What tolerances can you achieve in CNC post-machining after casting?
CNC post-machining can hold tighter functional dimensions than the preceding casting operation, but no single tolerance applies to every feature. The achievable result depends on the dimension type, datum reference frame, casting stability, machining allowance, fixturing, number of setups, tool access, wall stiffness, production quantity and measurement method. A tolerance should be committed only after those conditions are reviewed and then demonstrated on representative parts.
The tolerance belongs to a feature and datum chain
A machined hole diameter, its position, the flatness of a sealing face and the relationship between two faces are different control problems. Diameter may be generated in one tool path, while position also includes how the casting locates in the fixture. A dimension between features machined in separate setups accumulates re-location and datum-transfer effects. The drawing must define what each feature references before a supplier can assess it.
Cast surfaces are not automatically stable machining datums. Parting-line mismatch, draft, local flash, distortion and surface variation can change how the component seats. A fixture may use machined locating pads, defined cast datum targets or a staged operation in which the first setup creates reliable datums for later work. The chosen method should constrain the part without forcing a distorted casting into an artificial shape that springs back after release.
What controls cast-machined accuracy
| Control factor | How it affects the result | Useful review evidence |
|---|---|---|
| Casting condition | Distortion, parting mismatch, local hardness and porosity can change locating and cutting behavior | Representative casting measurements, section review and trial machining |
| Machining stock | Too little stock may leave uncleaned areas; excessive or uneven removal can release stress or expose voids | Stock map tied to the die and machining datum plan |
| Fixture and clamping | Locator repeatability and clamp force influence position, flatness and thin-wall deformation | Fixture concept, locating sequence and unclamped verification |
| Setup strategy | Re-clamping adds transfer error between features on different orientations | Operation drawing identifying features made in each setup |
| Cutting process | Tool deflection, wear, runout, heat and burr formation affect size and finish | Tool plan, offsets, wear limits and in-process checks |
| Measurement | Datum simulation, resolution, access and thermal condition affect the reported result | Gauge or CMM method, fixture, calibration status and measurement study where warranted |
These factors interact. A capable machining center cannot correct an unstable locating surface without an appropriate fixture and sequence. A precise gauge cannot make an uncontrolled process capable. Conversely, a thoughtful datum and stock strategy may make a demanding local feature practical without applying the same tolerance to the whole part.
Casting integrity can limit the machining plan
Machining removes the casting skin and may expose subsurface porosity. This matters at threads, sealing lands, bearing bores and fluid passages. The risk depends on wall geometry, metal flow, overflow and vent strategy, local solidification and how deeply the cut enters the casting. A blanket instruction to add more stock can make matters worse by increasing material removal and approaching regions with greater discontinuity risk.
Define functional zones before tool design. If a sealing face cannot tolerate a connected void, the team may change gate or overflow intent, move the face, alter wall geometry, control stock, choose a different sealing architecture or specify a relevant leak and internal-integrity check. The measurement plan then needs to address both dimension and function. A bore can meet size while failing pressure containment.
Geometry and stiffness matter after unclamping
Thin walls, broad faces, long spans and asymmetric rib patterns can move under clamp load or material removal. Measuring the part while constrained may hide springback. For a flat sealing surface, specify whether flatness is evaluated free-state, under assembly restraint or in a defined fixture. That choice should reflect how the component functions.
Tool access affects achievable geometry as well. A deep small-diameter feature can increase deflection and chip evacuation risk. An interrupted cut across a casting edge can shorten tool life. A feature close to a thin wall may burr or distort. Threads require decisions about cast pilot condition, drilling, tapping or thread milling, depth, gauge method and treatment of incomplete entry threads. These details deserve feature-specific review.
How to establish a defensible tolerance
Start with the assembly requirement and calculate the permitted variation across mating parts. Allocate tighter control only to characteristics that affect fit, seal, motion, optical alignment or another stated function. Mark those characteristics and their datum relationships on the drawing. Avoid carrying extra decimal places onto unrelated dimensions; they add inspection and process burden without improving the product.
Next, ask the supplier to propose the casting datum targets, machining setups, stock, fixture concept and inspection method. The post-machining route should identify whether related features can be cut in one setup and how the first reliable datums are generated. Resolve conflicts with parting lines, ejector marks, draft and cosmetic zones before tool release.
Then verify trial parts. First-article measurements show whether the route can produce conforming samples, but a small sample does not by itself prove long-run capability. For repeated production, collect data from the agreed process, cavities, fixtures and measurement system over a representative interval. Any capability target, sampling rule or 100-percent inspection requirement should be specified by contract and linked to product risk.
What to include in an RFQ
Nominal dimension, tolerance type, datum reference frame and feature location in the 3D model.
Material grade, heat or treatment condition if applicable, and relevant wall thickness around the cut.
As-cast surfaces, machining stock, no-porosity functional zones and finish allowance.
Mating component, assembly restraint, load and environmental condition.
Order quantity, cavity or fixture traceability need and expected inspection frequency.
Required gauge or measurement standard, report format and process-capability condition.
The die casting tolerance planning guide provides additional context for separating as-cast and machined controls. The final achievable tolerance still must be confirmed against the drawing, datum and fixture plan, representative castings, machine route and measurement evidence. Without that chain, a quoted number is only an assumption.
