Dimensioning

01/28/09

How many things are wrong with

this drawing?

Working Assembly Drawing

How many things are wrong with

this drawing?

Dimensioning of Holes

Counter Bored

Holes are shown

as concentric

circles in the plan

view, and

dimensioned

using the

standard symbols

Dimensioning of Holes

Countersunk holes

are dimensioned by

Callout the diameter

of the thru hole, the

diameter of the to of

the counter sink and

the included angle of

the countersink.

Spot Faced Holes

The symbols for

spot faced holes are

the same used for

countersunk holes.

Conventional Breaks

These are used to shorten very long objects

that have the same cross section throughout

or a uniform taper.

Cylindrical object are “broken” with a “S”

shape

Rectangular and other non-cylindrical object

are “broken” with a jagged heavy weight

irregular line, (.6mm), always drawn freehand.

Conventional Breaks

The length of these parts will not be

shown to scale, but will be dimensioned

as if the break was not there.

Dimensioning of Chamfers

Dimensioning of Knurl Finishes

A knurl is a roughened surface intended to provide a better hand grip or press fit between two parts

Coordinate Dimensioning

The use of Coordinate or baseline dimenions

eliminates the effect of “accumulation of

tolerances”, and is useful to the machinist

using a positioning table.

Tabular Dimensions

Used to show dimensions of similar parts, tabular dimension reduce the number of drawings required for a series of nearly identical parts

Mating Parts

In instances where a feature on a part

must fit into or onto another part. The

“mating dimension” must be identified.

Mating Parts

Mating Dimensions

Most often the dimensions of the mating

features will not be the same, and a

factor will be added to the dimension to

allow for the type of fit intended by the

Designer, (clearance, interference,

etc…)

The Cad Technician must choose the

dimensions keeping the type of fit and

tolerances in mind.

Key Ways

The dimensions for standard key ways are found in The Machinists Handbook

The distance from the center of the recess to the edge and the width of the recess are always provided.

The distance from the bottom of the shaft to the bottom of the keyway is always provided, rather then depth of the recess

Key Ways

Machine Pattern and Forging Dimensions

Parts that are machined from castings, will have a set of dimensions that the casting pattern maker will work from, and a set of dimensions that the machinist will work from.

When the part is not complicated enough to require two drawings, the dimensions for pattern maker and the machinist may be included on the same drawing.

Since the machinist will refer to the drawing more often then the pattern maker the drawing should be arranged in a manner that is convenient for the machinist.

Machine and Pattern Dimensions

Forged and Machined Parts

Separate drawings are typically created

for the forged part detail and the

machined part details.

Finish Marks

A Finish mark is used to identify those

surface that are to be machined or

finished, (as on a casting of forged part)

To the diemaker or pattern maker it

indicates that additional material should be

provided to allow for finishing.

If a part is to be finished on all faces a

note such as FINISH ALL OVER would be

used rather than the finish symbol

Finish Marks

Finish marks are only shown on the

edge view of a finished surface, and are

repeated in all views that show the

finished edge..

If a part is to be finished on all surfaces,

the notation “Finish All Over” or “FAO”

should be used in lieu of a Finish Mark

Finish Marks

Surface Finishes

The need for higher performance power

transmission components and other

devices that can withstand heavy loads

and high speeds has increased the need

for accurate control of surface finishes

Surface Finishes

The surface finish symbol alone is not

enough to specify a finish for a high

performance component.

Symbols for Surface Texture, Lay, and

Roughness are used to dimension

surface finish.

Surface Finish Symbols

Surface Finish Specifications

Lay Symbols

Surface Roughness

Designations

Process Range of Roughness

in Microinches

Range of

Roughness in

Micrometers

Flame

Cutting

500 – 1000 12.5 - 25

Milling 32 – 250 .80 – 6.3

Grinding 4 – 63 .10 – 1.63

Electro

Polish

4 – 32 .10 -.80

Welds

Welding Symbols

Most Commonly used welds

Fillet

Bevel

V Grove

V Grove Butt Welds

Bevel and Butt Welds