Machining Production Methods

Machining production methods may be divided into processes with geometrically defined cutter (turning, milling, drilling，sawing) and geometrically undefined cutler (grinding, honing，lapping). The machinery, frequently special equipment has to finish the object to the extent that only little postoperation, mostly manual in nature (polishing， lapping, and finishing) ,is left

Modem tooling machines for mold making generally feature multiaxial CNC controls and highly accurate positioning systems. The result is higher accuracy and greater efficiency against rejects. The result of a survey shows NC machining as having just a 25% share compared to 75% for the copying technique, but this does not  hold  true for modem tool shops and Ihe fabrication of large molds.

Nowadays, heat-treated workpieces may be finished to final strength by milling (e.g. Rm up to 2000 MPa)Various operations, e.g. cavity sinking by EDM，can be replaced by complete milling operations and the process chain thus shortened. Furthermore, the thermal damage to the outer zone that would otherwise result from erosion does not occur Hard milling can be used both with conventional cutting-tool materials, such as hard metals, and with cubic boron nitride (CBN), For plastic injection molds, hard metals or coated hard metals should prove to be optimum cutting-tool materials.

Machining frees existing residual stresses. This can cause distortion either immediately or during later heat treatment. It is advisable, therefore, to relieve stresses by annealing after roughing. Any occurring distortion can be compensated by ensuing finishing, which usually does not generate any further stresses.

After heat treatment, the machined inserts are smoothed, ground    and polished to obtain a good surface quality, because the surface conditions of a cavity are, in the end, responsible for the surface quality of a molding and its ease of release.

Defects in the surface of the cavity are reproduced to different extents depending on the molding material and processing conditions. Deviations from the ideal geometrical contour of the cavity surface, such as ripples and roughness, diminish the appearance in particular and form “undercuts”，which increase the necessary release forces.

There are three milling variants:

• three-axis milling,
• three-plus-two-axis milling and
• five-axis milling (simultaneous).

Competition has recently developed between high-speed cutting (HSC) and simultaneous five-axis milling. HSC is characterized by high cutting speeds and high spindle rotation speeds. Steel materials with hardness values of up to 62 HRC can also be machined with contemporary standard HSC millers .HSC machining can be carried out as a complete machining so that the process steps of electrode manufacturing and eroding can be dispensed with completely. In addition, better surface quality is often achieved, and this allows drastic reduction in manual postmachining.

For the production of injection and die-casting molds, a combination of milling and eroding may also be performed. The amount of milling should be maximized since the machining dmes are shorter on account of higher removal capability. However, very complex contours, filigree geometries and deep cavities can be produced by subsequent spark-erosive machining. Often* field electroctes are used. The electrode can, in turn, be made from graphite or copper by HSC .