ISO 1302 vs ASME Y14.36 Surface Roughness Standards for Carbon Steel

Surface roughness is one of the most frequently misunderstood specifications on carbon steel machining drawings. Buyers receive quotes with “Ra 3.2” on one drawing and “32 microinches” on another, and they are not sure whether these are the same, different, or even compatible. The confusion stems from the fact that two major standards govern surface texture: ISO 1302 (international) and ASME Y14.36 (North American).

This guide compares the two standards side by side, explains how to read surface finish callouts on both types of drawings, provides conversion tables for common roughness values, and offers practical guidance for specifying surface finish on carbon steel machined parts.

1. What Each Standard Covers

ISO 1302:2002 (“Geometrical Product Specifications — Indication of surface texture in technical product documentation”) defines how surface texture requirements are specified on drawings using graphical symbols and parameter values. It was technically revised and absorbed into ISO 21920-1:2021, but most drawings still use the ISO 1302 symbols.

ASME Y14.36-2018 (“Surface Texture Symbols”) is the American national standard for surface texture specification on engineering drawings. It defines the checkmark surface texture symbol and the placement of roughness, waviness, and lay parameters.

Both standards reference the same measurement parameters (Ra, Rz, Rmax, etc.) as defined in ISO 4287 (international) and ASME B46.1 (American). The difference is in how the requirements are communicated on drawings.

2. Surface Texture Symbols Comparison

Feature	ISO 1302	ASME Y14.36
Basic symbol	Checkmark (open triangle)	Checkmark (V-shape, same geometry)
Material removal required	Checkmark with top bar	Checkmark with horizontal bar
Material removal prohibited	Circle inside checkmark	Circle inside checkmark
Ra value placement	Above the symbol (a-position)	Left of the checkmark
Machining allowance	Below the symbol (c-position)	Not specified in symbol
Manufacturing method	Can be specified above symbol	Specified in a note
Lay symbol	Below the symbol (d-position)	Right of the checkmark
Sampling length	Can be specified	Not commonly specified

3. Ra Value Systems: Micrometers vs Microinches

The most common source of confusion is the unit system. ISO drawings use micrometers (micrometers, Ra); ASME drawings use microinches (microinches, Ra). The conversion is:

1 microinch = 0.0254 micrometers

1 micrometer = 39.37 microinches

Ra (micrometers)	Ra (microinches)	ISO grade number (N)	Typical carbon steel application
0.4	16	N3	Bearing journals, precision shafts
0.8	32	N4	Sealing surfaces, gauge surfaces
1.6	63	N5	Fitting bores, gear teeth
3.2	125	N6	General machining, shafts
6.3	250	N7	Non-critical surfaces, chamfers
12.5	500	N8	Rough machined, as-forged
25	1000	N9	Flame cut, rough cast

The ISO grade number (N3-N12) is a roughness grading system defined in ISO 1302. It is rarely used on modern drawings but still appears in older specifications and some supplier catalogs.

4. Reading ISO 1302 Surface Callouts

On an ISO 1302 drawing, the surface texture symbol is placed on the surface line, and values are positioned around the symbol:

Position a (above): Ra value in micrometers or the parameter name + value (e.g., “Ra 3.2”)
Position b (above, below a): Manufacturing method (e.g., “milling”, “grinding”)
Position c (below): Machining allowance in millimeters
Position d (below, right): Lay symbol (=, perpendicular, X, M, C, R, P)
Position e (below, right of d): Sampling length in millimeters

Example: A checkmark with “Ra 3.2” above, “milling” below “Ra 3.2”, “0.5” below the checkmark, and “=” to the right means: surface roughness Ra 3.2 micrometers, produced by milling, with 0.5 mm machining allowance, lay parallel to the drawing plane.

5. Reading ASME Y14.36 Surface Callouts

On an ASME Y14.36 drawing, the surface texture symbol is the same checkmark, but values are placed differently:

Left of checkmark: Ra value in microinches (e.g., “32”)
Below the checkmark: Ra cutoff/sample length (e.g., “0.8”)
Right of checkmark: Lay symbol
Above the checkmark: Maximum waviness height

Example: A checkmark with “32” to the left, “0.8” below, and “M” to the right means: surface roughness 32 microinches Ra (0.8 micrometers), 0.8 mm sampling length, multidirectional lay.

