Die Casting Tolerance Capability
As-cast tolerances to ISO 8062-3 CT5 and CT6, machined features held to ±0.05mm, and ±0.02mm on critical diameters and bores. Capability studies with CMM data on the table before you commit tooling, so the tolerance you specify is the tolerance you receive.
Free Consultation24h
Rapid Quote
500 pcs
MOQ
±0.05mm
Machined Tolerance
±0.02mm
Precision Features
Materials
| Grade | Properties | Typical Applications |
|---|---|---|
| Aluminum A380 | Tensile strength 324 MPa, density 2.71 g/cm³, stable dimensions in thin walls, good pressure tightness and dimensional repeatability shot to shot | Housings, brackets, covers, connector bodies where cast tolerance CT6 is enough |
| Aluminum A356-T6 | Tensile strength 275 MPa as cast and 310 MPa after T6, heat treatment raises strength but adds 0.1 to 0.3 mm of growth that must be planned | Structural parts, suspension brackets, load-bearing frames needing strength plus tolerance control |
| Aluminum A413 | Tensile strength 296 MPa, best castability and pressure tightness, thin-wall capable, low shrinkage variation | Pump bodies, valve housings, transmission cases where sealing faces need flatness control |
| Zinc Alloy Zamak 3 | Tensile strength 283 MPa, density 6.60 g/cm³, excellent dimensional stability, fine feature replication and long die life | Precision hardware, locks, small mechanisms, connector shells needing tight as-cast detail |
| Zinc Alloy Zamak 5 | Tensile strength 328 MPa, higher hardness and creep resistance than Zamak 3, casts fine detail with slightly lower ductility | Gearbox covers, precision mechanisms, tool bodies, threaded inserts in cast bosses |
| Magnesium AZ91D | Tensile strength 230 MPa, density 1.81 g/cm³, thin-wall capable, more sensitive to thermal distortion than aluminum | Thin housings, brackets and frames where weight matters more than the widest tolerance band |
Process Parameters
| Parameter | Standard Range |
|---|---|
| As-Cast Tolerance (ISO 8062-3) | CT5 to CT6, quoted per dimension from the standard table |
| Machined Feature Tolerance | ±0.05 mm (ISO 2768-f) |
| Critical Bore and Diameter Tolerance | ±0.02 mm on request |
| Flatness (Machined Face) | 0.05 mm per 50 mm |
| Positional Tolerance (Machined Holes) | ±0.03 mm |
| As-Cast Surface Roughness | Ra 1.6 - 3.2 μm |
| Machined Surface Roughness | Ra 1.6 μm, Ra 0.8 μm finish pass |
| Draft Angle | 1 - 3° per side, by surface depth |
| Wall Thickness Range | 0.8 - 5.0 mm |
| Process Capability Target | Cpk ≥ 1.33 on called-out features |
| CMM Measurement Uncertainty | about 2 μm |
| Die Casting Machine Tonnage | 280 - 1600 T (cold chamber) |
Surface Finishes
As-Cast (No Secondary Operation)
Surface at Ra 1.6 to 3.2 μm straight from the die, with parting line and ejector marks left in place. The cheapest route when tolerances follow ISO 8062-3 and cosmetic requirements are modest, and it keeps the shortest lead time.
CNC Machining of Critical Features
Datums, bores, sealing faces and threaded bosses machined after casting to ±0.05 mm, or ±0.02 mm where a fit demands it. Only the features that need it are machined, so cost stays close to as-cast while the functional dimensions tighten.
Flatness Correction by Face Milling
Large mating and gasket faces milled to 0.05 mm per 50 mm after the casting has cooled and stabilised. Fixturing on the as-cast datum keeps the correction consistent from part to part across a production run.
Powder Coating with Masking
Film 40 to 80 μm applied on non-critical surfaces with threads, bores and sealing faces masked. Coating build is excluded from tolerance stack-ups by masking rather than by machining after coating.
Anodizing and Plating on Machined Areas
Type II or Type III anodize for aluminum and nickel chrome for zinc, applied after machining so coated surfaces do not disturb the machined tolerance. Build is reserved in the drawing where a coated dimension is called out.
Straightening and Stress Relief
Heat treatment or mechanical straightening applied when a long thin casting drifts outside flatness after cooling. The correction is measured and recorded rather than assumed, so the report shows the part as shipped.
