Geometric Dimension and Tolerance

Geometric Dimension and Tolerance (GD&T)

is a framework for characterizing and imparting designing resistances. It utilizes emblematic language on designing drawings and PC created three-dimensional strong models that unequivocally depict ostensible math and its reasonable variety. It tells the assembling staff and machines what level of exactness and accuracy is required on each controlled component of the part. GD&T is utilized to characterize the ostensible (hypothetically great) math of parts and congregations, to characterize the suitable variety in structure and conceivable size of individual highlights, and to characterize the permissible variety between highlights.

• Measurement details characterize the ostensible, as-displayed or as-expected math. One model is a fundamental measurement.

• Resistance details characterize the reasonable variety for the structure and perhaps the size of individual highlights, and the admissible variety in direction and area between highlights. Two models are direct measurements and highlight control outlines utilizing a datum reference (both appeared previously).

There are several standards available worldwide that describe the symbols and define the rules used in GD&T. One such standard is American Society of Mechanical Engineers (ASME) Y14.5. This article is based on that standard, but other standards, such as those from the International Organization for Standardization (ISO), may vary slightly. The Y14.5 standard has the advantage of providing a fairly complete set of standards for GD&T in one document. The ISO standards, in comparison, typically only address a single topic at a time. There are separate standards that provide the details for each of the major symbols and topics below (e.g. position, flatness, profile, etc.).

Symbols And Conventions Used In Welding Documentation

The symbols and conventions used in welding documentation are specified in national and international standards such as ISO 2553 Welded, brazed and soldered joints -- Symbolic representation on drawings and ISO 4063 Welding and allied processes -- Nomenclature of processes and reference numbers. The US standard symbols are outlined by the American National Standards Institute and the American Welding Society and are noted as "ANSI/AWS". Due in part to the growth of the oil industry, this symbol set was used during the 1990s in about 50% of the world's welding operations. An ISO committee sought to establish a global standard during this decade.

In engineering drawings, each weld is conventionally identified by an arrow which points to the joint to be welded. The arrow is annotated with letters, numbers and symbols which indicate the exact specification of the weld. In complex applications, such as those involving alloys other than mild steel, more information may be called for than can comfortably be indicated using the symbols alone. Annotations are used in these cases.

• Component Elements

In the US, the component elements of the weld specification are:

1. The reference line - the body of the arrow which is the baseline for the specification.
2. The arrow tip which goes at an angle to the reference line, pointing to the joint to be welded.
3. The tail which goes at the other end of the reference line.
4. The basic welding symbol which goes on the reference line to indicate the shape of the weld such as a fillet or plug. The symbol is placed on the arrow side or other side of the line to indicate which side of the joint the weld goes.
5. The dimensions and other numbers such as the length of the weld or number of spot welds go above and below the reference line.
6. Supplementary symbols go at the junction of the reference line and the arrow tip. One such symbol is a circle to indicate an all-around weld, which goes on every side of the joint.
7. Finish symbols go above the reference line to indicate the surface contour or finish of the weld such as flush, convex or concave.
8. Letters indicating the welding process are placed at the tail end, such as AHW for atomic hydrogen welding. Further examples include:

• Abbreviations For Welding Process

If a particular welding process needs to be indicated in addition to the symbols, the following abbreviations are commonly used in North America:

Designation	Welding process
CAW	Carbon-arc welding
DB	Dip brazing
FB	Furnace brazing
FW	Flash welding
GMAW	Gas metal-arc welding
GTAW	Gas tungsten-arc welding
IB	Induction brazing
OAW	Oxy-acetylene welding
OHW	Oxy-hydrogen welding
PGW	Pressure gas welding
RB	Resistance brazing
SAW	Submerged arc welding
TB	Torch brazing
UW	Upset welding