Process guide

Shell CNC Optical Module: 7 Careful Steps From Blank to Clean Part

A shell CNC optical module is an enclosure first and an optical housing second. This guide walks through the seven steps we run on a machined shell, with the tolerances, cut depths and inspection points that decide whether the part passes. Written for engineers and buyers who need to judge a process before they release a drawing.

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±0.005 mm toleranceRa 0.2–0.8 μm finish16 five-axis centers100% inspection

shell CNC optical module machined on a 5-axis CNC machining center

On this page

1
Key takeaways
2
Reading the Drawing Before the shell CNC optical module Runs
3
Datum Strategy and Fixture Design
4
Roughing Parameters for Thin Shells
5
Bores, Window Openings and Sealing Faces
6
Inspection and Cleaning Before the Shell Ships
7
Step by Step: Shell CNC Optical Module Process
8
Which Process Fits Which Shell

Quick answer

Key takeaways

Fixtures decide the partThin shells move under clamping force, not under the cutter. Plan the fixture before the tool path.

Two setups beat fourOne five-axis setup plus one face operation holds datums far better than four three-axis re-clamps.

Finish is a process, not a passA spring pass at 0.05 mm radial cut keeps Ra 0.8–1.6 μm stable across a long pocket.

Clean before you measureDebris in a bore reads as a size error. Clean, then inspect at 20 °C ± 1 °C.

Not every shell suits CNCHigh-volume shells with no tight bore should go to die casting instead.

Step 1–2

Reading the Drawing Before the shell CNC optical module Runs

Most optical module shells look simple on paper: a box with a lid face, a few bores, a window opening. The trouble sits in the notes. Ask which surfaces actually carry the optical axis. A connector face machined to ±0.05 mm is fine. A photodiode seat at ±0.01 mm with a parallel callout is a different job and needs a different setup plan.

Start by splitting every dimension into three groups. First, the optical datums: bore positions, seat flats, window aperture. Second, the sealing and mating surfaces: O-ring grooves, gasket lands, screw bosses. Third, cosmetic and clearance faces. The first group drives the machine choice, the second group drives the finish, the third group can be roughed and left as machined.

Then check wall thickness. A 6061 shell with 1.2 mm walls behaves very differently from a 3 mm wall version of the same part. Under a 40 mm vise with normal clamping, a 1.2 mm wall can deflect 30–60 μm before the tool even touches it. If the drawing shows thin walls next to a tight bore, flag it in DFM.

One more check before quoting: material. 6061-T6 and 7075 cut clean and hold a bore well. 316L and beryllium copper are chosen for sealing and thermal reasons but move more during roughing. For a shell CNC optical module that must stay flat, 6061-T6 or 7075 is usually the safer start.

1
Optical datumsBores, seat flats, window aperture — these set the setup order.
2
Sealing facesO-ring grooves and gasket lands need Ra 0.8–1.6 μm or better.
3
Cosmetic facesRough, then leave as machined to save cycle time.
4
Wall checkAnything under 1.5 mm wall gets a fixture review before quoting.

Step 3

Datum Strategy and Fixture Design

Pick one primary datum and keep it through every operation. On a shell, the best candidate is usually the largest flat face that also carries a bore pattern. Machine that face first, in the same setup as the bores if the geometry allows, so the relationship is cut rather than re-clamped.

For a two-setup plan, use a soft jaw or a dedicated fixture plate that locates on the machined face and two edges. Do not locate on a raw casting face. Raw faces vary by 0.2–0.5 mm and every downstream dimension inherits that error.

Clamping force is the quiet killer. On a 1.5 mm wall, a 4 kN vise load can ovalize a Ø30 mm bore by 15–25 μm. Use low-pressure clamping, side clamps on a thick boss, or vacuum fixturing for large flat panels. If a shell is 4,000 mm long and thin, vacuum plus light toe clamps is the only practical route.

Add a witness mark or a small datum feature on the outside of the shell. When the part comes back from anodizing or plating, the mark lets the inspector re-establish the same datum in seconds instead of hunting for an edge.

1
Primary datumLargest flat face that also carries the bore pattern.
2
Locating edgesTwo machined edges, not raw stock.
3
Clamp pressureKeep vise load low on walls under 2 mm.

Step 4

Roughing Parameters for Thin Shells

Roughing a shell is about removing material without loading the wall. On 6061-T6, a 12 mm three-flute carbide end mill at 8,000 rpm, 2,400 mm/min feed and 6 mm axial depth takes material fast in the thick sections. Bring axial depth down to 2–3 mm once the wall thins below 2 mm.

