Lens Housing CNC Machining: Concentricity, Fine Threads and Anodizing Allowance

A lens housing may look like a simple cylindrical part. In practice, lens housing CNC machining depends on how several precision features relate to one functional axis.

The lens seats, internal bores, locating diameters, retaining-ring threads, mounting threads, and seating shoulders may all meet their individual size tolerances while remaining misaligned with one another.

These risks apply to optical tube machining, CNC-machined camera lens housings, and other precision optomechanical components. A second setup can shift the mechanical axis, while fine threads and lens seats can change after black anodizing.

This guide explains how to define the datum axis, align inner and outer diameters, machine fine threads, control radial and axial runout, compensate for anodizing, reduce internal reflections, and select suitable inspection methods.

Quick Answer: What Should Be Defined Before Machining a Lens Housing?

Before requesting a quote, define:

• The functional datum axis and lens seats
• Internal and external thread fits
• Radial and axial runout
• Anodizing thickness and masking
• Final inspection condition
• Prototype and expected production quantity

Use three principles:

1. Base the datum system on the features that locate the optics and finished assembly.
2. Machine alignment-critical rotational features in one setup whenever practical.
3. Define critical fits in their final post-anodize condition.

Notes such as “concentric,” “precision bore,” or “fine thread” do not fully communicate the design intent. Identify the datum axis, controlled surface, required relationship, and final acceptance stage.

For a meaningful quote, send both the STEP model and controlled drawing. Include the mating thread information, anodizing specification, prototype quantity, and expected repeat volume.

Why Is Lens Housing CNC Machining More Difficult Than Standard Tube Machining?

A standard turned tube may need only an inside diameter, outside diameter, length, and general-purpose thread.

A precision lens barrel must position optical elements relative to the mechanical assembly.

Depending on the design, this may require control of:

• Lens decenter
• Lens tilt
• Axial lens spacing
• Bore-to-bore alignment
• Thread-to-bore alignment
• Shoulder orientation
• Camera or sensor interface runout
• Internal stray-light surfaces

The housing does not create the optical axis by itself. The installed optics define the optical axis.

However, the housing determines where those optics sit.

Lens diameter and barrel geometry influence how the lens seats inside the mount. Edmund Optics notes that controlled lens diameter helps the optic seat correctly and supports alignment between the optical axis and the mechanical assembly.

A bore can therefore pass its diameter inspection and still fail in assembly if:

• Its axis is offset from the mounting diameter
• Its seating shoulder is tilted
• Adjacent lens seats use different axes
• The retaining thread is misaligned
• Anodizing removes the required fit clearance

The main challenge is not producing accurate individual dimensions. It is preserving the functional relationship between them.

How Should the Optical Axis and Mechanical Axis Be Related?

The machining supplier controls mechanical locating features rather than the final optical axis of the assembled system.

The drawing must translate the optical design into a practical datum structure.

The Installed Optics Define the Optical Axis

The optical axis depends on the geometry, centration, and installed position of the lenses.

The CNC manufacturer machines the features that locate those elements and connect the barrel to the rest of the system.

These features may include:

• Lens seating bores
• Lens seating shoulders
• Spacer locations
• Retaining-ring threads
• External locating diameters
• Camera or sensor interfaces

Misalignment between these features can introduce lens decenter, tilt, or inconsistent spacing.

Define the Mechanical Axis with Functional Datums

The primary datum axis should come from the feature that locates the finished housing in the actual assembly.

Possible datum features include:

• A primary lens seating bore
• An external locating diameter
• A camera-mount pilot
• Two separated cylindrical locating surfaces
• A customer-defined assembly interface

The correct choice depends on how the finished part is constrained.

An exterior cosmetic diameter is a poor primary datum when the assembly is located by an internal bore.

Do Not Use “Concentric” as a Standalone Requirement

In everyday engineering communication, “concentric” often means that two features should share one center or axis.

That wording does not define how the relationship will be controlled or measured.

Under ASME Y14.5-2018, the concentricity characteristic was removed. Position, circular runout, total runout, or another control may communicate the functional requirement more clearly.

Determine whether the design needs to control:

• Axis location
• Surface variation during rotation
• The full cylindrical surface
• Roundness at individual sections
• Alignment between several separate bores

For related guidance, see how to specify true position for CNC-machined parts.

