TM 1-1500-204-23-8 3-12           Change 1 (c) Welds  should  be  made  in  one  pass whenever possible. (3) Striking the arc.     The following procedures are for striking the electric arc when arc welding alumi- num. (a) Brush  electrode  over  surface  of  base metal  like  striking  a  match,  as  shown  in  figure  3-5. Attempts to strike an arc by touching electrode to the piece to be welded will usually cause sticking. NOTE A  short  arc  (1/8  to  3/16  inch)  is  most desirable. (b) When arc goes out, restart arc 1/2-inch back of preceding crater to avoid burning through  sheet. e.  Aluminum   Spot   Welding.  The   following   para- graphs describe aluminum spot welding practices and procedures.  Confine  spot  welding  to  alloys  1100  and 3003, and Alclads 2219, 5052, 5083, 5086, 5454, 5456 and 7075. (1) Preparation.     Position equipment and con- trols to prevent current flow until force is applied to the electrodes. Figure  3-5.  Brush Method of Starting Electric Arc.   (2) Spot spacing.    Spacing will not exceed 1/8 inch  between  welds  in  the  same  row  and   1/16  inch between welds in adjacent rows. (3) Joint  thickness.      Joint  thickness  must  not exceed four times the thickness of the thinner material. NOTE Do  not  join  more  than  two  sections  of  the material by one spot weld. (4) Machine  settings.       Machine  settings  are given in the applicable maintenance manual. (5) Weld sequence.    Start all welds at center of sheet and work toward each end. 3-6.  Magnesium  Welding.      Welding  of  magnesium can be accomplished by the use of gas, arc, or spot weld, depending on the alloy, nature of the parts, and the end usage. The following paragraphs cover cleaning proce- dures and procedures for gas, arc, and spot welding. WARNING The vapors from some chlorinated solvents (e.g., carbon tetrachloride , trichloroethylene, and perchloroethylene) break down under the ultraviolet radiation of an electric arc to form a toxic gas. Avoid welding where such vapors are   present.   Furthermore,   these   solvents vaporize  easily  and  prolonged  inhalation  of the vapor can be hazardous. These organic vapors should be removed from the work area before welding is begun. a.  Cleaning.  Clean oil , grease, and dirt  from area to be  welded  using  acetone  or  denatured  alcohol[.  After cleaning, remove oxide or chemical treatment using steel wool. Sand, mill, or file abutting edges to give a clean, smooth surface and to remove edges left by shearing. b.  Magnesium Gas Welding.   Oxyacetylene, oxyhy- drogen, or oxycarbohydrogen gas may be used for this method of welding. Oxycarbohydrogen is the preferred gas. Limit gas welding to butt or corner joints because the flux is an active corroding agent and must not become entrapped   in   the   weld.   The   following   paragraphs describe magnesium gas welding practices and proce- dures.