Pickling Formerly pickling castings was largely employed, but of recent years, by reason of the improved facings used, the practice is not so much followed. Nevertheless there are places about castings from which the sand is not properly removed by the ordinary processes, and again some machine shops prefer pickled castings, as the cutting edges of their tools are not injured so quickly, by reason of the entire removal of the sand. This process is also followed where the castings are to be galvanized or tinned, as it leaves clean metallic surfaces. For pickling, either sulphuric or hydrofluoric acid is used, the former more commonly. The acid solution must be weak; one part of ordinary vitriol to four or six parts of water attacks the iron rapidly, whereas the undiluted acid has no effect. In diluting the acid, care must be taken to pour the acid into the water, and not the water into the acid. Dilute sulphuric acid dissolves the iron in contact, thereby loosening the sand. The action is more rapid with warm than with hot solution. This solution, when applied to castings, will loosen the sand scale in from one to twelve hours, depending upon the thickness of the scale. The acid solution is kept in a lead-lined wood vat. The vat should be about two feet deep, the other dimensions varying with the amount of castings to be treated. At the bottom of the vat is a wooden grating fastened together by wood dowels. The grating is held down by lead weights. It must be high enough above the bottom of the vat for the sand to drop through. Upon this grating the castings rest as they are immersed. After remaining in the bath the requisite length of time, they are removed and thoroughly washed with hot water. The acid must be completely removed or they will rust. It is a good plan to dip them in a strong solution of lye or soda before washing. Another practice is to place a lead-lined platform so that one edge may overhang one end of the vat; the platform inclining a couple of inches toward the vat, and having the remaining edges raised two inches, so that all the drainage may be into the vat. Upon this platform is placed a wood grating, and the castings on the grating. The pickle is then dipped from vat with an iron bucket and poured over the castings. They are washed thoroughly with the pickle, so that there may be no sand surface which has not been saturated. It may be necessary to repeat the operation more than once. When the sand scale begins to loosen, the castings are removed and washed as before. The washing may be done with a hose while the castings are on the bed, but in such case provision must be made to carry off the water in a trough so that it may not enter the vat. The strength of the solution must be kept up by addition of fresh acid from time to time. Hydrofluoric Acid Where this acid is used for pickling, the solution should be one part of 48 per cent acid to 30 parts of water. Hydrofluoric acid dissolves the sand instead of acting on the iron. The treatment of the castings is the same as with the vitriol, but the sand must be removed from below the grating, otherwise the acid will be rapidly neutralized. The workmen should be cautioned in handling either of these acids as they cause severe burns, if they come in contact with the flesh. Where acid is spilled on the flesh or clothing, wash the parts freely with water and then with dilute ammonia. Raw linseed oil applied to burns produces a soothing effect. Hydrofluoric acid leaves the surface of the castings bright and clean, and is, therefore, best for electroplating. W = weight of casting in pounds; L = T B D = = = length of casting in inches; thickness of casting in inches; breadth of casting in inches; outside or large diameter in inches. To find the weight of hollow cylinders, multiply the small or inside diameter plus the thickness, by the length, by the thickness, by 0.817: W = (d+T) TL × 0.817. FIG. 223. |