Machine Design #64: Surface Treatment — Select It by Function, Environment, and Tolerance Stack

1. Start with function and evidence

Before choosing a dimension or opening a supplier catalog, write what the surface treatment must do, where the load or environment comes from, how many cycles it must survive, and what counts as failure. “Use the old drawing” hides assumptions about process, access, maintenance, and safety. A testable requirement states the input condition, operating condition, expected result, acceptance limit, and measurement method. Internal standards are a starting point; the final decision must be checked against the real model, material, process, and installation.

2. Review four layers together

Function and load

Separate nominal, start-up, impact, misalignment, and fault cases. Trace the load through the complete assembly, not only the attractive CAD section. Include inertia, thermal movement, cleaning, vibration, and the number of cycles that actually matters. State the boundary and the reason for the safety margin.

Material and manufacturing

The same geometry behaves differently after cutting, forming, welding, machining, coating, or heat treatment. The drawing must describe what a workshop can make and measure. A tight tolerance cannot compensate for an unknown process, an unprotected datum, or an inspection method that cannot reach the feature.

Interfaces and assembly

Define datums, direction, clearance, contact, fastening, tool access, and mistake-proofing. If variants exist, use geometry or markings to prevent a wrong part or orientation. Record the force, sequence, and evidence expected from the operator.

Operation and maintenance

Check access, replacement time, re-location, cleaning, adjustment, and the measurement that returns the machine to baseline. A design that works only with one experienced fitter is not repeatable.

3. Core checks for this topic

Function: wear, corrosion, friction, appearance, or electrical contact: decide the function first — wear, corrosion, friction, appearance or electrical contact — because one coating rarely serves two of them well.
Environment, chemical exposure, temperature, and cleaning method: state the environment, the chemicals, the temperature and the cleaning method the part will meet in service, not just in the factory.
Coating thickness, dimensional growth, and masking boundary: give the coating thickness and the dimensional growth it causes, and mark the masking boundary on the drawing rather than describing it in words.
Adhesion, substrate preparation, edge coverage, and defects: specify substrate preparation and check adhesion and edge coverage, since coating failures usually start at an edge or a hole.
Post-treatment fit, thread condition, flatness, and measurement: check fits, thread condition and flatness after treatment, and state how they are measured once the coating is on.
Supplier process, sample coupon, inspection, and rework rule: agree the supplier process, the sample coupon, the inspection and the rework rule before the first batch, not after a rejection.

The checklist is useful only when every line has evidence. “Reviewed” is not “calculated”, and “calculated” is not “tested at the boundary”. Keep an evidence ID next to each requirement so an engineering change can be audited.

3b. A lookup table for the common coatings

The three surface treatments most often seen on machine parts differ in the point that matters most to whoever issues the drawing: how thick the layer is and whether it destroys the tolerance.

Criterion	Anodising	Plating (zinc, nickel, chrome)	Powder coating
Suitable base material	Aluminium and its alloys	Steel, copper, some metals	Most metals
Mechanism	An oxide layer grown from the base itself	A metal layer deposited on the base	A polymer powder applied then cured
Typical layer thickness	Thin (usually 3 to 10 micrometres; hard anodising is thicker)	Thin (zinc 3 to 20, nickel and chrome 5 to 20 micrometres)	Thick (40 to 120 micrometres)
Effect on tolerance	Very small and predictable	Small, needs an allowance	Large, needs a substantial allowance
Wear resistance	Good; hard anodising is very good	Medium to good	Poorer
Surface conductivity	Insulating, except special types	Conductive	Insulating
Cost	Medium	Medium to high	Low

For a steel part needing a hard surface without much distortion, nitriding is a different route from all three above: it creates the hard layer from the steel base itself, distortion after treatment is usually small, and it suits sliding contact positions — but the hard layer is thin, so contact pressure and impact load have to be checked before choosing it.

Effect on the tolerance chain — where it goes wrong most often

A coating does not simply sit on top of the surface. With anodising, the oxide film grows partly into the base and partly outward, so pin holes, bush bores, keyways and bearing seats all change size after treatment. With powder coating, the layer is over 100 micrometres and uneven — never apply it to a precision mating face.

The compact rule: a functional face with a tight tolerance either gets a thin, predictable coating or is masked during coating. There is no third option.

