A Comparative Study of Helical Polygonal Turning and Helical Peripheral Milling in a Turn Mill
Authors: Pandya Madhur , Agarwal Mohit , Srivastava Alok , Narayan Dhruv , Bhatnagar Naresh 
Published: August 2024

Traditionally, the milling process is adopted to machine polygonal geometries from a cylindrical bar on a Sliding Head Turn-Mill Center. In the case of titanium implants, machining helical geometries with polygonal cross sections is time-consuming. However, to meet increasing demand, faster cycle times become a crucial parameter for mass production. With the complexity involved in the geometry of the medical screws with polygonal cross sections that assist bone-forming methods, polygonal turning has been a better alternative to the traditional milling approach. This study investigates trilobed helical geometry manufactured using fractional speed ratios and synchronized feed of polygonal turning and compares it with C-axis enabled peripheral milling. To understand the characteristics of the machined surface, the work-piece spindle and the polygon tool are desynchronized. The raw material Titanium alloy (Ti-6Al-4V) bar is fed axially in synchronization with the workpiece spindle speed to generate a helical trilobed shape. Fractional speed ratios could be achieved with workpiece and tool desynchronization during polygonal turning. To achieve the desired helix angle for the polygonal helical geometry, an optimum interplay between the fractional speed ratio and synchronized feed is required in the case of helical polygonal turning. In this work, two different helix angles are taken as fixed parameters, while the cutting parameters, such as speed ratio (K) and feed (f) (mm/rev), are varied to obtain the required helix angle. A similar part is also machined with peripheral milling by enabling C-axis interpolation with varying milling tool speed and feed rate (°/min). A qualitative assessment of the surface morphology under a Field emission scanning electron microscope (FESEM) was conducted to understand the machined surface topography. In the case of helical polygonal turning, the machining time was reduced by nearly 77% as compared to helical milling. At higher cutting parameters for milling at 5000 rpm and at feeds 1800 °/min, 3600 °/min and 7200 °/min, the machining time evaluated to machine a 32° helical polygon was 18.98s, 12.91s, and 9.96s respectively with Areal roughness (Sa) varying between 0.669μm to 0.805μm. Helical polygonal turning at a speed ratio of 1.0013 with workpiece speed of 3000 RPM and feed 0.026mm/rev clocked a machining time of 9.3s with Areal roughness (Sa) at 1.32μm to machine a 32° helical trilobe. Although helical polygonal turning shows an advantage over helical milling in terms of machining time, the Areal surface roughness (Sa) obtained from milling is better than the helical polygonal turned surface. It was observed through FESEM micrographs that on a helical polygonal turned surface, distinct V-patterns with alternate peaks and valleys were observed perpendicular to the feed direction during the entry and exit of the cut. However, helical milling produces a wavy-rippled pattern on the surface in the direction of the feed. Most of the medical implants for osseointegration are subjected to surface treatment post-machining. Therefore, there are no stringent limiting criteria for surface finish. Hence, helical polygonal turning could be a promising solution for lobed medical screw manufacturing applications in the future.