**IMPLIED RIGHT (90°) ANGLES** **OR PERPENDICULARITY**

The two lines are said to be implied 90° or contain perpendicularity when they are drawn at 90° on the drawing.

The tolerance for the implied 90° is mostly present on the title or else in the notes section in some of the drawings.

There are basically two short comings:

- The tolerance zone is fan shaped; it increases as it gets away from the origin.
- There is no datum reference involved hence the measurement can be done on either side.

Implied Right Angles

As shown in the above figure, the part could be inspected in two different ways.

**DEFINITION OF PERPENDICULARITY**

The condition when a surface, axis or center plane is exactly 90° to a datum is known as perpendicularity.

A perpendicularity control basically limits the amount a surface, axis or center plane varies from being perpendicular to the datum.

**PERPENDICULARITY TOLERANCE ZONE**

- Two parallel planes.
- A cylinder.

**PERPENDICULARITY APPLICATIONS**

- Application of perpendicularity to a surface.
- Application of perpendicularity to a planar FOS.
- Application of perpendicularity to a cylindrical FOS.

Perpendicularity applied to a surface

In the above figure, a perpendicularity control is applied.

This is the most basic and common application of this control.

The following conditions apply when a perpendicularity control is stated.

- Two parallel planes are the shape of the tolerance zone that are perpendicular to the datum plane.
- The distance between the tolerance zone planes is the tolerance value.
- All the elements must be in the tolerance zone.
- Flatness is limited by the perpendicularity tolerance zone.

Perpendicularity with Two Datum references

In the above figure, a perpendicularity control is stated on a surface.

There are 2 datum references mentioned.

When 2 datum references are used, the tolerance zone is perpendicular to the two datum planes, and all the conditions are applicable as mentioned above.

In the below example, a perpendicularity control with the MMC modifier is mentioned on the planar FOS.

Perpendicularity applied to a Planar FOS

To ensure the function of assembly such type of geometric control is used.

The applicable conditions when perpendicularity control is stated are mentioned below:

- Two parallel planes are present which is perpendicular to the datum plane.
- Tolerance values states the distance between the two parallel planes.
- Bonus tolerance is permissible.
- Perpendicularity control is verified by the fixed gage.

In the below example, a perpendicularity control with MMC modifier is mentioned to a cylindrical FOS.

Perpendicularity applied to a Diameterical FOS

The following conditions are applicable when this control is stated:

- Cylinder is the tolerance which is perpendicular to the datum plane.
- Tolerance value is the diameter of the tolerance zone cylinder.
- Axis of diameter must be within the tolerance zone.
- Bonus tolerance is permissible.
- Perpendicularity control is verified by the fixed gage.

**INDIRECT PERPENDICULARITY CONTROLS**

There are a number of geometric control which affect the perpendicularity control indirectly.

Profile, runout and tolerance of position can limit the perpendicularity control.

To inspect the perpendicularity, a perpendicularity control should be specified.

The tolerance value should be less than the tolerance value of any indirect perpendicularity control.

**INSPECTING PERPENDICULARITY**

There checks must be done for inspecting perpendicularity.

- Size of the FOS.
- Rule 1 boundary.
- Perpendicularity requirement.

Rest the part on the datum.

A precision square is placed on the datum plane which comes in contact with the part.

The gap between the part surface and precision square is the perpendicularity error.

A gage wire having a diameter equal to tolerance value is placed in the gap.

If the gage wire does not fit in then the surface is within perpendicularity tolerance value.