A coating is not the final dimension
In almost every project, the coating is applied oversize relative to the target dimension, with a "grinding allowance". This excess accommodates the surface roughness and minor irregularities produced during coating; grinding is the essential final step that brings the coated surface to the target dimension both geometrically and functionally. Without grinding, a coating is "applied" but not "ready for service".
Why is "just coating" not enough?
- Surface roughness — On seal, bearing or gasket contact surfaces, roughness directly affects sealing and wear performance. An as-sprayed coating surface is generally too rough for functional contact.
- Geometric tolerances — Tolerances such as cylindricity, diameter and run-out cannot be reliably achieved without grinding, owing to the natural variability of the spraying/welding process.
- Dimensional consistency — Part-to-part dimensional consistency within the same batch can only be guaranteed by controlled grinding.
How is the grinding allowance determined?
The grinding allowance is determined by engineering calculation based on the typical thickness tolerance of the coating process, the target final dimension of the part and the required surface roughness. In typical applications it ranges from 100-500 microns — a narrower allowance is sufficient for thin, precise HVOF coatings, while a wider allowance is left for thick PTA overlays.
Carbide coatings are ground differently
Very hard coatings such as tungsten carbide (1100+ HV) behave differently from conventional steel surfaces during grinding — they cannot be processed with standard grinding wheels and parameters. They require diamond or CBN wheels, low feed rates and appropriate cooling strategies. Incorrect grinding parameters can cause micro-cracking or localised heat damage in the coating.
Final inspection
Final inspection after grinding should include dimensional measurement, surface roughness (Ra) measurement and, where required, verification of coating thickness. If this step is skipped, a defect discovered in the field will cost far more than one caught in production.
Frequently Asked Questions
Does every type of coating require grinding?
Almost every coated part with a functional contact surface (seal, bearing, mating surface) requires grinding. Decorative or non-contact protective coatings may not need grinding.
How much grinding allowance should be left — should you leave extra "to be on the safe side"?
No — an excessive allowance means wasted material and time, as well as a risk of internal stress from unnecessary coating thickness. The allowance should be optimised by engineering calculation.
Which is right for your part?
Send us photos and operating conditions, and we will evaluate the process selection together with you.
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