6. Common Surface Finish Specifications for Carbon Steel

Carbon steel part	ISO Ra (micrometers)	ASME Ra (microinches)	Typical process
Shaft journal (bearing)	0.4-0.8	16-32	Grinding
Sealing surface (O-ring)	0.4-0.8	16-32	Grinding, lapping
Thread surface	1.6-3.2	63-125	Turning, threading
Bore (interference fit)	0.8-1.6	32-63	Boring, reaming
Gear tooth flank	0.8-1.6	32-63	Hobbing, grinding
General turned surface	3.2-6.3	125-250	Turning
Milled surface	3.2-6.3	125-250	Milling
Drilled hole	3.2-12.5	125-500	Drilling
As-forged surface	12.5-25	500-1000	Forging
Flame cut edge	12.5-50	500-2000	Oxy-fuel cutting

7. Measurement Methods and Instruments

Surface roughness is measured using two main instrument types:

Stylus Instruments (Contact Method)

The most common method for carbon steel parts. A diamond-tipped stylus traces the surface, and the vertical displacement is converted to a roughness profile. Standards:

ISO 4287: Defines Ra, Rz, Rmax, Rq parameters
ASME B46.1: American equivalent, defines same parameters with slight calculation differences
Typical stylus radius: 2-5 micrometers (2 micrometers for precision, 5 micrometers for general use)
Measurement force: 0.75 mN (ISO), 0.5-1.0 mN (ASME)
Sampling length (cutoff): 0.25 mm for fine surfaces (Ra below 0.8), 0.8 mm for general surfaces (Ra 0.8-6.3), 2.5 mm for rough surfaces (Ra above 6.3)

Optical Instruments (Non-Contact Method)

Laser confocal microscopy and white-light interferometry for non-destructive measurement of soft or delicate surfaces. Less common for carbon steel but useful for measuring very fine surfaces (Ra below 0.1 micrometers) or surfaces where stylus contact could damage the finish.

8. Practical Issues When Importing Carbon Steel Parts

Issue 1: Drawing Mismatch

When a Chinese shop receives an ASME drawing with “32 Ra” and interprets it as 32 micrometers (instead of 32 microinches = 0.8 micrometers), the part will be 40 times rougher than specified. Always include the unit explicitly: “32 microinches Ra” or “0.8 micrometers Ra.”

Issue 2: Ra vs Rz Confusion

Ra (arithmetic mean roughness) and Rz (average maximum height of the profile) are different parameters. Rz is typically 4-10 times larger than Ra for the same surface. A drawing specifying “Rz 16” is not the same as “Ra 16.” Always confirm which parameter is required.

Issue 3: Sampling Length

If the sampling length is not specified, ISO defaults to the value from ISO 4288 Table 1 based on the expected Ra range. ASME defaults to 0.8 mm. Mismatched sampling lengths produce different Ra readings on the same surface. Specify the cutoff length on the drawing.

Issue 4: Measurement Direction

Roughness varies with measurement direction relative to the lay. Always specify the measurement direction: perpendicular to the lay gives the highest Ra, parallel gives the lowest. If the drawing does not specify, measure perpendicular to the lay (worst case).

9. Conversion Quick Reference

ISO Ra (micrometers)	ASME Ra (microinches)	ISO grade	Rz approx (micrometers)
0.2	8	N1	1.2
0.4	16	N3	2.5
0.8	32	N4	5.0
1.6	63	N5	10
3.2	125	N6	20
6.3	250	N7	40
12.5	500	N8	80

FAQ

Is ISO 1302 the same as ASME Y14.36?

No. ISO 1302 is the international standard and ASME Y14.36 is the American standard. They use the same checkmark symbol but place parameters differently around the symbol. The measurement parameters (Ra, Rz) are the same, but the unit systems differ (micrometers vs microinches).

How do I convert 32 microinches Ra to micrometers?

Divide by 39.37: 32 / 39.37 = 0.813 micrometers. So 32 microinches Ra is approximately 0.8 micrometers Ra.

What Ra should I specify for a carbon steel shaft journal?

For bearing journals on carbon steel shafts (SAE 1045, 4140), specify Ra 0.4-0.8 micrometers (16-32 microinches). This requires cylindrical grinding after heat treatment. For non-bearing shaft surfaces, Ra 1.6-3.2 micrometers (63-125 microinches) is typical from turning.

Does the standard affect the cost of carbon steel machining?

No. The cost is driven by the Ra value itself, not which standard specifies it. Tighter Ra requires additional operations (e.g., grinding after turning) which adds 15-40 percent to machining cost per surface. The unit system (metric vs imperial) does not affect cost.

Can I use both ISO and ASME symbols on the same drawing?

Technically yes, but it creates confusion. Best practice is to use one standard consistently. If the drawing uses ISO dimensions (millimeters), use ISO 1302 surface symbols. If the drawing uses ASME dimensions (inches), use ASME Y14.36 symbols.

Conclusion

ISO 1302 and ASME Y14.36 are two parallel systems for specifying surface roughness on engineering drawings. The key differences are the symbol layout (where parameters are placed around the checkmark) and the unit system (micrometers vs microinches). By understanding both systems and including explicit units on drawings, buyers can avoid the most common surface finish errors when sourcing carbon steel machined parts from Chinese suppliers.

Contact Huaxia-Steel for precision-machined carbon steel components with surface finishes to ISO 1302 or ASME Y14.36. We supply shafts, flanges, fittings, and custom parts with Ra from 0.4 to 12.5 micrometers, including measurement reports.