CNC Machining Guide

CNC Precision Finishing: Roughing vs Finishing Parameters Explained

1. Roughing, Semi-Finishing and Finishing

Roughing is the first major machining operation. Its main
purpose is to remove most of the excess material as quickly and safely as possible.
Roughing prioritizes material removal rate, tool strength and process stability.

Semi-finishing is the transition between roughing and finishing.
It corrects uneven stock and leaves a consistent allowance for the final finishing pass.

Finishing is the final light-cutting operation after roughing
or semi-finishing. Its purpose is to achieve the specified final dimension,
geometry and surface roughness.

2. Roughing vs Finishing Parameters

Roughing and finishing normally use opposite cutting strategies.
Roughing uses a stronger cutting load, while finishing uses a smaller
cutting depth and feed rate to control dimensional accuracy and surface quality.

3. Why May Finishing Use a Higher Cutting Speed?

Increasing cutting speed during finishing may seem counterintuitive.
However, finishing normally uses a much smaller cutting depth and feed rate,
which reduces the cutting load on the tool.

When the machine, workholding and tool system are stable, a suitable increase
in cutting speed can improve cutting consistency and reduce the risk of built-up
edge in an unsuitable speed range.

Cutting speed has a significant effect on tool temperature and tool life.
However, the speed should never be increased blindly. Always begin with the
cutting data provided by the insert or end mill manufacturer.

4. Three Important Finishing Requirements

1. The stock allowance must be even.
   After roughing, uneven stock forces the finishing tool to cut both heavy
   and light areas in the same pass. This may cause deflection, chatter and
   inconsistent dimensions.
2. Tool runout must be controlled.
   For precision milling, low radial runout is important. In some applications,
   a runout range around 0.002–0.005 mm may be required. Hydraulic and shrink-fit
   holders may be suitable when low runout is a priority.
3. The machining environment must be stable.
   For precision work, a controlled environment near 20 ± 2°C can help reduce
   thermal variation. The required control level depends on the tolerance,
   part size and inspection method.

5. Material-Based Finishing Reference

The following values are general starting references only. Actual parameters
depend on the cutting tool, insert grade, tool diameter, machine rigidity,
workholding, coolant and material condition.

6. Three Common CNC Finishing Mistakes

• Using a roughing tool for the final pass:
  a worn or rounded cutting edge may rub instead of cutting cleanly,
  resulting in poor surface quality.
• Using too much finishing depth:
  excessive cutting depth can overload thin walls, create deflection
  and produce chatter marks.
• Leaving uneven stock after roughing:
  if one area has only 0.05 mm of stock while another area has 0.6 mm,
  the final dimensions may be inconsistent.

7. A Three-Step Finishing Workflow

1. Measure the stock after roughing.
   Use a suitable measuring method to confirm the remaining allowance.
   If the allowance is uneven, add a semi-finishing pass before finishing.
2. Install and verify the finishing tool.
   Check the tool length, holder condition, cutting edge and radial runout.
   Keep the tool overhang as short as practical to improve rigidity.
3. Use a stable finishing tool path.
   Use appropriate climb milling where applicable, smooth arc lead-in and
   lead-out movements, controlled feed and a clear surface roughness target,
   such as Ra 1.6 µm.

Final Takeaway

Good CNC precision finishing begins before the finishing tool touches the part.
Roughing must leave an even allowance, semi-finishing must stabilize the geometry,
and the final pass must use a sharp, low-runout tool with a controlled cutting load.

When these conditions are aligned, the finishing operation can achieve better
dimensional consistency and surface quality without simply taking another cut.