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Surface Finish Estimate Calculator

Work out surface finish estimate instantly with clear inputs, formula shown and shareable results.

About Surface Finish Estimate Calculator

The theoretical roughness left by a round-nosed turning tool is f² / (32 r): the scallops between successive feed marks. Because feed appears squared, halving the feed cuts roughness to a quarter, while doubling the nose radius only halves it, so feed is the more powerful lever.

Formulas

Theoretical roughness

Ra = f² / (32 r)

f
Feed per revolution
r
Tool nose radius

Peak to valley

Rz is roughly 4 x Ra for a turned surface

Worked example

With the default values below, Surface Finish Estimate Calculator returns Theoretical roughness Ra of 64.52. Change any input above and every figure here is recalculated in your browser.

Inputs used

Feed per revolutionminimum 0 in/rev
0.008 in/rev
Tool nose radiusminimum 0.001 in
0.031 in

Results

Theoretical roughness Ra
64.52 µin
Roughness in metric units
1.639 µm
Peak-to-valley height Rz
258.1 µin
Finish description
General machined finish

Step by step

  1. 1Values used
  2. 2Theoretical roughness
  3. 3Peak to valley
  4. 4Theoretical roughness Ra= 64.52 µin
  5. 5Roughness in metric units= 1.639 µm
  6. 6Peak-to-valley height Rz= 258.1 µin
  7. 7Finish description= General machined finish

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Surface Finish Estimate figure worked out?

Ra = f² / (32 r). The theoretical roughness left by a round-nosed turning tool is f² / (32 r): the scallops between successive feed marks.

Why is the measured finish usually worse?

The formula covers only the geometric feed marks. Built-up edge, vibration, tool wear and material tearing all add roughness that geometry cannot predict.

What values do I need to enter?

This calculator takes 2 inputs: Feed per revolution, Tool nose radius. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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