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.
Norton | Saint-Gobain
www.nortonabrasives.com
About the author: David Graham, an applications engineer at Norton | Saint-Gobain, can be reached at [email protected]
Creep-feed grinding
An alternative to machining, creep feed grinding uses diamond rolls to dress the grinding wheel for accurate form and finish.
Steering rack
Project requirements: High volumes, low cycle times
Traditional process stages
Bar stock turned, cut
Drilling, milling
Rack teeth broached
Heat treatment
Straightening, polishing
Process challenge
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
Tooling cost: Grinding 18% of broaching
Setup time: Grinding 38% lower than broaching
Total process costs: Grinding 53% of broaching
Secondary benefits: Improved quality, process stability, surface finish; tooth-spacing variance cut in half
Stainless steel exhaust manifold
Project requirements: Reduce tool cost, simplify multiple machining stages
Machining processes
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
Process challenges
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
Peel grinding
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
Results
Metal removal rate: 13in³ of HRc 57 material
Grind time: 16 min.; equivalent of 0.020″ depth of cut (DOC) at 500fpm, 0.006ipr
Large bearings
Project requirements: Switch to peel grinding from fard turning
Process challenges
Manufacturer struggled to achieve required form and finish on rollers with hard turning prior to finishing,
Grinding process
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″
Results
Abrasive cost: $0.18/in3 of material removed; >5% of total operation cost