Leave 0.3 mm radial stock on optical bores and 0.15 mm on sealing faces. That is enough for a finishing pass to clean up without a long spring pass. Leaving 0.05 mm sounds safer but often results in rubbing, heat and a torn surface.

Heat is the other risk. A shell has low mass and little heat sink, so a heavy roughing cut raises the part temperature 5–10 °C. If you measure right after roughing, a Ø20 mm bore can read 8 μm small. Let the part cool, or finish in a separate operation after a 30-minute rest.

Use air blast or minimum quantity lubrication rather than flood coolant on deep pockets. Flood coolant pools in the pocket, chips float, and a recut chip scores the wall. Air or MQL clears the pocket and keeps the chip load predictable.

1
Roughing stock0.3 mm on bores, 0.15 mm on sealing faces.
2
Thin-wall ruleBelow 2 mm wall, cap axial depth at 2–3 mm.
3
CoolingAir blast or MQL in deep pockets.

Step 5

Bores, Window Openings and Sealing Faces

Optical bores are where the shell earns its tolerance. Hold ±0.005 mm on a reamed or bored hole with a boring head, not with an end mill interpolation if the diameter is under Ø12 mm. For Ø6–12 mm, drill 0.2 mm under, then bore or ream to size with a sharp tool and a 0.1 mm finish allowance.

Sealing faces need flatness more than they need a mirror finish. A gasket land at Ra 0.8–1.6 μm with 0.01 mm flatness seals better than a polished face with a 0.04 mm wave in it. Face mill with a fine feed, then check flatness on a surface plate or with a dial indicator on a height gauge.

Window openings are the hardest feature on most shells. Thin walls around a rectangular aperture distort when the cutter exits. Use a helical entry, keep the tool diameter small, and support the far wall with a plug or sacrificial backing. If the window edge carries an optical reference, machine it before the surrounding walls are thinned.

Chamfer or break every edge that a technician will touch during assembly. A 0.3 mm × 45° chamfer on the bore entry prevents a burr from lifting a gasket or scratching a lens during insertion.

1
Bore tolerance±0.005 mm via boring head or reamer, not interpolation.
2
Sealing flatness0.01 mm flatness matters more than Ra.
3
Window supportBack the far wall before the cutter breaks through.

Step 6–7

Inspection and Cleaning Before the Shell Ships

Measure after the part reaches room temperature. A shell that was finished ten minutes ago can still be 3–5 °C warm, and that is enough to shift a 100 mm dimension by 5–6 μm in aluminum. We inspect at 20 °C ± 1 °C with the part rested.

Check in this order: datums and overall size, bore diameters and positions, flatness on sealing faces, wall thickness at the thinnest point, then surface finish on the optical faces. Bore position is checked with a coordinate measuring machine or a pin gauge and height gauge combination, depending on the feature.

Cleaning is part of the process, not a favor. Machined aluminum shells trap fine chips in blind holes, thread roots and O-ring grooves. Ultrasonic clean in a compatible detergent, rinse in deionized water, dry with filtered air, then inspect under magnification. For optical use, follow with an isopropanol wipe on the optical faces only.

Before packing, cap every optical bore and plug threaded holes. A shell that arrives with a chip lodged in a bore costs the customer a full re-clean and sometimes a scrapped lens. We ship with protective caps and a certificate of inspection on request, and 100% of parts are inspected before they leave.

1
Thermal soakInspect at 20 °C ± 1 °C, never straight off the machine.
2
CleaningUltrasonic, deionized rinse, filtered air dry.
3
PackingCap bores and plug threads before boxing.

How to run it

Step by Step: Shell CNC Optical Module Process

Parameters are starting points for 6061-T6 and 7075. Adjust for 316L and copper alloys.

