1. What ISO 8062 Is and Why It Matters for Your BOM

ISO 8062 is the international standard for casting dimensional tolerances and geometrical deviations, applicable to all major casting processes including sand casting, investment casting (lost-wax), die casting, and shell molding. The standard defines tolerance grades CT1 through CT16, where CT1 is the tightest and CT16 is the loosest. Investment casting typically operates in the CT4-CT9 range depending on the casting size, the wax pattern tooling precision, and the alloy material.

Why ISO 8062 matters for your BOM is the cost-precision linkage. Each CT grade corresponds to a tighter-or-looser tolerance range, and each CT grade tighter increases the manufacturing cost on three dimensions: tooling, process control, and yield. The BOM impact is the cumulative effect across the production volume. A CT5 specification on a 5,000-piece annual order will add significantly more cost than the same CT5 specification on a 500-piece prototype order. The OEM should select the CT grade based on the post-casting operation, not on a “tighter is always safer” reflex.

2. The CT Grade Framework — CT5, CT6, CT7 in Detail

CT5 through CT7 is the practical working range for OEM investment casting orders. The table below summarizes the typical linear tolerance values at four nominal dimensions:

The trap is reading the CT grade as a single number. Each CT grade corresponds to a different tolerance range at every nominal dimension, and the range widens as the nominal dimension grows. CT7 at 10 mm is roughly half of CT7 at 200 mm. The OEM should specify the CT grade on the technical drawing and verify that the supplier’s process capability supports that grade at the largest dimension on the part, not the average. A practical verification approach is to ask the supplier for their Cpk data at the largest nominal dimension across the critical-to-quality features; this avoids the common failure mode of a supplier holding tolerance on small features but losing it on large features where the process margin is thinner.

3. BOM Cost Impact — Why Each CT Grade Tighter Costs More

Tighter CT grades (lower number) increase BOM cost on three dimensions. The cumulative effect is significant and predictable across the production volume:

The trap is treating per-piece cost as the only BOM impact. Tooling cost is amortized across the production volume, so a CT5 specification on a 500-piece annual order adds roughly 4-5x the per-piece amortized tooling cost compared to a 5,000-piece annual order. The OEM should evaluate the CT grade selection across the full annual volume, not just the per-piece unit cost. For low-volume orders, the higher CT grades are often economically justified even when the as-cast tolerance is not strictly required. The inverse is also true: a CT7 specification on a 50,000-piece annual order saves significant cost compared to CT6, and the savings can justify a feature-by-feature tolerance audit to find CT7-eligible features that were over-specified at CT6 in the original design.

4. CNC Machining Allowance — Stock Required for Post-Casting Operations

The CNC machining allowance (also called machining stock or machining allowance) for investment casting depends on the dimensional feature and the post-casting operation. The allowance provides the cutter with consistent stock to remove without exposing casting surface defects, and it ensures the final machined dimension is achieved even at the tolerance edge of the casting.

The trap is treating the allowance as a fixed number across the casting. Different features on the same casting need different allowances based on the post-casting operation. The OEM should specify the allowance per feature on the technical drawing, with explicit notes for surfaces that will and will not be machined. A casting drawing with a single “0.5 mm machining allowance” note is a common source of confusion between the buyer and the casting supplier. The OEM can also specify a “machining allowance envelope” on the drawing that defines the minimum and maximum stock for each feature; this gives the casting supplier a target range rather than a single number, allowing the supplier to optimize their process within the envelope.

5. CT Grade Selection — The Decision Rule by Post-Casting Operation

The CT grade selection rule is driven by the post-casting operation:

The trap is selecting CT5 as a default out of caution. CT5 adds 20-40% tooling cost and 10-15% per-piece cost compared to CT7. If the part will be machined on functional surfaces, the CT5 precision is wasted and adds cost without benefit. The OEM should specify CT6 or CT7 unless the post-casting plan explicitly requires as-cast functional surfaces. The selection logic also interacts with the alloy material — softer alloys (aluminum, brass) achieve tighter as-cast precision than harder alloys (stainless steel, tool steel); the CT grade selection should consider both the post-casting operation and the alloy. A CT5 specification on stainless steel is significantly harder to achieve than the same CT5 specification on aluminum, even with equivalent process control.

