Surface Finish of Machined Parts

When it comes to the surface finish of machined parts, most people are typically concerned only with average roughness of a part, Ra; however, there are many more roughness parameters which can be specified.



Surface Finish usually takes on the lay of its manufacturing process.  For example, if a part is saw-cut, it will have a linear lay following

the travel of the saw teeth.  On the other hand, if a part is rotary-ground it’s finish will have a circular lay.

There are several specification guiding the detailing of surface finish.  The Europeans use BS EN ISO 1302:2002 Geometrical product specifications (GPS).  In the US, ASME Y14.36M is commonly used.  The symbols in each standard are quite similar:

The metric-to-inch conversion is:

a-μm – 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50
a– μinch 1 2 4 8 16 32 63 125 250 500 1000 2000

Various machining operations have different typical surface finishes inherent to their respective manufacturing process, as indicated in the table below:

Machinability of Metals

Machinability is the defined as the ease with which a material can be machined. The American Iron and Steel Institute has determined that AISI 1112 Steel has a machinability of 100%. Other materials are compared below:


Carbon Steels

  • 1015 – 72%
  • 1018 – 78%
  • 1020 – 72%
  • 1022 – 78%
  • 1030 – 70%
  • 1040 – 64%
  • 1042 – 64%
  • 1050 – 54%
  • 1095 – 42%
  • 1117 – 91%
  • 1137 – 72%
  • 1141 – 70%
  • 1141 annealed – 81%
  • 1144 – 76%
  • 1144 annealed – 85%
  • 1144 stress-proof – 83%
  • 1212 – 100%
  • 1213 – 136%
  • 12L14 – 170%
  • 1215 – 136%

Alloy Steels

  • 2355 annealed – 70%
  • 4130 annealed – 72%
  • 4140 annealed – 66%
  • 4142 annealed – 66%
  • 41L42 annealed – 77%
  • 4150 annealed – 60%
  • 4340 annealed – 57%
  • 4620 – 66%
  • 4820 annealed – 49%
  • 52100 annealed – 40%
  • 6150 annealed – 60%
  • 8620 – 66%
  • 86L20 – 77%
  • 9310 annealed – 51%

Stainless Steels and Super Alloys

  • 302 annealed – 45%
  • 303 annealed – 78%
  • 304 annealed – 45%
  • 316 annealed – 45%
  • 321 annealed – 36%
  • 347 annealed – 36%
  • 410 annealed – 54%
  • 416 annealed – 110%
  • 420 annealed – 45%
  • 430 annealed – 54%
  • 431 annealed – 45%
  • 440A – 45%
  • 15-5PH condition A – 48%
  • 17-4PH condition A – 48%
  • A286 aged – 33%
  • Hastelloy X – 19%

Tool Steels

  • A-2 – 42%
  • A-6 – 33%
  • D-2 – 27%
  • D-3 – 27%
  • M-2 – 39%
  • O-1 – 42%
  • O-2 – 42%

Gray Cast Iron

  • ASTM class 20 annealed – 73%
  • ASTM class 25 – 55%
  • ASTM class 30 – 48%
  • ASTM class 35 – 48%
  • ASTM class 40 – 48%
  • ASTM class 45 – 36%
  • ASTM class 50 – 36%

Nodular Ductile Iron

  • 60-40-18 annealed – 61%
  • 65-45-12 annealed – 61%
  • 80-55-06 – 39%

Aluminum / Magnesium Alloys

  • aluminum, cold drawn – 360%
  • aluminum, cast – 450%
  • aluminum, die cast – 76%
  • magnesium, cold drawn – 480%
  • magnesium, cast – 480%

Galvanic Corrosion and the Compatibility of Metals – Anodic Index

Designs sometimes cause dissimilar metals to be in contact within an assembly.  Increased variance of the two metals’ anodic index cause accelerated corrosion of the more anodic metal.  This is called galvanic corrosion.  The variance in anodic index can be controlled somewhat through coatings (paint, epoxy, powder coat, etc) and plating (zinc, chrome, etc).

The table below provides a generalize guide for predicting the anodic corrosion two dissimilar metals are prone to.  The service environment also plays a role:

  • For harsh environments: outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the “Anodic Index”. For example; gold – silver would have a difference of 0.15V being acceptable.
  • For normal environments: storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the “Anodic Index”.
  • For controlled environments: where the temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions.

Anodic Index

Metallurgy Index (V)
Gold, solid and plated, Gold-platinum alloy 0.00
Rhodium plated on silver-plated copper 0.05
Silver, solid or plated; monel metal. High nickel-copper alloys 0.15
Nickel, solid or plated, titanium an s alloys, Monel 0.30
Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys 0.35
Brass and bronzes 0.40
High brasses and bronzes 0.45
18% chromium type corrosion-resistant steels 0.50
Chromium plated; tin plated; 12% chromium type corrosion-resistant steels 0.60
Tin-plate; tin-lead solder 0.65
Lead, solid or plated; high lead alloys 0.70
Aluminum, wrought alloys of the 2000 Series 0.75
Iron, wrought, gray or malleable, plain carbon and low alloy steels 0.85
Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type 0.90
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate 0.95
Hot-dip-zinc plate; galvanized steel 1.20
Zinc, wrought; zinc-base die-casting alloys; zinc plated 1.25
Magnesium & magnesium-base alloys, cast or wrought 1.75
Beryllium 1.85