Thin-Wall Aluminum Optical Base: How We Held a 45 mm Deep Bore at 0.015 mm on 3.5 mm Walls

A Φ8 H7 bore at 45 mm depth, sitting inside a 3.5 mm aluminum wall. Standard vise clamping put the first batch at a 30% reject rate. We rebuilt the process from clamping through reaming and ran the next 10 pieces at 100% first-pass yield. The full sequence used 5-axis CNC milling, vacuum fixturing, and three-pass reaming.

The part is the base of a Swiss optical inspection instrument. Everything in the assembly mounts to it: lenses, sensors, positioning modules. Bore position error at the base propagates as angular misalignment up through the whole optical column. The customer’s spec left no room: ≤ 0.02 mm true position per ASME Y14.5-2018 on 12 bore groups, datum face flatness ≤ 0.025 mm, bore wall Ra ≤ 0.6 µm with zero chatter marks.

1.The part and the problem it has to solve

Material: Al 6061-T6. Envelope: 220 × 180 × 50 mm. Wall thickness: 3.5 mm. 12 precision bore groups across the top face. Core locating bore: Φ8 H7, tolerance 0 / +0.015 mm per ISO 286-1, at 45 mm depth. That puts the depth-to-diameter ratio at 5.6:1.

On a thin-wall optical base carrying lenses and sensors, CNC-machined aluminum beats a casting at this quantity and tolerance. Casting porosity and as-cast surface variation would push the bore and flatness out before machining even starts. On a thick rigid housing where wall flex is not a concern and quantities reach hundreds, a casting blank saves roughing time. This base needed 10 pieces with full ISO 9001 documentation: 6061-T6 material cert, CMM report, and roughness record. For a breakdown of how ISO tolerance bands apply to CNC parts, see our tolerances and standards reference.

2.What the standard single-setup sequence produced

We ran the standard approach first: vise side-clamping on a vertical machining center, rough and ream in one setup, no stress relief step. Three problems stacked up.

Clamping stress. The vise pressed the 3.5 mm wall from the side. The wall deflected under clamping load. When the clamp released, the part sprang back and the datum face moved with it.

Residual billet stress. Roughing pulled a large amount of material off the rolled billet in one pass. Internal stress locked in the rolled stock released unevenly. The flatness kept drifting during the finish pass.

Deep bore chatter. The Φ8 bore at 45 mm depth ran straight through on a long overhang. The tool vibrated. Every hole came out with spiral chatter marks on the wall and a position error that stacked across the 12-group pattern.

CMM result on the first batch: flatness hit 0.048 mm, bore positions ran past 0.025 mm across the board. Reject rate: 30%. No Swiss optical customer accepts that.

Engineer note “The 3.5 mm wall is the hardest part. The vise looks like it is holding the plate flat. The moment you release, it springs back and the flatness is gone. One dimension out of tolerance and the whole batch gets rejected.” “6061 builds up stress during roughing. The 4-hour ambient rest is what we landed on after several iterations. Cut it short and the datum face keeps drifting during the finish pass. There is no shortcut.”

3.Four changes that held the tolerance

1. Full-face vacuum clamping. Replaced vise side-clamping with a full-face vacuum chuck plus locating pins through process holes. Load distributed evenly across the plate face. No side pressure on the 3.5 mm wall. Repositioning repeatability held at ≤ 0.02 mm across all setups.

2. Stress-relief sequence. Rough mill, then a 4-hour ambient rest, then finish mill the datum face, then drill and ream precision bores. The datum face is locked before the bore pattern is cut. Any residual stress that moves the face after roughing has settled before bore position is set.

3. Three-pass deep bore reaming. On every Φ8 bore: drill to leave 0.2 mm stock at 800 rpm, 0.08 mm/rev; rough ream to 0.05 mm remaining; finish ream the final 0.02 mm with a single-edge pass at 400 rpm, 0.04 mm/rev. Low speed, low feed, steady chip load on a long overhang. Chatter gone. Wall came out clean. This is the same approach we detail in our write-up on tight tolerance machining for CNC features.