Use a Datum Structure That Reflects Assembly

A lens barrel drawing might use:

• Datum A: Primary locating bore or outside diameter
• Datum B: Main lens seating shoulder or mounting face
• Datum C: Clocking flat, radial hole, or alignment feature

This is only an example.

The datum order must match how the finished housing is located, oriented, and inspected.

How Do You Keep Inner and Outer Diameters on the Same Axis?

Machine the most alignment-sensitive rotational features without releasing the part whenever geometry allows.

Every relocation introduces another opportunity for axis transfer error.

Why Separate Setups Create Risk

Suppose the supplier machines the outside diameter first, removes the part, and then machines the main bore.

The second setup can introduce:

• Chuck runout
• Soft-jaw location error
• Chips on the locating surface
• Incomplete part seating
• Clamping deformation
• Datum transfer error

Both diameters may pass their individual size checks while their axes remain offset.

This risk increases in optical tube machining because long, thin-walled parts can also deform under chuck pressure.

Keep Related Rotational Features Together

Where practical, machine these features on the same spindle axis:

• Main internal bore
• External locating diameter
• Lens seats
• Seating shoulders
• Datum faces
• Related internal threads
• Related external threads

Not every housing can be completed in one operation.

Radial holes, mounting flats, opposite-end details, and inaccessible features may require additional setups. The priority is to preserve the relationship between alignment-critical features.

CNC turning services are well suited to lens barrels dominated by bores, outside diameters, shoulders, grooves, and precision threads.

Control the Second Setup

When a second setup is unavoidable, locate the part from a previously machined functional feature rather than an unqualified raw surface.

Possible workholding methods include:

• Machined soft jaws
• Precision collets
• Expanding mandrels
• Custom locating plugs
• Sacrificial support features

The process can also leave finishing allowance on critical features and complete them after the final relocation.

Verify the relationships most likely to change:

• Bore-to-OD runout
• Shoulder face runout
• Opposite-end bore alignment
• Thread-to-bore runout
• Tube straightness

Prevent Thin-Wall Clamping Distortion

A thin lens barrel can become temporarily oval inside a chuck.

The bore may measure correctly while clamped and change after release.

Evaluate workholding based on:

• Wall thickness
• Tube length
• Material
• Slot and cutout locations
• Roundness
• Required runout
• Surface condition

Jaw geometry and contact area often matter as much as clamping force.

Why Do Lens Seating Steps Need More Than a Diameter Tolerance?

A lens seat normally combines two functional surfaces:

• A cylindrical bore that controls radial position
• A shoulder that controls axial position

Both influence optical alignment.

The Bore Controls Radial Position

Clearance between the lens outside diameter and the seating bore affects:

• Lens decenter
• Assembly force
• Adhesive thickness
• Thermal expansion
• Lens removal
• Risk of edge damage

The smallest possible clearance is not always the best specification.

Select the fit based on:

• Lens diameter tolerance
• Housing bore tolerance
• Lens material
• Housing material
• Operating temperature
• Adhesive or retaining method
• Required assembly movement

Glass and aluminum respond differently to temperature. A fit that works during room-temperature assembly may become unsuitable at the operating limits.

The Shoulder Controls Axial Position and Tilt

The seating shoulder affects:

• Axial lens position
• Spacing between optical elements
• Lens tilt
• Retaining-ring load distribution

Relevant controls include:

• Axial runout
• Perpendicularity to the datum axis
• Shoulder location
• Contact width
• Surface condition
• Burr control

A shoulder can meet its axial position tolerance while remaining tilted relative to the bore axis.

Corner Relief Prevents False Seating

The intersection between the bore and shoulder needs deliberate design.

A tool nose radius, burr, or trapped chip can prevent the lens from contacting the intended seating surface.

Review:

• Lens edge chamfer
• Machined internal radius
• Relief groove
• Shoulder width
• Adhesive groove
• Deburring method

Provide enough clearance for the lens edge without removing unnecessary support area.

Control Only Functional Surfaces

The lens seating bore and shoulder may need controlled roughness or tool-mark direction.

Unrelated interior surfaces do not require the same finish.

For an optical barrel, a highly reflective interior may also work against stray-light control.

What Makes Fine Internal and External Threads Difficult?

Lens barrels often use fine threads because they provide compact engagement and small axial movement per revolution.