6. Tolerance Verification — Supplier Audit Scope Before Placing the Order

Supplier tolerance verification covers four dimensions. The audit should be conducted before the first purchase order is confirmed, with on-site review of the supplier’s inspection data and capability analysis:

The trap is accepting the supplier’s claim of “we can hold CT6″ without verifying the dimensional data on a comparable part. The capability evidence is the on-site dimensional data, not the verbal claim. The OEM should request the inspection data before the order is placed, and should validate that the data corresponds to a part comparable in size and geometry to the new order. A supplier that holds CT6 on a 50 mm simple block may not hold CT6 on a 200 mm complex geometry. The capability index Cpk is the standard metric to compare across parts and suppliers; a Cpk > 1.33 on a comparable part is the minimum baseline for OEM-grade investment casting capability.

7. Tolerance Selection Matrix — CT Grade by Feature and Operation

A consolidated tolerance selection matrix that maps the CT grade to feature type and post-casting operation. Each row is a tolerance decision; the right column is the recommended grade.

The tolerance selection matrix is the OEM’s decision framework for specifying CT grades across the technical drawing. Each feature on the drawing maps to a row in the matrix, and the recommended grade drives the casting supplier’s process control. A consistent CT grade across all features on the drawing is a common simplification; the precision-cost optimization is feature-specific. The matrix is also useful for supplier cost analysis — a request for quote that maps each feature to a specific CT grade lets the supplier price per feature rather than per casting, often surfacing significant BOM savings that a single-CT-grade drawing would obscure. The feature-by-feature CT grade approach is now standard practice in OEM-tier investment casting sourcing for buyers with annual volumes above 10K pieces, where the per-feature cost savings compound into meaningful annual BOM impact across the production lifecycle.

8. Buying Checklist, Compliance Reference & FAQ — CT5-CT7 Tolerance Selection Before Sign-Off

A combined pre-order buying checklist and FAQ for the OEM running the investment casting tolerance conversation today. The checklist captures the documentation to request; the FAQ addresses the questions most often raised in XinYe Metal Works’ consulting engagements.

FAQ 1 — What is ISO 8062 for investment casting?
A. International standard for casting dimensional tolerances. Defines CT1-CT16 grades. Investment casting typically operates in CT4-CT9. CT5-CT7 is the practical OEM range.

FAQ 2 — What does CT5-CT7 mean in mm?
A. CT5 ≈ ±0.10 mm at 10 mm. CT6 ≈ ±0.15 mm at 10 mm. CT7 ≈ ±0.20 mm at 10 mm. Tolerance widens with nominal dimension (up to ±0.6 / ±0.8 / ±1.0 mm at 200 mm).

FAQ 3 — How do CT grades affect BOM cost?
A. One CT grade tighter = tooling +20-40%, per-piece +5-15%. Tooling amortizes across volume. Low-volume orders make CT5 economically questionable.

FAQ 4 — CNC machining allowance for investment casting?
A. External diameters 0.5-1.0 mm. Internal bores 0.8-1.5 mm. Milled surfaces 1.5-3.0 mm. Threaded features 0.3-0.5 mm pitch. Specify per-feature on drawing.

FAQ 5 — CT5 vs CT6 vs CT7 selection rule?
A. CT7 if fully machined (CNC adds precision). CT6 if partially machined (as-cast + machined). CT5 if as-cast on functional surfaces.

FAQ 6 — Typical tolerance for CT6 at 100 mm nominal?
A. CT6 ≈ ±0.60 mm at 100 mm. CT5 ≈ ±0.45 mm. CT7 ≈ ±0.75 mm. Select per ISO 8062 Table 1.

FAQ 7 — Supplier tolerance verification?
A. Four dimensions: comparable part CMM data + CMM equipment and calibration + PPAP Cpk/Ppk + engineering change capacity (2-4 weeks).

9. Compliance Reference Table — Standards Anchored to Investment Casting Tolerance Selection

The compliance reference table above lists the standards that anchor the investment casting tolerance conversation. For an OEM specifying investment casting tolerances, the binding reference is ISO 8062 with its CT grade framework. For OEM specifying post-casting machining allowances, the binding reference is ISO 8062-3. For OEM running statistical capability analysis on first-article inspection, the binding reference is ISO 21747. The casting supplier typically provides the dimensional inspection data and capability indices; the OEM is responsible for the CT grade selection on the technical drawing.

Keywords: investment casting tolerances ISO 8062, CT5 CT6 CT7 lost wax casting, BOM cost impact, CNC machining allowance, lost-wax casting supplier audit.