4. 100% CMM inspection. Full CMM check on all 10 pieces. Datum face flatness and all bore positions measured. Roughness measured on every bore wall. FAI package includes 6061-T6 material cert, CMM report, Ra records, all under our ISO 9001 and ISO 13485 quality system.

On a deep bore in a thin-wall plate, three-pass reaming at low speed beats single-pass finish reaming. The long overhang magnifies any side load into chatter, and the 0.02 mm final pass is too small to generate enough heat or load to restart it. On a short bore in a stiff block, single-pass reaming is faster and the wall holds.

4.Measured results on this 10-piece production run

All numbers below are measured on this production run. None are pulled from material data sheets or process capability studies on other parts. For reference values on 6061-T6 yield strength and hardness, see our metal strength chart for CNC materials.

Metric	Requirement	We measured (this part)	What it means
Core bore Φ8 H7	0 / +0.015 mm	8.002 to 8.012 mm (all 10 pcs)	Lens module seats on the bore wall. Oversize or out-of-round creates tilt.
Bore position	≤ 0.02 mm	0.009 to 0.015 mm	Every sensor and lens lands where the drawing says. No shimming.
Datum face flatness	≤ 0.025 mm	0.008 to 0.016 mm	Optical stack sits on a true plane. Tilt at the base multiplies up the column.
Bore wall finish	Ra ≤ 0.6 µm	Ra 0.40 to 0.55 µm	No spiral tool marks. No press-fit interference from surface peaks.
First-pass yield / Cpk	100% / Cpk ≥ 1.65	100%, Cpk ≥ 1.65 (FAI passed)	All 10 shipped without rework or tolerance negotiation.

Client outcome The Swiss customer received 10 bases with full FAI documentation. All 10 passed CMM inspection on the first run. No rework, no tolerance negotiation, no redesign. The bases went into assembly on the customer’s original schedule.

5.This process was proven on a prior optical lens base

We had seen the same failure mode before. On a 6061-T6 optical lens base, 250 × 160 × 45 mm, 4 mm wall, same multi-bore pattern, the customer asked for a fast prototype and we ran the standard sequence to save tooling cost. CMM result: flatness hit 0.048 mm, bore positions ran past 0.025 mm, bore walls full of fine chatter marks. Straight reject.

We pulled the process apart. Rebuilt it with full-face vacuum clamping, a 4-hour ambient rest after roughing, and low-speed three-pass reaming. Three remake samples: flatness held at 0.012 mm, Ra stable below 0.5 µm. The customer confirmed on the spot. Long-run Cpk settled above 1.67 on that lens base. The same sequence ran on this Swiss instrument base.

6.What to watch on your next precision base

On a thin-wall plate, replace vise side-clamping with full-face vacuum. The vise bends the wall before the cutter touches it.

Cut the datum face before the bores. Any residual stress that moves the face will take the bore positions with it.

On deep bores with depth-to-diameter above 4:1, plan for three-pass reaming. At 5.6:1 on this base, every hole we cut with a single finish pass came out with chatter marks.

Rest the part after roughing before finishing. Four hours at ambient is enough for 6061-T6 at this wall thickness; skipping it puts the flatness back in play.

Measure every piece on the CMM before shipment. On a 10-piece optical instrument run, sampling is not a quality plan. For more on how wall thickness, boss depth, and bore aspect ratio shape the drawing, read our DFM design guidelines for CNC parts.

7.Project summary

Client region	Switzerland
Industry	Optical inspection instruments (precision lens and sensor mounting, micrometer-level assembly)
Scope	10-piece Al 6061-T6 optical base; 100% first-pass yield, Cpk ≥ 1.65, full FAI documentation
Processes	5-axis CNC milling | Full-face vacuum clamping | Three-pass deep bore reaming | Ambient stress relief | 100% CMM inspection | Ra measurement | FAI

Working on a similar part? Working on a precision base, multi-bore plate, or thin-wall optical housing? This project ran as a low-volume CNC production batch with full FAI. Send us your drawing. Tell us your tightest bore tolerance and depth-to-diameter ratio, your datum face flatness requirement, and whether this is a prototype or production batch. We will come back with a DFM note and a quote within 48 hours. No commitment.

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