How to Choose CNC Machining & Injection Molding Tolerances for Production (Not Prototypes)
2.2K views · 2026-01-07
Industrial Design
How do you choose tolerances for production? Start with loose general tolerances, then tighten only the few features that control function (fits, sealing, alignment). For CNC, tighter tolerances increase machining + inspection time. For injection molding, shrink, cooling, and tool behavior make “machining-level tolerances” expensive and slow.
Typical CNC tolerance approach (production):
- Use a sensible general tolerance for non-critical features (e.g., ~±0.125 mm on small features, scaling with size).
- Tighten only for: mating parts, sealing surfaces, bearing bores, precision locators.
- Specify fits (example: H7 hole with g6 shaft) instead of guessing.
- Keep GD&T practical: align datums with how the part is used; reserve ultra-tight callouts for true performance needs.
- Match inspection to risk: calipers/go-no-go for general, gauges/mics for interfaces, CMM for a few CTQs.
Typical injection molding tolerance approach (production):
- Expect most molded features around ±0.10 to ±0.30 mm.
- Tighten locally to ±0.05–0.10 mm only for CTQs like snap fits, alignment pins, sealing edges, optical interfaces.
- Design to protect tolerances: uniform walls, ribs ~40–60% wall, fillets, adequate draft.
- Plan tool tuning: steel-safe strategy + T0/T1 sampling, measure a meaningful sample, then cut steel to center the mean.
- Agree capability targets for CTQs (example: Cpk 1.33) to avoid over-inspection.
Takeaway: Default loose, prove tight. Design the fit, constrain what matters, and align inspection and capability to risk.
#cnc #injectionmolding #Tolerances #dfmei #GDT
Chapters:
00:00 Prototype → production tolerance mindset
00:15 CNC tolerances: where tight matters
00:32 Fits + GD&T + surface finish
01:20 Inspection strategy
01:35 Injection molding tolerances: shrink + design rules
02:48 Steel-safe + T0/T1 + Cpk/CTQs
03:44 Wrap-up
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Protolabs is the fastest and most comprehensive digital manufacturing service in the world, combining in-house quick-turn factories and premium manufacturing partners at Protolabs Network. Users can easily upload their design, instantly receive a quote, and start production at the click of a button.
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Typical CNC tolerance approach (production):
- Use a sensible general tolerance for non-critical features (e.g., ~±0.125 mm on small features, scaling with size).
- Tighten only for: mating parts, sealing surfaces, bearing bores, precision locators.
- Specify fits (example: H7 hole with g6 shaft) instead of guessing.
- Keep GD&T practical: align datums with how the part is used; reserve ultra-tight callouts for true performance needs.
- Match inspection to risk: calipers/go-no-go for general, gauges/mics for interfaces, CMM for a few CTQs.
Typical injection molding tolerance approach (production):
- Expect most molded features around ±0.10 to ±0.30 mm.
- Tighten locally to ±0.05–0.10 mm only for CTQs like snap fits, alignment pins, sealing edges, optical interfaces.
- Design to protect tolerances: uniform walls, ribs ~40–60% wall, fillets, adequate draft.
- Plan tool tuning: steel-safe strategy + T0/T1 sampling, measure a meaningful sample, then cut steel to center the mean.
- Agree capability targets for CTQs (example: Cpk 1.33) to avoid over-inspection.
Takeaway: Default loose, prove tight. Design the fit, constrain what matters, and align inspection and capability to risk.
#cnc #injectionmolding #Tolerances #dfmei #GDT
Chapters:
00:00 Prototype → production tolerance mindset
00:15 CNC tolerances: where tight matters
00:32 Fits + GD&T + surface finish
01:20 Inspection strategy
01:35 Injection molding tolerances: shrink + design rules
02:48 Steel-safe + T0/T1 + Cpk/CTQs
03:44 Wrap-up
🔔 Subscribe for more 🔔
https://www.youtube.com/channel/UC1nzmktP1usjdqewrBDN_uQ
Protolabs is the fastest and most comprehensive digital manufacturing service in the world, combining in-house quick-turn factories and premium manufacturing partners at Protolabs Network. Users can easily upload their design, instantly receive a quote, and start production at the click of a button.
Order custom parts online today: https://identity.protolabs.com/
https://twitter.com/Protolabs
https://www.facebook.com/Protolabs/
https://www.instagram.com/protolabsmfg/
https://www.linkedin.com/company/proto-labs/