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GDT Flatness and Straightness

Flatness and straightness are geometric form tolerances that limit the deviation of a feature from its perfect geometry. Flatness controls how much a surface deviates from a perfect plane, while straightness restricts the variation of a line or an axis relative to a theoretical straight line. Both tolerances are specified in the feature control frame per ISO 1101 and ASME Y14.5 standards, and are independent of any datum, defining only the form of the feature. Their application ranges from machine base plates to transmission shafts, ensuring proper operation and assembly of mechanical components.

Symbol and Representation

Section titled “Symbol and Representation”

The standardized symbols for flatness and straightness are indicated in the first compartment of the feature control frame. The graphical representation follows ISO and ASME conventions.

Characteristic ISO/ANSI Symbol Description
Flatness A slanted parallelogram (two parallel oblique lines) Applied to a surface, it defines a tolerance zone between two parallel planes.
Straightness A horizontal straight line Controls the form of a line on a surface or an axis. If the zone is cylindrical, the tolerance value is preceded by the diameter symbol (⌀).

The typical frame includes, from left to right, the characteristic symbol, the tolerance value (in millimeters or inches) and, if applicable, the datum letter for axis straightness.

Tolerance Zone

Section titled “Tolerance Zone”

The tolerance zone defines the volume or area within which the actual feature must lie.

  • Flatness: Zone bounded by two parallel planes separated by a distance equal to the specified tolerance value (e.g., 0.05 mm / 0.002 in). The entire surface must reside between these planes.
  • Straightness of a surface line: Zone in the projection plane bounded by two parallel straight lines separated by the tolerance.
  • Straightness of an axis: Cylindrical zone with a diameter equal to the tolerance value (preceded by the ⌀ symbol). The actual axis of the cylinder must remain inside that cylindrical volume.

General Tolerances per ISO 2768-2

Section titled “General Tolerances per ISO 2768-2”

When no form tolerance is explicitly indicated on the drawing, the general tolerances of ISO 2768-2 apply. The following tables show the values for flatness and straightness based on nominal length and tolerance class (H – fine, K – medium, L – coarse). The values are identical for both characteristics.

Nominal length range (mm / in) Tolerance class H (mm / in) Tolerance class K (mm / in) Tolerance class L (mm / in)
up to 10 mm / 0.394 in 0.02 mm / 0.0008 in 0.05 mm / 0.002 in 0.1 mm / 0.004 in
over 10 up to 30 mm / 0.394 – 1.181 in 0.05 mm / 0.002 in 0.1 mm / 0.004 in 0.2 mm / 0.008 in
over 30 up to 100 mm / 1.181 – 3.937 in 0.1 mm / 0.004 in 0.2 mm / 0.008 in 0.4 mm / 0.016 in
over 100 up to 300 mm / 3.937 – 11.811 in 0.2 mm / 0.008 in 0.4 mm / 0.016 in 0.8 mm / 0.031 in
over 300 up to 1000 mm / 11.811 – 39.37 in 0.3 mm / 0.012 in 0.6 mm / 0.024 in 1.2 mm / 0.047 in
over 1000 up to 3000 mm / 39.37 – 118.11 in 0.5 mm / 0.02 in 1.0 mm / 0.039 in 2.0 mm / 0.079 in

Note: The nominal length for flatness is the longer side of the surface; for straightness it is the total length of the controlled feature.

Achievable Tolerances by Manufacturing Processes

Section titled “Achievable Tolerances by Manufacturing Processes”

Machining processes allow obtaining different degrees of form precision. The following table, derived from the typical capabilities of each process, shows indicative flatness/straightness values per 100 mm (3.94 in) of length. The IT tolerance grades are based on ranges from the source engineeringtoolbox.com.

