Site Acceptance Testing (SAT) for CNC Machines: Post-Installation Verification, Acceptance Criteria, and Sign-Off

What decision this actually supports

The real question behind Site Acceptance Testing (SAT) is: “Is this CNC machine installed correctly, capable of meeting my production requirements, and ready to be formally accepted as a fully functional asset?”

SAT is not a perfunctory checkmark. It is the final, binding step in the CNC machine procurement lifecycle that separates “a machine that arrived at your shop” from “a machine that you own and are responsible for.” Before SAT sign-off, the supplier typically retains ownership of the machine, is liable for defects, and may be obligated to fix issues at no cost. After sign-off, ownership transfers, warranty terms activate, and you assume full operational responsibility.

This decision has cascading impacts: it triggers payment milestones (often 10–20% of the total machine cost), locks in warranty coverage, and defines the baseline for future performance expectations. For example, if you sign off on a machine that later fails to hold a tolerance required for your aerospace parts, you cannot retroactively claim the supplier was responsible for a defect that should have been detected during SAT.

How it works in practice

SAT is a structured, on-site verification process that occurs after the supplier’s installation team has uncrated, assembled, aligned, and calibrated the CNC machine. It combines two core activities: post-installation verification (confirming the machine was set up correctly) and acceptance criteria testing (confirming the machine performs as specified).

Core Mechanics

The process follows a standard sequence:

1. Pre-SAT Preparation: Both parties align on the test plan, which outlines all tests, pass/fail thresholds, required tools, and personnel.
2. Post-Installation Verification: The supplier’s installation team confirms the machine’s physical and electrical setup meets pre-agreed standards.
3. Acceptance Criteria Testing: The machine is tested against its technical specifications and your production requirements.
4. Defect Resolution: Any failed tests are documented, and a resolution plan is agreed upon.
5. Sign-Off: If all tests pass, both parties sign the SAT document, formalizing acceptance.

Typical Numbers and Personnel

SAT timelines vary by machine complexity:

• Small, single-spindle machines (e.g., GA-VT540 drill/tap center): 1–2 days
• Medium vertical machining centers (e.g., GA-MV856): 2–3 days
• Complex multi-spindle or 5-axis machines (e.g., GA-MVD2560 dual-spindle vertical machining center, GA-UHD500 high-precision 5-axis machining center): 3–5 days

Personnel roles are clearly defined:

• Supplier’s SAT Lead: A service engineer with expertise in the specific machine model, responsible for executing tests, calibrating the machine, and documenting results.
• Buyer’s SAT Representative: A cross-functional team including a CNC programmer, maintenance technician, and quality engineer, responsible for observing tests, verifying results, and confirming alignment with production needs.
• Third-Party Inspector (Optional): For high-value or regulated machines, an independent metrologist may be hired to verify test results.

Example SAT Test Workflow for a GA-MV856 Vertical Machining Center

1. Post-Installation Verification: The supplier confirms the machine is level within 0.02 mm per meter (a standard requirement for most machining centers), electrical connections meet local safety codes, and all auxiliary systems (coolant, chip conveyor, tool changer) are functional.
2. Geometric Accuracy Testing: Using a laser interferometer, the supplier verifies the machine’s positioning accuracy (specified as 0.010 mm for the GA-MV856) and repeatability (0.006 mm). For example, the machine is commanded to move to 10 random positions along the X-axis; each position is measured, and the maximum deviation from the commanded position must be ≤0.010 mm.
3. Spindle Performance Testing: The spindle is run at maximum speed (12,000 rpm for the GA-MV856) for 30 minutes to check for vibration, temperature rise, and power output. Vibration is measured with a accelerometer; a typical pass threshold is ≤2.5 mm/s RMS for vertical machining centers.
4. Tool Changer Testing: The tool changer is cycled 50 times to verify it meets the specified tool change time (2.0 s for the GA-MV856) and does not drop or misalign tools.
5. Production Part Test: A representative part is machined using the buyer’s standard program and tooling. The part is inspected for dimensional accuracy, surface finish, and tolerance conformance.

