The time required for the process is usually about a day, and the solution should be poured over the castings from four to five times.
Forgings may be pickled by immersing in a solution of sulphuric acid and water of 30° specific gravity (Beaume hydrometer) for a period of from 3 to 12 hours, according to hardness of scale.
When either castings or forgings are pickled, they should be thoroughly washed off with hot water, as this will wash out sand and remove the acid better than cold water. The water may be conveniently heated for this purpose by injecting steam into the cold water pipe.
Cutter Close to End of Spindle. In all milling operations, especially the heavier ones, care should be taken to have milling cutters as near the nose of the spindle as practicable. This will reduce to a minimum any possible vibration and spring of the arbor. It also brings the table close to the face of the column and ensures additional rigidity. Other valuable points about cutters have been taken up in Chapter VI, and it may be well to review these previous to starting to operate a machine.
Fastening Cutter on Arbor. See that the ends of the collars and washers are clean, for particles of dirt or chips between them will cause the arbor to be sprung when the nut is tightened. Small cutters can be held securely by the mere clamping effect of the collars on each side when the nut is tightened, but medium and large cutters should always be keyed to the arbor to prevent slipping.
Manner of Driving and Supporting Arbors. Milling machine arbors are driven in several different ways, some of which are shown in Fig. 58. In A, the arbor has a tenon at the small end of the taper that fits a slot at the end of the taper hole in spindle, thus giving a positive drive. The method of driving arbors B and C is similar to that of driving face milling cutters, described on page 98. The arbor is provided with a clutch, and has threaded hole in end of shank; the clutch fits into recess in end of spindle, and the arbor is drawn into place and held securely by the drawing-in bolt, (see Fig. 58.) The method of driving collets is the same as that of arbors.
All milling machines are equipped with some support for the outer end of the cutter arbor. The adjustable centre shown at A is one form that is used for lighter classes, or work where an arbor with a flat tenon is employed. The centre serves to support the outer end of the arbor and helps to keep the flat tenon in place in the slot in the spindle. Another form of support is shown at B. This support is a bronze bushing mounted in the arm that extends down from the overhanging arm, and is used where an arbor with clutch drive is employed. An
