New milestones in microfinishing--Precision engineered finish for auto parts

By Niraj M Wanikar, Chief Editor, CNCTimes.com, 2016-12-14 14:51:00

Greenhouse emissions and energy efficiency are the critical aspects pushing the limits of powertrain engineering in automotive industry. ‘Lightweighting’ and ‘friction reduction’ are two key strategies in this aspect. Deploying both strategies makes designers of engines resort to stringent surface finish parameters on critical powertrain components such as Crankshafts, Camshafts, Transmission shafts etc. Microfinishing is a mission critical process required to achieving these specifications. Over the last few years microfinishing technology has crossed several milestones and made possible key advances in engine performance.

New Milestones in Micro-Finishing
Process of Microfinishing with film backed abrasives was developed to achieve consistently uniform finishes faster and more efficiently. Microfinishing generates lubricating characteristics at surface contact through a highly engineered surface finish so that the surface provides maximum bearing area to take up heavy loads and at the same time retain enough oil in the surface for adequate lubrication.
Milestones in this technology in the past few years include the process capability of achieving highly precision engineered finishes, as well as advancements to process more and more variety of components using film backed abrasives. Major improvements in machine technology include flexible manufacturing systems capable of multi variants and compacter more energy efficient machines with ultra user-friendly interfaces.

What is Micro-Finishing?
Micro-finishing is a method or process to remove the surface material of a dimensionally finished part to provide a precise fit to the mating part. An examination of the texture left on a metal surface as a result of machining operation will reveal tool marks, fragmented metal, chatter, etc. It is the micro-finishing operation that must be used to generate the final surface texture required for service life, performance and reliability of a product.
A non-microfinished surface can be compared to a snow covered lake. A person’s weight will produce foot prints in the snow whereas, the ice will even provide support for the concentrated pressure of a person on ice skates. When a metal or steel part is machined, the surface becomes fragmented and also annealed if a heat generating grinding process is used. This type of surface will not support high bearing loads. Microfinishing is the process that removes the fragmented and annealed surface, leaving base metal for greater bearing loads.

Meeting Advanced Specifications of State-of-the-Art Crankshafts
Crankshaft is the heart of an engine. This mission critical component transmits power from the combustion chamber to the powertrain. Specifications on a Crankshaft have undergone major advances globally in the past few years. While Friction Reduction has demanded finer and finer finishes, lightweighting (and the resulting downscaling of engines to 2 and 3 cylinder engines instead of 4 cylinder engines) has led to more bearing loads on the crankshafts features. At the same time user experience demands higher speeds, longer engine life and lowest NVH (Noise, Vibration and Heat) have all led to tighter and tighter expectations from the crankshaft.

Reducing friction requires following features on the crankshaft:

• Surface finish Ra values to be lesser than typically Ra 0.1 microns on main and pin journals, finish Ra values to be lesser than typically Ra 0.15 microns on thrust walls, tighter oil seal finish requirements required to use thinner oils
• Bearing area Htp requirements on all journals
• Tighter geometrical tolerances including roundness, straightness, cylindricity. New specifications on profiles, crowning, lobing
• Consistent Stock Removal – Maintaining size tolerances
• Most modern automotive crankshafts are microfinished using multiple Level processes, such as Level II or Level III microfinish. Some of the case studies below will illustrate the results achieved on such crankshafts.

Case Study: Achieving parameters on automotive crankshafts for passenger car
Qualifying higher energy efficiency and emission norms had generated stringent requirement on journal surface so that engine NVH performance could improve. Tribology experts and engine designers came to a conclusion that to achieve the stringent requirements the bearing ratio parameters needed major enhancements, along with improvements in other specifications.
Microfinishing with Level II process with optimization for various process parameters led to achieving remarkable results with consistency as indicated below:
Bearing Ratio
5-20% Htp: 0.15 microns Depth
5-80% Htp: 0.25 microns Depth
5-90% Htp: 0.35 microns Depth
Surface Finish: Ra 0.1 microns

The above parameters were achieved with a consistency indicated by statistical process control parameters Cp (achieved Cp > 2.0 and Cpk > 1.67)
Besides finish, the corrections in geometry such as roundness correction by 50% below 3 microns, cylindricity controlled within 3 microns also indicate the process capability of microfinishing.

In one specific realization for passenger car crankshafts, a special size control feature with online gauging was deployed on a machine to achieve diameter tolerance ±3 µm with PP 2.19 and PPK 2.16.

Case Study: Microfinishing replaces stone finishing technology for camshafts
Stone finishing is a traditional technology for finishing of shaft like components, including camshafts. Microfinishing with film backed abrasives gives several advantages over stone such as consistency of results, lesser maintenance and adjustments, lower energy consumption etc. Due to above reasons, microfinishing has replaced conventional Stone lapping in majority of shaft type components. In one particular instance, a global leader in engine technology used combination of stone and film to achieve very stringent surface finish specifications, in plants including American facility.

The earlier 3 level finish process was replaced with a 2 level process using film alone. The Surface finish achieved as below
Ra < 0.067 microns with Cpm > 1.67
Rtm < 0.412
Wt < 0.45
Wc < 0.12
The resulting consistency and low-energy operation as well as lower cycle time truly delighted the customer.

Microfinishing Automotive Transmission Components
Automotive transmission components such as geared shafts (input shaft, output shaft, main shaft) and synchrocones, differential housings etc. were traditionally having only surface finish specifications such as Ra values. While not as critical as engine components, specifications on transmission parts have also been upgrading over the years, necessitating the use of microfinishing technology instead of superfinishing. Apart from Ra values, many components are now specified with Rk, Tp, Rmr and Lead angle specifications. Geometry specifications such as roundness, cylindricity and size are also being tightened on such components.

Microfinishing is distinguished from Superfinishing in the type of contact between the abrasive film and component as illustrated by the figures below.
Microfinishing gives better consistency in surface finish, bearing ratios much quicker than superfinishing. Geometry correction is not possible with Superfinishing, whereas it is guaranteed in Microfinishing. Several successful projects including Microfinishing for Balancer Shafts, Differential Gear Housing, Main Shaft etc illustrate the benefits.

Conclusion – Milestones in Micro-finishing
Microfinishing is a key process technology in automotive powertrain for achieving engine performance in terms of energy efficiency and emissions. Capability to achieve superior finishes enables reduced friction in the powertrain and low NVH. In future specifications on finish for components such as crankshafts, camshafts and various powertrain parts are likely to get more and more stringent. Advances in microfinishing technology enable achieving these parameters with consistency.

About the Author:

Sameer Kelkar, Silver Medallist and Graduate of IIT Bombay, and MS in Robotics from University of California, Berkelet, is Executive Director of Grind Master Machines Pvt. Ltd, pioneers Robotic Metal Finishing Technologies. “Explorer” at heart, Sameer is venturing into new horizons of Robotic Applications such as Grinding, Deflashing, Fettling, Machining and many others. This article is written using research, data and application experience over the past 5 years.

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