There is a ratio between the average surface roughness and a quantiny I've never seen before (Ri) and it equals 1.26 for cast iron. Can someone tell me what is this equation refering to, I couldn't find anything on the web.
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Statisticool
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1where did you see that equation? – jsotola Sep 07 '23 at 23:41
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1Welcome to Engineering! Can you give us more information, such as what calculation uses that ratio? Please [edit] your question with this information. – Wasabi Sep 09 '23 at 15:12
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It looks to me like they've messed up the equation a bit.
I'd guess what they're trying to do is convert Ra (average surface roughness) to RMS surface roughness (which I have seen abbreviated as Ri, though not very often). But 1.26 would be a somewhat reasonable number for RMS/Ra, so to convert from Ra to RMS you'd multiply rather than dividing.
For example: 140 microinch Ra * 1.26 = ~176 microinch RMS roughness.
Note that any number you plug in there is an estimate, but 1.26 is at least reasonably close to the numbers you typically see for this conversion. You probably see 1.11 more often, but that's for more ductile materials like steel. If memory serves, you'd expect a lower constant with greater ductility (and vice versa).
Jerry Coffin
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