What Is a Breaching CNC Machine?
A breaching CNC machine produces controlled internal openings, notches and channels that a drill or standard end mill cannot reach. This page explains the cutting mechanics, the machine requirements and the cases where breaching is the wrong choice. Written for engineers and buyers who need to judge fit before they send a drawing.
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How a breaching cnc machine removes material
A breaching cnc machine is not a separate class of machine tool. It is a CNC mill, usually 4-axis or 5-axis, set up and programmed to cut an opening that starts inside the part rather than at an edge. The spindle still turns a rotating cutter. What changes is the path, the entry method and the way the tool is supported while it cuts.
The most common entry method is helical interpolation. The cutter ramps down in a spiral instead of plunging straight, so the load stays on the side of the flutes. On a 12 mm carbide end mill in 6061 aluminium, a helix of 2–3° per revolution with a 0.5–1.0 mm radial step keeps chip load even and avoids the corner wear that a straight plunge causes.
Once the tool is through, the machine walks the contour of the opening with a radial depth of cut that the tool can survive. In hardened or gummy material that means light radial passes and higher surface speed. In soft aluminium it means a deeper axial cut. Either way the shape of the opening is defined by the interpolated path, not by the tool diameter.
That last point matters for design. A breaching operation can produce a rectangular port, a keyway, a cross-slot or a curved channel with a corner radius as small as the cutter allows. It cannot produce a sharp internal corner, because the tool is round. Draw the corner radius you can actually accept.
- 1Helical entrySpiral ramp, not a straight plunge. Protects the flute corners.
- 2Trochoidal pathsUseful in hard alloys where full-width cuts cause chatter.
- 3Tool diameter sets the cornerSmallest internal radius equals cutter radius.
- 4Through or blindBlind breaches need a relieved tool or a flat-bottom cutter with a small corner radius.
What the machine and fixture must provide
Breaching puts the cutter in a weaker position than an edge cut. The tool is often long and slender, reaching into a pocket or through a wall. Deflection is the first thing that kills accuracy. Keep the tool stick-out under four times its diameter where the geometry allows. When the reach must be longer, reduce the radial engagement and accept a slower feed.
Rigidity of the setup matters as much as the spindle. A part held on a vise with 15 mm of unsupported overhang will move under a side load. The fix is usually a custom soft jaw or a machined fixture plate that supports the wall where the cutter exits. On thin-wall parts, support on both sides of the cut, or the wall will spring back and pinch the tool.
Coolant and chip evacuation decide the surface finish inside the opening. Through-spindle coolant is the cleanest option for deep pockets. High-pressure air works well in aluminium and plastics, where coolant can stain or contaminate a medical or food-contact part. In titanium and Inconel, flood coolant is normal because the heat has to leave with the chip.
The control is the last piece. Look-ahead and jerk control on a modern controller let the machine hold feed through a tight corner radius. An older control will slow down or overshoot, and the opening will be out of position by 0.02 mm or more. On a 4,000 mm bed machine, thermal drift over a long cycle also has to be managed.
Material behavior during breaching
Aluminium is the easy case. Grades 6061 and 7075 both breach cleanly with high surface speed and good chip clearance. The risk is built-up edge on the cutter, which leaves a torn finish on the wall. Sharp, polished flutes and a 6–8% cobalt tool solve most of it. Keep Ra 0.8–1.6 μm as a realistic as-machined target inside an opening.
Stainless 304 and 316 work-harden if the cutter rubs instead of cuts. Feed per tooth below about 0.03 mm makes that worse, not better. Keep the chip load up and the radial step light. In 17-4PH the same rule applies, and a finishing pass with a fresh tool is worth the tool change because the wall finish inside a port is hard to repair later.
Titanium Ti-6Al-4V and Inconel generate heat fast and conduct it poorly. The tool takes almost all of it. Breaching these grades means conservative radial engagement, generous coolant and a rigid setup. Tool life is measured in minutes of cut, not hours, so a tool-change strategy belongs in the program from the start.
Plastics such as POM, PEEK and ABS breach easily but melt and burr at the exit. Climb milling with a two-flute cutter and air blast gives a clean edge. PEEK and carbon fibre need sharp carbide and no coolant, since contamination on a medical or aerospace part is a reject, not a cosmetic issue.
Specifying a breaching operation on a drawing
The drawing decides whether the part is easy or expensive. Start with the corner radius. If the opening is 8 mm wide and the print calls a 0.5 mm internal radius, the cutter has to be Ø1 mm, which is fragile and slow. Widening the tolerance to a 2 mm radius lets a Ø4 mm cutter do the job in a fraction of the time.
Next, define the position tolerance from a real datum. Openings tied to a bolt pattern or a mating face should be dimensioned from that face, not from a rough cast surface. On a casting, the first operation should establish the datum, then the breach is cut in the same setup wherever possible. Moving a part between machines adds stack-up error.
Depth and exit clearance come third. A blind breach needs the tool to clear the bottom without rubbing, so specify a flatness and a corner radius that the cutter can produce. A through breach needs somewhere for the chips to fall. If the exit is closed, the chips recut and the finish fails.
