 mm

Hole32.080 +0.080

- 0.080Shaft

31.800 +0.080 0.080

Question: loose running fitDetermine the medium drive force fit tolerances for a shaft and hole that have a basic diameter of 32 mm. 32H7/32s6Tolerance Grade Hole Shaft

Upper deviationLower deviationMax DiameterMin Diameter

Average Diameter

0.025 mm0.000 mm32.025 mm32.000 mm32.013 mm

0.059 mm0.043 mm32.059 mm32.043 mm32.051 mm

Max ClearanceMin Clearance

Dmax-dmin= - 0.018 mmDmin-dmax= - 0.059 mm

Example

Types of measurement and instrumentMeasurement Instrument Sensitivity

mLinear Steel rule,

vernier caliper, micrometer

0.5 mm252.5

Angle Bevel protractor with vernierSine bar

5 min1

Comparative length Dial indicatorGauge blocks

10.05

Straightness Autocollimator 2.5Flatness Interferometry 0.03roundness Dial indicater circular tracing 0.03Profile Dial indicator

Optical comparator1125

GO NOT GO

Limit gaugesThe limits for GO and NOT GO gauges for an internal

diameter component are found as follows:

The workpiece tolerance is 0.200 mm. From the column 4 of Table 1, the limits for GO gauges are:

+ 0.021, +0.012, therefore, the size of the GO gauge is:+75.021 mm, + 75.012 mm.

Limit gaugesThe limits for GO and NOT GO gauges for an internal

diameter component are found as follows:

The workpiece tolerance is 0.200 mm. From the column 5 of Table 1, the limits for NOT GOgauges are:+ 0.0, -0.009, therefore, the size of the NOT GO gauge is:+75.200 mm, + 74.991 mm.

Limit gaugesThe limits for GO and NOT GO gauges for a shaft are

found as follows:

The workpiece tolerance is 0.040 mm. From the column 6 of Table 1, the limits for GOgauge are: -0.002 mm, - 0.005 mm+ 0.0, -0.009, therefore, the size of the GO gauge is:+44.928 mm, + 44.925 mm.

Limit gaugesThe limits for GO and NOT GO gauges for a shaft are

found as follows:

The workpiece tolerance is 0.040 mm. From the column 7 of Table 1, the limits for NOT GOgauge are: +0.003 mm, - 0.000 mmtherefore, the size of the NOT GO gauge is:+44.893 mm, + 44.890 mm.

References

1. S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006.

2. E. Paul Degarmo, J. R. Black, R. A. Kohser; Materials and Processes in Manufacturing, 9th edition, John Wiley & Sons, Inc, 2003.

3. R. L. Timings, S. P. Wilkinson: Manufacturing Technology, 2ndedition, Pearson Education Limited, London, 2000.

4. J. F. W. Galyer, C. R. Shotbolt: Metrology for Engineers, Cassell & Co. Ltd, 3rd edition, 1977.

5. Data Sheet BS 4500A: 1970