CNC Inlay Tech: How Two Materials Become One Part

• 1
  What the process actually does to a part
• 2
  Why five-axis motion changes what an inlay can be
• 3
  Pocket depth, wall thickness and the bond line
• 4
  Which material pairs behave, and which fight you
• 5
  Holding ±0.005 mm across two materials
• 6
  Where the process stops working
• 7
  Inlay versus the alternatives

An inlay is a pocket plus a second material. The pocket is machined into the base body, the insert is cut to match, and the two are joined by adhesive, press fit, thermal fit or a mechanical undercut. After joining, the face is skimmed so both materials sit on the same plane. That last pass decides most of the visible quality.

Engineers reach for this approach instead of paint or plating because of wear. A laser-marked logo fades. A bonded-in stainless strip on an aluminum handle keeps its edge because the wear surface is metal all the way through, not a coating a few micrometres thick.

Function beats decoration in most real jobs. A copper pad inlaid into an aluminum busbar keeps contact resistance low at the joint. A ceramic or PEEK island in a steel fixture stops heat from bleeding into a clamped workpiece. The second material is there to do work, and the pocket is just the holder.

Decorative work still exists. Watch bezels, tool handles and instrument panels use contrasting metals for appearance. The machining problem is identical either way, so the same rules on pocket depth, wall thickness and bond line apply.

A three-axis machine can only approach the pocket from above. That limits you to straight walls, flat floors and shapes the tool can reach without collision. Undercuts, curved pocket floors and inlays that wrap around a corner are off the table.

Five-axis machining adds two rotary axes, so the tool tilts as it cuts. On a Ø400 mm rotary table we can machine a pocket wall with a draft angle that matches a molded insert, or cut a curved floor that follows the contour of a handle. The tool stays normal to the surface, which keeps chip load even along the whole path.

The practical gain shows up in corner geometry. A tilted tool reaches into a pocket corner with a smaller effective radius, so the insert corner does not need a large relief. That keeps the visible seam thin, often under 0.1 mm, without hand fitting.

GreatLight runs 16 simultaneous 5-axis machining centers among 127 high-precision CNC machines. Maximum processing size is 4,000 mm, and travel envelopes cover 4,000 × 400 × 150 mm down to 500 × 310 × 200 mm, so both long extrusions and small instrument bodies fit the same process.

Pocket depth sets how much material you remove and how much the base can still carry. A shallow pocket of 0.5–1.0 mm is enough for a wear strip. Deeper pockets of 3–5 mm suit structural inserts that carry load. Past that, check the remaining wall under the pocket before you commit.

Wall thickness around the pocket matters more than depth. Below roughly 1 mm on aluminum, the wall deflects during the finishing pass and the seam opens up. On stainless and titanium, 1.5–2.0 mm is a safer floor because cutting forces are higher.

Bond line thickness depends on the joining method. Structural epoxy wants 0.05–0.15 mm for a reliable film. Press fits target a light interference, and thermal fits rely on a calculated shrink gap that closes as the part cools. A gap that is too tight starves the adhesive and leaves voids.

Surface prep is not optional. Degrease, then abrade or grit-blast the pocket floor so the adhesive has something to key into. Anodized aluminum should be masked at the bond area, because a hard anodic layer is a weak interface and will fail before the adhesive does.

Metal into metal is the common case, and thermal expansion drives the choice. Aluminum in steel moves at roughly twice the rate when the part heats up, which puts shear stress on the bond line. Small inserts survive it. Long strips do not, unless you leave clearance or switch to a mechanical retention.

Metal into plastic works well when the insert is small and the plastic is stiff. POM, PEEK and PC hold a press fit without creeping. Softer plastics such as PP and HDPE relax over time and the insert loosens, so plan on adhesive or a molded-in rib.

Dissimilar metals need a corrosion check. Copper in aluminum, or stainless in magnesium, creates a galvanic couple wherever moisture sits. A thin insulating adhesive layer or an anodized base helps, but the joint should still be kept dry in service.

We machine aluminum grades 6061, 7075 and 2024, stainless 303 through 17-4PH, titanium TC4, copper alloys such as C110 and C36000, PEEK, POM and carbon fibre. When a pair looks risky, we say so before cutting, not after.

Tolerance on an inlay is a stack, not a single number. The pocket position, the pocket size, the insert size and the joining shift all add up. If each contributes 0.005 mm of error, the visible seam can move 0.02 mm before any thermal effect.

Cutting both parts on the same machine in the same setup removes most of that stack. That is why we machine the pocket and the insert blank in one fixturing cycle where the geometry allows, then split them for finishing. Shared datums beat tight individual tolerances.

GreatLight holds ±0.005 mm (±0.0002 in) on machined features. Surface finish ranges from Ra 0.2–0.8 μm for fine sealing faces to Ra 1.6–3.2 μm as machined. A finer finish on the pocket floor gives the adhesive a better wet-out, but too fine and the surface has nothing to grip.

Measure the seam, not just the parts. Optical comparison or a profilometer trace across the joint shows the real step height. A step of 0.01 mm is usually invisible. Above 0.05 mm it reads as a line under raking light, which is often what the customer is actually rejecting.

Deep, narrow pockets are the first limit. When depth exceeds about four times the tool diameter, the tool shank rubs the wall, chatter rises and the floor finish degrades. A 3 mm cutter should not be asked for a 15 mm deep pocket without a step-down strategy or a smaller neck.

Large flat inlays on thin plates are the second limit. Thin stock moves as material is removed, so the pocket floor is no longer flat when the insert arrives. Stress relief before the finish pass, or clamping on a sacrificial backing, is the usual fix.

High-service temperature joints are the third. Adhesive strength falls off above roughly 120 °C for most structural epoxies, and differential expansion grows with temperature. If the part sees 150 °C in service, a mechanical key or a welded insert is the honest answer.

Very small features push the other way. Laser marking and engraving need a minimum character height of 1.5 mm to stay legible. If the design needs finer detail than that, an inlay is the wrong tool. Choose etching, not machining.