Surface Finish Converter
Convert Ra between micrometers and microinches, and cross-reference RMS, Rz, VDI 3400 and SPI mold grades, with the processes that reach each value.
Ra is the arithmetic mean deviation of the profile.
Common callouts
Exact conversions
Estimates, profile dependent
Rz and RMS are not true conversions
Both depend on the shape of the surface profile, not on Ra alone. Two surfaces with the same Ra can differ considerably in Rz. If a drawing requires Rz, measure Rz rather than converting from Ra.
Mold finish
Published SPI charts disagree. This uses the SPI grade bands (A-1 at 0.012 to 0.025 um through D-3 at 3.2 to 18 um); some published charts give a simplified one value per grade chart instead. State the SPI grade on the tool drawing rather than an Ra, so the finishing METHOD is what is agreed.
Processes that reach this finish
Ra converts exactly between micrometers and microinches, and VDI 3400 follows from Ra by formula. RMS and Rz do not: they depend on the shape of the profile, so this converter shows them as estimates with a range rather than a single number. SPI mold grades map as published Ra bands, and each result lists the processes that reach it.
How to read the result
Ra is the arithmetic mean deviation of the profile from its mean line. It is the most quoted roughness parameter and the least descriptive, because very different surfaces share an Ra: a fine turned surface and a shot-blasted one can measure the same while behaving nothing alike in sealing, fatigue or coating adhesion.
That is also why Rz cannot be converted from Ra with any single factor. Rz measures peak to valley height, so the ratio between them depends on whether the profile is periodic, random or skewed. Four to seven times Ra covers most machined surfaces, and a drawing that genuinely needs Rz should call out Rz and be measured for Rz.
For molded parts, specify the finish on the tool, not on the part. The part reproduces whatever the cavity surface is, so an SPI grade on the mold drawing is the controllable requirement. VDI 3400 does the same job for EDM-textured cavities and is defined directly from Ra.
Frequently asked questions
- How do I convert Ra to Rz?
- You cannot do it exactly. Rz is peak to valley and Ra is an average deviation, so the ratio depends on the profile shape. For ordinary machined surfaces Rz falls between about four and seven times Ra, with five times a reasonable estimate. If a requirement is stated in Rz it should be measured in Rz, not converted.
- What is a standard machined surface finish?
- Ra 3.2 um (126 uin) is a normal as-machined finish and 1.6 um (63 uin) is the usual default when a drawing says nothing. Getting to 0.8 um (32 uin) needs a deliberate finishing pass, and below about 0.4 um (16 uin) you are generally into grinding, lapping or polishing, with cost rising sharply.
- What is the difference between Ra and RMS?
- Ra is the arithmetic mean of the profile deviations and RMS, properly Rq, is the root mean square of the same deviations. RMS weights larger excursions more heavily, so it reads slightly higher: about 1.11 times Ra for a near-Gaussian machined profile. Older US drawings often quote RMS in microinches where modern ones quote Ra.
- How do SPI and VDI 3400 relate to Ra?
- VDI 3400 is defined from Ra by a formula, so it converts exactly: the reference number equals 20 times the base-10 logarithm of Ra in micrometers times ten. SPI grades are finishing procedures rather than a formula, each with a published Ra band, so a given Ra can legitimately fall in more than one grade reached by different methods.
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