Design for Manufacturing (DFM)
- Specify tight tolerance only on functional features; every dimension pulled in below ISO 8062-3 CT6 adds machining and cost
- Assign ±0.02 mm to machined bores and diameters, not to as-cast surfaces, since the die cannot hold it reliably over tool life
- Use GD&T datums that match the machining setup, otherwise position tolerance is measured from a different origin than the one used to cut the part
- Avoid tolerancing features across a parting line; a dimension that spans two die halves inherits the mismatch between them
- Keep flatness callouts on faces that can be milled in one clamping, since re-fixturing adds stack-up error
- Provide 0.2 to 0.5 mm machining allowance on any face that will be cut, and a flat datum pad for clamping
- Expect 0.1 to 0.3 mm growth on A356 parts after T6, and place critical machining after heat treatment
- Keep wall thickness uniform around bores; uneven sections cool at different rates and pull the bore oval
- Put ejector pins on non-functional surfaces so pin marks do not sit inside a tolerance zone
- Upload STEP or IGES files and get a free DFM report within 24h stating which tolerances are achievable as cast and which need machining
Tooling & Capacity
| Type | Lead Time | Suitable For |
|---|---|---|
| Prototype Die (Soft) | 2 - 3 weeks | 500 to 2,000 pcs, tolerance validation before production tooling is cut |
| Production Die (H13) | 4 - 6 weeks | 5,000+ pcs, stable dimensions across 50,000 to 100,000 shot die life |
| Multi-Cavity Die (4 - 8) | 6 - 8 weeks | High volume with short cycle times, cavity-to-cavity tolerance checked at T1 |
| Machining Fixture for Tight Features | 1 - 2 weeks | Any volume where ±0.02 mm features need repeatable clamping |
Quality Assurance
- Capability study at tooling trial, with Cpk calculated on the features called out in the drawing
- CMM full dimensional report for the first article, measured against the 3D model
- Statistical process control on tight-tolerance features during production, not only at first article
- Cavity-to-cavity comparison on multi-cavity dies to confirm every cavity holds the same band
- Gauge repeatability checked before a dimensional result is accepted as valid
- Flatness and parallelism measured on a granite plate after the part stabilises at room temperature
- Incoming alloy verified with mill certificate and EN 10204 3.1 material certificates on request
- X-ray porosity grading on critical sections, since internal voids distort a bore during machining
- Third-party inspection by SGS, Bureau Veritas or TUV arranged at any stage on request
- Batch dimensional report and material certificate shipped with every production lot
Applications
- Automotive sensor and mounting brackets
- Precision gearbox and motor housings
- Optical and lighting frames with flatness callouts
- Industrial valve bodies with machined sealing faces
- Power tool housings with bearing bores
- Electronic enclosures with connector cut-outs
- Pump and compressor end plates
- Instrument frames and mounting plates
- Robotics joint and link brackets
Why Choose FHY Precision
Tolerance capability stated as CMM data from a capability study, not a catalogue number
Integrated CNC machining tightens cast tolerances only where function needs it, keeping cost in check
Heat treatment and machining sequenced so T6 growth does not consume the tolerance band
GD&T review at DFM stage catches datums that cannot be fixtured before steel is cut
Cavity-to-cavity verification on multi-cavity dies, which is where tolerance complaints usually start
MOQ 500 pcs with prototype dies for tolerance validation before production tooling
Documented quality system with third-party inspection and EN 10204 3.1 certificates on request
Factory-direct in China with no trading layer
FAQ
1. What tolerance can die casting hold without machining?
As-cast tolerances follow ISO 8062-3 at CT5 to CT6 depending on dimension and alloy. For most aluminum and zinc parts that is a practical band of a few tenths of a millimetre on a 50 mm dimension, and it widens as the dimension grows. Anything tighter than CT5 as cast is quoted honestly as not repeatable over die life, and we propose machining instead.
2. Why can a die casting not hold ±0.02mm as cast?
Four effects move an as-cast dimension: thermal contraction varies with section thickness, the two die halves shift slightly under injection pressure, tool wear changes the cavity over 50,000 shots, and ejector and draft geometry adds variation. Each is small, but together they exceed 0.02 mm. Machining removes the variation because the cutter references a fixed datum.
3. What is ISO 8062-3 and how does it set casting tolerances?
ISO 8062-3 assigns casting tolerance grades CT1 to CT16 by process, with die casting typically at CT5 to CT6. Each grade maps a basic dimension to a tolerance band in a table, so a 50 mm dimension and a 200 mm dimension get different allowances at the same grade. We quote against that table and mark on the DFM report where a callout falls outside the achievable grade.
