On a drawing there is a little triangle with a number next to it: 3.2 · 1.6 · 0.8. That is roughness, and it defines how "rough" the surface is allowed to be. Ignoring it costs money in two ways: by falling short and having to rework, and by overshooting and spending three times as long on a finish nobody asked for.
Ra and Rz are not the same thing
Ra is the arithmetic mean of the profile's deviations from the mean line. It is the most widely used parameter because it is stable and easy to measure, but it has a flaw: it averages. A surface with one deep scratch and the rest perfect can still give a good Ra.
Rz measures the mean distance between peaks and valleys across several sampling lengths. It is more sensitive to isolated defects, which is why it is specified when a surface has to seal, slide or carry fatigue loads.
As a rough guide, Rz is usually between four and seven times Ra on machined surfaces. It is not an exact conversion and should not be used as one, but it tells you whether you are in the right territory.
What finish each process gives
- Rough turning or milling: Ra 6.3 to 12.5 µm.
- Finish turning or milling: Ra 1.6 to 3.2 µm.
- Fine finishing with a large nose radius: Ra 0.8 µm.
- Grinding: Ra 0.2 to 0.8 µm.
- Fine grinding, lapping, superfinishing: below Ra 0.1 µm.
The practical conclusion is blunt: if the drawing calls for Ra 0.4, you are not going to get there by milling. You have to plan for grinding and leave stock. Knowing this when you read the drawing, before you program, avoids a nasty surprise on the last operation.
The seven things that change the finish
1. Feed per revolution or per tooth. This is factor number one in turning. Theoretical roughness grows with the square of the feed: halve it and roughness drops to a quarter. Before you touch anything else, drop the feed.
2. The nose radius of the insert. A larger radius spreads the mark and improves the finish at the same feed. But it raises the radial force, so on slender parts it can bring on vibration. It is a balance.
3. Cutting speed. Cutting too slowly in steel produces a built-up edge: material that welds to the cutting edge, breaks away and leaves the surface torn. Raising the speed usually fixes an ugly finish in steel.
4. The condition of the edge. A worn tool does not cut, it crushes. If the finish gets steadily worse through the batch, look no further: it is wear.
5. The rigidity of the whole set-up. A long tool or part overhang, a loose clamp, and waves appear. No cutting parameter compensates for a lack of rigidity.
6. Coolant. In aluminium it stops the material welding to the edge; in steel it controls the temperature and keeps the built-up edge away. Enough flow, pointed in the right place.
7. The finishing strategy. One light, even final pass with no abrupt changes of direction leaves a better surface than finishing with whatever the roughing left. And entering tangentially avoids the mark at the entry point.
How it is measured
With a surface roughness tester: a stylus travels a few millimetres across the surface and calculates the parameters. You always measure perpendicular to the machining marks, which is where roughness is highest; measuring parallel to them gives optimistic numbers and does not count.
Where there is no tester, visual and tactile comparison specimens are used. It is not measuring, but on the shop floor it is enough to tell a 3.2 from a 0.8.
The mistake of asking for more finish than you need
Going from Ra 3.2 to Ra 0.8 can triple the time of the operation. And from Ra 0.8 to Ra 0.2, add a whole grinding operation with its own set-up and alignment. A finer finish than the function calls for is money thrown away, and on a part that will be painted or anodised it often does not even show.
If the drawing does not specify it, ask before deciding it yourself. And if you want to go back over how the dimensions beside that little triangle are read, there is the guide to tolerances and fits.
Need a part machined? The directory of Spanish machine shops lists more than 700 workshops across Spain with their processes, materials and certifications verified, filterable by province. And if you are learning, the CNC Path takes you from G-code to your first part.