BIZOMAT MAKEOS · Manufacturing
Turning Feed for Surface Finish (Ra)
What surface finish does this feed and nose radius give, or what feed achieves the target Ra?
Theoretical arithmetic-mean roughness Ra and peak-to-valley Rt in turning from feed and tool nose radius, or the feed for a target Ra.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Calculate | choice | Find Ra from a feed, or the feed for an Ra. |
| Tool nose radius | mm | Insert nose radius. |
| Feed per revolution | mm/rev | Feed per revolution (Ra mode). |
| Target Ra | µm | Required arithmetic mean roughness from the drawing (feed mode). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Calculate
- What it is
- Whether to work out the theoretical roughness from a feed you plan to use, or the feed that gives a roughness you need.
- Units and format
- Choose one of 2 options from the list: Ra from feed; Feed for a target Ra.
- Example
- The form opens with “Feed for a target Ra” selected as a worked example. Choose the option that matches your case.
- Get it right
- Only the input for the chosen direction is used: feed in the first case, target Ra in the second.
What this means: Tool nose radius
- What it is
- The radius of the rounded corner of the turning insert that generates the surface, read from the insert designation.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 25 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger nose radius gives a lower Ra at the same feed, or allows a higher feed for the same Ra.
- Get it right
- Use the radius, not the diameter, and the insert actually fitted.
What this means: Feed per revolution
- What it is
- The distance the tool moves along the work for each revolution of the spindle; used when finding Ra from a feed.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 10 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Theoretical Ra rises with the square of feed, so halving the feed quarters the roughness.
- Get it right
- Enter millimetres per revolution, not per minute.
What this means: Target Ra
- What it is
- The arithmetic mean roughness the surface must reach, usually taken from the drawing; used when finding the feed.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0.000001 µm to 100 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The allowable feed rises with the square root of the target Ra.
- Get it right
- Enter micrometres. Real surfaces are usually rougher than the theoretical value, so leave a margin.
Method and formulas
- Ra ≈ 0.031553 f² ÷ R; Rt ≈ f² ÷ 8R (= 0.125 f² ÷ R).
- Feed for a target Ra: f = √(Ra × R ÷ 0.031553), with Ra and R in the same length unit.
- Example (handbook case): Ra 1 µm with a 1 mm nose radius needs a feed of 0.17802 mm/rev.
Limitations
- Theoretical geometric finish; built-up edge, vibration, tool wear and material effects make the real Ra higher.
- Wiper inserts give better finish than this relation predicts.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a finishing feed for a drawing Ra
- Trade feed against finish and cycle time
- Explain finish problems
Who it is for
- Machinists
- CNC programmers
- Manufacturing engineers
What you get
- Theoretical Ra
- Theoretical Rt (peak to valley)
- Feed
Interpreting the result
- Start from the theoretical feed and reduce it if measured finish is too rough.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1125–1131. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Useful alongside · BIZOMAT original book
Surfaces, scatter and proof
In Metal Fatigue by the Numbers
What it covers: How surface finish is specified and inspected at fatigue-critical points, how Weibull analysis reads a fleet's failures, and how tests and field checks prove a design's life.
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