Process Typical dimensional tolerance grade (IT) Indicative form tolerance per 100 mm (mm / in)
Lapping / Honing IT4 0.0025 mm / 0.0001 in
Cylindrical / Surface grinding IT5 – IT6 0.005 mm / 0.0002 in
Diamond turning / Precision boring IT6 – IT7 0.01 mm / 0.0004 in
Broaching IT7 – IT8 0.02 mm / 0.0008 in
Reaming IT8 0.03 mm / 0.0012 in
Conventional turning IT9 – IT10 0.05 mm / 0.002 in
Milling IT9 – IT11 0.08 mm / 0.003 in
Drilling IT11 – IT12 0.2 mm / 0.008 in
Planing / Filing IT11 – IT13 0.3 mm / 0.012 in

Real-World Application Examples

Section titled “Real-World Application Examples”
  • Base plate of a machining center: The support surface is specified with a flatness tolerance of 0.02 mm / 0.0008 in per 500 mm / 19.69 in to ensure the geometric stability of the machine.
  • Precision transmission shaft: Axis straightness is applied with a cylindrical zone ⌀ 0.01 mm / 0.0004 in over its entire length, preventing vibrations and premature wear.
  • Shims and alignments: Contact surfaces on welding fixtures require a flatness of 0.05 mm / 0.002 in over the entire surface to ensure repeatable positioning.
  • Linear guides: The straightness of sliding faces is controlled with 0.01 mm / 0.0004 in per 300 mm / 11.81 in.

ISO vs ANSI Comparison

Section titled “ISO vs ANSI Comparison”
Aspect ISO 1101 ASME Y14.5
Flatness symbol Identical (parallelogram) Identical
Straightness symbol Straight line Straight line
Use of diameter modifier Rarely used for line straightness; not explicitly required Mandatory (⌀) when the tolerance zone is cylindrical
Indication of tolerance zone in axis straightness Cylindrical zone is assumed if the tolerance is associated with the axis ⌀ must be shown before the value
Interpretation of Rule #1 (Envelope) No equivalent universal rule; depends on ISO 8015 Rule #1: Perfect form at MMC
General tolerances ISO 2768-2 defines default form tolerances No direct equivalent; must be specified on the drawing
Application on curved surfaces Flatness applies only to nominally flat surfaces Same criterion

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the typical flatness tolerance for a 200 mm × 200 mm support plate?

Section titled “What is the typical flatness tolerance for a 200 mm × 200 mm support plate?”

A 200 mm / 7.87 in side plate usually receives a flatness tolerance of 0.05 mm / 0.002 in for precision seats, a value that falls within ISO 2768-2 class H for that length.

What does a straightness value of ⌀ 0.03 mm / 0.0012 in mean on a shaft?

Section titled “What does a straightness value of ⌀ 0.03 mm / 0.0012 in mean on a shaft?”

It means that the actual axis of the cylinder must lie entirely within a cylindrical zone of 0.03 mm / 0.0012 in diameter over its entire length of 150 mm / 5.91 in.

How is the flatness of a surface of 0.2 mm / 0.008 in measured with conventional means?

Section titled “How is the flatness of a surface of 0.2 mm / 0.008 in measured with conventional means?”

For a tolerance of 0.2 mm / 0.008 in, a dial indicator mounted on a magnetic base can be used, sliding it over a granite surface plate and covering an area of 300 mm × 300 mm / 11.81 in × 11.81 in.

What practical difference exists between applying surface straightness or flatness on a guide rail?

Section titled “What practical difference exists between applying surface straightness or flatness on a guide rail?”

The straightness of each rail line individually controls 0.01 mm / 0.0004 in over 500 mm / 19.69 in, while flatness would control the entire top surface of 500 mm × 20 mm / 19.69 in × 0.79 in with a single zone of two parallel planes.

Can flatness and straightness tolerances be combined on the same plane?

Section titled “Can flatness and straightness tolerances be combined on the same plane?”

Yes, it is common to specify a straightness of 0.03 mm / 0.0012 in for a generating line and simultaneously a flatness of 0.05 mm / 0.002 in for the entire surface of 250 mm / 9.84 in length.

What machining processes guarantee an axis straightness less than 0.01 mm / 0.0004 in?

Section titled “What machining processes guarantee an axis straightness less than 0.01 mm / 0.0004 in?”

Precision cylindrical grinding or diamond turning can achieve an axis straightness of ⌀ 0.005 mm / 0.0002 in over a length of 100 mm / 3.94 in.

References

Section titled “References”