Calculating Surface Roughness in Turning Operations
Jun 14, 2024
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Calculating Surface Roughness in Turning Based on Cutting Radius and Feed Rate
The surface roughness in turning operations depends on the feed rate and the nose radius of the cutting insert. Lower feed rates and larger nose radii generally improve surface finish.
In the formula, the feed rate is raised to a power, making it more impactful on surface roughness. When you need to improve surface quality, start by reducing the feed rate. K is a constant, with different values for metric and imperial systems (explained later).
=K×
This formula provides the best theoretical surface roughness. Actual surface quality also depends on factors like stability and cutting edge wear.
Metric System
= 46 × 
- Feed rate [mm/rev]
r - Nose radius [mm]
- Surface roughness [µm]
Imperial System
= 31,675 × 
- Feed rate [IPR] (inches per revolution)
r - Nose radius [inches]
- Surface roughness [µin] (microinches)
Surface Finish Conversion Table
|
Ra (µm) |
Ra (µin) |
Rq (µm) |
Rz (µin) |
N (ISO) |
|
0. 025 |
1 |
1.1 |
1 |
1 |
|
0.05 |
2 |
2.2 |
2 |
2 |
|
0.1 |
4 |
4.4 |
4 |
3 |
|
0.2 |
8 |
8.8 |
8 |
4 |
|
0.4 |
16 |
17.6 |
16 |
5 |
|
0.8 |
32 |
32.5 |
32 |
6 |
|
1.6 |
63 |
64.3 |
63 |
7 |
|
3.2 |
125 |
137.5 |
125 |
8 |
|
6.3 |
250 |
275 |
250 |
9 |
|
12.5 |
500 |
550 |
500 |
10 |
|
25 |
1000 |
1100 |
1000 |
11 |
|
50 |
2000 |
2200 |
2000 |
12 |
In the table below, you can find the minimum surface roughness (best surface finish) that can be achieved by the main machining processes.
|
Machining Process |
Ra (µin) |
Ra (µm) |
|
3D Printing |
125 |
3.1 |
|
Drilling |
63 |
1.6 |
|
Milling |
32 |
0.8 |
|
Turning |
16 |
0.4 |
|
Grinding |
4 |
0.1 |
|
Honing |
2 |
0.1 |
/
Conversion Table
to 
Metric
|
Ra (µm) |
Rz Range [μm] |
|
0.1 |
0.4 - 2.5 |
|
0.2 |
0.8 - 4 |
|
0.5 |
2 - 9 |
|
1 |
4 - 14 |
|
1.5 |
6 - 20 |
|
2 |
7 - 25 |
|
3 |
11 - 33 |
|
5 |
17 - 49 |
|
10 |
34 - 84 |
|
15 |
50 - 115 |
|
20 |
65 - 140 |
to 
Metric
|
Rz (µm) |
Ra Range [μm] |
|
1 |
0.03 - 0.2 |
|
5 |
0.3 - 1.3 |
|
10 |
0.6 - 3 |
|
20 |
1.5 - 6 |
|
50 |
5 - 15 |
|
100 |
13 - 32 |
|
150 |
21 - 49 |