metrology limits and fits

1. Metrology
Module 1
Lecture 1

2. Introduction
• Metrology is the study of measurements
• Measurements are quantitative observations;
numerical descriptions

3. LIMITS
• 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.

4. TERMINOLOGY
• NOMINAL SIZE: It is the
size of a part specified in
the drawing.
• BASIC SIZE: It is the size of
a part with respect to
which all limits of variation
are determined.
• ACTUAL SIZE: It is the
actual measured
dimension of a part.
Nominal and basic size are
often the same.

5. • Tolerance is defined as the total permissible
variation of a size.
Tolerance = maximum limit of size - minimum
limit of size.
• Allowance: It is the difference between the
basic sizes of the mating parts.
Allowance = basic size of hole - basic size of
shaft

6. ACCURACY
• Accuracy is closeness of an measured value to
the true or accepted value
• Accuracy can be measured as
% error = True value – measured value X 100%
True value
• True value refers to the measurement where
there is not any error involved in its
measurement

7. PRECISION
• Precision is the consistency of a series of
measurements
• PRECISION CAN BE EXPRESSED as Standard
deviation of measurements
• Take the mean (average) of measurements
• Calculate how much each measurement deviates from
mean
• Take an average of the deviation, so it is the average
deviation from the mean

8. • Bullseye charts representing precision and bias of
a measurement instrument

9. DEVIATION
• It is the algebraic difference between any given
size and basic size.
• LOWER DEVIATION: It is the algebraic difference
between the minimum limit of size and the basic
size.
• UPPER DEVIATION: It is the algebraic difference
between the maximum limit and the basic size.

10. LIMITS, FITS AND TOLERANCE
• ZERO LINE : It is the straight line
corresponding to the basic size. The deviations
are measured from this line.

12. FITS
• There are three general types of fit between the
mating parts
1. Clearance fit: A clearance fit is one having limits of
size are so prescribed that a clearance always
results when mating parts are assembled.
2. Interference fit: An interference fit is one having
limits of size so prescribed that an interference
always results when mating parts are assembled.
3. Transition fit: A transition fit is one having limits of
size so prescribed that either a clearance or
interference may always result when mating parts
are assembled.

13. Types of fits

14. • Fundamental deviation: The fundamental
deviation is that limit deviation, which is the
nearest to the nominal size