A custom magnet can pass a dimensional check and still create an assembly problem if the buyer and supplier measured different surfaces. Coating can add material to a finished dimension, while a drawing may define a substrate size, a coated size or a functional fit. At Nibboh, we recommend agreeing the measurement convention and coating allowance before approving samples or production.
Define what the dimension includes
For every critical feature, mark whether the tolerance applies to the magnetic substrate, the finished coated part or a functional assembly. Call out length, width, thickness, diameter, hole size, counterbore, chamfer and flatness separately where they affect fit. A note such as “NiCuNi coating” is not enough to define the dimensional requirement. The drawing should state the datum surfaces and the inspection direction.
Make the coating allowance explicit
Ask for the coating system and the agreed thickness range or acceptance rule for the order. Then state how that thickness interacts with the drawing: is the substrate machined undersize so the finished part fits, or is the nominal dimension measured after coating? Do not turn a generic coating name into an assumed micrometre value. Nibboh’s official sintered NdFeB page lists zinc, nickel-copper-nickel, chromium and epoxy as examples of protective coatings, but it does not establish a universal thickness or tolerance for every custom part.
Agree the measurement method
Specify the instrument, resolution, contact or non-contact method, datum setup, number and location of readings, temperature and rounding rule. For a suitable non-magnetic coating on a magnetic substrate, a magnetic coating-thickness method may be relevant; the official ISO 2178 page identifies ISO 2178:1982 as withdrawn and points to ISO 2178:2016. ISO’s 1982 abstract limits the method to reasonably flat specimens, so do not treat ISO 2178 as universally valid for every permanent magnet, coating system, geometry or magnetization state. A competent laboratory should assess suitability, calibration and the actual reading plan; otherwise request a validated, agreed method before accepting results.
Do not confuse thickness with dimensional size
A coating-thickness reading answers how thick the coating is at a test point. It does not automatically prove the overall length, diameter, hole or assembly fit. Use dimensional gauges or coordinate measurement for the dimensions they are intended to evaluate, and document the datum scheme. NIST explains that every measurement is an estimate and that an uncertainty statement communicates the bounds around the result. For a tight fit, ask the supplier to state the method and uncertainty or capability information that supports the tolerance decision.
Build the agreement into sample approval
Before approving a sample, place the inspection table beside the drawing. For each characteristic, record nominal, tolerance, included coating state, datum, method, instrument and acceptance rule. Ask for readings at agreed locations rather than only “OK” marks when the feature is safety- or fit-critical. If the drawing changes, re-issue the inspection plan; do not carry old coating or measurement assumptions forward by habit.
RFQ questions to send
- Are dimensions specified before coating or on the finished part?
- Which surfaces and datums define each critical measurement?
- What coating system and thickness acceptance range apply?
- Which instrument and method will measure dimensions and coating?
- How many points are measured, at what temperature, and how are results rounded?
- What record accompanies the lot, and how is a revision change identified?
We invite buyers to send the drawing, quantity, operating temperature and coating requirement for review. If your project needs a particular coating thickness or tight fit, ask us to confirm the proposed inspection convention in writing before production; the public product page is not a substitute for an order-specific report.