Streamline OEM Production with Value-Added CNC Machining Services

TL;DR

In-house value-added CNC machining services such as assembly and pressure testing let OEMs buy ready-to-install, fully tested components from a single partner. That means fewer suppliers to manage, cleaner process flow, shorter lead times, tighter quality and traceability, and smoother launches. If your current CNC machining partner only ships bare parts, it may be time to reassess whether they’re really supporting your program goals.

If you manage purchasing or supply chain for an OEM, you already know the story: one supplier for castings, one for machining, another for assembly, and then there’s the issue of pressure testing. Sometimes it’s done by the customers. Sometimes by the CNC shop. Sometimes not at all.

Every extra handoff adds freight cost, administrative time, and risk. Also, when a part shows up late or fails in the field, it’s harder to pinpoint what went wrong and who’s responsible.

That’s why value-added services in CNC machining are sought after. When the same CNC partner that machines your part can also assemble and pressure test it, you gain efficiencies that streamline program management and protect timelines.

As two of the most impactful CNC value-added services, assembly and pressure testing merit a closer look. We’ll cover how they work, why in-house capabilities matter, and what to look for in a CNC machining partner.

What Are Value-Added CNC Machining Services?

Value-added CNC machining services are steps beyond core machining that move your part closer to a ready-to-install component. Instead of shipping a bare machined casting, the shop delivers an assembly that’s been pressure tested, washed, and marked for full traceability.

Typical CNC value-added services include:

• Assembly and sub-assembly
• Pressure testing
• Data matrix and traceability
• Broaching
• Balancing
• Washing and cleanliness control

CNC shops often provide many of these services in-house instead of using outside subcontractors. This distinction is important. Machining, assembly, and pressure testing all happening under one roof results in:

• Fewer shipments and vendors to manage
• Shorter lead times (no delays caused by other shops’ schedules)
• Tighter quality control and clearer accountability

What Assembly Looks Like in a CNC Machine Shop

Assembly is one of the most advantageous value-added CNC machining services because it transforms a set of parts into something ready for immediate use on a production line.

4 characteristics of a CNC shop’s approach to assembly

Experienced CNC machining partners are prepared to support simple sub-assemblies and complex, multi-level BOM assemblies. They generally have a well-defined approach that includes:

1. Clearly defined assembly cells near machining lines
2. Custom fixtures and automation designed in-house
3. Work instructions and quality checks aligned with customer requirements
4. Operators able to assemble, deburr, inspect, and package in a tight loop

Integrating assembly into the overcall CNC operation results in better process flow. Parts don’t stack up waiting for transport to the next vendor. They move directly into assembly and testing.

How does in-house assembly benefit purchasing and supply chain managers?

For purchasing and supply chain teams, in-house assembly provides:

• Streamlined management of vendors and ERP system data
• One finished assembly for receiving and inspection instead of disparate parts
• Clear accountability for fit, function, torque values, and component selection
• Less risk of mismatched revisions between machined parts and purchased components

5 questions that reveal a CNC shop’s assembly capabilities

When evaluating a CNC partner’s design capabilities, ask these 5 critical questions:

1. What range of assembly complexity is supported (from basic to multi-level BOMs)?
2. Are assembly fixtures or stations designed and built in-house when needed?
3. How is poka-yoke or automation incorporated to prevent assembly errors?
4. How are assembly steps documented, audited, and shared with customers?
5. Can both prototype/low-volume and full production builds be executed?

The answers quickly reveal if the CNC shop truly treats assembly as a core value-added service, a side task, or an afterthought.

Pressure Testing: Verifying Performance Before Parts Ship

Pressure testing is non-negotiable if parts carry oil, fuel, coolants, or compressed air. It’s a critical part of quality assurance, especially for complex CNC machined castings.

Why is pressure testing important?

Pressure testing helps catch potential part defects, namely:

• Porosity in castings
• Leak paths through walls, ports, or interfaces
• Assembly issues such as missing seals or incorrect plugs

Without reliable pressure testing, parts may pass dimensional inspection but fail in the field, leading to:

• System performance issues
• Internal corrosion or surface defects over time
• Warranty claims, rework, and potential litigation

Common Methods of Pressure Testing

Most complex CNC parts are pressure tested using one or more of these methods:

1. Air Decay

   • The part is filled with pressurized air
   • A control unit monitors pressure over time and measures any drop
   • If the loss stays within acceptable limits, the part passes

2. Mass Flow

   • Air is introduced and a gauge measures any flow needed to maintain pressure
   • Even very small leak rates can be quantified and compared against spec

3. Air Under Water

   • The part is pressurized, sealed, and submerged
   • Inspectors watch for bubbles to locate leak locations
   • It’s simple and effective, especially for large parts or specific leak paths

A CNC shop proficient in all three pressure testing methods can readily recommend the right approach based on a part’s design and requirements.

