In several projects for Israeli customers, aluminum CNC machining is outsourced to China, while precision grinding and tightly controlled finishing remain local. The local process may involve qualification, inspection, or proprietary requirements that are not fully disclosed to the machine shop. In this situation, installing thread inserts after surface treatment is often the most controlled approach.
We machine the receiving holes and STI threads, then ship the matching wire thread inserts separately with the CNC-machined parts. After finishing, the customer can use the supplied inserts or purchase the same part number or an approved equivalent locally.
When Delayed Installation Makes Sense
The decision to install thread inserts after surface treatment is not a universal rule. If the finishing process is fully defined and the finishing supplier confirms that installed stainless steel inserts can remain in place or be reliably masked, installing them earlier may be acceptable.
When the process is unknown, however, the machine shop cannot determine whether chemicals, cleaning media, temperature, or coating materials will affect the insert. Installed hardware may also complicate masking, trap liquid or debris between the insert and the parent thread, or be exposed to grinding, workholding, and aggressive cleaning.
Questions about thread inserts after anodizing are common, but the same sequencing review also applies to chemical treatments, specialty coatings, and precision grinding. The approved process specification and the finishing supplier should determine the final sequence.
Surface Treatment Can Change an STI Tapped Hole
A coating that enters the receiving thread can reduce available clearance and change the effective thread geometry. This may increase installation torque, prevent the insert from reaching the specified depth, or cause the coil to deform or skip a thread.
The drawing or process instructions should therefore state whether the STI hole must be masked. If coating is allowed inside the hole, the receiving thread should be verified after finishing. Insert manufacturers also recommend repeating STI thread inspection after plating or coating because the added material can affect installation and the finished internal thread.
Inspection performed by the CNC supplier confirms only the pre-finish condition. It cannot guarantee fit after an undisclosed coating has entered the hole.
Running a tap through the hole after finishing should not be the default solution. Chase tapping can remove the protective layer, expose the aluminum, or change the original thread limits. It should be used only when the drawing and finishing specification allow it.
An STI Thread Is Not a Standard M6 Thread
Consider an M6 × 1.0 wire thread insert. After installation, the screw engages a standard M6 × 1.0 internal thread. Before installation, however, the aluminum part contains an M6 × 1.0 STI receiving thread rather than a standard M6 tapped hole.
The STI thread normally has the same pitch as the final screw thread but a larger diameter to accommodate the insert. It must be produced with the correct STI tap for the selected insert system. A standard M6 tap cannot create the required receiving thread, and an M6 screw cannot be used to inspect the empty STI hole.
Preparing the Part for Later Installation
The insert specification should be fixed before machining. This includes the applicable standard or full part number, nominal thread size, pitch, length, material, free-running or locking design, and tanged or tangless configuration. A drawing note that says only “M6 insert” does not fully define the insert, tap, or installation tool.
The drill size, entry chamfer, and full tapping depth should follow the insert manufacturer’s data. We cut the receiving thread with the matching STI tap and use pin gages to check the minor diameter. Because a tap is a cutting tool rather than an inspection gage, it does not replace a dedicated STI GO/NO-GO gage. If the customer requires documented thread-class verification, the gaging method or qualified third-party inspection should be agreed upon before production.
Before shipment, the holes should be deburred, cleaned, and protected from damage and contamination. After local finishing, the customer removes masking and residue, performs any required post-finish inspection, and installs the inserts to the specified depth.
Define the Manufacturing Handoff
The drawing, purchase order, or project instructions should identify:
- The final screw size and pitch
- The insert standard or complete part number
- Insert length, type, and installed depth
- STI thread class and inspection requirements
- Whether the STI holes must be masked
- Responsibility for post-finish inspection
- The party responsible for insert supply, thread insert installation, and final acceptance
Defining these requirements prevents the machine shop, finishing supplier, and assembly team from applying different acceptance criteria to the same hole.
Final Takeaway
When aluminum CNC machining is completed in China and specialized finishing remains with the customer, installing thread inserts after surface treatment can be a practical way to control process risk. The key is to select the insert in advance, machine the correct STI thread, and clearly assign responsibility for masking, post-finish inspection, and final installation.
This approach gives the customer control over the local finishing process while providing CNC-machined parts with receiving threads already matched to the specified inserts and ready for final assembly.