Reciprocating Grinding: Cycle Times Cut

In the automotive sector, reciprocating grinding is the way to go for manufacturing ball nuts in steering sytems. By Markus Isgro, marketing communications, Emag

Few areas of car design are currently attracting more attention from developers than the steering system. There are several reasons for this: Firstly, complex assistance systems to support the driver are a huge trend in automotive manufacturing. Secondly, customers are increasingly demanding highly precise steering.

Both these requirements ultimately give rise to new technological solutions. Purely electrical steering systems, in particular, are becoming more important. They are easier to adapt to different types of vehicles, require less energy (and therefore less fuel) than hydraulic systems, and generally enhance driver comfort.

Refining Production Technology

These developments, however, are also focusing the attention of the designers on the production of the required components. It often needs to become more precise, and needs to do that with maximum efficiency as well.

These conditions are leading to changes in production technology, and a novel approach called reciprocating grinding is able to machine the ball nut of the steering system very finely and reliably.

Emag recently developed a vertical grinding machine utilising reciprocal grinding. Called the VLC 250 PDS, processing time required was reduced by around 30 percent compared to alternative techniques. This approach shows a lot of promise, and other automotive components could benefit from such reciprocating grinding in future.

Extreme Precision And Resilience

Ball screws are one of the most demanding production tasks in the world of metal machining, requiring extreme precision. Only then can the circulating ball bearings roll with minimal friction between the screw shaft and the ball nut, ensuring motion with high efficiency and low driving torque.

At the same time, if the assembly is used in a steering system, for example, it must be able to safely withstand the stresses of several thousand hours of driving. Hardening of the part is first followed by external grinding.

Then begins the demanding process of grinding the ball track to generate the profile with the quality parameters required. However, this process is relatively time-consuming, especially for ball nuts, because various preparatory steps are needed. Before starting the grinding process itself, the pre-machined inner profile of the part has to be aligned precisely in the machine so that the grinding spindle can be applied to the runout of the thread.

This increases the machine’s non-productive time and drags out the cycle time per part. “It was exactly this situation that led us to collaborate with a German automotive supplier about three years ago to develop a new production solution for the grinding of ball nuts, in the form of the VLC 250 PDS vertical grinding machine,” said Marina Manger from technical sales at Emag.

“Our design is based around a fully automated process with two pick-up spindles and two feeding devices. They make it possible for two parts to be active in parallel, but with each of them going through a different step of the process. That saves a great deal of time,” Ms Manger explained.

Positioning And Grinding In Parallel

Ball screws are one of the most demanding production tasks
in the world of metal machining, requiring extreme precision.
Reciprocal grinding is a promising approach that saves time.

The design engineers call this approach reciprocating grinding. Specifically, their design uses the following arrangement: The two pick-up spindles move alongside each other on the same X-axis, but take parts from two different feeders, each of which is located at one end of that axis.

This allows, for example, pick-up spindle A to take a part from the feeding device assigned to it on one side and transport it to the corresponding measuring probe on the machine for positioning, while pick-up spindle B is already in the machining area, transporting a part either to the first grinding spindle for preliminary grinding or to the second grinding spindle for finish grinding.

In addition, both the grinding spindles are installed on a rocker that allows the contact angle of the tool to be adjusted according to the lead angle of the ball track. When the two-stage grinding procedure is completed, the part is conveyed out of the machine via the feeder. Directly after that, pick-up spindle A transports its previously aligned workpiece into the machining area from the other side.

Perfectly Synchronised

The whole process is then performed again in the reverse direction, forming a perfectly synchronised system in which the positioning of one part and the machining of another part always take place in parallel. Non-productive idle time arising from the alignment process is thus minimised. In practical terms, this means that the cycle time for grinding a ball screw falls by about 30 percent compared with the production processes used previously.

Additional speed and high productivity are also contributed by the machine components used: The pick-up spindles are arranged on very rigid compound slide rests and their axis motion is perfectly matched to the specific work task. The powerful linear motor in the X-axis ensures high acceleration.

At the same time, the typical Emag approach of using just one clamping operation for the machining process safeguards workpiece quality and process reliability, as it minimises the risk of clamping errors. Operator comfort and ease of access were important priorities for the developers, so large doors allow easy access to the machining area. Tools, clamping equipment, and measuring probes can always be changed quickly.

New Market Potential

German grinding specialist Emag has seen multiple sales of this machine design since its introduction in 2013. So far it has been used exclusively for machining ball nuts of various sizes at several of the customer’s sites.

But the future beckons with new areas of application, according to those in charge of the project at Emag. Ms Manger is optimistic, saying, “This approach could be used successfully for many two-stage grinding processes. The innovative synchronisation with two pick-up spindles always results in exceptionally low non-productive times and a drop in per unit costs. We want to introduce these benefits more extensively in new areas of application in the years to come.”

APMEN Machine Tools, Dec 2016

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  • Last modified on Friday, 23 December 2016 07:18
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