Understanding Industrial Blueprints 165
If tolerances are not actually specified on a drawing, certain assumptions can be made regarding
the anticipated accuracy by applying the following principles: For dimensions ending in a fraction of an
inch, such as 1/8, 1/16, 1/32, or 1/64, the required accuracy will be to the nearest 1/64 inch. When the
dimension is given in decimal form, the following principles should be followed: If a dimension is given
as 2.000 inches, the accuracy expected is ±0.005 inch; if the dimension is given as 2.00 inches, the ac-
curacy expected is ±0.010 inch. The ±0.005 is called in shop terms “plus or minus five thousandths of
an inch.” The ±0.010 is called “plus or minus ten thousandths of an inch.”
Fillets and Rounds.
Fillets are concave metal corner (inside) surfaces. In a cast, a fillet normally increases the strength
of a metal corner because a rounded corner cools more evenly than a sharp corner, thereby reduc-
ing the possibility of a break. Rounds or radii are edges or outside corners that have been rounded
to prevent chipping and to avoid sharp cutting edges. Figure 7.4 shows an example of fillets and
rounds.
Slots and Slides.
Slots and slides are used to mate two specifically shaped pieces of material and securely hold them to-
gether, while allowing them to move or slide. Figure 7.5 shows two common types: the tee slot and the
dovetail slot. A tee-slot arrangement is usually used on a milling-machine table, while a dovetail slot is
often used on the cross-slide assembly of an engine lathe.
Keys, Keyseats, and Keyways.
These terms denote several types of small wedgelike metal objects designed to fit mating slots in a shaft
and the hub of a gear or pulley to prevent slippage and provide a positive drive between them. Parts can
be keyed together by means of a flat piece of steel that is partly seated in a recessed shaft or groove
called a keyseat. Figure 7.6 shows three types of keys, a keyseat, and a keyway. A keyseat is a slot or
groove on the outside of a part into which the key fits. A keyway is a slot or groove within a cylinder, tube,
or pipe. A key fitted into a keyseat will slide into the keyway and prevent movement of the parts. Draw-
ings usually include a height dimension that states the measurement from the circumference to the
depth to which the keyway and keyseat are machined.
Coordinate Measuring Machines.
Coordinate measuring machines (CMM) use software and sensors to compare computer prototype
drawings to the finished prototype. Tool and die makers must use accurate prototypes to tool a produc-
tion line that will produce identical quality components.
Casting.
It is the production of metal components by pouring molten metal into moulds and allowing it to solidify.
It can also be a metal component produced by casting.