b'Car Trouble ers are machining parts that will berial cost, die castings are made near assembled in-house and sold to thenet shape so that only a moderate customer as a finished sub-assembly. amount of machining is demanded.drive out the normal forms of waste,Automakersthe companiesSmall deviations in workpiece lo-such as tool changes and non- with their names on the carsdocation because of chips or misload-cutting moves, from the process.not make many parts themselvesing can send the process out of Therefore, little waste is left. and ask their suppliers to providecontrol. Results like unmachined Aerospace parts normally are as- components that are ready to beareas are easy to find and fix. Other sociated with close-tolerance ma- bolted onto vehicles. Therefore,times, a coordinate measuring ma-chining, which is understandable.the component maker must ma- chine will report incorrect dimen-Automotive machining does notchine the various parts and piecessions on features made across mul-share that stereotype, but automo- from the required materials, whichtiple machines, which is the worst tive machining tolerances are oftencan vary greatly. A steering gear ispossible situation. I have seen engi-as tight as those in aerospace ma- a good example. A single gear canneersmyself includedspend chining. Not only do automotivehave die-cast aluminum, steel bar,days chasing a few microns and try-parts have very close tolerancesplastic, bronze and rubber compo- ing to recover a setup or repair athe parts have the added pres-sure of statistical process control requirements. SPC specifies con- I have had the opportunity to work trol limits, which essentially close upat different shops and havethe tolerance band, necessitatingfound automotive machininghighly developed processes that are both accurate and repeatable. to be the most challenging.Entire books have been devoted to SPC tools, so a full definition here would be impossible. SPC sets upper and lower control limits to ensure that the process delivers dimensions that are as close to nominal as pos-sible. There are statistical tools likenents, all of which may be manufac- fixture because a die casting was Cand Pthat quality profession- tured in-house, creating a diversenot locating properly.pk pkals use to measure accuracy and re- set of machining requirements. Every machine shop feels the peatability of the machining process,Complicatingmattersevenpressure of delivery, but automo-with an ideal value being 1.66 or bet- more, many of these materials cantive manufacturing occurs at a fran-ter. Designing a machining processbe difficult to machine. Some peo- tic pace compared with all other in-that achieves the desired 1.66 Ces- ple might argue that steel, alumi- dustries. Cars move nonstop down pksentially eliminates most of the toler- num and plastics are not hard tothe assembly line, and the compo-ance band. So if a 10 mm hole has amachine, and thats a valid argu- nents needed for assembly must tolerance of 0.03 mm, achieving 1.66ment. However, its not the spe- flow at the same rate because au-Cwould require a process that con- cific material that usually presentstomakers hold only a few hourspksistently delivers a hole that deviatesthe challengeits the form of theyes, hoursworth of inventory. from the 10 mm target by only 0.01workpiece that complicates the ma- This means that there is very lit-mm (estimated). It is possible to drillchining. Aluminum die castings aretle time for a shop to recover when holes that consistently maintain thethe best example. To control mate- something goes wrong, so failures 0.03 tolerance, but achieving 1.66that are minor at most shops can be capability probably would need anabout the authorcatastrophes for automotive shops. additional reamer or a custom drill.SoTime is the catalyst that turns minor SPC adds another layer of complexityChristopher Tateproblems into major incidents.to machining process development. is manufacturingAll these challenges are com-Workpiece materials contributeengineering man- mon, and alone they do not seem a third layer of difficulty to automo- ager at Unifiedinsurmountable. But when com-tive machining. Materials have a tre- Brands Inc. inbined, they create the unique man-Vicksburg, Mis-mendous amount of variation be- sissippi. Contactufacturing environment that is auto-causemost automotive manufactur- him at [email protected]. motive machining.CTE54APRIL 2023AutoMaterials.indd 54 3/16/23 1:57 PM'