Shanghai Jiao Tong University 1

GEOMETRIC DIMENSIONING & TOLERANCING (GD & T)

ME 250: Design & Manufacturing I

School of Mechanical Engineering

Shanghai Jiao Tong University 2

Outline Tolerance

Definitions Datums Form Controls Rules for Tolerancing Location Control Orientation Controls Runout Controls Profile Controls

Surface Finish

Shanghai Jiao Tong University 3

Rules for Tolerancing RULE-1: For features-of-size, where

only a size dimension is specified, the surfaces shall not extend beyond a boundary (envelope) of perfect form at MMC.

“perfect form” means perfect flatness, straightness, circularity, cylindricity.

Shanghai Jiao Tong University 4

Rule-1 for Tolerancing

Shanghai Jiao Tong University 5

Rule-1 for Tolerancing Rule-1 applies to all feature-of-size (and only

to feature-of-size) on a drawing. Rule-1 says that a feature-of-size must have

perfect form if the feature-of-size is at MMC. If the part was at less than MMC, a form error

equal to the amount of departure would be permissible.

If the part was at LMC, a form error equal to the amount of departure would be permissible.

Shanghai Jiao Tong University 6

The Term of RFS The Regardless of Feature Size (RFS) is

when a geometric tolerance (or datum) applies independent of the feature size. That is, the geometric tolerance is limited to the stated amount regardless of the size of the feature-of-size.

Shanghai Jiao Tong University 7

Rules for Tolerancing

Shanghai Jiao Tong University 8

Rules for Tolerancing

Shanghai Jiao Tong University 9

Exercise in Class If the block was at MMC, what would

its straightness be limited to? the flatness of surface A be limited to?

If the block was at LMC, the flatness of surface A would be limited to and the flatness of surface B would be limited to . Could both of these flatness conditions exist simultaneously? Why?

If the block was at LMC and the flatness of surface A was 0.3, what would the flatness of surface B would be limited to?

Shanghai Jiao Tong University 10

Location Control

Shanghai Jiao Tong University 11

Boundary for Surface of Hole

Shanghai Jiao Tong University 12

Location Control

Shanghai Jiao Tong University 13

Location Control

Shanghai Jiao Tong University 14

Orientation Controls Perpendicularity Angularity Parallelism

Always uses a datum surface

Shanghai Jiao Tong University 15

Perpendicularity

Shanghai Jiao Tong University 16

Perpendicularity

Shanghai Jiao Tong University 17

Runout Controls Circular runout Total runout

Always uses a datum surface

Shanghai Jiao Tong University 18

Runout Tolerance

Shanghai Jiao Tong University 19

Circular runout

Shanghai Jiao Tong University 20

Total runout

Shanghai Jiao Tong University 21

Profile Controls Profile of a line Profile of a surface

May uses a datum surface

Shanghai Jiao Tong University 22

Profile of a Surface

Shanghai Jiao Tong University 23

Profile of a Surface

Shanghai Jiao Tong University 24

Surface Finish We control the sizes and shape (GD&T)

of parts in order to make them fit. We control surface textures in order to

make the parts work Fatigue Fiction Wear

Shanghai Jiao Tong University 25

Surface Roughness Surface Roughness refers to the fine

irregularities (scale smaller than that of waviness) on the surface and is a result of the manufacturing process used.

It is measured (by surface profilometers) in terms of height, width and distance on the surface.

Shanghai Jiao Tong University 26

Surface Finish Surface finish impacts:

How well a surface will withstand pressure, friction and wear

How well it will hold and disreibute a lubricant How it will change over use How it will seat or seal How it will conduct electricity, reflect light, accept

paint/plating, etc How it will change by further manufacturing

processes

Shanghai Jiao Tong University 27

Terminology Ra (AA): Arithmetic mean/average Rq (RMS): Root mean square

Shanghai Jiao Tong University 28

Considerations for Surface Roughness

Reasons/ Considerations for different surface roughness in design Precision requirements on mating surfaces (seals,

fittings, bearings, etc.) frictional considerations (effect of roughness on

friction, wear, lubrication) Fatigue and notch sensitivity Resistance (electrical/thermal contact resistance

corrosion resistance) Subsequent processing (painting, coating, etc.) Appearance Cost