It is always the aim of a production engineer to obtain high quality work with minimum expenditure. The following tips would be helpful in achieving this objective.
In selecting milling cutters, it should be remembered that, in most cases, milling operations performed with face- milling cutters are more economical than those with cylindrical slab-milling cutters. For instance, it is not always necessary to cut grooves with slot milling cutters or end mills.
In many cases, a run-out can be provided for the cutter so that it is possible to make use of the more efficient side milling cutters permitting considerably higher feed rates and shorter working times. In many cases, the ease or otherwise with which the work may be clamped plays a decisive role.
The condition of the milling machine is of primary importance for successful milling. The cutter spindle and feed screw should be checked periodically for wear and play. Similarly the guides of the table for tightness, and the driving mechanism for proper running should also be checked from time to time.
The arbor should be properly supported. As far as possible, the cutter should be mounted close to the spindle nose. If it is not possible, then the cutter arbor can be supported by the steady bracket immediately adjacent to the cutter to prevent deflection of the arbor. If it is not done, then cutter edge may be blunted or damaged. The cutter should be a close fit on the arbor, but it should not be a drive or force fit as this may lead to breakage.
In fitting the cutter and the spacer rings on the arbor, care should be taken to keep them free from particles of dirt or chips.
Before introducing the arbor into the spindle, the tapers of both arbor and spindle should be wiped with a clean cloth. The taper surface of arbor should be coated with a thin film of grease before fitting into the spindle nose in order to prevent difficulty in removing the cutter arbor from the machine after the completion of milling operations.
The form of teeth of cutter must be such that discharge of the cuttings is ensured. Use of proper clamping devices and fixtures speeds up the milling operation.
Selection of proper speed, feed, depth of cut are very important and need to be properly selected depending on material of cutter, workpiece and condition of milling machine.
The maintenance of milling cutters directly affects the quality of work. The use of dull cutters results in waste of power, inaccurate work, and poor surface finish. A sharp milling cutter should never be placed on a hard table or tool-rest as this might result in injury to the cutting edge.
Cutters should be correctly sharpened, providing desired rake and clearance angles. During grinding, cutters should never the forced against the grinding wheel under an excessive pressure to expedite the grinding.
During milling, the flow of coolant should be so directed as to wash away the cuttings adhering to the tooth faces, to prevent them from being carried round by the cutter.
Optimum cutting conditions (maximum speed, feed and depth) depending on the power of the machine and rigidity of machine should be used to get maximum utilisation of the milling machine. Throwaway, tipped cutters are usually run at higher speeds than brazed tip cutters. The wear allowed on throwaway tips is upto 1.2 mm and on brazed-tip cutters, it is restricted to 0.8 mm.
Feeds of the order of 0.1 to 0.5 mm per tooth are employed, being minimum for form cutters and in increasing order for end mills, slotting and side mills, slab mills and maximum for face mills. The value of feed used also depends on the material being milled. Maximum feeds are used with materials like aluminum alloys and magnesium alloys and least with alloy steels, stainless steels and titanium alloys.