Multi-function machining for hollow electric drive components

High-torque electric motors put added strain on powertrain components such as shafts and splines. The additional forces, combined with the need for higher rotational speeds and greater concentricity to reduce noise, vibration, and harshness (NVH), have made specs for electric vehicle (EV) and hybrid powertrain suppliers extremely challenging. Lightweight requirements for all components heighten the challenge as automakers request hollow designs for many rotating parts.

DVS Production in Krauthausen, Germany, maker of 20 million transmission parts per year for the international automotive industry, meets those demands with hard-fine machining of complex hollow shafts. Using flexible, multi-function turrets on its machines, the company produces parts with dozens of machined features: OD grinding, hard turning, surface facing, milling, and seal-seat grinding (see Fig. 1, below).

The supplier, a member of Germany’s DVS Technology Group, relies on DVS UGrind series machines from group partner Buderus Schleif- technik in Asslar, Germany. Developed for the hard-fine machining of small- to medium-sized series of shafts and lining parts, the machine series combines grinding, turning, milling, and measuring in one clamping.

Complex manufacturing challenge

Cubic boron nitride (CBN) high-performance cutting materials are reducing grinding process times by permitting faster cutting speeds while increasing grinding tool service life. When manufacturing small- to medium-sized batches, which are usual for the series production of powertrain components for electric vehicles, optimizing processes relies less on tool service lives and more on reducing non-machining times, such as changeovers between the first workpiece clamping and the final finishing processes.

A multi-functional turret on DVS’ UGrind machines can reduce machining time for multi-featured components up to 50%. Compact components minimize non-cut travel times for grinding, turning, milling, and measuring. An integrated measuring probe controls machining to achieve the required final dimensions, eliminating successive feeding and re-measuring steps.

Hollow, heavily machined shafts

More stringent shape and position tolerances for EV components require maximum manufacturing precision. First, a measuring probe on the UGrind’s turret with a 270° radius of action establishes a level surface to determine the exact axial and rotation position of the workpiece. The control system then calculates optimum feeding and forwarding parameters, after which the key surface is manufactured using a CBN hard milling tool. Then, a combined turning steel holder with two blades performs high-precision hard turning of various flat surfaces on the left and right, followed by CBN grinding of several ODs to micron tolerances. For precise grinding of the long outer diameter, plunging takes place at several points, followed by peel grinding from left to right.

An extremely fine grit CBN grinding disc finishes the hollow shaft’s seal seat, achieving a surface roughness of Rz<1µm. In-process measuring equipment measures a reference OD, and other ODs are traced following this reference. Executing all machining operations in a single clamping reduces setup times and the number of manual handling steps, avoiding re-clamping faults. Using high-quality, high-performance tools and short travel paths can minimize cycle times.

Robotic automation

A 6-axis robot with 1.6m range manages the machine. The robot loads and unloads pre-positioned hollow shafts from a conveyor system. A camera system permits precise part detection using a dot matrix code – offering the customer exact traceability of each hollow shaft. For worker safety, a sensor-controlled system shields the working chamber using optical barriers, guaranteeing the loading and unloading process comes to a standstill if an object enters the protected area.

Buderus Schleiftechnik GmbH
www.buderus-schleiftechnik.de