Situation	How to handle it on the drawing
Precision holes and tapped holes on an anodised aluminium part	Call out masking for the precision holes and the threads
A mating face on a powder-coated part	Mask the mating face before coating, state the masked area
A part needing electrical contact or earthing	Mask the contact area, or use a conductive treatment
A dimension that matters after coating	State whether it is measured before or after treatment

A workable drawing note reads like: Material A5052 / Surface treatment: black anodising / Mask precision holes and threads / Critical dimensions apply after treatment. The Japanese equivalent on a drawing is: 材質 A5052 / 表面処理黒アルマイト / 精度穴・ねじ部マスキング / 重要寸法は処理後寸法.

Without those notes, the coater will follow their own internal standard, and a part that was correct when machined may no longer fit after coating.

Masking costs money, so design to need less of it

Masking is manual work: the more faces have to be masked, the higher the cost and the longer the lead time. Grouping the functional faces on one side of the part is the cheapest way to cut surface treatment cost — and it is the designer's decision, not the coater's.

4. Tolerance stack and variation

Build the stack from the functional datum to the characteristic that must be protected. Separate nominal, worst-case, and statistical values only when the process is stable enough to justify it. Include flatness, squareness, coating or treatment thickness, clamping deformation, temperature, field assembly error, and wear.

If assembly succeeds only because a technician nudges a part, the design has no reliable capability. Assign each contributor a source and identify whether it is controlled by the supplier or verified at incoming inspection. For surface treatment, pay particular attention to wear, corrosion, adhesion, masking, thickness variation, and post-treatment fits.

5. Failure modes before release

Ask: “If this is wrong, what will the machine show, and what evidence will reveal the cause?”

Failure mode	Machine symptom	Verification direction
Boundary selected too optimistically	Passes a demo, fails at speed, temperature, load, or cleaning	Test min/max and the defined fault case
Tolerance not tied to function	Difficult assembly, play, bind, noise, or leak	Rebuild the stack from functional datums
Manufacturing step omitted	Distortion, burr, crack, drift, or poor finish	Review the process and measure after each critical step
Inspection cannot reach feature	Supplier report looks complete but function is unknown	Define a reachable method and a sample plan
Maintenance not designed	Long replacement or wrong restoration	Run a maintenance trial and restoration check
Documents out of sync	Correct name but wrong revision or setting	Baseline BOM, drawing, process, and work instruction

6. Drawing and record discipline

Specify only requirements that have functional meaning and can be inspected. For a special requirement state the scope, datum or measurement location, condition, and acceptance limit. Notes such as “accurate machining” or “assemble carefully” are not instructions. Keep the calculation or rationale, source revision, assumptions and limits, review comments, prototype or FAT result, and the point to recheck after change.

7. Design review on the real production route

Start at the input and follow the load or environmental attack to the output. Name the surface, edge, joint, thread, hole, treatment, or contact that carries the function. Mark where friction, clearance, temperature, chemicals, operator adjustment, or cleaning can change the result. If a parameter is unknown, assign an owner and measurement plan instead of hiding it in a large factor.

Ask the fabricator which operation creates the most variation, how the feature is inspected, and what happens after deburring, welding, heat treatment, coating, or washing. Compare capability with the stack. If the process cannot hold the drawing, change the process or design before ordering.

On the assembled machine, run start-up, normal duty, stop, restart, and a controlled fault. Record force or torque, temperature, vibration, noise, motion, visual marks, and replacement time as relevant. These observations often reveal an assumption that a drawing review missed.

8. Boundary conditions and calculations

Write lower and upper values for every influential parameter and include combinations that can occur together. Keep units consistent, show the equation or reference, and record catalog test conditions and correction factors. Compare at least one analysis result with a hand calculation or a measured prototype.

For surface treatment, a useful sheet has columns for input, nominal, lower bound, upper bound, source, result, pass/fail, and “what would invalidate this assumption?” This makes the decision robust when a supplier changes material, cycle time, temperature, chemical, or inspection route.

9. Supplier and incoming inspection

The supplier package should include drawing revision, material condition, special process, inspection points, sample record, and rework rule. Do not outsource design intent. If a supplier proposes a different process, compare function, durability, cost, lead time, and inspection capability before approval.

At incoming inspection, measure the features that protect the function rather than every dimension with equal effort. Record actual values and instrument ID. A part can be within a drawing tolerance and still fail because of the wrong surface, burr, fit, treatment, lubricant, or orientation. Link the result to the lot or serial used on the machine.

