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In the tough market of automotive manufacturing a slight decrease in the rejection rate can save the company millions of dollars. One mid sized transmission component manufacturer was experiencing excessive rework and rejection rates over 6.5% due to grinding induced surface burns and micro-cracks. This raised production costs and threatened delivery schedules and customer satisfaction. This case study examines how one manufacturer achieved a rejection rate below 0.8% in less than six months using CBN abrasives from Zhengzhou Ruizuan Diamond Tool Co., Ltd.
The Problem: Why Aluminum Oxide Wheels Were Failing
This medium-sized, supplier of automotive transmission gears was finishing 62-64 HRC case-hardened gears and shafts on alumina wheels. Alumina wheels were actually less expensive, however, the alumina grains dulled rapidly and produced abundant heat at the grinding zone. This heat often caused:
Surface burns visible by dye penetrant examination.
Micro cracks and thus shorter service life of the final part.
Frequent dressing and thus high down times and tool costs for the grinding wheels.
Surface finish variation, thereby introducing problems with subsequent assembly stages.
The rejects in the shop averaged 6.5% (5%-8%). Quality managers put annual losses due to defects at over $400,000.
The Solution: Switching to Engineered CBN Wheels
The plant consulted with application engineers at Zhengzhou Ruizuan and agreed to do an experimental trial using custom CBN wheels. Ruizuan recommended metal-bonded CBN wheel that was formulated for the machining of hardened gear steels. Specifically:
CBN grit size: B151 (approximately 100/120 mesh)
Bond type: Metal matrix actually provides superior grit retention.
Concentration: 150%, actually enables faster cutting.
Operating speeds (compatible with current grinders) ranging from 45 to 60 m/s
Unlike aluminum oxide, the extreme hardness of CBN kept the abrasive grains sharp throughout the entire grinding cycle. The metal bond actually provides heat sink, efficiently removing heat from the part. Ruizuan also provided extensive grinding data including optimal wheel speed, feed rate, and coolant.
The Results: Defect Rates Drop to 0.8%
After two week trial on one of their gear grinding lines it was confirmed:
Defect reduction: there were practically no burns or micro-cracks found, rejects fell from 6.5% during the trial to 1.2% after some refinement. This had steadied at 0.8%.
Wheel life extension: each CBN wheel was making 4500+ parts per dress as opposed to 300 or less with aluminum oxide. Tooling costs have been decreased by over 70% per year.
Dressing frequency: Aluminum oxide required dressing after 50-100 parts whereas, with CBN, it is once every 1500 parts, allowing for machine usage elsewhere.
Surface finish improvement: Ra values fell from 0.8 m to 0.4 m, the second honing operation was eliminated.
Plant manager stated over $600,000 saving per year-a substantial figure compared to original estimates.
Why Zhengzhou Ruizuan’s CBN Wheels Made the Difference
The transition to CBN wheels was made successful by combination of:
Application - specific customization: Ruizuan tailored its CBN wheels to the specific gear material, hardness, machine parameters and coolants used by the gear maker.
Strict quality control: The wheels met ISO 9001 manufacturing standards and actually came with individual test reports.
Technical support: Ruizuan engineers actually provided training in the new grinding parameters and monitored the initial run.
Certifications: The wheels were CE, SGS and TUV certified, meeting the actual plant's automotive standards (IATF 16949).
Summary
Switching to custom metal bonded CBN wheels helped this gear plant achieve significantly improved surface finish and reduction in grinding defects. The company saved more than $600,000 annually through lower scrap, longer tool life and less machine downtime. Since 2014, Zhengzhou Ruizuan Diamond Tool Co., Ltd. has been helping industrial customers reduce grinding costs and improve quality with full line of diamond and CBN abrasives. Contact Ruizuan for your own grinding process audit.