Machining allowance for die cast parts is local stock cast for later CNC removal. For many small to mid-size aluminum die castings, 0.3–0.8 mm is a useful start, not a rule for every surface.
A sealing face, bearing bore, locating hole or mounting pad may need secondary machining; other walls can stay as-cast. The die casting machining allowance must cover casting and fixture variation without removing excess metal.
It is not casting tolerance or shrinkage allowance. Tolerance is allowed variation in the raw part; shrinkage is built into die design; machining stock is cut away.
Where Is Machining Allowance Needed, and How Much Is Enough?
Machine only where function demands it. Each extra cut adds setup time, inspection and cost.
| Feature | Why it may need machining | What must be defined |
|---|---|---|
| Sealing or mating face | Flatness, contact and surface finish | Stock on each face |
| Bearing or shaft bore | Diameter, roundness and fit | Radial or diameter stock |
| Datum or mounting pad | Stable location and assembly | Datum relationship |
| Precision or locating hole | Size and position | Cast pilot or drilled hole |
| General wall or cosmetic face | Often meets the need as-cast | Usually no stock |
MRT Castings gives these minimum starting points for pressure die castings. They aid review but are not a universal aluminum die casting standard.
| Maximum casting size | Starting reference |
|---|---|
| Under 150 mm | 0.375 mm |
| 150–300 mm | 0.50 mm |
| 300–450 mm | 0.625 mm |
| 450–600 mm | 0.75 mm |
Size alone cannot complete a machining allowance calculation. Also review as-cast flatness, distortion, datums, fixture repeatability, wall thickness, finish and pore risk. A long thin housing can need different local stock from a compact bracket of the same length.
Stock direction matters. A 0.5 mm face allowance raises the cast surface by 0.5 mm. For a finished Ø20 mm bore, 0.5 mm radial stock gives about a Ø19 mm cast pilot; 0.5 mm total diameter stock gives Ø19.5 mm. “Leave 0.5 mm” is incomplete.
Why More Machining Stock Is Not Always Safer
Too little allowance may leave low areas uncut. A face can miss its flatness or finish target, and a bore may not reach its full form. Rework or scrap can follow.
Too much allowance creates a different set of problems:
- longer CNC cycle time and more tool wear;
- more chips and a higher finished-part cost;
- reduced wall or boss thickness after cutting;
- removal of more of the dense die cast surface layer;
- exposure of gas or shrinkage pores below the surface.
That last risk matters on fluid housings, threaded ports and sealing faces. A sound-looking casting can leak after a deep cut opens a connected pore. Extra stock cannot correct poor metal flow or porosity. For pressure parts, review machining depth with our guide to leak-tight aluminum die cast parts.
The target is the smallest local allowance that reliably cleans up the feature. Base it on process capability, not a large safety number copied from another part.
How Should Machining Allowance Be Shown on the Drawing?
A finished 3D model does not show which faces need stock. ISO 8062-3:2023 covers casting tolerances and machining allowance grades, but not a 3D CAD model with no indicated dimensions. NADCA guidance also calls for machining location, depth, finish and gaging to be agreed in advance.
For a clear machining allowance on a casting drawing:
- identify machined and as-cast surfaces;
- state the final size, tolerance and required surface finish;
- state whether stock is per face, per side, radial or diametric;
- relate each machined feature to the correct datums;
- keep the 2D drawing and 3D model at the same revision;
- label separate raw-casting and finished-part models clearly.
For example: “Finished bore Ø20 H7; cast pilot Ø19.0 mm nominal; 0.5 mm radial stock; position from datums A and B.” The supplier must still confirm that the pilot, wall and die design are practical.
This callout does not replace project review. Use the die casting DFM checklist to close tooling risks and the die casting RFQ checklist to define quote scope. Check machined T1 parts against the approved finished drawing, not only the raw casting.
Review Casting and Machining Before Tooling
Yongzhu Casting supports custom aluminum parts from drawing review and mold making through die casting, CNC machining, finishing and inspection. Send the 2D drawing and 3D model with final dimensions and machined faces marked. We can review what stays as-cast, where local stock is needed and what must be agreed before tooling.
Frequently Asked Questions
Can an existing die be modified if machining stock is insufficient?
Sometimes. An outer cavity may be enlarged, or a core pin changed. Feasibility depends on the feature, tool structure, wall thickness and metal flow. Inspect the die before promising a correction.
Should ADC12 and A380 use the same machining allowance?
Not by default. Machinability matters, but part size, local geometry, casting variation, datums and the required cut can have more influence. Confirm the value for the actual part.
Does machining allowance affect the quoted part weight and CNC cost?
Yes. Added stock increases shot weight and metal removal. A small local change may have little effect, but broad or deep stock can add material, cutting time, tool wear and inspection.
What changes if casting and CNC machining use different suppliers?
Both suppliers need the same raw and finished definitions, datums, stock direction and inspection plan. The handoff must assign responsibility for missing stock, distortion and machining setup.
