2. Individually manufactured parts and components eventually are assembled into
products. We take it for granted that when a thousand lawn mowers are
manufactured and assembled, each part of the mower will mate properly with its
For example, when we have to replace a broken or worn bolt on an old machine, we
purchase an identical bolt. We are confident from similar experiences in the past that
the new bolt will fit properly in the machine. The reason we feel confident is that the
bolt is manufactured according to certain standards and the dimensions of all similar
bolts will vary by only a small, specified amount that do not affect their function. In
other words, all bolts are manufactured within a certain range of dimensional
tolerance; thus, all similar bolts are interchangeable.
Dimensional Tolerance: Dimensional tolerance is defined as the permissible or
acceptable variation in the dimensions (height, width, depth, diameter, and angles)
of a part. Tolerances are unavoidable, because it is virtually impossible and
unnecessary to manufacture two parts that have precisely the same dimensions.
Fit and Tolerance
3. Basic size: Dimension from which limits of size are derived with the use of
tolerances and allowances.
Nominal size: An approximate dimension that is used for the purpose of general
Standard size: Nominal size in integers and common subdivisions of length.
Zero line: Reference line along the basic size from which a range of tolerances
and deviations are specified.
Fundamental deviation: the distance from the basic size where the tolerance
zone is situated.
Limit dimensions: The maximum and minimum dimensions of a part; also called
Fit: The range of looseness or tightness that can result from the application of a
specific combination of allowance and tolerance in the design of mating-part
5. Unilateral tolerance: Deviation from the nominal dimension in one direction
only. Or when the tolerance zone is only one side of zero line.
Bilateral tolerance: Deviation (plus or minus) from the basic size. Or when
tolerance zone is both side of zero line.
Various methods of assigning tolerances on a shaft: (a) bilateral tolerance, (b)
unilateral tolerance, and (c) limit dimensions.
6. Hole-basis system: Tolerances based on a zero line on the hole; also called standard hole practice or
basic hole system.
Shaft-basis system: Tolerances based on a zero line on the shaft; also called standard shaft practice or
basic shaft system.
Maximum material condition (MMC): The condition whereby a feature of a certain size contains the
maximum amount of material within the stated limits of that size.
7. Clearance: The space between mating parts.
Clearance fit: Fit that allows for rotation or sliding between mating parts. In
other words If lower limit of hole is larger than the upper limit of shaft.
Interference: Negative clearance.
Interference fit: A fit having limits of size so prescribed that an interference
always results when mating parts are assembled. In other words if maximum size
of hole is smaller than the minimum size of shaft then force has to be applied to
make the assembly.
8. Allowance ( = minimum clearance or maximum interference): The specified
difference in dimensions between mating parts; also called functional dimension
or sum dimension. It is defined as the difference between maximum material
limit of hole and shaft.
Transition fit: A fit with small clearance or interference that allows for accurate
location of mating parts. This type of fit appears when there is a overlaps in
tolerance zone. Some of the assembly can be made without the application of
force and for some of the assemblies force will be required.
9. Inspection Gauges
°PLUG GAGES: Plug gage are commonly used for holes (Figs. a and b). The GO
gage is smaller than the NOT GO (or NO GO) gage and slides into any hole that has
a dimension smaller than the diameter of the gage. The NOT GO gage must not go
into the hole. Two gages are required for such measurements, although both may be
on the same device-either at opposite ends or in two steps at one end (step-type
gage). Plug gages also are available, for measuring internal tapers (in which
deviations between the gage and the part are indicated by the looseness of the gage),
splines, and threads (in which the GO gage must screw into the threaded hole).
RING GAGES: Ring gages (Fig. c) are used to measure shafts and similar round
parts. Ring thread gages are used to measure external threads. The GO and NOT
GO features on these gages are identified by the type of knurling on the outside
diameters of the rings.
SNAP GAGES: Snap gages (Fig. d) commonly are used to measure external
dimensions. They are made with adjustable gaging surfaces for use with parts that
have different dimensions. One of the gaging surfaces can be set at a different gap
from the other, thus making the device a one-unit GO-and-NOT-GO gage.
10. (a) Plug gage for holes, with GO and NOT GO on opposite ends of the gage. (b) Plug gage with GO and
NOT GO on one end. (c) Plain ring gages for gaging round rods. Note the difference in knurled surfaces to
identify the two gages. (d) Snap gage with adjustable anvils.