Common applications include:

• Lens retaining rings
• Focus adjustment
• Spacer positioning
• Camera interfaces
• Sensor mounts
• Closely spaced optical elements

Commercial lens-tube systems commonly use fine internal threads for adjustment and component positioning.

Thread Fit Is More Than Major Diameter

A functional thread depends on:

• Thread form
• Pitch
• Pitch diameter
• Major diameter
• Minor diameter
• Lead
• Flank condition
• Engagement length
• Burrs
• Mating-part variation

A caliper measurement cannot verify the complete fit.

A thread can still bind because of pitch-diameter error, damaged flanks, coating buildup, or lead error.

Internal Fine Threads Create Tooling Limits

A fine internal thread becomes more difficult when the design combines:

• A small bore
• Long engagement
• Deep thread location
• Limited relief
• A nearby lens shoulder
• Thin surrounding walls

The tool needs enough stiffness and clearance to enter, cut, and retract without damaging adjacent features.

Define:

• Thread designation
• Pitch and fit
• Full usable engagement
• Incomplete-thread allowance
• Relief location
• Entry chamfer
• Mating component

Do not treat the full modeled thread length as usable engagement unless thread runout has been included.

Thread Start Position Is a Separate Requirement

A standard fine thread controls fit. It does not control the final rotational position of the mating component.

Additional requirements may be needed when the assembly must align:

• Engraved scales
• Cable exits
• Radial holes
• Focus marks
• Mechanical stops
• Multi-start threads

A normal thread callout will not make a retaining ring or focus sleeve stop at one exact angle.

Fine Threads Are Sensitive to Finishing

Fine threads are sensitive to burrs and anodic growth.

A thread that passes before finishing may bind after black anodizing. Final acceptance should reflect the delivered surface condition.

Thread decision: Define the mating component, usable engagement, and post-anodize acceptance method before quotation.

When Does Turn-Mill Machining Improve Lens Barrel Accuracy?

Turn-mill machining is useful when a lens barrel combines rotational features with radial or non-rotational details.

Its primary benefit is datum continuity.

Turning Establishes the Rotational Geometry

Turning can produce:

• Internal bores
• Outside diameters
• Lens seats
• Axial shoulders
• Grooves
• Internal threads
• External threads

These features are naturally related to the spindle axis.

Milling Adds Non-Rotational Features

Driven tools or milling operations may add:

• Mounting flats
• Radial holes
• Set-screw holes
• Alignment slots
• Cable clearances
• Wrench features
• Sensor pockets

A conventional route may require moving the part from a lathe to a separate milling machine.

A turn-mill machine can produce more of these features before the part is released.

The Main Benefit Is Datum Continuity

Keeping the part in one machine helps relate non-rotational features to the same axis used for the bores and locating diameters.

This is valuable when:

• A radial hole must intersect an internal groove
• A mounting flat must align with a lens seat
• A clocking feature must relate to the optical assembly
• A sensor pocket must maintain position relative to the barrel axis

Single-setup machining reduces datum-transfer risk, but it does not eliminate spindle error, chuck distortion, tool deflection, thermal drift, or calibration error.

Inspection must still verify the required relationships.

What Is the Difference Between Radial and Axial Runout?

Runout evaluates surface variation while a part rotates around a defined datum axis.

It often communicates lens-barrel function more clearly than a general concentricity note.

Radial Runout

Radial runout applies to cylindrical or circular surfaces measured in the radial direction.

Lens-housing examples include:

• Lens seating bores
• External locating diameters
• Focus sleeves
• Camera-mount pilots
• Cylindrical surfaces associated with threads

Excessive radial runout can make a mounted feature wobble or shift sideways relative to the datum axis.

Axial or Face Runout

Axial runout applies to a face measured parallel to the datum axis during rotation.

Relevant surfaces include:

• Lens seating shoulders
• Mounting flanges
• Sensor interface faces
• Retaining-ring contact faces

Excessive face runout can tilt the installed lens or mating assembly.

Circular Runout and Total Runout

Circular runout evaluates individual circular elements.

Total runout evaluates the complete controlled surface as the indicator moves along its length or area.

A long locating diameter may pass at one cross-section and still vary farther along the surface. Total runout provides broader control when full-length behavior matters.

Match the Control to the Failure Mode

Lens bore shifts from the datum axis

Use position, radial runout, or another defined axis control.

Lens shoulder causes tilt

Use perpendicularity or axial runout.

Long locating diameter wobbles along its length

Consider total runout.

