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Basics of GD & T: Geometric Dimension and tolerancing

The use of dimensional and geometrical tolerance in combination is known as Geometric Dimension and tolerancing.

In Engineering, Dimension measurement deals in Design, Manufacturing, and Inspection.

In design, there is an ideal form of the product with the specification.

This specification used to design the product called the work-piece. The work-piece is a real form of the product, we have to measure it.

manufacturing process

In measurement, we inspect the work-piece by the reference of Specification given in the Design of the product.

As we know it is impossible to manufacture the exact dimension of the product every time, so we try to manufacture a product within tolerance.

for example, XYZ company manufacturing a rod of diameter 10 +/- 1 mm i.e. the work-piece of diameter between 9 to 11 mm is acceptable, this +/- 1 mm is called tolerance.

According to ISO, tolerance is the difference between upper and lower limits.

There are two factors of accepting or rejecting the manufacturing product

  1. Tolerance
  2. Geometry

See the below example

GD and T

The use of dimensional and geometrical tolerance in combination is known as Geometric Dimension and tolerancing.

In the above image; Let consider the example of Inspection of a circle having a diameter of 27.5 mm with tolerance 0.5 mm(the acceptable range is 27 mm to 28 mm). Product A is within the tolerance limit and the same geometry. On the other side Product B also within tolerance limit but not in our required geometry.

There are many standards of Geometric Dimension and tolerancing like National, International as follow(the most common is ISO)

International Standard

ISO 1101:2017 (Geometrical product specifications (GPS) — Geometrical tolerancing — Tolerances of form, orientation, location, and run-out)

Read more aboutISO

National Standards

-United States

ASME Y 14.5:2009

-United Kindom

BS 8888:2013

-Germany

DIN Standards

The first thing is to know which standard your industry using, Most common is ISO standard due to its global reach.

The reason behind the development of this standard is to build a common understanding between Designer, Manufacturer, and inspector.

I.e. Common language of AutoCAD Designer, Production engineer, and quality engineer.

Learn more about GD & T: Join our online course

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