4. Can you machine a casting to ±0.02mm?
Yes, on machined features such as bores, spigots and turned diameters, where the cutter works from a machined datum. We hold ±0.02 mm on those and verify with CMM. The limit is not the machine but the datum chain: if a feature is measured from an as-cast surface, the cast variation enters the result, so we machine the datum first.
5. How do you prove tolerance capability before we commit to tooling?
At the tooling trial we measure the T1 samples, calculate Cpk on the features in your drawing, and send the raw measurement data with the report. If a feature lands below Cpk 1.33, we either adjust the die, change the machining allowance or tell you the tolerance needs relaxing. You see the numbers before authorising a production run.
6. Does tighter tolerance always mean higher cost?
No, and that is why the DFM review matters. Tightening a machined bore from ±0.05 to ±0.02 mm may add nothing if the feature is already being cut. Tightening an as-cast surface forces an extra machining operation, extra fixturing and slower inspection, which can double the cost of that feature. Cost follows the process change, not the number on its own.
7. How does draft angle affect achievable tolerance?
Draft of 1 to 3 degrees is needed to release the part from the die, so a drafted wall is never truly vertical. On a 20 mm tall wall, 1 degree of draft shifts the surface about 0.35 mm from top to bottom, which is far more than a 0.05 mm tolerance. Vertical tolerance callouts therefore belong on machined faces, not on drafted as-cast walls.
8. What flatness can you hold on a die cast face?
As-cast flatness is typically 0.1 to 0.2 mm on a 100 mm face, depending on gating and cooling. Milling the face after casting brings it to 0.05 mm per 50 mm, measured on a granite plate after the part reaches room temperature. For gasket sealing faces we machine and then verify flatness on every first article and on periodic samples.
9. How does wall thickness variation affect dimensional results?
Thick sections cool slower and shrink more than thin ones, so a part with uneven walls distorts after ejection and pulls bores out of round. Keeping walls uniform between 0.8 and 2.5 mm limits that distortion. Where a design needs a thick boss, we core it or add a machining allowance rather than let shrinkage set the final dimension.
10. Can you hold position tolerance on holes cast in place?
Holes cast in place inherit draft, parting line shift and die wear, so positional accuracy is roughly ±0.15 to 0.3 mm depending on size. Where position matters for mating parts, we cast the hole undersize and machine or drill it to ±0.03 mm from a machined datum. The DFM report states which holes need that treatment.
11. What is the effect of heat treatment on casting dimensions?
Solution treatment and ageing on A356 grows the part by roughly 0.1 to 0.3 mm and can add slight bowing on long parts. We plan for it by heat treating before final machining, and by leaving machining allowance on faces that will be cut afterwards. Parts that stay as cast are measured after the full heat treat cycle, not before.
12. Do you run SPC on production parts or only inspect the first article?
Both. The first article establishes the baseline with a full CMM report, then tight-tolerance features are monitored during production with SPC so drift is caught before parts ship. Cavity-to-cavity comparison is run on multi-cavity dies, because one cavity drifting while the others hold is a common and invisible failure mode.
13. How is a tolerance measured and reported?
Critical features are measured on a CMM with roughly 2 μm uncertainty, flatness on a granite plate, and surface roughness with a calibrated profilometer. The report lists actual measured values, the tolerance band and the pass or fail status per feature, rather than a single statement that the part is within tolerance.
14. Can you supply parts to a drawing with GD&T?
Yes. Send the drawing with datums and feature control frames and we review whether each datum can be fixtured and measured. Some drawings reference datums on as-cast surfaces, which makes position results unstable, so we propose a machined datum and confirm the change with you before quoting the machining scope.
15. What is the minimum order for tight-tolerance die casting?
MOQ is 500 pieces, the same as our standard die casting minimum, and tight-tolerance work does not raise it. A soft prototype die supports 500 to 2,000 pieces for tolerance validation, and an H13 production die takes over at 5,000 pieces. Machining fixtures for ±0.02 mm features take 1 to 2 weeks to build.
16. What information do you need to quote a tolerance-critical part?
A 3D model in STEP or IGES, a 2D drawing with tolerances, datums and surface callouts, the alloy, quantity per release, and which features are functional versus cosmetic. If some tolerances look tighter than the process allows, we quote two options: the drawing as specified, and a relaxed version that meets function at lower cost.