How Dod CNC Machines Cut: The Shear Zone Behind Every Pass
['A CNC machine does not slice material. It drives a hardened edge into the workpiece until the metal yields along a narrow shear plane, and the chip that curls off the rake face is the only proof the cut worked.', 'This page is written for engineers and buyers who need to judge a process, not a brochure. You will see which parameters set the shear zone, how 3-axis and 5-axis motion change the approach angle, and when a cut should be moved to a different machine.']
In this article
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Key takeaways
The shear zone: where the chip is actually born
Push a wedge into metal and the material does not part cleanly along the edge line. It deforms first. A narrow band ahead of the tool tip, usually 0.1–0.4 mm wide, takes almost all the strain. Once the shear stress in that band passes the material's shear strength, the metal slides along the plane and a chip begins to flow up the rake face.
The angle of that shear plane depends on the rake angle of the tool and the friction between chip and rake face. A positive rake tool, say 10–15°, pushes the shear plane forward and lowers cutting force. That suits aluminum and soft brass. A negative rake tool, around –5 to –10°, has a blunter edge that survives interrupted cuts in hardened steel, but it needs more spindle power.
Heat splits between the chip, the tool, and the workpiece. In high-speed aluminum milling, most of it leaves with the chip, which is why 6061 can run dry or with a light mist. In titanium and Inconel, heat concentrates at the edge because the chip is thin and moves slowly. Flood coolant is not optional for those alloys.
- 1Shear plane widthTypically 0.1–0.4 mm; wider in soft, ductile metals.
- 2Chip thickness ratioChip is thicker than the undeformed cut depth, often 1.5–3×.
- 3Built-up edgeForms at low speed on ductile steel; causes a rough Ra 3.2 μm or worse finish.
Surface speed, feed per tooth, and depth of cut
Surface speed (Vc) is how fast the cutting edge travels past the material, in m/min. It comes from spindle rpm and tool diameter: Vc = π × D × n / 1000. Feed per tooth (fz) is how far the tool advances per cutting edge, usually 0.05–0.25 mm for a 10 mm carbide end mill. Multiply fz by the number of teeth and the rpm to get table feed in mm/min.
Depth of cut splits into axial (ap) and radial (ae). For roughing aluminum with a 10 mm end mill, a common starting point is ap 5–10 mm, ae 2–4 mm, and 4,000–8,000 rpm. For roughing 4140 steel, drop to ap 1–2 mm, ae 3–5 mm, and 1,200–2,500 rpm. These are starting numbers, not recipes. The machine's spindle torque and the fixture stiffness decide the ceiling.
Radial engagement matters more than most people expect. Climb milling at 5–10% radial engagement turns the cut into a high-speed, low-load pass. It reduces radial force and tool deflection, which is how a 12 mm tool can hold ±0.005 mm on a thin wall. Full-width slotting does the opposite: the tool rubs at the centerline, heat climbs, and the wall pushes away.
- 1Vc for 6061-T6300–500 m/min with carbide, uncoated or ZrN.
- 2Vc for 316L80–150 m/min; work-hardens if the feed is too light.
- 3Vc for Ti-6Al-4V40–60 m/min with flood coolant and high-pressure through-tool delivery.
How dod cnc machines cut on 3, 4, and 5 axes
A 3-axis machine moves X, Y, and Z. The tool axis stays vertical. That is fine for prismatic parts with features reachable from one direction: plates, housings, brackets, heat sinks. The limit is undercut geometry. A side pocket with a curved floor, or a port that enters at 30°, needs a second setup or a different tool.
A 4-axis machine adds a rotary table, usually around X or Y. The part turns while the tool cuts. This suits shafts, cam profiles, and parts with features spaced around a bore. GreatLight runs 12 four-axis mills and a Ø400 mm rotary table, which covers most cylindrical work that does not need a tilted tool.
A 5-axis machine adds two rotary axes, so the tool can tilt. The payoff is constant effective engagement on a curved surface. On a turbine blade, a 3-axis pass changes the contact angle from leading edge to trailing edge, and the surface finish varies with it. A tilted tool holds the same lead angle across the whole pass, which keeps Ra 0.8–1.6 μm consistent and avoids a polished band next to a torn one.