Bore is not round

Use roundness or cylindricity.

Several lens seats must share one axis

Reference all controlled features to a clearly defined datum axis.

The final control must follow the GD&T standard specified on the drawing.

Upload the drawing and mark the optical axis, mechanical datum axis, lens seating surfaces, thread fit, and runout requirements. We can review whether the tolerance scheme matches the proposed machining and inspection plan.

Review Optical Tolerances

How Does Black Anodizing Affect Lens Housing Dimensions?

Black anodizing can change bores, threads, locating diameters, and mating fits.

Include the finishing process in the dimensional strategy before releasing the machining drawing.

Why Lens Housings Are Black Anodized

Black anodizing may provide:

• Corrosion resistance
• Wear resistance
• Dark appearance
• Electrical isolation
• More consistent internal color

It can also reduce visible internal reflections when combined with suitable surface texture and baffle geometry.

“Black anodized” does not fully define the process.

The specification may also need to state:

• Anodizing type
• Coating thickness
• Dye or color requirement
• Sealing
• Masking
• Cosmetic acceptance
• Final dimensional requirements

Anodizing Changes Bores and Threads

Anodizing converts part of the aluminum surface into aluminum oxide.

Part of the oxide penetrates the original surface, while part grows outward. The final dimensional change depends on the anodizing type, alloy, pretreatment, and process conditions.

Bore changes occur on both sides of the diameter.

Affected features include:

• Lens seating bores
• Fine internal threads
• Fine external threads
• Slip-fit diameters
• Retaining-ring fits
• Camera-mount pilots
• Small holes
• Precision locating diameters

A single generic anodizing allowance is not suitable for every feature or finishing supplier.

Separate Machined and Final Dimensions

Identify whether each critical tolerance applies:

• After machining
• After anodizing
• After anodizing and final cleaning
• At both process and final inspection

For a critical lens seat, the manufacturer may machine a compensated pre-anodize diameter and verify the final bore after finishing.

Agree on the compensation with the anodizing supplier before production.

Specify Coating Thickness

Anodizing type alone may not define the final film thickness.

Specify the required thickness, permitted variation, masking, and final inspection condition for black anodizing aluminum parts.

Anodizing decision: Compensate or mask only after the coating type, thickness, fit, and final acceptance condition are agreed.

Should Fine Threads and Lens Seats Be Masked During Anodizing?

Masking preserves a machined surface but adds cost and another source of process variation.

Base the decision on function.

Consider Masking When

Masking may be suitable for:

• A fine thread with insufficient coating allowance
• A precision sliding fit
• A lens seat that must remain bare aluminum
• An electrical ground
• A locating diameter validated in its machined condition
• A mating part that cannot accommodate anodic growth

Do Not Mask Automatically

Masking can create:

• Additional handling
• Irregular transition boundaries
• Incomplete coverage near edges
• Exposed aluminum
• Cosmetic variation
• Higher unit cost

The bare area can also behave differently from the surrounding anodized surface in corrosion, wear, and reflection.

Use a Defined Decision Process

1. Confirm the required final fit.
2. Specify the anodizing type and thickness.
3. Estimate or test the dimensional change.
4. Choose machining compensation or masking.
5. Inspect the final finished feature.
6. Test the actual mating component where required.

Record the approved pre-anodize dimensions and finishing process for repeat production.

How Do Internal Grooves and Black Surfaces Control Reflections?

A black interior does not automatically eliminate stray reflections.

Reflection also depends on geometry, surface texture, light angle, contamination, and whether bright surfaces remain visible to the optical path.

Black Color Is Only One Part of the Control

Potential reflection sources include:

• Smooth turning marks
• Thread crests
• Sharp shoulders
• Bare masked areas
• Retaining rings
• Fastener heads
• Oil residue
• Dust and fibers

A surface can look black under room lighting and still reflect unwanted light at a shallow angle.

Internal Baffle Threads and Grooves

Internal grooves or baffle threads interrupt direct reflection paths and block the line of sight to brighter surfaces.

Possible designs include:

• Repeated shallow grooves
• Thread-like baffles
• Stepped internal diameters
• Separate baffle rings
• Local shadowing features

Keep Baffle Geometry Manufacturable

Review:

• Groove pitch
• Groove depth
• Included angle
• Tool access
• Burr risk
• Cleaning access
• Distance from the lens seat
• Anodizing coverage

Very deep or tightly spaced grooves can require fragile tooling and trap chips or finishing residue.

