Manufacturing Processes, GD&T, and Measurement Techniques in Engineering Drawing

Manufacturing

The process of transforming materials into useful products.

Formative manufacturing

Manufacturing by causing material to take a desired shape, such as casting or forming.

Subtractive manufacturing

Manufacturing by removing material from a workpiece.

Additive manufacturing

Manufacturing by adding material to build a part.

Engineering drawing

A technical drawing used to communicate information needed to manufacture, inspect, and understand a part.

Visible line

A line showing an edge of a part that is visible from the current view.

Hidden line

A line showing an edge or feature that is not visible from the current view.

Center line

A line showing the center of a circular feature or axis of symmetry.

Orthographic view

A 2D representation of a 3D object from a particular direction.

Common orthographic views

Front, top, and right-side views.

Why choose orthographic views

To provide the most information while minimizing hidden lines.

Sectional view

A view that exposes internal features of a part.

Section line

A heavy dark line used to divide a part view and expose internal geometry in a sectional view.

Half section

A sectional view showing one half of an object in section.

Offset section

A sectional view where the cutting plane is bent or offset to pass through important features.

Removed section

A section shown away from the main view to show the cross-sectional shape.

Auxiliary view

A view used to show the true shape of an inclined or angled surface.

Dimension

A numerical value describing the size or location of a feature.

Overall dimension

A dimension showing the overall size of a part.

Dimension line

A line with arrows that indicates the distance being dimensioned.

Extension line

A line extending from a feature to indicate where a dimension starts or ends.

Leader line

A line with an arrow pointing to a feature and providing information about that feature.

Datum

A theoretical exact point, axis, or plane used as a reference for establishing dimensions or geometric characteristics.

Datum feature

A physical feature, usually a surface or axis, used to establish a datum.

Primary datum

The first datum; a planar surface with a minimum of 3-point contact.

Secondary datum

The second datum; a planar surface with a minimum of 2-point contact.

Tertiary datum

The third datum; a planar surface with a minimum of 1-point contact.

Datum memory trick

Primary = 3, Secondary = 2, Tertiary = 1.

Three-plane datum system

A system using primary, secondary, and tertiary datums to establish a part's position and orientation.

Baseline dimensioning

Dimensioning where multiple features are measured from the same datum.

Chain dimensioning

Dimensioning where each feature is dimensioned from the previous feature.

Why baseline dimensioning is preferred

It avoids accumulating dimensional variation through a chain of features.

Basic size

The theoretically exact preferred size of a feature.

Tolerance

The allowable amount of variation in a dimension.

Limits of size

The maximum and minimum acceptable sizes of a feature.

Total tolerance

The total amount of dimensional variation allowed.

Allowance

The intended difference in size between mating parts.

Limit dimensioning

Dimensioning that specifies only the maximum and minimum acceptable sizes.

Bilateral tolerance

A tolerance that allows variation in both directions from the basic size.

Equal bilateral tolerance

A tolerance with equal positive and negative variation, such as 3.375 ± 0.005.

Unequal bilateral tolerance

A tolerance with different positive and negative variations.

Unilateral tolerance

A tolerance that allows variation in only one direction from the basic size.

Why tolerances are necessary

Manufacturing processes are imperfect, so parts cannot be made exactly to a theoretical dimension.

Why not use extremely tight tolerances

Tighter tolerances usually require more precise and expensive manufacturing processes.

Tolerance strategy

Choose the widest tolerance possible while still meeting functional requirements.

Clearance fit

A fit where the hole is always larger than the mating shaft or cylinder.

Transition fit

A fit that may result in either clearance or interference.

Interference fit

A fit where the hole is always smaller than the mating shaft or cylinder.

Clearance fit memory

Hole > shaft.

Transition fit memory

Hole may be > or < shaft.

Interference fit memory

Hole < shaft.

GD&T

Geometric Dimensioning and Tolerancing; a system used to control geometric characteristics of parts.

Form tolerances

Tolerances controlling the shape of a feature.

Four form tolerances

Straightness, flatness, circularity, and cylindricity.

Straightness

A form tolerance controlling how straight a line or surface must be.

Flatness

A form tolerance controlling how flat a surface must be.

Circularity

A form tolerance controlling how circular a feature must be.

Cylindricity

A form tolerance controlling the overall cylindrical form of a feature.

Orientation tolerances

Tolerances controlling the orientation of features relative to references.

Three orientation tolerances

Angularity, perpendicularity, and parallelism.

Angularity

A tolerance controlling an angled orientation relative to a reference.

Perpendicularity

A tolerance controlling whether a feature is 90° to a reference.

Parallelism

A tolerance controlling whether a feature is parallel to a reference.

Location tolerances

Tolerances controlling the location of features.

Position tolerance

A GD&T tolerance controlling the location of a feature.

Profile tolerance

A GD&T tolerance controlling the shape/profile of a feature.

Runout

A GD&T control of variation as a feature rotates.

Circular runout

Runout controlling variation at individual circular cross-sections.

Total runout

Runout controlling variation over the entire surface as a feature rotates.

Metrology

The science of measurement.

Measurement

Comparing an unknown quantity to a known standard.

Inspection

Examining a part to determine whether it conforms to its design requirements.

Gaging

Determining whether a part or feature meets design specifications.

Resolution

The smallest change that an instrument can detect or display.

Accuracy

How close a measurement is to the true or actual value.

Precision

How consistently repeated measurements agree with one another.

Calibration

Checking a measurement device against a known standard.

Gauge blocks

Known standards that can be used for calibration.

Gauge pins

Known standards that can be used for calibration.

NIST

A government standards organization referenced as a basis for measurement standards.

Inspection by variables

Inspection that provides a numerical measurement of a characteristic.

Inspection by attributes

Inspection that determines whether a characteristic meets requirements, often as a yes/no result.

GO/NO-GO gauge

A gage used for attribute inspection; GO should fit and NO-GO should not fit.

Measuring vs gaging

Measuring provides numerical information; gaging is faster but generally provides less information.

Measurement variability

The fact that all measurements contain some amount of variation.

Measurement error

The difference between a measured value and the actual value.

Repeatability

Variation when the same operator uses the same instrument to measure the same characteristic of the same part.

Reproducibility

Variation between different operators using the same instrument to measure the same characteristic.

Repeatability memory

Same operator + same instrument.

Reproducibility memory

Different operators + same instrument.

Gage R&R

A method used to evaluate repeatability and reproducibility of a measurement system.

Crossed Gage R&R

A Gage R&R approach generally used for nondestructive testing.

Nested Gage R&R

A Gage R&R approach generally used for destructive testing.

P-value in Gage R&R

If P ≤ alpha, there is evidence that at least one operator's mean measurement differs.

P-value greater than alpha

If P > alpha, the operator means are not significantly different.

Accuracy problems

Can be caused by poor calibration or thermal effects.

Stability

Variation in measurement performance over time.

Drift

Change in measurement behavior over time.

Causes of instability

Calibration intervals that are too long, instrument wear, or environmental changes.

Linearity

The accuracy of an instrument across its entire measurement range.