- 13-axisBest for flat-bottom pockets, drilled hole patterns, and open faces.
- 24-axisBest for turned features plus milling in one setup; shortens cycle time.
- 35-axisBest for impellers, blades, medical bone plates, and deep cavities with draft.
Tool material and coating decide which metals you can cut
Carbide covers most production work. It holds an edge at 300 m/min in aluminum and at 60 m/min in titanium. Cobalt HSS still has a place in long-reach tools and in shops that need a tough edge on an old machine with limited rpm. Diamond (PCD) is for aluminum and composites at very high speed, and for non-ferrous work where a mirror finish is required. It is a poor choice for steel because carbon diffuses into iron at cutting temperature.
Coatings change the friction and the thermal barrier. TiN is general purpose and cheap. TiAlN works at higher temperature and suits stainless and hardened steel. AlCrN resists heat better still and is common on tools for Inconel and tool steel. For aluminum, an uncoated polished carbide or a ZrN coating avoids the built-up edge that dulls a TiAlN edge.
Tool geometry has to match the operation. A 3-flute end mill clears chips in aluminum without chatter. A 4-flute is stiffer and better for steel. Variable helix and unequal index spacing break the resonance that causes chatter on thin walls. For finishing a Ra 0.2–0.8 μm surface, a 6-flute tool with a wiper flat and a small corner radius does more than raising rpm.
- 1Aluminum2–3 flutes, polished rake, PCD for volume runs.
- 2Stainless 316L4–5 flutes, TiAlN, positive rake, never dwell in the cut.
- 3Titanium4–6 flutes, AlCrN, sharp edge, through-coolant above 70 bar.
Reading the cut: chips, sound, and finish
The chip is the cheapest diagnostic you have. A 6061 chip should come off as a short comma shape with a bright silver surface. If it is a long string, the feed per tooth is too low for the speed, and the tool is rubbing. If it is a fine powder, the material is being crushed rather than sheared, which usually means a dull edge.
Sound tells you about stability. A steady hum with a rhythmic beat is normal. A high-pitched squeal means chatter: reduce radial engagement, shorten the tool, or add a support under the part. A dull thud on entry means the tool is hitting the part before the programmed feed is established. Use a lead-in arc instead of a straight plunge.
Surface finish is the last check. A Ra 1.6–3.2 μm as-machined finish is normal for a roughing or semi-finish pass. To reach Ra 0.8–1.6 μm, use a sharp finishing tool, keep radial engagement under 5% of diameter, and hold the feed per tooth constant through corners. Corner slowdown is the most common cause of a visible mark on an otherwise good surface.
- 1Blue chip on steelSurface speed too high, or coolant is not reaching the edge.
- 2Tapered wallTool deflection; shorten the gauge length or reduce ap.
- 3Torn finish on stainlessBuilt-up edge; raise speed 15% or change to a sharper rake.
Step by step: setting up a cut that holds tolerance
- 11. Read the drawing for the tightest featureFind the tolerance that drives the process, usually a bore or a mating face. If it is ±0.005 mm, plan for a finishing pass with a separate tool and a spring pass. If the tightest callout is ±0.05 mm, one roughing and one semi-finish pass will do.
- 22. Pick the machine by geometry, not by habitCount the setups a 3-axis machine would need. If the part has features on four sides plus a tilted port, a 5-axis center removes three setups and the positional error that comes with them. Check the work envelope: our largest travel is 4,000 × 400 × 150 mm.
- 33. Choose the tool and the holderKeep the tool length-to-diameter ratio under 4:1 for finishing. Beyond that, use a shrink-fit or hydraulic holder and reduce ap by 30%. A long tool in a sidelock holder will deflect and cut a tapered wall.
- 44. Set work zero and verify itTouch off X, Y, and Z, then re-check with a dial indicator on a known datum. A 0.02 mm error at zero becomes a 0.02 mm error on every feature. For 5-axis work, also verify the rotary centerline with a test bar before the first pass.
- 55. Run a first article at reduced loadCut the first part at 60–70% of the calculated feed and speed. Listen for chatter and watch the chip color. Silver or straw chips are fine. Blue chips on steel mean the surface speed is too high; powdery chips on aluminum mean the feed per tooth is too low.
