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Surface Finish Ra to N Grade Conversion Chart

Drawings in the US usually call surface finish in microinches Ra while many other regions use N grades or micrometres. This chart lets an inspector convert between them in seconds and see which process normally produces each finish.

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N gradeRa (um)Ra (uin)Typical processesInspection note
N1
0.025
1
Lapping, superfinishing
Optical or gauge block class surfaces; needs a stylus with low noise floor
N2
0.05
2
Lapping, polishing
Mirror finish; use a lab profilometer, not a portable one
N3
0.1
4
Lapping, fine honing
Typical for precision sealing and bearing faces
N4
0.2
8
Honing, fine grinding
Hydraulic rod and cylinder bore range
N5
0.4
16
Grinding, honing
Common for bearing journals and seal contact surfaces
N6
0.8
32
Fine grinding, fine turning
Common fitted shaft and bore finish
N7
1.6
63
Finish turning, finish milling, reaming
Standard general machined finish on many drawings
N8
3.2
125
Milling, turning, drilling
Visible tool marks; typical when no finish is called out tightly
N9
6.3
250
Rough milling, rough turning, planing
Non-mating surfaces
N10
12.5
500
Rough machining, flame cutting
Compare with a visual and tactile roughness comparator plate
N11
25
1000
Sand casting, saw cutting
As-cast or as-cut surfaces
N12
50
2000
Rough casting, forging scale
Coarsest N grade on the ISO scale

Ra is the arithmetic mean roughness. N grades follow the ISO 1302 series and map to Ra values by definition. Process columns are typical ranges, not guarantees. Confirm the cutoff length and evaluation length on the drawing before measuring.

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Optional: we can review how your drawing characteristics map to ERP inspection plans and suggest a cleaner structure.

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How to use the chart

Type the value you have into the search box, for example 63 or N7, and the matching row is shown. Read across to the equivalent Ra in the other unit and to the processes that usually achieve it. Export the table to Excel or PDF if you want a controlled copy at the inspection station. The Ra values are rounded as they appear in the standard series, so a measured value of 1.58 um is read as N7 compliant against a 1.6 um limit.

How roughness is checked

Ra is measured with a stylus profilometer over an evaluation length that normally spans five sampling lengths. The cutoff must suit the expected roughness: shorter cutoffs for fine finishes, longer for coarse ones. Always compare the measured Ra to the maximum on the drawing, and check whether the drawing gives a single limit (maximum) or a range, since a lower limit means a too smooth surface is also rejectable.

  • Measure across the lay direction, not along it
  • Clean the part before measuring
  • Record the cutoff and evaluation length with the result
  • Use a comparator plate only for rough screening

Common mistakes

The most frequent error is mixing Ra with Rz or Rmax. They are different parameters and cannot be converted with a fixed factor. Another is measuring one spot on a surface whose finish varies, or judging by eye. For ERP users, store the required finish as a characteristic on the inspection plan with the unit stated, so receiving inspection and the shop floor read the same limit.

Frequently Asked Questions

What is the difference between Ra and Rz?

Ra is the arithmetic average of profile deviations over the evaluation length. Rz is based on peak to valley heights over sampling lengths. Rz is more sensitive to isolated scratches and peaks. They are different parameters, so you cannot reliably convert one to the other.

Is N7 the same as 63 microinch?

Yes for Ra. N7 corresponds to 1.6 micrometres Ra, which is 63 microinches in the usual US call-out. The microinch figure is a rounded conversion of the metric value.

Can I use a roughness comparator plate for final acceptance?

Comparator plates are fine for quick screening on coarser finishes. For finishes tighter than about N8 or where the drawing states a measured limit, use a stylus profilometer and record the numeric result.

Does the chart replace the drawing requirement?

No. The drawing governs, including the roughness parameter, cutoff and whether a minimum applies. The chart only helps convert units and understand typical processes.

If surface finish and other characteristics live in drawings and spreadsheets, Netray can help pull them into your ERP inspection plans so results are recorded against the right limit.

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