5 Manufacturing Controls to Ensure Macor Parts Pass Inspection (tolerances, measurement & FAI)

Macor machinable glass-ceramic is prized for its ease of machining, vacuum compatibility and high-temperature stability — but its brittle nature means quality assurance must be deliberate. This article is a hands-on Macor CNC machining guide focused on the five manufacturing controls that reliably raise first-pass yield: design for inspectability, cutting and fixturing strategy, material handling and bake-out, measurement & inspection, and troubleshooting.

1. Design for inspectability and smart tolerance allocation

Start inspection planning at the CAD stage. Make critical features standard geometric primitives (planes, cylinders, holes) and call out datums explicitly so both machinist and inspector share a reference.

Practical tolerance guidance for typical applications:

Critical assembly holes: ±0.01 mm (use these for positioning or press fits).

General form/position: 0.01–0.05 mm depending on size.

Cylindrical/rotational features: ±0.001–0.01 mm where rotated symmetry is essential.

Surface roughness: Ra0.01 µm for structural; Ra0.002 µm for optical supports.

Label only functional faces with tight tolerances; relax non-functional faces to reduce scrap. Include inspection datums on the drawing and, when possible, design a simple measurement jig footprint into the part for repeatable CMM setups.

2. Cutting strategy, tool choice and fixturing to reduce fracture risk

Macor machines well with carbide tooling, but brittle fracture, chipping and tool-induced microcracks are common failure modes if parameters are wrong.

Best-practice machining rules:

Tooling: use sharp carbide end mills with polished flutes; avoid worn cutters.

Cut strategy: prefer multiple passes (rough → semi-finish → finish) with shallow depths of cut and modest radial engagement. Higher spindle speed + lower feed per tooth often reduces chipping.

Coolant: use minimal or spray cooling. Excessive liquid can introduce thermal shock; for high-finish operations consider dry or micro-lubrication.

Fixturing: avoid point clamping that concentrates stress. Use soft pads, vacuum fixtures for thin/fragile shapes, or bespoke multi-point supports to distribute clamping loads.

For micro features (e.g., Macor micro-hole drilling 0.1mm), use specialized micro-drills, peck-drill cycles, and slow feed rates to prevent breakage.

3. Material control, cleaning and bake-out for vacuum/UHV parts

If parts are destined for vacuum systems — e.g., a Macor UHV compatible vacuum feedthrough insulator — incoming material control and thermal conditioning are mandatory.

IQC checklist:

Verify material batch and visually inspect for transport damage (chips, corner knocks).

For UHV use, perform a validated cleaning protocol (ultrasonic solvent wash where appropriate) followed by controlled bake-out. Typical starting point: 150–200°C for several hours, but final temperature/time must match the system’s requirements and adhesives/fasteners used.

Record bake-out temperature profile, vacuum level, and elapsed time as part of FAI documentation to demonstrate low outgassing.

4. Measurement & inspection: methods that catch real problems

A structured measurement plan prevents surprises. Choose measurement equipment that matches your claimed tolerances (for Macor precision tolerances around ±0.01 mm, your CMM should have repeatability in the 2–5 µm range).

Recommended inspection equipment and workflow:

CMM (coordinate metrology): primary tool for dimensional and geometric tolerances; export deviation maps for customers.

Optical profilometer or interferometer: for flatness and optical-quality surfaces.

Surface profilometer: verify Ra values at designated points.

Vision systems / stereo microscope: for micro-hole inspection and visual crack/chip detection.

FAI (first article inspection): perform 100% measurement of the first part — document every dimension, GD&T callout and surface measurement. Embed photos and CMM plots into the FAI report.

Process controls:

1. FAI — measure the first produced part completely; approve program and fixture.

2. IPQC — in-process checks after critical operations (after drilling, after finish pass, post-clean).

3. FQC — final checks before shipment — surface, dimensional, and bake-out sign-offs.

Provide a templated FAI checklist with fields: drawing version, material lot, measurement instrument IDs, measured vs nominal values, operator and inspector signatures, and photo attachments.

5. Common defects, root causes and corrective actions

Anticipate typical Macor failures and define corrective actions to shorten response time.

Chipping / edge breakage. Root cause: concentrated clamping stress or aggressive cutting. Fix: redesign clamps, reduce depth of cut, add radii, add sacrificial features for fixturing.

Micro-cracks / fractures. Root cause: thermal shock or excessive cutting force. Fix: lower feed rates, increase support, review tool runout/vibration.

Dimensional drift / out-of-tolerance holes. Root cause: tool wear, thermal drift of machine, wrong datum usage. Fix: implement tool-life replacement, control shop temperature, and standardize datum references on fixtures.

Poor surface finish. Root cause: chatter, wrong spindle speed, inadequate tooling. Fix: optimize speeds/feeds, use finish passes, polish where necessary.

Set quality gates: first-part 100% inspection, initial batch 100% inspection (first 5–10 parts), then sample-based IPQC (e.g., 1 in 10) with immediate rollback to 100% on any nonconformance.

Practical Macor CNC machining guide for engineers: tooling, cutting parameters, vacuum bake-out, FAI checklist and concise FAQ. Download the full 30-Q PDF + Excel FAI template