Back or backing and melt-thru welds are frequently used with groove welds to strengthen the weld on the opposite side of the joint.

A back weld is distinguished from a backing weld only by its sequence of application with respect to the face side of the groove weld. In the case of a back weld, the groove weld is made first and then the back weld is applied. In contrast to this, the backing weld is applied first and then the groove weld is made. Actually, the back and backing welds are the same and are shown by the symbol: О Either is speci­fied by including a reference in the tail of the welding symbol, Figure 14.1. A distinction may also be made by use of multiple reference lines, Figure 14.5(d).

The melt-thru symbol is included with the study of the back or backing symbol because of its similarity in shape and the way in which it is applied. It differs from the backing symbol in that it is shown as a solid: .

When used with groove welds, the back­ing, back, and melt-thru symbols are placed on the side of the reference line opposite the groove weld symbol, Figure 14.2.

SIZE OF BACKING AND MELT-THRU WELDS

Dimensions that refer to backing and back welds are not applied to these symbols; how­ever, a height of reinforcement dimension may be applied to the melt-thru symbol. It is placed to the left of the symbol, Figure 14.3. Generally, when it is necessary to specify dimensions for these welds, they are shown on a detail drawing or are included in the tail of the welding symbol.

CONTOUR AND FINISHING

The contour symbols and finish abbreviations are added to the groove, backing, back, and melt-thru symbols as shown in Figure 14.4.

ctr

'HEIGHT OF REINFORCEMENT

FIGURE 14.3 ■ Application of height of reinforcement dimension.

/-Ч / BACKING

/ WELD

H

V X /back

^ 4 WELD

G X

FIGURE 14.4 ■ Application of contour and finish symbols to backing, back, and melt-thru symbols.

When a backing weld or a melt-thru weld is to have a mechanically finished convex contour, this is indi­cated by adding the convex contour symbol and the finish abbreviation to the backing, back, or melt-thru symbol, Figure 14.4.

To this point, melt-thru symbols have been used in conjunction with groove symbols. However, this symbol is also used with the edge weld symbol, Unit 17.

SELECTED APPLICATIONS OF BACK OR BACKING SYMBOLS

Figure 14.4 shows several applications of the back and backing symbols. Note that Figure 14.4(a) and Figure 14.4(b) show the distinction between a back and a backing weld. Note also that a back or back­ing weld is easily distinguished when a multiple reference line is used, Figure 14.4(d). Observe that in all cases a reference is included in the tail to specify either a back or backing weld except when multiple reference lines are used.