What level of precision can be achieved with custom CNC turning parts?

Quick Answer

Standard CNC turning achieves ±0.002" tolerances routinely, with precision turning reaching ±0.0002". Surface finish of 16 Ra is standard, and 8 Ra is achievable with fine finishing passes. The actual precision depends on machine condition, tooling quality, material, and part geometry.

Standard vs Precision Tolerances

Standard turning (±0.002" to ±0.005"): Suitable for most mechanical parts where general fit is acceptable. Achievable on any well-maintained CNC lathe. Precision turning (±0.0005" to ±0.001"): Requires quality tooling, proper feeds and speeds, and temperature-controlled environment. Ultra-precision turning (±0.0001" to ±0.0002"): Needs specialized machines, diamond tooling, and strict environmental control. Used for aerospace bearings, hydraulic components, and precision shafts.

Surface Finish Capabilities

Surface finish is measured in Ra (roughness average). Standard turning: 32-63 Ra. Fine turning: 16-32 Ra. Precision turning: 8-16 Ra. Ultra-precision: 4-8 Ra. Achievable finish depends on feed rate, nose radius, material, and whether a dedicated finishing pass is performed. SOMI uses programmed finishing passes and quality inserts to consistently achieve 16 Ra or better.

Factors Affecting Precision

Machine rigidity and condition: Newer, well-maintained machines hold tighter tolerances. Tool quality: Premium carbide inserts with proper coatings reduce wear and maintain consistency. Material stability: Some materials (aluminum, brass) are more stable than others (stainless, titanium). Thermal expansion: Parts machined in a temperature-controlled environment hold tighter tolerances. Part size and geometry: Longer parts are harder to hold tight tolerances on due to deflection.

Why Choose SOMI Custom Parts

SOMI Custom Parts maintains a fleet of modern CNC lathes capable of holding tolerances as tight as ±0.0002" on critical diameters. Our quality team uses CMM inspection to verify every critical dimension, and we provide full dimensional inspection reports with each shipment. Whether your project requires standard commercial tolerances or ultra-precision aerospace-grade specifications, SOMI has the capability and expertise to deliver.

Case Study

A hydraulic valve manufacturer required precision spools with ±0.0003" diameter tolerance and 12 Ra surface finish. SOMI programmed finishing passes with wiper geometry inserts and performed in-process gauging to maintain consistency. The 5,000-piece order was completed with 100% dimensional compliance and a Cpk of 1.67 across all critical features.

Industry Data

The Precision Machined Products Association reports that the average CNC turning shop maintains ±0.002" as their standard tolerance. Only about 15% of shops are equipped for ±0.0005" or tighter (PMPA 2025 Benchmarking Survey). This makes precision turning capability a key differentiator when selecting a machining partner.

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