The objective is to control light paths, not to create the deepest possible texture.

Cleanliness Is Part of Reflection Control

Oil films, water spots, loose particles, and anodizing residue can create bright points or contaminate the optics.

Define:

• Cleaning level
• Permitted residue
• Particle requirements
• Packaging method
• Protection of internal surfaces

Which Inspection Method Should Be Used for Each Requirement?

No single instrument is suitable for every lens-housing characteristic.

Match the inspection method to the feature, tolerance, datum structure, and functional risk.

Thread Plug and Ring Gauges

GO/NO-GO gauges provide practical acceptance checks for standard threads.

• A thread plug gauge checks an internal thread.
• A thread ring gauge checks an external thread.

These gauges confirm whether the thread falls within its specified functional limits.

They do not prove:

• Thread-to-bore alignment
• Thread-axis runout
• Shoulder orientation
• Actual pitch-diameter trend
• Final rotational stop position

Roundness Instrument

A roundness instrument is well suited to rotational geometry.

Depending on the system and setup, it can evaluate:

• Roundness
• Cylindricity
• Coaxial relationships
• Radial runout
• Axial runout
• Squareness to an axis

It is particularly useful when several bores, outside diameters, and shoulders must be evaluated around one rotational datum.

CMM Inspection

A coordinate measuring machine is useful for:

• Multi-feature datum systems
• Radial holes
• Mounting flats
• Slots and pockets
• Feature position
• Relationships between rotational and non-rotational features

The inspection result depends on:

• Datum alignment
• Probe selection
• Number of measurement points
• Point distribution
• Feature-fitting method
• Part restraint
• Measurement uncertainty

Use the same datum logic in the inspection program and the drawing.

Functional Mating Checks

Final verification may also use:

• Approved retaining rings
• Master camera mounts
• Focus sleeves
• Mating barrels
• Assembly torque checks
• Travel checks

Functional mating checks can reveal problems not captured by one isolated measurement.

They supplement rather than replace dimensional inspection.

Mobile Inspection Guide

Fine internal thread

Use a thread plug gauge.

Fine external thread

Use a thread ring gauge.

Bore roundness

Use a roundness instrument.

Multiple bore alignment

Use a roundness instrument or a defined CMM strategy.

Radial and axial runout

Use a roundness system or controlled rotational setup.

Radial holes and mounting flats

Use a CMM.

Final mating fit

Use an approved mating component.

For a broader view of measurement planning and documentation, review the CNC machining quality-control process.

Inspection decision: Specify how each critical requirement will be verified before accepting the quote.

Should Critical Dimensions Be Inspected Before or After Anodizing?

Critical features may need both process inspection and final inspection.

The two stages answer different questions.

Pre-Anodize Inspection

Machining-stage inspection can identify:

• Bore-size errors
• Roundness problems
• Excessive runout
• Thread damage
• Incorrect compensation
• Burrs
• Features unsuitable for finishing

This prevents an already nonconforming part from proceeding to anodizing.

Post-Anodize Inspection

Final inspection confirms:

• Finished lens-seat dimensions
• Final thread fit
• Coating-related bore changes
• Masking boundaries
• Cosmetic condition
• Cleaning and residue
• Final mating performance

For critical features, use a two-stage inspection plan:

1. Inspect the machined condition.
2. Control the anodizing process.
3. Inspect the final finished condition.
4. Verify threads and mating features.
5. Complete functional checks where required.

State which stage controls final acceptance.

How Can Lens Housing Machining Cost Be Reduced Without Losing Alignment?

Cost reduction should simplify manufacturing and inspection without weakening the functional axis.

Use One Functional Datum System

Avoid assigning unrelated datum structures to different features.

A consistent datum strategy reduces setup interpretation, programming complexity, and inspection alignment work.

Group Critical Rotational Features

Producing related bores, seats, shoulders, locating diameters, and threads in one setup can reduce machining risk and downstream inspection.

Tighten Runout Only Where It Matters

Reserve strict runout requirements for:

• Lens seats
• Functional locating diameters
• Optical mounting interfaces
• Critical shoulders
• Focus mechanisms

Decorative diameters and clearance features rarely require the same control.

Use Standard Fine Threads Where Possible

Standard threads are more likely to have readily available:

• Cutting tools
• Thread gauges
• Retaining rings
• Mating parts

A custom thread can require dedicated tooling, programming, gauges, and fit validation.

