# The Machining of Hard Mold Steel by Ultrasonic Assisted End Milling
## 1 Introduction  
Stavax (modified AISI 420) stainless mold steel is highly resistant to corrosion, it is also tough and wear resistant and can be polished to a high finish. A fine polished finish is a basic requirement for plastic molds because it directly affects the appearance of the product. Molds made of Stavax (modified AISI 420) are usually machined by end milling in computer numerical control (CNC) machining stations. The roughness of the finished mold surface will determine the amount of work that will be needed in the final grinding and polishing process to give the kind of smooth surface needed for a good mold. In other words, a milling process that gives the least rough surface to the steel mold surface is desirable. Polishing is an essential step in the manufacture of high-quality molds and dies. It is labor intensive, very time consuming and can account for 30%-50% of the time taken by the entire die making process [1]. Therefore, any improvement in the surface roughness of the final machined mold will reduce the amount of labor and time needed to complete the work and also reduce cost. There have been a number of studies made on the use of ultrasonically assisted machining aimed at improving the process. [PERSON] et al. [2] reported on the effect of the eccentricity, helix angle and feed on the surface roughness during side milling operations with cylindrical tools. A model was developed to predict various roughness parameters and surface topography. The roughness topography was acquired from a specific tool with eccentricity and grinding errors from distinct helix angles. [PERSON] et al. [3] studied a hydrodynamic polishing process under semi-contact lubricating conditions used to increase the efficiency of surface irregularity reduction. Several investigations ofthe variations in the magnitude of workpiece surface irregularity were carried out during the fine finishing process. These experiments indicated that the magnitude and wavelength of the surface irregularities of the workpiece are related to the surface irregularity conduction. [PERSON] et al. [4] investigated ultrasonic vibration-assisted micromilling (UVAM) using longitudinal vibration of the workpiece. The major difference between ordinary micromilling and UVAM lies is the trajectory of the cutter tooth. A mathematical model has been developed to simulate this, and a series of experiments were performed to investigate the surface roughness of Al6061 processed by micromilling with and without ultrasonic vibration. [PERSON] and [PERSON] [5] investigated the effects of frequency in UVAM using axial vibration of the cutter. A set of face-milling experiments under dry conditions were carried out on AISI 316L, an alloy that is widely used in the industry. The finished surface roughness was studied along with basic consideration of frequency in UVAM (20-40-60 kHz) under a range of cutting conditions and compared with the tool wear in conventional milling. Cross sections of the metal were examined and measurements of excess surface stress were used to investigate the latent effects of UVAM. [PERSON] and [PERSON] [6] investigated the cutting performances of four solid ceramic end milling tools, including Ti(C,N), Si\({}_{3}\)N\({}_{4}\), LT55 and SG4 for machining hardened AISI H13 steel (HRC 60-62). The results showed that the cutting force was lower for ceramic end milling tools than for cemented carbide cutters, and ceramic tools such as Ti(C,N)-cermets, Si\({}_{3}\)N\({}_{4}\), and LT55 generate better surface quality and these tools have much longer life. [PERSON] and [PERSON] [7] suggested that the simplest and most effective way to improve the machining efficiency of ultrasonic milling was to first improve the design of the tool. The main purpose of the study was to make use of rotary ultrasonically assisted machining (RUM) on brittle material, investigate the application of milling on brittle materials and innovative manufacturing methods such as automated rotary ultrasonic machining.