10. Commissioning and maintenance feedback

Create a baseline at commissioning: cycle time, force or torque, temperature, vibration, noise, position, surface condition, or appearance as relevant. Define the alarm limit and the action when it is reached. After the first service interval, compare the trend with baseline and update the maintenance instruction. Replacement should restore the baseline, not merely make the machine move again.

A coating changes the size and can make it brittle: do not just write "plate/anodize"

Surface treatment is not "transparent" to tolerances. A coating adds dimension, and some processes can make the part brittle. Writing only "zinc plate" or "anodize" is not enough.

Dimensional build-up. Plating adds thickness on top of the surface; anodizing grows both outward and into the base, so the final size shifts. For a fitted feature, decide whether to measure before or after coating, and allow for the layer thickness.
Masking. Threads, datum faces, and interference-fit bores usually must be masked so the build-up does not spoil the fit — state which areas are coated and which are masked.
Hydrogen embrittlement. Acid pickling and electroplating can charge hydrogen into high-strength steel and cause delayed cracking; those parts need a de-embrittlement bake after plating, per the standard.
Adhesion depends on pretreatment and roughness; a good-looking coating on a dirty or over-polished surface still peels.

Omission	Consequence	Correct note
Layer thickness not accounted for	Tight fit or bind after coating	State post-coating dimensions or allow for thickness; say before/after
Threads/datums not masked	Threads do not engage, datum shifts	State masked and coated areas
High-strength steel, electroplated	Delayed cracking from embrittlement	Require a de-embrittlement bake per the standard

11. MINATA release checklist

[ ] Function, load path, duty cycle, and fault or environment boundary are written.
[ ] Material, process, surface condition, and inspection method are agreed.
[ ] Functional datums and tolerance stack are visible.
[ ] The six topic checks above have evidence and pass/fail criteria.
[ ] Assembly, tool access, orientation, and mistake-proofing were tried on hardware.
[ ] Failure modes have an owner, evidence, and re-test condition.
[ ] Supplier and incoming records link to the revision and lot.
[ ] Commissioning baseline and maintenance response are defined.

Frequently asked questions

Does surface treatment disturb the tolerance?

It does, and by very different amounts depending on the type. Anodising is thin and predictable; plating is thin but needs an allowance; powder coating at 40 to 120 micrometres destroys a fit tolerance. On a functional face, either choose a thin coating with an allowance or mask the face.

Should dimensions apply before or after surface treatment?

It has to be stated on the drawing, because both are legitimate and the supplier cannot guess. On a part with important mating faces, the usual choice is that critical dimensions apply after treatment, together with masking instructions for the areas that must not be coated.

What is the difference between anodising and plating?

Anodising is an oxide layer grown from the aluminium base itself and is only for aluminium and its alloys. Plating deposits a metal layer onto the base and is used for steel and copper. Because the mechanisms differ, so do the effect on dimensions, the conductivity and the selection logic.

How is a part that needs earthing treated?

Anodising and paint both insulate, so the electrical contact area has to be masked during treatment or a conductive treatment used. This is a common mistake on electrical cabinets and base plates: the part looks good, and only after assembly does it turn out that it cannot be earthed.

Is writing "plated" enough?

Not really. Zinc plating is inexpensive corrosion protection, nickel plating gives corrosion resistance and an undercoat, hard chrome gives wear resistance for shafts and sliding faces — three different purposes. Writing only "plated" leaves a design decision open.

12. Closing note

Good mechanical design is a chain of explicit assumptions that survives manufacturing, assembly, operation, maintenance, and change. For surface treatment, the right question is not only “will it work?” but “what evidence will show it continues to work when the boundary moves?” That is the standard MINATA uses to turn a drawing into a dependable machine.

Frequently asked questions, continued

Why does a plated part fit tighter than designed?

Because the coating adds thickness to the surface (and anodizing grows on both sides). A fitted feature must account for the layer: allow for it, or state clearly whether the tolerance applies before or after coating, and mask the areas that must keep their size.

When is a de-embrittlement bake needed after plating?

For high-strength steel that has been acid-pickled or electroplated, hydrogen can enter and cause delayed cracking. Those parts need a de-embrittlement bake after plating per the applicable standard; put the requirement on the drawing rather than leaving it to the plater.

Conclusion

Surface Treatment is not a matter of picking a single number. Good design ties function, load, material, process, tolerance, assembly and maintenance into one contract that can be checked. An internal standard preserves the experience already accumulated; the designer's job is to understand the conditions under which it applies and turn it into a clear decision on the product at hand.

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