Chapter 10 cont.Dimensioning and Tolerancing

Dimensioning Features (cont.)Hexagons and polygons

Should be dimensioned across the flats in a view that shows it in it’s true shape

Arcs Dimensioned with a leader in a view

where shown as it’s true shape Diameters-dimensioned with symbol

before the value Radii-dimensioned with capitol R

before the value Leader should theoretically extend

through the center of the arc Leader can physically extend through

the center of the arc to help locate the arc center

Center Marks Center marks should be placed on all

diameter features Center marks should only be placed

on radii that require center location

Spherical features Spherical radii dimensioned with an SR

before dim value Spherical Diameters dimensioned with

an S before the dim value

Contours Shapes that are not defined as arcs Dimensioned by locating points along the

contour

Locating a Point Method of locating points that are not

part geometry but are necessary to describe the shape

Commonly referred to as “point of intersection” or PI

Often PI will be used to note point of intersection

PI

Notes for size featuresHoles

Holes-dimensioned in views where they appear as circles or in a sectional view

Dimensioning blind holes

Blind holes-a hole that does not go thru the part

Depth must be given in one of two ways

Counterbore

One small thru hole for a fastener to pass through

One large hole to recess the head of a fastener below the surface of the part

You will need to know how to place one style of these notes on a drawing

Countersink

One small thru hole for a fastener to pass through

Large hole has angled sides to accept a countersink headed fastener

Fastener head sits flush with top of the part

You will need to know how to place one style of these notes on a drawing

Counterdrill Combination

counterbore and countersink

You will need to know how to place one style of these notes on a drawing

Spotface

Similar to a counterbore accept for the depth of the large hole

Large hole is a machined surface to provide a flat bearing surface for a fastener to clamp against

Depth is determined by machinist based on how rough the surface of the part is

Seen commonly on cast parts

You will need to know how to place one style of these notes on a drawing

Slotted Holes Must define length and width (and

location) Choose a method based on what is

important for part function

Knurls A pattern machined into parts to provide

grip Not shown in true detail on drawings Defined by a note with a leader to show

the area to be knurled

Necks and grooves Areas on cylindrical parts where the

diameter changes for a certain length Appear on turned parts

Repetitive Features When a part has more than one feature of the

same size, they can be dimensioned with a note that specifies the number of features

Should only be used when it is obvious which features are the same size