KEVOS® MAKEOS · Manufacturing
Surface Finish Change with Tool Nose Radius (Colding Rule)
If I change the tool nose radius while keeping the same equivalent chip thickness and speed, how much will the surface roughness Ra change in finishing and in roughing, and what nose radius would reach my target Ra?
The handbook’s rule of thumb from the machining econometrics section for the effect of tool nose radius on surface finish at the same ECT and speed: Ra1 ÷ Ra2 = (r2 ÷ r1)^0.5 in finishing, with roughing values 1.5 times the finishing values, compared with the inverse proportion of the simple geometric rule that the handbook says gives large errors, and the nose radius that would give a target Ra.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Ra measured with the present nose radius (finishing) | µm | Roughness measured in finishing with the present insert. |
| Present nose radius r1 | mm | Nose radius of the present insert. |
| New nose radius r2 | mm | Nose radius of the insert you are changing to. |
| Target Ra (0 = none) | µm | The Ra you need, if you want the nose radius for it. |
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: Ra measured with the present nose radius (finis…
- What it is
- Arithmetic mean roughness measured with the insert you use now.
- 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 1.6 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The new finish is in proportion to it.
- Get it right
- Measure the finishing cut with a profilometer across the feed marks.
What this means: Present nose radius r1
- What it is
- Corner radius of the insert used for the measured finish.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A smaller present radius gives a larger gain from the change.
- Get it right
- Read it from the insert designation, e.g. 08 is 0.8 mm.
What this means: New nose radius r2
- What it is
- Corner radius of the insert you are changing to.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Ra falls with the square root of the radius ratio.
- Get it right
- Read it from the insert designation, e.g. 16 is 1.6 mm.
What this means: Target Ra (0 = none)
- What it is
- Roughness the drawing or the process needs at the finished surface.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 100 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Gives the nose radius that would reach it.
- Get it right
- Enter the Ra from the drawing, or 0 to skip the check.
Method and formulas
- Finishing, same ECT and speed: Ra2 = Ra1 × (r1 ÷ r2)^0.5.
- Roughing: Ra = 1.5 × the finishing value at the same ECT and speed.
- Simple geometric rule for comparison: Ra2 = Ra1 × r1 ÷ r2.
- Nose radius for a target finish: r = r1 × (Ra1 ÷ Ra target)².
- Example: changing from 0.8 mm to 1.6 mm takes a finish of 1.6 µm to about 1.1314 µm.
Limitations
- The rule holds for common practical ECT values and speeds; the work material, tool geometry and cutting conditions also affect the finish.
- Keep the equivalent chip thickness and speed the same when you compare; a larger nose radius at the same feed changes the ECT.
- Measure the finish after the change; this is an estimate.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate the finish after changing to an insert with a larger nose radius
- Choose a nose radius for a target Ra from a measured finish
- Compare the empirical and geometric nose radius rules
Who it is for
- CNC machinists
- Manufacturing engineers
- Process planners
What you get
- Ra with the new nose radius in finishing and roughing
- Ratio of the finishes
- Ra by the simple geometric rule for comparison
- Nose radius for the target Ra
Interpreting the result
- The geometric rule (Ra in proportion to feed squared over nose radius) overstates the gain from a larger nose radius, as the handbook explains.
- Roughing finishes are about half as good again as finishing at the same ECT.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1143–1144. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Useful alongside · KEVOS® original book
Turning: feed, depth of cut, removal rate and surface finish
What it covers: Choose feed and depth of cut within the insert's working range, calculate metal removal rate and machining time, and predict the surface finish a feed and nose radius can give.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Surface Finish Change with Tool Nose Radius (Colding Rule) calculator