You have no idea what a 250μ milled finish is, until you see it and feel it. These gauges are relatively cheap, durable, and a quick reference. Having one around will make your drawings more accurate and ultimately save machining time on your parts.
Below is a handy reference chart from the Wikipeida article on surface finish.
Here are some gauges you can order from Amazon. The less expensive one is cast and not calibrated. This means that it’s not standards compliant, and not as accurate as the calibrated scales. In my experience, this calibration is generally unnecessary with surface finish gauges. Learn about surface finish calibration at the National Institute of Standards & Technology’s website.
What other scales and gauges do you keep around?
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I’ve always had a surface finish gauge, drill & tap gauges, radius gauges, micrometer/ calipers, slip gauges & bore gauges – sitting at my desk…
…Thats why starting out in a machne shop is usually the best place for a engineer…
I just purchased the economy set – McMaster-Carr was not clear if it was made out of metal or other. It is plastic. It may be fine for the most entry level inquiry, but it is not very good for differentiating between, say, 125 and 250 on a shape turn process. If you need to use this for any kind of inspection, get the $60 or so metal set.