Analysis of advanced transpedicular screw machining technologies

Abstract

The purpose is to improve the machining efficiency of titanium alloy transpedicular screws on highperformance machine-tools based on the selection of advanced technological approaches, and to analyze the current manufacturing technology of implants on CNC machines of the semi-automatic longitudinal turning lathe type. The efficiency is assessed using the following criteria: process performance determined by the machine time and the quality of implant processing (surface roughness, geometric accuracy, mechanical properties). It is found that semi -automatic longitudinal lathes equipped with a collet feed system and drive heads for thread whirling allow processing the implants of the transpedicular screw type in a single set-up with maximum efficiency. It is shown that the machining technology of transpedicular screws is largely determined by the features of their design. The type and shape of the thread have the greatest influence on the used cutting tool and cutting modes. The analysis of screw breakages revealed that the main failure reasons are design defects and poor machining quality of the threaded part. It is determined that the use of the thread whirling method makes it possible to obtain the thread in one cutting pass and, therefore, significantly increase the machining performance compared to the traditional technology without any loss of quality. Additional advantages of this method are the reduction in the number of tools used and follow-on finishing deburring operations. Based on the conducted analysis the manufacture of transpedicular titanium alloy screws is recommended to perform using advanced cutting tools, primarily thread whirling cutters ensuring 4 times increase in machining performance without any loss of the processed item quality and 2 times reduced surface roughness. In this case the temperature in the cutting zone decreases, which has a positive effect on processed product service life. The condition for the effective use of the cutters is equipping of the machine-tools involved in the technological process with special drive heads.

Studying the Structure and Properties of the Material of Screws for Spinal Osteosynthesis

“…Titanium alloys are hardly processible [15,16,17,18]; high-tech equipment is required. The transpedicular polyaxial screw prototypes were made on a Cincom K16E-VII Swiss-type automatic lathe designed to manufacture small precision parts.…”

“…The transpedicular polyaxial screw prototypes were made on a Cincom K16E-VII Swiss-type automatic lathe designed to manufacture small precision parts. A vortex thread-cutting head was installed in the lathe [16]. Drawings and a digital 3D model of the screw were previously developed in the Siemens NX computer-aided design system.…”

“…Drawings and a digital 3D model of the screw were previously developed in the Siemens NX computer-aided design system. In the manufacture of screws, the 3D model allowed correcting the control program and the thread-cutting head settings and obtaining the thread parameters specified [16,19,20,21,22].…”

Technological Characteristics of Production of Implants from Titanium and Alloys Based on it

“…The main constructions of pedicle screws are screws with lateral fixation of their body, which combine the attachment fixture with the vertical placement of the body. The latter can be both monoaxial, that is, with a rigid thread axle, and polyaxial, that is, with a variable angle of the threaded portion [11].…”

“…Therefore, the question of choosing the right equipment and tool has still been relevant. Automatic lathes with a movable headstock (Swiss type) were developed specifically for the production of small precision parts [11,12]. During machining, the workpiece (bar) is fed through the machine spindle and held by a collet system.…”