Where the money or risk is

SAT involves both direct costs and potential risks that can impact your bottom line.

Cost Drivers

1. SAT Labor Costs: The supplier’s service engineer time is typically included in the machine’s purchase price, but if SAT is delayed due to buyer-preparedness issues (e.g., no power available, no foundation ready), the buyer may be charged a daily rate of $500–$1,500 for the engineer’s time.
2. Third-Party Inspection Costs: Independent metrology services cost $1,000–$3,000 per day, depending on the machine’s complexity.
3. Production Downtime Costs: For small shops, a 3-day SAT delay can result in $2,000–$10,000 in lost production revenue, depending on the machine’s utilization rate.

Risk Drivers

1. Payment Milestone Risk: Most CNC machine contracts tie 10–20% of the total purchase price to SAT sign-off. For a $150,000 GA-UHD500 high-precision 5-axis machining center, that is $15,000–$30,000 held back until sign-off. If SAT is delayed, this payment is deferred, which can impact the supplier’s cash flow and your relationship.
2. Warranty Activation Risk: Warranty coverage typically starts on the date of SAT sign-off, not the date of delivery. A 2-week SAT delay means you lose 2 weeks of warranty coverage.
3. Performance Gap Risk: If you sign off on a machine that does not meet your production requirements, you may incur costs to modify the machine (e.g., adding a higher-precision spindle, upgrading the tool changer) or replace it. For example, if you sign off on a GA-MV856 that cannot hold the 0.005 mm tolerance required for your parts, you may need to invest $20,000–$50,000 in a spindle upgrade to meet the requirement.
4. Regulatory Compliance Risk: For machines used in regulated industries (aerospace, medical devices), a failed SAT can result in production delays and non-compliance penalties. For example, if a GA-UHD500 used to machine medical implants fails SAT, you may be unable to meet FDA production requirements, resulting in fines of up to $100,000 per day.

Common mistakes

SAT is a high-stakes process, and even small mistakes can lead to significant costs.

Buyer Mistakes

1. Skipping Pre-SAT Preparation: Many buyers assume the supplier will handle all SAT preparations, leading to delays. For example, if you do not have a foundation ready for a GA-MVD2560 dual-spindle vertical machining center (which requires a 300 mm thick reinforced concrete foundation), the supplier cannot install the machine, and SAT is delayed.
2. Not Defining Production Acceptance Criteria: Buyers often rely solely on the machine’s technical specifications, not their own production requirements. For example, a GA-DV750 vertical machining center has a specified positioning accuracy of 0.003 mm, but if your production requires holding a 0.002 mm tolerance on a specific feature, you must include that in your acceptance criteria.
3. Signing Off Too Quickly: Buyers often sign off on SAT to avoid payment delays or production downtime, even if there are minor defects. For example, if you sign off on a GA-VT540 drill/tap center that has a 1.5 s tool change time (slower than the specified 1.2 s), you cannot later claim this as a warranty defect.
4. Not Documenting Defects: Buyers often fail to document defects in detail, leading to disputes. For example, if you note that the GA-MV856’s spindle is “vibrating,” but do not measure the vibration level or document the conditions under which it occurred, the supplier may refuse to repair it under warranty.

Supplier Mistakes

1. Rushing SAT to Meet Delivery Deadlines: Suppliers often rush SAT to meet contractual delivery deadlines, leading to incomplete testing. For example, a supplier may only test the GA-MVD2560’s tool changer 10 times instead of the required 50 times, leading to tool misalignment issues after sign-off.
2. Not Calibrating the Machine for Local Conditions: Suppliers often calibrate machines in their factory, but do not adjust for local conditions (e.g., temperature, humidity) on-site. For example, a GA-UHD500 calibrated in a 20°C factory may have a positioning accuracy of 0.006 mm, but in a 25°C shop, the accuracy may drop to 0.010 mm due to thermal expansion.
3. Hiding Defects: Suppliers may hide minor defects to avoid rework costs. For example, a supplier may adjust the GA-MV856’s positioning accuracy by modifying the machine’s control parameters, leading to long-term wear issues.