- 66. Measure, then adjust one variable at a timeCheck the critical dimension and the surface finish. If the finish is rough, raise the surface speed by 10–15% or switch to a wiper insert. If the dimension drifts, check tool wear before touching the offset. Change one parameter per run so you know what caused the shift.
- 77. Lock the offsets and record the setupOnce the part is in tolerance and repeatable across three pieces, save the program, tool offsets, and fixture position. The next run should not need a new first article. Inspection reports are available on request for each lot.
Matching the cut to the machine and the material
Starting parameters for a 10 mm carbide end mill. Adjust for tool length, holder, and fixture stiffness.
| Material | Surface speed (m/min) | Feed per tooth (mm) | Best machine type |
|---|---|---|---|
| 6061-T6 aluminum | 300–500 | 0.10–0.20 | 3-axis or 5-axis, high rpm |
| 7075 aluminum | 200–350 | 0.08–0.15 | 3-axis, rigid fixture |
| 304 / 316L stainless | 80–150 | 0.05–0.12 | 4-axis or 5-axis, flood coolant |
| 4140 steel (28–32 HRC) | 90–160 | 0.06–0.12 | 3-axis with 4th-axis index |
| Ti-6Al-4V | 40–60 | 0.04–0.10 | 5-axis, through-tool coolant |
| Inconel 718 | 25–40 | 0.03–0.08 | 5-axis, AlCrN tool, low ap |
| POM / PEEK plastic | 200–500 | 0.10–0.25 | 3-axis, sharp single-flute for POM |
| Brass C36000 | 200–400 | 0.08–0.20 | 3-axis, high rpm, light mist |
Questions engineers ask before releasing a cut
How dod cnc machines cut a part without deforming it?
The tool removes material in small, controlled increments rather than one heavy pass. Radial engagement is kept low, often 5–10% of the tool diameter, which keeps radial force small. The fixture and the toolpath work together: supports sit under thin floors, and the tool approaches along an arc instead of plunging straight in.
For thin-wall parts, we often leave 0.2–0.3 mm of stock for a finishing pass and cut it after the wall has been stress-relieved by the roughing cycle.
What tolerance can a CNC cut actually hold?
Our standard capability is ±0.005 mm (±0.0002 in) on critical features, measured in a temperature-controlled inspection room. That is a capability, not an automatic result on every feature. A 200 mm long bore and a 10 mm bore do not hold the same tolerance.
If a callout is tighter than the process can hold reliably, we flag it during DFM review and propose a change: a different datum, a ground finish, or a two-stage cut.
When should a part move from 3-axis to 5-axis?
When the geometry forces more than two setups, when the surface is curved in two directions, or when the tool has to approach at an angle to avoid a collision. A 3-axis machine can cut a complex shape, but the setups add positional error and cycle time.
Five-axis work is not automatically more accurate. It removes setup error and holds a constant contact angle. If the part is a flat plate with holes, 3-axis is faster and cheaper.
Why does the same program cut differently on another machine?
Spindle runout, machine rigidity, and thermal growth differ between machines. A 0.005 mm runout on a new spindle can become 0.015 mm on a worn one. The tool holder also matters: a shrink-fit holder repeats within 0.003 mm, while a worn collet can add 0.02 mm.
We treat the program as a starting point, not a fixed recipe. The first article on each machine is checked and the offsets are adjusted before the run continues.
How do you handle heat when cutting titanium?
Through-tool coolant at 70 bar or higher reaches the cutting edge where the heat is generated. Flood coolant alone often fails on deep pockets because the chip packs the flute and blocks the flow.
We also keep surface speed low, 40–60 m/min, and use a sharp AlCrN-coated tool. If the edge shows a white layer or a shiny wear scar after one part, the speed is still too high.
Can you cut a prototype and then run 10,000 parts?
Yes. There is no minimum order quantity, so a single prototype and a 10,000-part run use the same quoting path. The difference is the process plan: prototypes may be cut from billet on a 5-axis center, while volume runs may use a dedicated fixture, a 4-axis mill-turn setup, or die casting depending on the geometry.
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