CNC cutting mechanism essentials
This page covers how a cutting edge actually separates material, what governs the result, and where the limits sit. Written for engineers and buyers who need to judge a process before they release a drawing.
What happens at the tip of a cutting tool
The cut removes material through concentrated shear. The tool tip presses into the workpiece until local stress passes the material's shear strength, and a thin layer slides off along a shear plane. That plane sits ahead of the tip, not at the tip itself. If you picture the edge as a scraper, you will misread most of the signals on the machine.
Three zones matter more than spindle speed on the spec sheet. The primary deformation zone is where the chip forms. The secondary zone is the rubbing contact between chip and rake face. The tertiary zone is the flank rubbing the finished surface. Heat, tool wear, and surface finish mostly come from the last two.
Chip thickness is never equal to feed per tooth. The material compresses ahead of the edge, so the chip leaves thicker than the uncut layer. This is why a 0.1 mm feed per tooth can produce a 0.13 mm chip in mild steel. Ignoring that gap makes chip evacuation and load estimates wrong.
- 1Shear plane angleSteeper in hard materials, shallower in soft ones.
- 2Built-up edgeCommon in aluminium and low-carbon steel at low speed.
- 3Chip thinningReal effect on radial engagement, not a rounding error.
Tool geometry sets the limits before the machine does
Rake angle controls how much force goes into cutting versus pushing. Positive rake cuts freely and suits aluminium, brass, and most plastics. Negative rake is stronger and survives interrupted cuts in 4140 or Inconel, at the cost of higher spindle load.
Clearance angle has one job: stop the flank from rubbing. Too little and the tool burns the surface. Too much and the edge loses support and chips. On a Ø10 mm end mill in 6061, 10° to 12° clearance is a normal starting point.
Nose radius trades finish against vibration. A larger radius spreads load and improves Ra, but raises radial force. On long, thin parts, that force deflects the workpiece and shows up as taper. A smaller radius cuts cleaner on flexible setups.
- 1Sharp edgeBetter for soft, gummy materials like 304 stainless.
- 2Honed edgeBetter for hardened steel and interrupted cuts.
- 3Coated edgeTiAlN or AlTiN for dry or high-speed work.
Speeds, feeds, and depth of cut
Surface speed is the real control variable. Spindle rpm just scales it to diameter. In 6061 aluminium, 300 to 600 m/min is common with carbide. In 304 stainless, 120 to 180 m/min. In Ti-6Al-4V, 40 to 70 m/min. Push past the top of the range and edge life collapses.
Feed per tooth is set by chip load, not by how fast the table can move. Too light a chip rubs instead of cuts. Too heavy a chip overloads the edge. For a 3-flute carbide end mill in aluminium, 0.05 to 0.15 mm per tooth is a workable window.
Depth of cut should follow the tool's radial engagement, not the other way round. Adaptive toolpaths keep radial engagement at 5% to 10% of diameter and take axial depth up to 2× diameter. This spreads wear along the flute and lowers cutting temperature.
Coolant choice follows the same logic. Flood coolant handles deep pockets where chip evacuation is hard. Through-spindle coolant helps in holes deeper than 5× diameter. Minimum quantity lubrication works well in aluminium with high-pressure air, but poorly in titanium.
Where the mechanism stops working
Every cutting method has a size floor. Below a certain tool diameter, the edge cannot carry enough stiffness to cut without chatter. On a 5-axis machine, Ø1 mm end mills are practical; below Ø0.5 mm, deflection dominates and finish suffers.
Hardness is the other wall. Carbide cuts hardened steel up to about 45 HRC comfortably. Past 55 HRC, edges chip or wear flat in minutes. That is when EDM or grinding becomes the cheaper route, even with slower cycle times.
Thin walls are a third limit. A 0.5 mm wall in aluminium deflects under cutting force. Light passes, sharp tools, and reduced radial engagement keep the wall in place. If wall thickness falls below 0.3 mm, expect to fixture with support or switch to a non-contact process.
Cutting mechanism compared across common processes
Compare how material is separated and what that means for edge quality.
| Process | How material separates | Typical edge result | Best fit |
|---|---|---|---|
| CNC milling | Shear at the tool tip | Ra 0.8–1.6 μm | Prismatic parts, tight tolerance |
| CNC turning | Continuous shear on a single point | Ra 0.4–1.6 μm | Round symmetric parts |
| Laser cutting | Melt and vaporize | Small heat-affected zone | Sheet stock, thin gauge |
| Waterjet | Erosion by abrasive slurry | No heat-affected zone | Thick plate, heat-sensitive alloys |
| EDM | Spark erosion in dielectric | Ra 0.2–0.8 μm | Hardened steel, sharp internal corners |
| Plasma | Arc melt and blow-off | Rough cut edge | Structural plate, low tolerance |
Choose by the mechanism, not by the machine list
For tight tolerance and 3D geometry, use a shearing process like milling or turning. For thin sheet or heat-sensitive alloys, use laser or waterjet. Match the mechanism to the material, and the numbers follow.
Common questions about CNC cutting
Why does my surface finish get worse at higher spindle speed?
Past the material's ideal surface speed, edge temperature rises faster than the tool can dissipate it. The coating breaks down, the edge dulls, and the flank rubs the finished surface.
Drop surface speed by 10% to 15% and check again. If finish still drops, the issue is usually chip evacuation or tool runout, not speed.
When should I switch from milling to EDM?
Switch when the material exceeds about 55 HRC, or when the geometry has sharp internal corners that a rotating tool cannot reach.
EDM is slower per part, but it holds Ra 0.2–0.8 μm and does not care about hardness. For low-volume hardened tooling, it is often the cheaper path.
Does coolant always improve the cut?
No. In interrupted cuts on cast iron, dry cutting often works better because thermal cycling cracks the chip. In titanium, flood coolant helps; in aluminium with high-pressure air, MQL is enough.
Match coolant to the failure mode you are seeing. If the edge is chipping, coolant will not fix it.
How do I know if a part is too thin to machine?
Check the wall-to-tool-diameter ratio. Below 3:1, deflection starts to matter. Below 1:1, expect to add support or change process.
Send the model for a DFM review before quoting. Wall thickness, corner radii, and depth-to-diameter ratios all change the answer.
What tolerance can a shearing process actually hold?
On a rigid setup with carbide tooling, ±0.005 mm is achievable. That is 0.0002 in. Below that, thermal drift and tool wear set the floor.
For features smaller than Ø1 mm, expect ±0.01 mm unless the operation is dedicated and temperature-controlled.
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