Quality Control

Critical Tolerances for Custom Metal Products: What to Specify

Why tolerances are the hidden budget line

Most rework on custom metal goods is not caused by the artwork. It comes from missing tolerance rules: the pin looks close enough in a render, then the post is 0.7 mm off, the coin rim is too thin after polishing, or a magnet pocket is shallow enough to wobble. For procurement, the problem is that looks good is not measurable, so the factory fills in the gaps with its own defaults.

A practical spec separates critical dimensions from cosmetic ones. Critical dimensions are the ones that affect fit, symmetry, or function: overall size, hole centers, post position, thickness, and clasp clearance. Cosmetic dimensions are line width, micro texture, and tiny edge rounding. At ZheCraft, the cleanest orders are the ones where the buyer tells us which numbers must be held tight and which can float by a few tenths of a millimetre.

Set size tolerance by product type

The right tolerance depends on the product body and the metal system. Brass H65 or H68 usually holds finer edges than soft iron SPCC, while Zamak 3 zinc alloy is better for deeper 3D relief but is less forgiving on microscopic text. If you only care about the front face, do not waste money tightening hidden backs or internal cavities.

Product	Critical dimensions	Practical tolerance	Typical MOQ / lead time / FOB
Soft enamel pin	Overall size, post position, line width	+/-0.3 mm overall; post +/-0.25 mm	100-300 pcs; 10-15 days; USD 0.38-1.20
Challenge coin	Diameter, rim width, edge text depth	+/-0.2 mm diameter; +/-0.15 mm rim	100-200 pcs; 12-18 days; USD 1.10-3.50
Metal keychain	Outline, hole center, thickness	+/-0.5 mm outline; +/-0.2 mm hole center	300 pcs; 12-20 days; USD 0.75-2.40
Fridge magnet badge	Outline, flatness, magnet pocket depth	+/-0.5 mm outline; flatness <=0.3 mm over 50 mm; pocket +/-0.2 mm	300-500 pcs; 12-18 days; USD 0.60-2.10

For small promo orders, those numbers are usually enough. For retail or licensed goods, the more important point is to state which dimensions are inspected 100 percent and which are checked by sampling. If you put that in the RFQ, you get fewer arguments when the first article comes back.

Details that break first: line width, recess depth, and wall thickness

The narrowest line in the design is often the first place a mold fails. For enamel pins and badges, 0.20 mm is a realistic minimum line width if the factory is using good tooling and experienced polishing; 0.25 mm is safer when you need repeatability across 1,000 units or more. For soft enamel, cavity depth usually lands at 0.25-0.40 mm; for hard enamel, 0.15-0.25 mm is more common because the surface is polished flush.

Wall sections between color cells should usually stay above 0.20-0.25 mm, and any raised detail thinner than that is likely to round off or fill over. If you want readable edge text on a coin, keep the stroke at 0.30 mm or more and the letter height at least 1.2 mm. That is not a design rule; it is a manufacturing limit, and ignoring it is how buyers end up paying for a second tool revision.

This is also where material choice matters. Brass gives the best edge definition for fine lines, iron is cheaper but less forgiving after polishing, and zinc alloy works well for deep relief and heavier pieces. If the design uses tiny internal cutouts, ask the factory to tell you in writing which part will be hand-finished and where the polish allowance will eat into the line width.

Plating and color specs need different tolerances

Plating should be treated as a separate spec, not a decorative afterthought. On brass or iron, a nickel barrier of 1.0-2.0 um is a sensible target for decorative accessories, while gold flash is usually 0.05-0.10 um. Antique nickel, antique brass, and black nickel can all look close on a screen, but in real lighting they vary enough that you should approve a physical master sample before production.

Color and sheen also need their own acceptance rules. For enamel, state Pantone references and ask the supplier to match against a physical standard under D65 lighting at 50 cm viewing distance. If you need retail consistency, expect tighter plating and color inspection to add roughly USD 0.05-0.18 per piece on runs below 500 units, plus 2-4 extra days if a replate or re-polish is needed.

A useful shortcut is to write finish requirements in plain factory language. Instead of saying glossy antique gold, say antique gold plating on H65 brass, brushed face, no sharp polishing marks, no copper bleed, and even tone across the entire lot. That level of detail gives the plating line and QC team the same target, which is the real goal.

Attachment, holes, and moving parts must be dimensioned

Attachment specs matter because the front can be perfect while the product still fails in use. A standard butterfly clutch pin post is usually 0.8 mm in diameter and 7-10 mm long; 0.7 mm posts are lighter but bend more easily, while 0.9 mm posts make sense on larger badges or layered pieces. If you are using a magnetic back, define both the magnet diameter and the recess depth, typically 1.0-1.5 mm, with the magnet seated within +/-0.2 mm of the pocket depth.

For keychains, the hole must be large enough for the ring to move without chewing the plating. A practical rule is ring wire diameter plus 0.8-1.2 mm of clearance, with the hole center at least 2.0 mm from the edge on brass and 2.5 mm on zinc alloy. When the attachment is off-center by even 0.3 mm on a small charm, the whole piece reads crooked, so center location should be one of the dimensions called out on the drawing.

If the design includes moving parts, such as a spinner coin, linked charm, or rotating badge, add a clear function requirement. State the acceptable rattle, friction, or spin duration instead of assuming the factory will guess the feel you want. Mechanical fit is one of the easiest places for a buyer and supplier to talk past each other, especially when the order mixes multiple attachments in one lot.

How to write the QC clause

Inspection policy is where good specs become enforceable. For most promotional orders, AQL 2.5 major and 4.0 minor is a workable baseline; for retail, event-critical, or licensed goods, tighten that to AQL 1.0 major and 2.5 minor. Critical defects such as wrong size, missing attachment, or broken plating edge should be zero tolerance, not sampled away.

A strong QC clause also says how the lot will be measured. Use calipers for overall size, go/no-go gauges for hole and post fit, and a visual master sample for plating tone and enamel fill. If a buyer leaves this unspecified, the factory may inspect by appearance only, which is a weak method for attachment fit or flatness.

• Mark the three or four dimensions that cannot move, such as diameter, post center, and hole location.
• Attach a signed master sample or approved photo of the target finish and enamel fill.
• State the material grade, for example H65 brass, SPCC iron, or Zamak 3 zinc alloy.
• Define plating finish and thickness, such as nickel 1.0-2.0 um or gold flash 0.05-0.10 um.
• Set AQL by defect class and write zero tolerance for critical defects.
• Require pre-production approval after plating, not just before plating.
• Add functional tests, such as 1.5 kgf pin post retention for 10 seconds or a 3.0 kgf ring pull test.

What to do next

The fastest way to avoid rework is to turn the design into a measurable spec before asking for price. Start with three critical dimensions, one master sample, and one inspection standard, then ask the supplier to quote the base price, the cost of tighter tolerances, and the extra days needed for a first article. If you already work with ZheCraft, send artwork, material, finish, attachment type, and the one dimension you cannot afford to miss; the quote will be sharper and the sample cycle shorter.

Have a project? Send your artwork and target quantity and we’ll reply with a detailed quotation within 12 working hours.