Page:Practical Treatise on Milling and Milling Machines.djvu/113

Rh CHAPTER VII General Notes on Milling, together with Typical Milling Operations

Milling, as we have already explained, is a process that cannot be governed by any fixed set of rules, but there are a few general instruc- tions which, if carefully followed, will enable the machine to be more efficiently operated and largely influence the success that is attained. These we have collected in this chapter, ana, in addition, show illustrations of a number of common milling operations to give an idea of how various and widely different jobs can be set up.

Pickling Castings and Forgings. Due to the rapid cooling or chilling of the outside of castings and forgings, a tough, hard skin, or scale forms, that is very destructive to the cutting edges of the teeth of milling cutters. There is also considerable of the moulding sand left on castings, and this is likewise harmful to the cutting edges. The sand can be removed and scale softened to some degree by the process of pickling, and it is essential that this be done preparatory to milling. Castings are usually pickled in the foundry, but it is well to make sure that this has been done before attempting to mill them. It is also an advantage in some cases to have castings rattled after being pickled. Where they are small, and are to be finished rapidly, they should be annealed. For pickling castings, a solution of oil of vitriol, or sulphuric acid, reduced with water to a specific gravity of 25° (Beaume hydrometer) is recommended. The castings should be stacked on a bench over a vat containing the solution, and the solution poured over them. Castings should never be immersed in the pickling bath if they are to be painted, because the iron is more or less porous, and the acid that is absorbed in pickling will work out after the pieces are finished, causing the paint to flake off. Furthermore, the pickle works better when it is poured over the castings and then allowed to dry off before another application of the solution.