Honest limitations

SAT is a powerful tool, but it has inherent limitations that buyers must be aware of.

What SAT Cannot Fix

1. Long-Term Wear and Tear: SAT tests the machine’s performance at the time of sign-off, but it cannot predict how the machine will perform over time. For example, a GA-DV750’s spindle may have a vibration level of 2.0 mm/s RMS during SAT, but after 6 months of heavy use, the vibration level may increase to 3.0 mm/s RMS due to bearing wear.
2. Unforeseen Production Requirements: SAT tests the machine against the buyer’s current production requirements, but it cannot account for future changes. For example, a GA-VT540 may meet all SAT requirements for drilling aluminum parts, but if you later switch to drilling titanium parts, the machine may not have the required spindle torque (11.8/17.5 Nm for the GA-VT540) to handle the material.
3. Software Bugs: SAT tests the machine’s hardware performance, but it may not detect all software bugs. For example, a GA-MH500 horizontal machining center’s control system may have a bug that causes it to crash when running complex 5-axis programs, which is not detected during SAT if only simple programs are tested.

When Outside Help Is Needed

1. High-Precision Machines: For machines with positioning accuracy requirements of ≤0.005 mm (e.g., GA-DV750, GA-UHD500), independent metrology services are often needed to verify test results.
2. Regulated Industries: For machines used in aerospace or medical device production, third-party inspectors are required to ensure compliance with industry standards (e.g., AS9100, ISO 13485).
3. Complex Machines: For multi-spindle, 5-axis, or gantry machines (e.g., GA-MVD2560, GA-FMB3020D), specialized service engineers with expertise in these machine types may be needed to conduct SAT.

Worked-Numbers Comparison Table

The following table compares SAT requirements and costs for three common Gree CNC machine models:

Practical Checklist Before Committing

Before starting SAT, ensure you have completed the following:

1. Pre-Installation Preparation: Confirm the machine’s foundation meets the supplier’s requirements, power and compressed air are available, and the shop floor is clean and clear of obstacles.
2. Test Plan Alignment: Review and sign off on the SAT test plan, which includes all acceptance criteria, test methods, and pass/fail thresholds.
3. Personnel Availability: Ensure your CNC programmer, maintenance technician, and quality engineer are available during SAT.
4. Test Parts and Tooling: Prepare representative production parts, programs, and tooling for the production part test.
5. Measurement Tools: Ensure you have the required measurement tools (e.g., coordinate measuring machine, laser interferometer) to verify test results.
6. Defect Resolution Process: Agree on a process for documenting and resolving defects, including timelines for repairs and re-testing.

FAQ

1. What happens if a test fails during SAT? If a test fails, the supplier must document the defect, develop a resolution plan, and re-test the machine after the defect is fixed. The timeline for resolution depends on the defect’s complexity: minor defects (e.g., a misaligned tool changer) can be fixed in a few hours, while major defects (e.g., a faulty spindle) may take several weeks to resolve.

2. Can I modify the acceptance criteria after SAT starts? Modifying acceptance criteria after SAT starts is possible, but it requires mutual agreement from both parties. Changes may result in additional costs or delays, as the supplier may need to adjust the machine or re-run tests.

3. What is the difference between Factory Acceptance Testing (FAT) and SAT? FAT is conducted at the supplier’s factory before the machine is shipped, while SAT is conducted on-site after installation. FAT verifies the machine’s performance in the supplier’s controlled environment, while SAT verifies the machine’s performance in your shop’s specific conditions.

4. Do I need to be present during SAT? Yes, your presence is critical. You are responsible for verifying that the machine meets your production requirements and that all tests are conducted correctly. If you cannot be present, you must appoint a qualified representative to act on your behalf.

5. What happens if I refuse to sign off on SAT? If you refuse to sign off on SAT, the supplier may be required to fix the defects or take back the machine, depending on the contract terms. Refusing to sign off may result in payment delays, warranty disputes, and production downtime. It is important to document all reasons for refusing sign-off to avoid legal disputes.

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