1. Geometric Dimensioning and Tolerancing
Chapter 1, Introduction

2. History
during the early period of manufacturing there were seldom any drawings
one-off manufacturing
~1798 – concept of interchange parts
Eli Whitney won a contract to supply muskets to the United States government. The firearms manufacture were based on the concept of interchangeable parts.
He made a presentation to congress by building 10 guns and assembling and disassembling them claiming the same exact parts and mechanisms.
specifying tolerances became very important
Eli Whitney

3. Tolerances All dimensions require a tolerance.
A tolerance should be as large as possible without interfering with the function of the part to minimize production costs.
Consider how your part will be checked to see if it meets the tolerances.

4. Tolerance Notes

5. Plus/Minus (Limit) Tolerancing
Plus/minus tolerancing seemed to work well for many applications.

6. Plus/Minus (Limit) Tolerancing

7. Plus/Minus (Limit) Tolerancing

8. Limit Tolerancing
Is the hole horizontal position measured from a true vertical plane or from the as built face?
A .005” tolerance on the horizontal and vertical position of the hole means that the position could be off by as much as .007”.
max allowed error
for hole center
+.005
.007
perfect location
for hole center

9. Limit Tolerancing
Limit tolerances don’t have an origin or any orientation or location relative to datums.
The datums are usually implied.
The drawings are subject to different interpretations.
Plus/minus tolerancing works well for individual features of size (ex. diameter of a shaft), but does not control the relationship between individual features very well.

10. Limit Tolerancing

11. Limit Tolerancing

12. Limit Tolerancing

13. GDT Limit tolerancing can give the size of individual features.
GDT can provide:
relationships between features
circular tolerance zone for hole placement
GDT removes any ambiguity from the drawing specification

14. History of Dimensioning and Tolerancing Standards in the USA
Mil Std ’s
Mil Std 8A
Mil Std 8B
Mil Std 8C-1963
ASA-Y
USASI Y
ANSI Y
ANSI Y14.5M-1982
ASME Y14.5M-1994
ASME Y
The first dimensioning and tolerancing standards in the USA were military standards in the 50’s. There were revisions A,B and C. The standards have undergone name changes in the past, American Standards Association (ASA).
The current standard is the American Society of Mechanical Engineers (ASME Y ) The M in the title, which meant “metric compatible” was dropped because it was deemed no longer necessary. The standard can be used with both inch and metric values. It is a ASME standard with American National Standards Institute approval. Standards are always improved and work is taking place on the next revision. Additional information can be found at

15. Geometric Dimensioning and Tolerancing (GDT)
establish a reference coordinate system by defining datums
provide basic dimensions (perfect dimensions) relative to the datums
specify allowable tolerances

16. communication
symbols
ASME
clarity
replace
total
form
size & location
function & relationship
tolerance
tolerances & interchageability
size
plus/minus