Coating Allowance for Precision CNC Parts
How to control final dimensions after plating.
Why coating allowance must be planned before CNC machining
A part may be fully within tolerance before coating and still fail assembly after coating. This is especially common when the component includes a precision shaft, bore, sleeve, bushing, bearing seat, sealing surface, thread or press fit. A change of only a few micrometers can alter clearance, interference, alignment or motion.
The drawing should therefore make one point clear: does the specified dimension apply before coating or after coating? For functional features, JF Precision normally reviews the required final coated dimension first and then calculates the machining target before the part enters production.
Outside diameters normally become larger after coating
Internal bores normally become smaller after coating
Grooves and keyways may become narrower if their sidewalls are coated
External and internal threads may become tighter
Coated datum surfaces can change how the finished part is measured
Masking may be needed where coating would interfere with assembly, conductivity, grounding or sealing
The basic geometry of coating build-up
The following relationships describe the ideal geometric effect of a uniform coating, where t is the coating thickness deposited on one surface.
| Feature | Ideal Relationship | Why |
|---|---|---|
| Outside diameter | Final OD = Pre-coating OD + 2t | Coating builds on both sides of the diameter. |
| Internal bore | Final ID = Pre-coating ID − 2t | Coating grows inward from both sides of the bore. |
| One coated flat face | Final position change ≈ t | The coated surface moves outward by approximately one layer thickness. |
| Two coated flat faces | Overall thickness increase ≈ 2t | Both opposite faces contribute to the total thickness increase. |
These formulas describe ideal geometry. They are not a guarantee that every location on a real part will receive one mathematically exact thickness. Actual results depend on the coating specification, permitted thickness range, part geometry, edge condition, masking, surface preparation, coating access and measurement location.
Worked examples: outside diameter and internal bore
The two examples below are illustrative geometry calculations only, not guaranteed production specifications. Assume, only for this calculation, a target coating thickness of 0.005 mm on each side.
| Calculation Item | Outside Diameter Example | Internal Bore Example |
|---|---|---|
| Required final coated dimension | 20.000 mm | 30.000 mm |
| Assumed coating thickness per side | 0.005 mm | 0.005 mm |
| Total theoretical build-up / reduction | 0.010 mm increase | 0.010 mm reduction |
| Theoretical pre-coating dimension | 19.990 mm | 30.010 mm |
For the outside diameter, the theoretical center value alone is not enough to set a production machining target — the engineer must also consider the final diameter tolerance, the permitted coating-thickness range, the pre-coating machining tolerance and the measurement method used after coating. For a tight shaft fit, JF Precision would confirm whether the drawing requires the full shaft, only a bearing land or only selected wear surfaces to be coated.
Internal features require additional caution because coating access and inspection can be more difficult than on an open outside diameter. Deep holes, blind holes, small bores and internal transitions should be reviewed with the coating specialist before the machining target is released, and the inspection method — bore gauge, plug gauge, air gauge or CMM — should be defined early.
Why XADC® and Bi-Protec® need different allowance strategies
The coating system changes the size of the allowance that must be considered. Published supplier technical sheets list different deposition structures for XADC® diamond thin dense chrome and Bi-Protec® nickel-chrome composite coating.
| Coating System | Published Thickness Information | Allowance Implication |
|---|---|---|
| XADC® diamond thin dense chrome | Typical deposit: 0.0001–0.0003 in (2.54–7.62 µm); published effective range: 0.00005–0.0007 in (1.27–17.78 µm) | A comparatively thin precision coating. Allowance control is often focused on wear surfaces, shafts, guide faces and dimensions sensitive to small changes. |
| Bi-Protec® nickel-chrome composite | Electroless nickel: 0.0002–0.003 in (5.08–76.2 µm), plus thin dense chrome: 0.0001–0.0005 in (2.54–12.7 µm) | Both layers must be included in the dimensional chain. The wider nickel range makes thickness planning, masking and final inspection especially important. Compare Ni + NTDC vs Ni + XADC® for the two working-layer options. |
The thickness values above are published ranges from the coating supplier material. They are not universal process-capability guarantees for every feature or component. The actual target, permitted variation and inspection location must be confirmed for each order.
For a Bi-Protec® system, the engineer must consider the electroless nickel and the selected thin dense chrome top layer together. In some repair or restoration applications, a thicker nickel deposit may help recover a limited dimensional shortage, but this must be specified and controlled as an engineered process rather than assumed after machining.