Separate Baffles from Locating Features

An internal light-control groove does not need the same size, runout, or surface tolerance as a lens seat.

Specify each feature according to its function.

Define Anodizing Early

Adding black anodizing after the machining drawing is complete may force changes to:

• Bores
• Threads
• Fits
• Masking
• Inspection
• Mating parts

Finalize the finishing strategy before releasing the critical machined dimensions.

State Prototype and Production Quantities

Prototype and production orders may use different fixtures, inspection frequencies, and tooling strategies.

Providing the expected repeat volume helps the precision lens barrel manufacturer select a scalable process.

What Should a Lens Housing Drawing Clearly Identify?

A 3D model defines geometry. It does not communicate all axis relationships, fits, finishing boundaries, or acceptance requirements.

Axis and Datums

Identify:

• Functional mechanical axis
• Primary cylindrical datum
• Primary seating face
• Clocking datum where required
• Applicable GD&T standard and revision

Lens Seats

Define:

• Lens-seat diameters
• Seating shoulders
• Axial spacing
• Corner relief
• Fit or clearance
• Functional surface requirements

Threads

Include:

• Thread designation
• Pitch and fit
• Full engagement length
• Relief
• Entry chamfer
• Mating component
• Final acceptance condition

Runout and Alignment

State:

• Radial runout
• Axial runout
• Total runout where required
• Controlled surface
• Datum axis

Finish and Light Control

Specify:

• Black anodizing type
• Coating thickness
• Masking boundaries
• Internal baffle grooves
• Cosmetic areas
• Cleaning requirements

Quality and Delivery

Confirm:

• Critical characteristics
• Required inspection records
• Prototype quantity
• Repeat quantity
• Packaging protection

Complete information allows different suppliers to quote the same technical scope.

What Should You Ask a Precision Lens Barrel Manufacturer?

A capable supplier should explain how alignment will be created and verified.

Ask:

1. Which feature establishes the primary mechanical datum axis?
2. Which alignment-critical features remain in one setup?
3. Where is a second setup required?
4. How will thin-wall distortion be controlled?
5. Which GD&T control represents the required axis relationship?
6. Will critical threads and lens seats be compensated or masked?
7. How will each thread, bore, shoulder, and runout requirement be inspected?
8. Does final acceptance occur before or after black anodizing?

“We can hold the tolerance” is not a complete manufacturing response.

Look for an answer that identifies:

• Datum selection
• Machining sequence
• Setup transitions
• Anodizing allowance
• Inspection strategy
• Final acceptance condition
• Prototype and production differences

Control the Axis, Fit, and Final Finished Condition

Successful lens housing CNC machining depends on controlling the functional axis, thread fit, lens seating surfaces, and post-anodize dimensions as one connected system.

Individual diameter tolerances cannot guarantee optical alignment. The datum strategy and machining sequence must preserve the relationship between the lens seats, threads, shoulders, and mounting interfaces.

Define black anodizing before finalizing critical dimensions. Then match every runout, fit, and alignment requirement with a suitable inspection method.

Upload the drawing and mark the optical axis, lens seating surfaces, thread fit, and concentricity or runout requirements. We can review the datum strategy, machining sequence, anodizing allowance, and inspection plan before quotation.

Quote a Lens Housing

Need a tolerance review first? Mark the optical axis, mechanical datum axis, critical lens seats, and final thread fits when submitting the drawing.

Frequently Asked Questions

Should lens housing tolerances apply before or after anodizing?

State the acceptance stage on the drawing. Dimensions controlling lens fit, thread engagement, and final assembly normally require verification after anodizing. Pre-anodize measurements remain useful for process control.

Can a thread gauge verify that a fine thread is concentric with the lens bore?

No. A thread gauge verifies thread acceptance within specified functional limits. It does not verify alignment between the thread axis, lens bore, or another mechanical datum.

Is black anodizing enough to prevent internal reflections?

Not always. Reflection also depends on surface texture, viewing angle, internal geometry, baffle grooves, exposed masked areas, retaining rings, and cleanliness.

Why can a lens barrel pass diameter inspection but fail assembly?

Possible causes include bore misalignment, shoulder runout, pitch-diameter error, anodizing buildup, burrs, corner-radius interference, contamination, or variation in the mating component.