Static Accuracy Sheet

Definition

A Static Accuracy Sheet (静的精度表, seiteki seido-hyō) is a precision record submitted by the equipment manufacturer after machine run-off and process capability qualification. It documents the machine’s accuracy under static conditions — not under load or dynamic stress — establishing the baseline condition of the machine itself, independent of material, tooling, and cutting conditions.

The static accuracy sheet answers a specific question in problem solving: is the machine itself capable? By documenting the machine’s accuracy when it is known to be good, the static accuracy sheet provides the reference that isolates the machine variable from all other variables in the machining process.

When It Is Created

The static accuracy sheet is submitted after a specific qualification sequence:

1. The equipment manufacturer completes machine assembly and initial testing
2. Machine run-off is conducted — the machine produces parts under controlled conditions
3. A 30-part quality sample is produced and measured to verify the machine achieves a specified process capability (Cp/Cpk)
4. Once process capability is demonstrated, the equipment manufacturer documents the machine’s static accuracy measurements

At this point, the machine has proven it can produce good parts. The static accuracy sheet captures the machine’s condition at this proven state — the baseline against which all future measurements will be compared.

What It Measures

Static accuracy measurements are taken with the machine not under load — the spindle rotates but no cutting is taking place, no material stress is applied. This isolates the machine’s inherent precision from all process variables. Typical measurements include:

• Spindle run-out — radial and axial deviation as the spindle rotates, measured in microns
• Table surface parallelism — flatness and parallelism of the work table to the machine base
• Table faceplate run-out — deviation of the rotary table surface
• Center bore accuracy — precision of the spindle or table center bore
• Center line accuracy — measured using a test bar to verify true center
• Squareness — angular accuracy between table face and table base, between axes
• Axis parallelism — alignment of test bar center line to guideways and table surfaces
• Indexing accuracy — precision of rotary positioning (measured in arc-seconds)
• Repeatability — consistency of positioning when returning to the same point

Each measurement has a standard tolerance (the specification the machine must meet) and the actual measured value at the time of qualification. The static accuracy sheet records both, providing a clear picture of where the machine stands relative to its specification.

Role in Cutting Point Management

The static accuracy sheet is one of the four key shop floor documents that support Cutting Point Management. In the systematic diagnostic sequence for defect investigation:

1. Measure the part — compare actual dimensions to the quality standard
2. Check the Operation Drawing — verify stock removal and dimensions
3. Check the Tooling Layout Drawing — verify tooling specifications
4. Inspect the machine — compare current machine accuracy to the Static Accuracy Sheet

The static accuracy sheet is the fourth and final reference point in the diagnostic sequence. If the part is out of tolerance, the operation drawing is correct, and the tooling is correct, then the machine itself may have drifted out of specification.

How It Isolates the Problem

The power of the static accuracy sheet is in what it removes from the equation. Static accuracy measures the machine with material, tooling, and cutting conditions taken out. If the machine’s static accuracy is at standard, then the problem is not in the machine — it is in the material, method, or human dimension. The investigation moves to those variables.

If the machine’s static accuracy has degraded, the specific measurement that has drifted tells you exactly where to look. For example:

• Spindle run-out at standard (3 microns) but actual is 15 microns → isolate the problem to the spindle head, inspect bearings, check for wear or damage
• Axis positioning accuracy degraded → check the axis drive system, ballscrew, guideway wear
• Table parallelism out of spec → check table mounting, foundation, leveling

This systematic isolation is what enables Toyota employees to rapidly trace the probable root cause of variation in any machining line product — crankshaft, camshaft, cylinder head, engine block, connecting rod, piston. Rather than guessing or replacing components until the problem goes away, the static accuracy sheet lets you compare the current machine condition against its documented standard and identify which specific aspect of the machine has changed.

The Four-Document System

The static accuracy sheet completes the four-document system that is the foundation of Cutting Point Management and quality problem solving in Toyota machining:

Together, these four documents account for the major variables in machining quality. When a defect occurs, the diagnostic sequence works through them in order — part, process, tool, machine — comparing actual conditions against documented standards at each step. The problem is usually solved by identifying which variable has deviated from standard and restoring it.

These documents are the building blocks for “building in quality” (jidoka) at Toyota’s machining operations. They reside upon years of research and study into the variables that determine machining accuracy, and they enable systematic problem solving that is faster, more reliable, and less dependent on individual expertise than trial-and-error troubleshooting.

Periodic Verification

Machine accuracy degrades over time. Bearings wear, guideways develop play, thermal characteristics change, foundations settle. The static accuracy sheet provides the reference against which this degradation can be detected through periodic re-measurement.

When verification measurements show the machine has drifted beyond acceptable limits, maintenance or recalibration is required. The specific measurements that have degraded guide the maintenance response — rather than overhauling the entire machine, the maintenance team can target the components that have actually deteriorated.

This connects to Ownership Maintenance (the second pillar of the 3 Pillar Activity). While operators may not perform the precision measurements themselves, their daily observation of the machine — changes in vibration, sound, temperature, or positioning behavior — provides early warning of accuracy degradation that feeds into the maintenance planning process.

Common Mistakes

Not establishing a baseline at run-off. If the machine’s accuracy is not measured and recorded when it is new and qualified, there is no reference point for detecting future degradation. The static accuracy sheet must be created at machine acceptance — after the 30-part capability study confirms the machine can produce good parts.

Measuring only when problems occur. Reactive measurement — checking machine accuracy only after defects appear — means the machine was producing borderline parts for an unknown period before the problem was caught. Periodic verification catches degradation before it causes rejects.

Confusing static and dynamic accuracy. Static accuracy is measured without load — it reflects the machine’s inherent precision. Dynamic accuracy under cutting loads may differ due to deflection, vibration, and thermal effects. The static accuracy sheet establishes the baseline; dynamic behavior is managed through cutting conditions and process design.

Not using it during problem solving. The static accuracy sheet exists to support the diagnostic sequence. If defect investigation jumps to replacing tools or adjusting programs without checking whether the machine itself is still at standard, the actual root cause may be missed — and the new tools or adjusted program will produce the same defects.