Case Study: Achieving Micron-Level Tolerances on a SUS316 Bearing Sleeve for Automation Equipment
Cylindricity ±0.001 mm. Circular runout ±0.005 mm. Perpendicularity ±0.005 mm. These are the callouts most shops decline. Here is exactly how we held every one of them — in production, not just on a first article.
The Part and the Problem
A customer in the automation equipment sector came to us with a bearing sleeve drawing in SUS316 stainless steel. The geometry itself was not unusual — a precision cylindrical sleeve used to locate and guide a rotating shaft. What made this job atypical were the GD&T callouts on the drawing.
These are not tolerances that can be achieved by simply “running carefully.” Cylindricity of ±0.001 mm — one micron — means the bore must be round and straight to within the diameter of a fine human hair. It requires a controlled sequence of operations, the right machine platform, and careful management of the material itself before a single cut is made.
The core challenge: SUS316 is an austenitic stainless steel with relatively low thermal conductivity and a strong work-hardening tendency. Residual stress in the raw bar stock — introduced during the rolling and drawing process — is a primary cause of dimensional drift during and after machining. Any process that fails to account for this will produce a part that measures correctly on the machine but moves out of tolerance on the inspection table or on the customer’s assembly floor.
Why Standard Machining Would Not Be Enough
Before we committed to a process, our engineering team worked through the failure modes that would prevent compliance with the drawing. Three issues stood out:
1 — Residual stress in the raw material. SUS316 bar stock carries internal stress from the manufacturing process. When material is removed during machining, the stress equilibrium changes, and the part can spring, twist, or distort — sometimes by several microns — between operations. At a cylindricity target of 0.001 mm, even minor stress release is disqualifying.
2 — Machine capability at this tolerance class. Standard CNC lathes have positioning and thermal repeatability in the ±0.005–0.010 mm range. Achieving ±0.001 mm cylindricity requires a machine platform with lower geometric error — specifically, a precision-grade turn-mill center where spindle runout and axis straightness are verified and within the required envelope.
3 — Single-operation finish machining is insufficient. At micron tolerances, a single grinding pass cannot remove enough material to correct geometry while also achieving the required surface finish without introducing grinding burn or re-introducing stress. A two-stage approach — coarse grinding to establish geometry, fine grinding to achieve finish and final tolerance — is standard practice for this tolerance class.
Four Steps That Made the Difference
The process we developed was not a single clever technique — it was a deliberate sequence where each step eliminated a specific failure mode.
Stress-Relief Annealing of SUS316 Raw Material
Before any machining, the raw SUS316 bar stock was put through a full stress-relief annealing cycle. The material was heated to the appropriate solution-annealing temperature and held to allow the internal stress state to equilibrate, then cooled in a controlled manner.
Why this matters: Skipping this step on a standard job causes no visible problems — the part looks fine, it measures fine, and the customer receives it. On a job with ±0.001 mm cylindricity, the stress release that happens during machining would push the bore geometry outside tolerance before the part ever reached the grinder. Annealing eliminates the variable before it can act.
Precision Turn-Mill Machining to Near-Net Shape
All primary features — OD profile, ID bore, face features, and any cross-features — were machined in a single setup on a precision-grade turn-mill center. Using a single setup eliminates the datum shift that occurs when a part is re-fixtured between operations.
Machine selection: We selected a machine from our turn-mill cell with verified spindle runout and axis straightness within the tolerance envelope required by this drawing. The combination of live tooling and turning in one setup also meant the perpendicularity relationship between the bore axis and the face datums was established in the same fixturing — with no re-referencing error accumulated between operations.
Two-Stage Grinding: Rough Grind + Finish Grind
After turn-mill machining, the OD and ID were left with a deliberate stock allowance for grinding. Grinding was performed in two distinct stages rather than in a single pass.
Rough grind: Removed the bulk of the grinding stock, corrected any remaining roundness error from the turning operation, and established the geometric baseline (cylindricity) within a tighter intermediate band.
Finish grind: Removed the final small stock allowance at light infeed to achieve the specified surface finish and bring cylindricity into the final ±0.001 mm window. Separating these two stages prevents the heat and cutting forces of rough grinding from affecting the final surface, and avoids the risk of burn or re-distortion at the finish stage.
100% Final Inspection with Ceramic Pin Gauges
Every one of the 30 parts was individually gauged using calibrated ceramic pin gauges. Ceramic pins are dimensionally stable and thermally inert — unlike steel gauges, their size does not change measurably with handling temperature. This matters at the ±0.001 mm level, where a steel gauge held in a warm hand can read a micron larger than when measured on the inspection table.
Rough grind: Removed the bulk of the grinding stock, corrected any remaining roundness error from the turning operation, and established the geometric baseline (cylindricity) within a tighter intermediate band.
Finish grind: Removed the final small stock allowance at light infeed to achieve the specified surface finish and bring cylindricity into the final ±0.001 mm window. Separating these two stages prevents the heat and cutting forces of rough grinding from affecting the final surface, and avoids the risk of burn or re-distortion at the finish stage.
All 30 Parts. Zero Rework. Zero Rejections
The complete batch of 30 bearing sleeves was delivered conforming to every callout on the drawing. No rework was required, no parts were rejected, and the customer confirmed successful fit and function in their assembly.
Cylindricity
All parts held within ±0.001 mm — verified at multiple bore positions with ceramic pin gauges.
Circular Runout & Perpendicularity
All parts within ±0.005 mm — confirmed against drawing datums on 100% of the batch.
Delivery
Batch of 30 pieces delivered complete, with inspection records and ceramic gauge traceability.
Customer Outcome
Parts fitted and functioned correctly in the automation assembly on first use — no field rework.
What This Case Demonstrates
The most common reason precision parts fail their drawing is not bad machining — it is an incomplete process. A shop with excellent machines and skilled operators can still produce out-of-tolerance parts if the process sequence does not account for the material’s behavior and the tolerance chain from fixturing through inspection.
The key lesson
For high-precision components, good equipment alone is not enough. You need a process that is designed around the material and the tolerance chain — from stress relief before the first cut to gauge selection at final inspection. Every step in this case existed to eliminate a specific failure mode, not to add cost.
The four steps in this process — annealing, precision turn-mill single-setup machining, two-stage grinding, and 100% ceramic gauge inspection — are a transferable template for any SUS316 or similar austenitic stainless component where GD&T callouts are in the single-digit-micron range. If your drawing has tolerance callouts at this level, the process approach described here is where the conversation should start.
Part Specifications
Key Challenges Overcome
- Residual stress in SUS316 bar stock causing dimensional drift after material removal.
- Machine capability gap: standard CNC lathes cannot hold ±0.001 mm cylindricity.
- Single-stage grinding insufficient — risk of heat distortion and re-introduced stress.
- Gauge thermal instability: steel gauges unreliable at 1-micron tolerance class.
Have a Similar Precision Requirement?
Share your drawing with us. We’ll review the GD&T callouts and tell you exactly how we’d approach it — including whether stress relief or multi-stage grinding is warranted.