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[Katharina Engelhardt]

For a mounting plate for a precision sensor (about 200 mm × 200 mm) I was going to call out a flatness of 0.05 mm, but my senior engineer says that’s overkill and will double the machining time/cost. The plate is 10 mm thick 6061‑T6. Is 0.05 actually “expensive” to hit on a standard mill, or is he just being old school? What’s a “safe” flatness for a general ground surface that won’t break the bank?

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[UlrichHartmann]

You’re not entirely wrong to be cautious. 0.05 mm flatness on a 200 × 200 mm plate is achievable, but it’s tight for a standard 3-axis mill. You’re talking extra passes, slower feeds, and possibly hand scraping or surface grinding if the machine isn’t perfectly tuned – as is quite often the case. Old school manual scraping is great.

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