DESIGN DATA ON PRODUCTION METHODS 261 



FOUR SURFACE PREPARATIONS FOR SPOT WELDING ALUMINUM 

 Method I. 



1. Dip parts in 50 per cent HNO3 cold for 15 sec* 



2. Rinse in cold water. * 



3. Dip parts in 5 per cent NaOH + 4 per cent NaF at 160°F. for 30 sec. 



4. Rinse in cold water. 



5. Dip parts in 50 per cent HNO3 cold for 15 to 30 sec. to remove black deposit from step 3. 



6. Rinse in cold water, f 



7. Rinse in boiling water. J 



8. Dry parts on steam coils or in sawdust. 



Method II. 



1. Dip parts in 50 per cent HNO3 cold for 15 sec* 



2. Rinse in cold water. * • ■ 



3. Dip parts in 10 per cent NaOH at 160°F. for 30 sec. 



4. Rinse in cold water. 



5. Dip parts in 50 per cent HNO3 cold for 15 to 30 sec. to remove black deposit from step 3. 



6. Rinse in cold water, f 



7. Rinse in boiling water. | 



8. Dry parts on steam coil or in sawdust. 



Method III. 



1. Paint area to be welded with gum tragacanth HF acid paste and leave on for 30 sec. 



2. Wash paste off with running cold water or with wet rags. 



3. Dry off water with dry rags. 



Gum tragacanth paste is prepared from : 



3 lb. gum tragacanth 

 10 gal. hot water 



Dissolve gum tragacanth in hot water, add one gallon of alcohol to water if necessary to dissolve 

 gum tragacanth. 



Add 10 lb. hydrofluoric acid to above solution. 



Material must be stored in parafHn-lined containers. 



Operators must wear rubber gloves and goggles to use this material. In partially assembled 

 parts precautions should be taken to keep acid out of joint. 



Method IV. 



Area to be spot welded may be cleaned by mechanical means. 



a. Rub with steel wool. 



h. Rub with fine emery cloth. 



c. Use fine wire brush. 



* Omit steps 1 and 2 for material relatively free from oil. 



t Repeat steps 3 to 6 for material having very heavy heat-treating film not removed in steps 1 to 6. 

 I Final hot water should be thoroughly free of dissolved salts and of organic matter which would tend to stain the 

 freshly cleaned parts. 