Finally, state the surface finish only where it matters. Ra 0.8–1.6 μm is a normal as-machined result inside an opening. Ra 0.2–0.8 μm needs a separate finishing pass and sometimes a different tool, and it adds cost. If the wall is a sealing surface, say so. If it is a clearance channel, leave it as machined.
Common failure modes and what causes them
Oversize openings that drift across a batch usually come from tool wear, not from the program. A cutter that has run 40 minutes in 4140 steel is no longer the diameter it was. Measure the tool or compensate in the control between parts. On a tight-tolerance job, a mid-batch tool change with a re-probe is normal practice, not a sign of a bad program.
Chatter inside a deep opening shows up as a rippled wall and a loud cut. The causes stack in a predictable order: too much tool stick-out, not enough fixture support, then wrong speed. Shorten the tool first. If the reach is fixed by the part, drop the radial engagement and raise the spindle speed slightly until the wall cleans up.
Burrs at the exit are a fixture and tool-geometry problem more often than a feed problem. A dull cutter pushes material instead of shearing it. So does a cutter with too small a helix angle for the material. On a through breach, a light chamfer pass at the exit removes the burr at the machine instead of at a deburring bench.
Dimensional error that appears only on the last parts of a long cycle points to thermal growth. A 4,000 mm machine can move measurably over a shift. Warm up the spindle, keep the shop temperature stable, and re-check the first part after the machine has been running for an hour.
Breaching compared with drilling and standard milling
Use this to pick the entry method before you pick the machine.
| Method | Best for | Main limit | Typical tolerance |
|---|---|---|---|
| Drilling | Round holes on a centerline | Round shape only; no internal contour | ±0.05 mm on position |
| Standard milling | Open pockets and edge cuts | Reach and rigidity fall off with depth | ±0.02 mm |
| Breaching | Internal ports, notches, keyways, channels | No sharp internal corner; needs tool reach | ±0.005 mm |
| Wire EDM | Through openings in hardened steel | Conductive parts only; slow | ±0.005 mm |
| Laser cutting | Flat sheet profiles | Thickness limit; heat-affected edge | ±0.1 mm |
When breaching is the right process, and when it is not
| Situation | Choose | Reason |
|---|---|---|
| Internal port in a machined housing | Breaching on a 5-axis mill | One setup, no secondary EDM |
| Sharp internal corner required | EDM or a design change | A round cutter cannot cut a sharp corner |
| Through slot in 2 mm hardened steel | Wire EDM | Tool deflection too high for a mill |
| Prototype, 1 to 5 parts | 3-axis or 4-axis breaching | Programming cost is low, no tooling |
| 10,000 parts with a fixed opening | Casting or stamping, then finish | Cycle time per part drops sharply |
| Opening in a thin unsupported wall | Breaching with a support fixture | Without support the wall springs and chatters |
The short answer
If the opening is internal, needs a defined position and the corner radius is realistic, a breaching cnc machine is the fastest route to a finished part. If the corner must be sharp, or the wall is thin and unsupported, change the design or use EDM instead.
Breaching cnc machine questions engineers ask
What materials can be breached on a CNC machine?
Aluminium 6061, 7075 and 2024, stainless 304, 316 and 17-4PH, alloy steels such as 4140 and 4340, titanium Ti-6Al-4V, Inconel, and plastics including POM, PEEK and ABS.
Hardened steel above roughly 45 HRC is usually a wire EDM job, because the cutting forces and tool wear make milling uneconomical.
What tolerance can a breaching operation hold?
On a rigid setup with a short tool, ±0.005 mm is achievable on position and size. That figure assumes the part is probed in the fixture and the tool is measured before the cut.
Deep openings with long tool reach typically land between ±0.02 mm and ±0.05 mm unless the radial engagement is reduced and a finishing pass is added.
How small can the internal corner radius be?
The corner radius equals the cutter radius, so a Ø2 mm cutter leaves a 1 mm radius. Smaller cutters exist, but they are fragile and cut slowly.
If the design allows a 2–3 mm radius instead of 0.5 mm, cycle time and tool cost both drop noticeably. This is the single biggest cost lever on a breaching job.
Is breaching done in one setup or several?
Where the geometry allows, one setup on a 5-axis machine is best, because it removes stack-up error between the datum face and the opening.
Parts with openings on multiple faces may need two setups. In that case the second setup should be located from a machined feature, never from a raw surface.
What surface finish comes out of a breaching cut?
As-machined walls typically read Ra 0.8–1.6 μm. A dedicated finishing pass with a fresh tool reaches Ra 0.2–0.8 μm.
Specify the fine finish only on walls that seal, slide or carry fluid. On clearance channels it adds cost with no functional gain.
Can breaching handle prototype quantities?
Yes. There is no minimum order quantity, so a single part and a 10,000-part run use the same process route.
For prototypes the cost is mostly programming and setup. For production, cycle time and tool life dominate, and the same opening may be better formed by casting or stamping before finishing.
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