How coating affects common CNC features
Each feature type responds to coating differently, and the planning method should match the geometry rather than apply one generic allowance everywhere.
| Feature | Typical Dimensional Effect | Planning Method |
|---|---|---|
| Outside diameter | Diameter increases | Machine below the required final diameter and confirm whether the full circumference is coated. |
| Internal bore | Bore diameter decreases | Machine above the required final bore and verify coating access and bore-inspection method. |
| Flat surface | Surface position moves outward | Confirm whether one or both faces are coated and review stack-up. |
| External thread | Major and pitch diameters may increase | Use masking, calculated allowance or project-specific thread verification. |
| Internal thread | Thread fit may become tighter | Masking is often considered; confirm with gauges after coating if required. |
| Groove or keyway | Usable width may decrease | Define whether the floor and sidewalls are coated. |
| Bearing seat | Clearance decreases or interference increases | Calculate the final fit, not only the final nominal diameter. |
| Press-fit surface | Interference can change significantly | Specify the required fit after coating and inspect the final surface. |
| Sealing face | Dimension and surface condition may change | Confirm coating compatibility, flatness and final finish requirement. |
| Datum surface | Measurement reference may move | State whether the datum is coated or masked before defining inspection. |
Threads, masking and selective coating
Threads and mating surfaces should not be treated as ordinary open surfaces. Coating an external thread can increase its effective size, while coating an internal thread can reduce usable clearance. Fine-pitch threads, gauge-controlled threads and threaded sealing features require a project-specific decision. Masking may also be required on:
Bearing seats and press-fit locations that must retain a precise uncoated fit
Electrical contact or grounding surfaces
Datum faces used for final inspection or assembly location
Threaded areas that must remain within an existing gauge class
Sealing surfaces where the selected coating is not part of the intended interface
Areas that must be welded, bonded or assembled after surface treatment
The drawing should show the coated and masked boundaries clearly. A note such as "coat where required" is usually not enough for a precision component. Better instructions identify the coating system, target thickness or range, final-after-coating dimensions and surfaces that must remain uncoated.
Coating thickness is a range, not one perfect number
A nominal coating thickness should not be treated as one mathematically exact value on every point of every surface. Precision planning must consider the complete dimensional budget.
| Dimensional Input | Why It Matters |
|---|---|
| Final dimension | The required dimension of the finished coated part. |
| Final tolerance | The acceptable dimensional range after coating. |
| Nominal coating thickness | The intended single-side or local deposit target. |
| Coating thickness range | The process variation permitted by the selected specification. |
| Pre-coating machining tolerance | The variation introduced during CNC machining before plating. |
| Masking and edge condition | Local geometry that can affect where and how the coating builds. |
| Measurement uncertainty | Variation associated with the gauge, CMM, bore gauge, measurement force and inspection location. |
The final tolerance must be able to accommodate both machining variation and coating variation unless a specially controlled coating and inspection process has been agreed. If the remaining tolerance budget is too small, the design, masking method, coating thickness or final finishing route may need to be reconsidered.
How to specify coated dimensions on a drawing
Clear drawing notes reduce quotation delays and prevent different interpretations between machining, coating and inspection teams. Useful examples include:
Ø20.000 ±0.005 AFTER COATING
FINAL BORE SIZE AFTER PLATING
COAT SURFACE A ONLY
MASK M10 × 1.0 THREAD
DO NOT COAT DATUM B
TARGET COATING THICKNESS: [CONFIRMED PROJECT VALUE]
INSPECT FINAL OD AND BORE AFTER COATING
The drawing, purchase order and coating specification should agree. If the coating type or target thickness changes, the dimensional chain should be reviewed again before production continues.
JF Precision's controlled coating-allowance workflow
At JF Precision, coating is reviewed as part of the manufacturing plan for custom CNC machined parts rather than as an unrelated finishing step. Depending on the project, our workflow can include:
Review the final 3D model and 2D drawing, including fits, datums, threads and functional dimensions
Confirm whether each critical dimension applies before or after coating
Identify coated areas, masked areas and surfaces that require special inspection
Confirm the coating system, target thickness and project-specific process requirements
Calculate theoretical pre-coating dimensions and allocate machining and coating tolerances
Complete CNC milling or turning and inspect critical pre-coating dimensions
Coordinate the specified coating through the applicable specialist process route
Inspect the finished OD, ID, threads, fits and surface condition, following our dimensional inspection process
Provide inspection records, coating documentation or material records when specified and available
This integrated approach is particularly useful for custom shafts, sleeves, bushings, pump and valve components, tooling, automation parts and other precision custom CNC turned parts where the coated surface is also a functional dimension.
FAQ: Coating allowance for CNC machined parts
Questions about ordering, lead times or shipping? Our FAQ page answers the most common questions.
Yes, a deposited coating adds material to the surfaces it covers. The functional effect depends on the coating thickness, feature geometry and whether the surface is internal, external or selectively masked.
A diameter includes two opposite surfaces. Under ideal uniform conditions, a coating thickness t on each side changes the full diameter by approximately 2t.
Only when the specified dimension is intended to apply before coating or when the coated feature is not dimensionally critical. If the drawing requires the final size after coating, the machining allowance should be planned first.
It depends on the thread class, coating system and function. Masking, calculated allowance or post-coating gauge verification may be required.
A controlled nickel deposit may support limited dimensional restoration in some applications, but the target thickness, adhesion, geometry and final inspection must be engineered and agreed in advance.
Send a 3D CAD file, a 2D drawing, material, quantity, final-after-coating dimensions, coating specification, coated and masked areas, surface requirements and inspection documentation needs.
Yes. Depending on the feature and project requirement, inspection may include micrometers, bore gauges, plug or thread gauges, CMM measurement and a dimensional report.
Continue reading
Ni + NTDC vs Ni + XADC®
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Nickel-Chrome Composite Coating
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XADC® diamond thin dense chrome for shafts, bushings and tooling exposed to severe wear.
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