When every component drawing is marked green and receiving inspection passes, it is easy to assume the build will go smoothly. Then the housing binds on the stack, a seal face opens a gap, or alignment drifts past the functional window. The usual cause is not a single “bad” piece. It is tolerance stacking: allowed variation on each print adds up across a mating chain until the clearance, preload, or alignment the assembly needs is gone.
This page is for sourcing and engineering teams who release die-cast, molded, or CNC component orders. It is not a capability table, not an RFQ checklist, and not a process-selection card. One idea: part OK is not the same as assembly OK.
What tolerance stacking means in practice
Each print carries nominal sizes and tolerances. On its own, a dimension inside those limits is “correct.” Assemblies care about the interface: the sum or difference of several dimensions that meet at a joint, seal, bearing seat, or cover-to-housing stack.
If three pieces each sit at the high end of their tolerance and the housing sits at the low end — or the reverse — the stack can leave too little clearance, too much free play, or a seal that no longer closes. Incoming inspection that only gates each print will not catch that. The stack shows up when parts are built together.
Coatings and secondary operations sit in that chain as well. Where surface finishing is coordinated as an outsourced step, coating thickness and machining stock after finish still belong in the fit discussion — not as an afterthought once parts are already on order.
Four checks before you release component orders
Before you release die-cast aluminum / magnesium / zinc parts, injection-molded pieces, or CNC secondaries into tooling or production that locks the interface, look past “each print is green”:
1. Functional clearance and fit intent
State what the assembly actually needs: gap, interference, preload, seal compression, or alignment window — not only nominal sizes on each separate drawing. If the functional window is never written down, suppliers will optimize to print limits, not to build success.
2. The tolerance chain at the interface
List which dimensions stack at the joint. Include finishes and secondary processes where they change size or location. Ask for mating context or assembly intent with the drawing package when the fit is critical. (MINDWELL does not publish a default stack-up worksheet; use your program’s chain.)
3. Critical-to-fit versus non-critical
Tighten what controls the fit. Blanket-tightening every dimension raises cost without fixing the interface. Mark critical-to-fit features so quoting and inspection effort go where they matter.
4. How the assembled result will be measured
Agree datums and a functional check before parts ship as “approved.” If approval is only piece-part inspection against each print, assembly surprises are still possible at the customer’s build line.
Illustrative mini-stack (not a MINDWELL job)
Illustrative example only — not a MINDWELL project, not a capability claim, not our measured data. Replace with your real stack.
- Housing depth: 30.00 ± 0.10 mm
- Three stacked pieces: 9.90 ± 0.05 mm each
Nominal clearance lands near 0.30 mm. Under worst-case extremes, that clearance can sit outside a functional window of 0.20–0.40 mm. These numbers are teaching aids only. Do not treat them as MINDWELL process capability, Cpk, scrap rate, or a quoted tolerance menu.
What this page is not
- Not a Mistakes / before-after failure story
- Not an RFQ attachment checklist
- Not a supplier-vetting questionnaire
- Not a full die-casting versus CNC decision guide
- Not the same as “where to mark criticals on a single print” (related FAQ topic; separate page)
Those belong elsewhere. This page stays on the fit chain across parts.
Quoting and DFM when fit matters
When fit matters, send mating context — or full assembly intent — with the drawing package. On a complete drawing package, we aim to return a quote within 24 hours. That timing depends on a usable package (prints, materials, quantities, and enough interface context), not on a promised tooling or production calendar.
MINDWELL (Xiamen Mindwell Precision Manufacture) supports aluminum, magnesium, and zinc die casting in parallel, in-house mold design and manufacturing, injection molding, and CNC machining. OEM / ODM / OBM work is available where the program needs it. Surface finishing is outsourced and coordinated when the RFQ calls for it — not described here as an in-house plating or coating line.
Next step
If assembly clearance or alignment is the risk, include the mating parts or fit intent with the prints so the stack can be discussed early. Start from mnwell.com or use the site inquiry form; for email, service@mnwell.com.
