By David Graham, Application Engineer, Norton | Saint-Gobain
First published in Today’s Motor Vehicles, March 2017
Success stories: when milling and machining are replaced with peel and creep-feed grinding.
While machining processes dominate metal removal for automotive parts manufacturing, trends toward smaller pieces with higher form and finish tolerances are leading manufacturers to consider alternatives. Advances in abrasive grinding materials and processes have made the technology more suited to a range of manufacturing applications.
Cost savings can come from many places – reduced capital expenditure, consumable tooling, logistics, and cycle time. In particular, creep-feed and peel grinding have applications suited to motor vehicle production. The following are examples of customers replacing machining processes with abrasive grinding.
An alternative to machining, creep feed grinding uses diamond rolls to dress the grinding wheel for accurate form and finish.
Project requirements: High volumes, low cycle times
Traditional process stages
- Bar stock turned, cut
- Drilling, milling
- Rack teeth broached
- Heat treatment
- Straightening, polishing
If the time to cut the teeth in the rack (about 30 sec.) is taken separately, then it would be difficult for grinding to complete the part. Broaches take up a lot of floor space, and large, sharp broaching tools require special care during handling. Shimming or manual adjustments by skilled operators are needed to get the required accuracy once a broach is in the machine. Once the setup is complete and production starts, part quality degrades as the tool dulls. As a rack tooth forms, pitch and surface finish change, then steering assembly performance suffers. Tool maintenance, storage, planning, and logistics add to the cost of each rack.
Switch to grinding results
Stainless steel exhaust manifold
Project requirements: Reduce tool cost, simplify multiple machining stages
- Rough-, finish-milling passes machined two flat surfaces – 70 sec. for large surface, 40 sec. for smaller surface
- Face mill tool life: 10 parts per corner adding time for indexing inserts, projected annual insert cost: <$2 million
Inclusions in the castings caused milling inserts to break unpredictably; flatness problems around ports due to tool deflection; large burrs around holes in manifold required additional operations
Switch to grinding results
- Large surface finished in same time as milling
- Improved part quality
- Reduced tooling costs, tool change time, de-burring time
- Total cost saving: $250,000 per year before accounting for fewer scrapped parts, reduced need for inspection
Narrow cubic boron nitride (cBN) or diamond wheels work in conjunction with high-wheel and work speeds for high removal rates.
Large shaft manufacturing
Project requirements: Peel grinding replaces hard turning
- Metal removal rate: 13in³ of HRc 57 material
- Grind time: 16 min.; equivalent of 0.020" depth of cut (DOC) at 500fpm, 0.006ipr
Project requirements: Switch to peel grinding from fard turning
Manufacturer struggled to achieve required form and finish on rollers with hard turning prior to finishing,
- 2.7" diameter roller x 3.25" ground at 0.004"DOC, 13ipm feed rate: 15 sec. cycle time
- Material: 62HRC, 52100 steel
- Removal rate: 0.46in³/min.
- Surface finish: <16µin Ra
- Profile, roundness: >0.0002"
Abrasive cost: $0.18/in3 of material removed; >5% of total operation cost