Why project the zone?
A small hole tilt can produce a larger offset farther above a thin base. Projected control helps a fixed fastener pass through its mate without enlarging the clearance hole just to accommodate that tilt.
DATUM / MODIFIER GUIDE
PROJECTED TOLERANCE ZONE
A projected tolerance zone controls a feature’s axis beyond the part surface, for example where a fixed fastener enters its mate. It limits the effect of hole tilt on assembly rather than checking only the portion inside the base.
INTERACTIVE APPLICATION
Projected Tolerance Zone: drawing and interpretation
DRAWING CALLOUT
Read the projected position callout
Position Ø0.20 Ⓟ 20 | A | B | C: the diameter sets the zone width; 20 sets its minimum height. Basic dimensions 20 and 15 locate the hole. All example lengths are in mm.
Separate diameter from height
Circled P and 20 define the projection. They do not change the Ø0.20 tolerance diameter. The top face is A; side faces B and C locate the hole.
Orange: projected axisPurple: cylindrical position zone
INTERPRETATION
Evaluate the extended axis
The axis must remain inside the Ø0.20 zone over at least 20 mm above A. The zone is cylindrical; these two dashed lines are its section. A larger pin or screw is shown for assembly context.
Why it helps assembly
The extended axis is checked where the fastener passes through the mate. This example covers a maximum 20 mm mating thickness; size and the mating clearance still need their own checks.
Choose the height from the assembly
Use it for threaded or press-fit holes that guide screws, studs or pins. Thick mates and long projections make tilt more significant.
For a bolted clearance joint, cover the maximum mating-part thickness. For a stud or pin hole on a detail drawing, use the maximum permitted installed projection. An assembly drawing controlling the installed stud or pin directly does not need this modifier just to control its protruding portion.
Specify the projection, not the hole depth
Drawing callout
This M8 hole uses projected position Ø0.20 over 20 mm above its mounting face A.
Steps to add it
- Choose the start surface, projection direction and required height from the assembly.
- Set the hole’s basic location and datum references.
- Place circled P after the tolerance value and any eligible material modifier; follow it with the minimum height.
- If direction needs clarification, show the height and direction in the view using a dimensioned chain-line indication beside the extended centreline.
How to read it
A orients the zone; B and C locate it. Ø0.20 is a diametrical position tolerance. The 20 mm height starts at A, is a minimum extent and is not hole depth or bonus tolerance.
Check the whole joint
Projection does not replace size limits or establish every mating clearance by itself. Check the fastener and mate as an assembly; add an in-part geometric requirement if needed.
What each value means
Ø0.20
Diameter of the projected position zone.
Ⓟ
Apply the zone beyond the indicated surface.
20
Minimum projection height in mm.
A | B | C
Primary orientation and secondary/tertiary location references.
ASME Y14.5-2018 (R2024)
ASME Y14.5 uses a projected tolerance zone modifier with a stated height where applicable.
ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.
LEARN BY DOING
Practise GD&T on engineering drawings
The INTERFAIC interactive GD&T course is coming soon.