1
Review the drawing and split dimensionsGroup every callout into optical datums, sealing faces and cosmetic faces. Flag any wall under 1.5 mm and any bore tolerance tighter than ±0.01 mm for a fixture review before the job is released.
2
Cut the primary datum faceFace the largest flat that also carries the bore pattern. Take 0.5 mm off in two passes at 2,500 rpm, 800 mm/min feed on a Ø50 mm face mill. This face becomes the datum for everything after.
3
Rough the shell with controlled depthUse a 12 mm three-flute end mill at 8,000 rpm and 2,400 mm/min. Keep 6 mm axial depth in thick sections, drop to 2–3 mm once the wall is under 2 mm. Leave 0.3 mm on bores and 0.15 mm on sealing faces.
4
Semi-finish and rest the partTake a 0.2 mm radial pass to even out the stock. Let the part sit for 30 minutes so the temperature drops. Measuring hot is the most common reason a good bore reads out of tolerance.
5
Finish optical boresDrill 0.2 mm under, then bore or ream to size with a 0.1 mm allowance. Hold ±0.005 mm on diameters under Ø12 mm. Break the entry edge with a 0.3 mm × 45° chamfer.
6
Finish sealing faces and window edgesFace mill gasket lands with a fine feed to reach Ra 0.8–1.6 μm and 0.01 mm flatness. Machine window apertures with a helical entry and back the far wall with a plug so the thin edge does not deflect.
7
Deburr, clean and inspectDeburr all edges by hand or with a light tumble. Ultrasonic clean, rinse in deionized water, dry with filtered air. Inspect at 20 °C ± 1 °C in the order: datums, bores, flatness, wall thickness, finish.
8
Cap and packInstall protective caps in every optical bore and plugs in threaded holes. Add the inspection report if requested, then pack with the machined faces separated from each other.

Judgment table

Which Process Fits Which Shell

Pick the process before you pick the tolerance.

Shell type	Best process	Why	Watch out for
Prototype, 1–50 pcs	3-axis or 5-axis CNC	No tooling cost, drawing changes are free	Setup count drives price
Thin wall under 1.5 mm	5-axis CNC with vacuum fixture	Fewer re-clamps, less distortion	Clamp pressure, chatter
Tight bore ±0.005 mm	CNC with boring head	Size held by a rigid bar, not interpolation	Thermal drift, chip recut
Volume over 5,000 pcs	Die casting plus CNC finishing	Lower piece cost at volume	Porosity, draft angles
Large flat panel to 4,000 mm	CNC on gantry travel	Single setup across the full length	Sag, fixture stiffness
Optical face Ra 0.2–0.8 μm	CNC plus polishing	Diamond or fine boring, then lap	Contamination after polish

FAQs

Shell CNC Optical Module Questions

What tolerance can you actually hold on a thin shell?

On a shell with walls above 2 mm, ±0.005 mm on bore diameters is routine and we inspect 100% of parts before shipment. On walls under 1.2 mm, the practical limit moves to about ±0.01 mm because clamping and cutting forces move the wall, not the tool.

If the drawing calls for ±0.005 mm next to a 1 mm wall, we usually ask for a fixture change or a small design change such as a local boss around the bore.

Do you need a 5-axis machine for every optical shell?

No. A simple box shell with bores on two perpendicular faces runs well on a 3-axis machine with two setups. A 5-axis machine earns its cost when the shell has angled optical faces, deep pockets on several sides, or thin walls that cannot survive a second clamp.

We have 16 simultaneous 5-axis machining centers, 12 four-axis mills and 27 three-axis machines, so the setup plan can follow the geometry instead of forcing the part onto one machine.

How do you stop chips from ending up inside the shell?

Air blast or MQL during cutting, a chip break in the program before each tool change, and ultrasonic cleaning after machining. Blind holes and thread roots hold chips longest, so they get a separate air blast and a brush pass before the ultrasonic tank.

Every optical bore is capped before packing. A chip that arrives with the part costs the customer more than the cap does.

Which materials work best for an optical module shell?

6061-T6 and 7075 are the common choices for aluminum shells: stable, machinable and good for anodizing. 316L and 304 are used where corrosion or sealing matters more than weight. Beryllium copper and copper alloys appear when the shell also has to move heat.

We machine all of these, plus titanium TA1, TA2 and TC4 when the application needs it. Material choice changes the roughing parameters, so tell us the alloy before quoting.

What does a shell need before anodizing or plating?

A clear datum mark, a deburred surface and a decision about which faces must stay conductive. Hardcoat anodizing adds 20–50 μm per surface and will close a tight bore, so bores that carry an optical fit are usually masked or machined oversize to compensate.

Laser marking has a minimum character height of 1.5 mm. If the marking is a serial number on a thin wall, place it on a flat boss rather than on the sealing land.

How fast can a shell move from quote to shipped part?

Quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts typically ship in 3–5 days. That timing holds when the drawing is complete and the material is in stock.

There is no minimum order quantity. One prototype and a 10,000-part run go through the same inspection process, and uploads stay confidential with an NDA available on request.