Machineability of Materials
Although steel is the most common material used in the process industries, there are various other materials employed, of which it is necessary to have some knowledge of their machineability. These consist of alloy steels, cast iron and non-ferrous metals. The following will give some indication of the points to note when cutting.
Mild Carbon Steel
The most frequently used free cutting material. A coolant, consisting of a mixture of soluble oil and water should be used. The turnings should be bright and curly. If turnings become smaller and discoloured, this is an indication of tool wear, resulting in the deterioration of surface texture.
Medium Carbon and Cast Steel
Of a much tougher nature than mild steel. Hardness increases under the tool when cutting. Turnings break into short chips and may be brown or blue. The surface finish will be rather glazed and tools should be kept sharp as continual use of a worn tool will form a hard skin on the workpiece surface.
High Speed Steel
Turns fairly easily and long chips are formed and should be blue or brown in colour. The scale emits a distinctive smell whilst turning.
A fairly easy cutting material. A hard crust or skin is found on the outside diameter from the moulding process and the first cut taken should be deep enough to cut under this skin. Material will be removed in black crumbly chips, light cuts will produce a black dust. Cast iron should be cut dry, and surfaces wiped clean to remove the black dust before checking.
Spun brass is very free cutting material and freshly ground tools should be used. A high cutting speed is required and swarf will be removed in a fine spray, and is cut dry. Cast brass is slightly darker in colour and the same methods of cutting are employed, chips will be crumbly and slightly curled.
A free cutting material but very tough; high speed steel tools may be used, but when large amounts of material are to removed, or to obtain clean cuts to close tolerances, particularly on long runs, tipped tools are preferred. Turnings will be long and curly. A glazed finish will indicate tool wear and burrs which are difficult to remove will be formed. Coolant will prolong the life of the tools, and a mixture of soluble oil and water is satisfactory. Care should be exercised when reaming holes, as the reamer will tend to "seize up" in the hole.
A very free cutting material. Care should be exercised when clamping in the chuck or by other workholding methods, as bruising or distortion may occur. Tools should have a large top rake (up to 40o). Turnings will be removed in a thin continuous ribbon. Cutting speeds should be sufficiently high to retain the generated heat in the swarf, rather than in the workpiece. When a bright finish is required, paraffin should be used as a cutting lubricant. When a mirror finish is not required, turning may be performed dry. It should be noted that the expansion rate of aluminium is very high and machined parts should be allowed to cool before measuring them.
An extremely light alloy, very free cutting, the speeds, feeds and tools used for aluminium are suitable for magnesium alloys. Usually machined dry. An important point to observe is the prevention of any tendency for the fine chips or powder to ignite, for this metal is liable to burn violently. Ordinarily there is little risk, as the percentage of magnesium in the alloy is very small, and high cutting speeds may be employed. Swarf will be removed in small chips or a powder form, depending on the depth of cut.
Most plastics are very free cutting, but have a very tough nature and high speed steel tools will lose their cutting efficiency fairly quickly, therefore the cutting edges must be kept in good condition. High speeds and feed give the best results, both as to finish and length of time between regrinds of tools. A high speed and coarse feed will throw the chips away from the workpiece, and will prevent the rubbing action, which dulls the tool quickly. No coolant is required. Drills tend to cut smaller in most plastics, making the hole slightly undersize, and this may be overcome by grinding the drill slightly off centre. Digging in may be minimised by stoning the cutting edge, to give a slight negative rake.