Have you ever encountered a situation where you received a test report for a critical shaft or gear with perfect dimensions. Subsequently, the parts are sent for hard chrome plating or nitriding treatment. A few weeks later, it was discovered during the assembly of the parts that they could not be installed. The bearing cannot be pushed in, and the gear is stuck due to meshing.
As a professional Precision Parts for SPM machine manufacturer, we are well aware that in order to achieve tolerances of ± 0.005mm or even stricter, CNC Machining Post Processing cannot be regarded as an additional process after the fact, but must be treated as a calculation variable in the engineering equation.
The Four Major Factors Causing Tolerance Failure
Why do parts fail after surface treatment? The reason is usually not just 'too thick coating'. To control the results, we must first understand the four variables that affect tolerances.
Edge accumulation
In electroplating processes such as galvanizing or hard chrome plating, the distribution of current density is not uniform. The current will naturally concentrate at sharp corners and edges.
For example, if you request a coating of 10 µ m, you may get 10 µ m at the center of the plane, but as high as 15-20 µ m at the corners.
Pre treatment acid corrosion (invisible size shrinkage)
Before electroplating, the parts must undergo acid washing to remove the oxide scale. Because strong acid cleaning can corrode the base metal. If the shaft is acid etched for too long, its diameter may decrease by 2-3 µ m before electroplating. Without plating thickness control that includes substrate loss calculation, the final size calculation will be incorrect.
Thermal deformation (stress release)
In processes such as carburizing or nitriding, steel is exposed to high temperatures. But even if the machining dimensions are perfect, the residual stress from CNC machining will still be released during the heating process, causing the parts to warp or grow. If not subjected to sufficient stress relief annealing, cutting perfect gears on our Kashifuji Gear Skiving Machine may result in elliptical shapes after heat treatment.
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Hydrogen Embrittlement
The infiltration of hydrogen atoms during the electroplating process can cause microcracks in high-strength steel. This may lead to unpredictable structural fracture of parts under high loads, even if the dimensions are qualified.
How to guarantee tolerances?
Hansheng introduces our four-step process based on our precision machining and surface treatment workflows. All calculation data in the following steps are simulated assumptions; actual data or calculations require confirmation with your supplier.
Design Collaboration and "Undersize" Reverse Calculation
If your drawing requires a final shaft of Ø20.000mm ±0.005mm and requires a 10µm Electroless Nickel plating, do not machine it to Ø20.000mm. For example:
Target finished size: 20.000mm
Plating thickness (diameter direction x2): +0.020mm
CNC machining target size: 19.980mm
We program the CNC machine to this "undersize" (pre-machined) target.
Precision Machining
Using our Seibu Wire Electric Discharge Machines (WEDM) and precision grinding machines, we achieve a surface finish (Ra) that reduces the peaks and valleys that can cause plating nodules.
For gears, our Kashifuji and Ningjiang equipment ensures that the tooth profile is perfect before hardening, minimizing post-heat treatment correction work.
Controlled Surface Treatment Process
Custom Racking: We design specialized fixtures to shield high current density areas and prevent edge buildup.
Conforming Anodes: For complex shapes, we use conforming anodes to ensure that deep holes and grooves receive the same plating thickness as the outer surface.
Post-Plating Finishing
For the most extreme precision requirements (±0.002mm), relying solely on plating control is risky. In this case, we will make the plating slightly thicker than required. Then, the part is brought back to our workshop for Final Precision Grinding or honing.
summary
As a confidential mechanical parts supplier, Hansheng specializes in handling components for Special Purpose Machines (SPM), especially high-precision components. Whether it is machining or surface treatment, these are our areas of expertise.
Final Dimensions: For example: Ø 50.000mm ± 0.005mm
Size status: Does the above size refer to before or after electroplating?
Plating Type&Spec: For example: Electroless Nickel, High Phosphorus, ASTM B733
Thickness Range: For example: 8-12 microns
Key mating surfaces: Please indicate on the drawing which surfaces must ensure mating (do other areas need to be covered?)
Post plating processing: Is post plating grinding necessary to achieve the final tolerance? Yes/No
FAQ
What should I do if the internal threads of my part are too precise and the screws won't screw in after surface treatment?
We have two solutions:
Oversize Tapping: During the CNC machining phase, we use enlarged taps (e.g., H7 or H11 tolerance classes) to reserve space for plating.
Masking: If the thread accuracy requirements are extremely high, we will use specialized plugs to cover the screw holes during the electroplating process, ensuring the threads remain in their original metallic state.
How do you verify that the coating thickness meets the requirements?
We provide X-Ray Fluorescence (XRF) spectroscopy test reports to measure coating thickness, or use magnetic thickness gauges.
How do you prevent hydrogen embrittlement for high-hardness steel (HRC > 40)?
We strictly adhere to the ASTM B850 standard. After electroplating, we immediately place the parts in an oven for Hydrogen De-embrittlement (hydrogen baking), typically within 4 hours. This step is crucial for ensuring the safety of high-stress components.