Learn more about the 3 Ways to Pressure Test Complex CNC Parts

When should pressure testing occur?

Pressure testing can occur

• After machining
• After assembly
• After vacuum impregnation

Depending on risk tolerance and requirements, testing may range from sample-based to all parts. 

Pressure testing after assembly has a key advantage. It verifies not just the casting, but the entire assembled unit. Poka-yoke and automation built into the test stand help ensure the process was followed correctly.

Why In-House Pressure Testing Is a True Value-Add

Subcontracting pressure testing can work, but it introduces familiar challenges OEMs want to avoid:

• Extra logistics and lead time
• More risk of handling damage
• Less visibility into test methods and data

When a CNC shop performs pressure testing in-house:

• Quality control stays under one roof
• Feedback loops are faster when a part fails a test
• Test stations can be located near machining or assembly cells, integrated into the machining cycle for maximum efficiency, or even custom-built for large or unique parts

5 questions to ask about a CNC shop’s pressure testing capabilities

When assessing current or potential CNC machining partners, these 5 questions help determine whether the shop can keep pace with a program throughout its lifecycle:

• Which pressure testing methods are supported (air decay, mass flow, air-under-water)?
• When can parts be tested: after machining, assembly, and/or impregnation?
• What test pressure ranges and leak-rate limits are accommodated?
• How are results recorded and tied to individual parts via data matrix or serial numbers?
• How quickly can adjustments be made to test parameters or capacity as volumes ramp up?

Long-term Cost and Efficiency Gains of Value-Added Services

When CNC machining and value-added services such as assembly and pressure testing occur in the same facility, the benefits compound:

• Cleaner PPAP activities and launches since there’s one source of truth for processes, data, and documentation
• Faster problem solving because cross-functional root cause analysis, process changes, and corrective actions are rolled out in a single loop
• Optimized engineering and value analysis applies across the entire process, not just spindle time

Make CNC Value-Added Services Work for You

Value-added CNC machining services shouldn’t be considered “extras.” Assembly and pressure testing are core tools for:

• Reducing vendor count and complexity
• Shortening lead times and cutting total landed cost
• Improving quality, traceability, and launch success

If your current machining partner can only ship bare parts, it may be time to reevaluate whether they’re helping or hindering your program goals.

Consider using our Value-Added Services Checklist as a structured comparison tool to evaluate suppliers objectively. The right CNC partner will welcome that level of scrutiny and be ready to show how their CNC value-added services can make your job easier. 

FAQs

What are value-added CNC machining services?
Value-added CNC machining services are processes beyond core machining, such as assembly, pressure testing, washing, balancing, and part marking. Done in-house, these services move a component closer to a ready-to-install state.

Why should pressure testing be done in-house at a CNC machine shop?
Having pressure testing done in-house at a CNC machine shop ensures parts can be confidently assembled without needing additional testing. By pressure testing where the parts are actually manufactured, issues such as leaks can be identified and resolved immediately, creating a faster feedback loop for improving part quality. This helps maintain higher yields and prevents quality problems that could otherwise slow down production and drive up costs.

How do assembly services from a CNC shop help OEM purchasing teams?
Assembly services reduce the number of suppliers, POs, and shipments, while consolidating accountability for fit, function, and torque specs. OEMs receive a ready-to-install assembly instead of multiple loose parts.

Which pressure testing methods are commonly used for complex CNC parts?
The most common methods are air decay, mass flow, and air-under-water testing. Each is suited to different part designs and leak-rate requirements. Experienced CNC machining partnerships are proficient at and can apply all three approaches.

About the Author

Started at SMC in late 2006 with no manufacturing experience outside of high-school shop classes. My first role was pressing valve seats and guides into small-engine cylinder heads in a value-added cell, machining features, installing components, and finishing critical dimensions. After some time in assembly, I transitioned into machining, and SMC supported my journeyman machinist apprenticeship. I worked as a machinist for several years before moving into engineering in 2023.