Quick Answer
Machining allowance for castings — this is the extra material left on a casting surface so it can be machined to final tolerance. Standard allowance is 1.5–3 mm for most cast surfaces; 2–4 mm for rough surfaces or large castings. Specify too little allowance and the part cannot be machined to drawing; specify too much and you waste material and machining time. Getting it right the first time saves money on every part.
What Is Machining Allowance?
A casting shrinks as it cools. A pattern must be made slightly larger than the final part to account for this shrinkage — typically 1–3% depending on the alloy. The result is a casting that is slightly oversize on every surface.
Machining allowance (also called machining stock or finish allowance) is the additional material specified on the drawing over and above the nominal dimension, to be removed by machining to achieve final tolerance.
Why it matters: If the allowance is too small, the machined surface may reveal underlying porosity, cold shuts, or other casting defects — and the part is scrap. If allowance is too large, the foundry may charge extra, and your machinist wastes time and tooling cutting material that should not have been there.
Standard Allowance by Casting Process
| Casting Process | Typical Allowance (mm) | Notes |
|---|---|---|
| Investment Casting | 1.0–2.0 | Good surface, tight control |
| Permanent Mold / Gravity Die | 1.5–2.5 | Moderate surface quality |
| Low Pressure Die Casting | 1.5–2.5 | Good surface, consistent |
| High Pressure Die Casting | 1.0–2.0 | Low porosity if vacuum-assisted |
| Sand Casting (green sand) | 2.0–4.0 | Rough surface, more variation |
| Sand Casting (resin sand/no-bake) | 1.5–3.0 | Smoother surface than green sand |
| Large Sand Castings (>500 mm) | 3.0–6.0 | Greater dimensional variation |
Allowance by Surface Type
| Surface Category | Allowance (mm) | Example Surfaces |
|---|---|---|
| Datum / Reference surfaces | 2.0–3.0 | Mounting faces, datum holes, register surfaces |
| Machined bearing/sealing surfaces | 1.5–2.5 | Bores, shaft seats, gasket faces |
| General machined surfaces | 1.5–2.5 | Flanges, bosses, chamfers |
| Threaded holes (tapped) | 1.0–1.5 over tap depth | All threaded holes |
| Hub/bore surfaces | 2.0–3.0 | Bores that will be honed or reamed |
| As-cast non-machined surfaces | 0 (no allowance) | Exterior cosmetic surfaces, ribs |
Material Effect on Allowance
Different casting alloys have different shrinkage rates and surface characteristics, which affect the minimum practical allowance:
| Material | Linear Shrinkage (%) | Surface Hardness | Allowance Impact |
|---|---|---|---|
| Aluminum alloys (A356, 6061) | 1.0–1.3% | Low–Medium | Standard allowance sufficient |
| Zinc alloys | 0.6–0.8% | Medium | Lower shrinkage, standard allowance |
| Gray iron | 0.8–1.0% | High (HB 150–250) | Standard; hard surface may need carbide tooling |
| Ductile iron | 0.8–1.0% | High (HB 160–300) | Standard; may need harder cutting tools |
| Carbon steel (investment) | 1.5–2.0% | Medium–High | Higher shrinkage; verify patternmaker’s rule |
| Stainless steel (investment) | 1.5–2.5% | High | Higher shrinkage; allowance on high side |
| Brass / Bronze | 1.5–2.0% | Medium | Standard; good machinability |
How to Specify Allowance on Your Drawing
There are two ways to specify machining allowance:
Method 1: Global Note
Add a general note on the drawing:
“Machining allowance: 2.0 mm on all surfaces requiring finish machining unless otherwise specified.”
This is simple but may not reflect actual needs for all surfaces.
Method 2: Surface-by-Surface Indication
Use a machining symbol or GD&T modifier on each surface requiring machining:
- ISO 21920 / ASME Y14.5 surface texture symbols
- Indicate datum surfaces separately (higher priority for accuracy)
- Use “CNC machined surface” callout on datum and fit surfaces
Method 3: Combined Approach (Recommended)
Use a global note for general surfaces, and add specific callouts for critical surfaces:
“General machining allowance: 2.0 mm. Datum faces and sealing surfaces: 2.5 mm minimum.”
What Happens When Allowance Is Too Small
If machining allowance is insufficient:
- Porosity exposure: Machining cuts into the casting skin and reveals subsurface porosity — part is scrap
- Hard spot exposure: Chilled spots or hard inclusions near the surface cause tool wear and dimensional error
- Datum shift: If the datum surface was cast undersize, machining shifts all subsequent features
- Leakage: Thin walls machined below minimum wall thickness — part cannot hold pressure
- Cost: Machine shop may charge for “thin cuts” or reject the part
What Happens When Allowance Is Too Large
Excess allowance costs money in two ways:
- Material cost: Casting weighs more; foundry charges by weight
- Machining time: Extra material means more cutting, more tool wear, longer cycle time
- Surface treatment cost: More surface area to shot blast, anodize, or coat
Rule of thumb: Every extra 0.5 mm of allowance across all surfaces adds roughly 1–3% to casting cost and 5–10% to machining cost.
Buyer Checklist: Allowance Review
- Does every machined surface have an allowance specified?
- Are datum surfaces given higher priority (more allowance) than general machined surfaces?
- Is allowance consistent with the casting process (check the tables above)?
- Has the foundry confirmed the allowance is achievable on this specific part geometry?
- Are there any surfaces where as-cast finish is acceptable (no allowance needed)?
- Are threaded holes specified with sufficient allowance over the full tap depth?
- Has the foundry flagged any surfaces where their process cannot reliably hold the specified allowance?
DFM: Allowance and Process Capability
Before finalizing allowance on your drawing, discuss with your foundry:
- Thin sections: Minimum wall may not hold full allowance if the section is too thin after machining
- Deep pockets: Internal cavities may need extra allowance for tool access and chip evacuation
- Draft angle conflicts: Areas with steep draft may have insufficient allowance on one side
- Parting line location: Surfaces near the parting line may have flash or cope/drag mismatch requiring extra stock
Allowance and Cost Trade-off
Allowance directly affects total part cost. Here is the trade-off:
| Allowance Strategy | Casting Cost | Machining Cost | Scrap Risk | Best For |
|---|---|---|---|---|
| Minimum allowance (1.0–1.5 mm) | Lowest (least material) | Lowest (least cutting) | Higher (porosity risk) | High-quality investment casting, thin sections |
| Standard allowance (1.5–2.5 mm) | Moderate | Moderate | Low | Most applications |
| Generous allowance (2.5–4.0 mm) | Higher (more material) | Higher (more cutting) | Very low | Sand casting, large parts, critical datums |
Before finalizing the sourcing decision, many OEM buyers also compare CNC Machining for Cast Parts, Casting Tolerances, DFM Review Service, and Sand Casting Process to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
FAQ
What is the standard machining allowance for investment casting?
Typically 1.0–2.0 mm per surface. Investment casting produces a smoother, more dimensionally consistent surface than sand casting, so lower allowance is acceptable.
Can allowance be different on different surfaces of the same casting?
Yes — and it should be. Datum and fit surfaces need more allowance than general machined surfaces. Use drawing notes or surface symbols to specify different allowances for different surfaces.
The foundry says allowance is fine but the machine shop says it is too tight. Why?
This is a common conflict. The foundry sees the as-cast dimension; the machine shop sees the actual machined surface after cutting. If your allowance is 1.0 mm but the casting skin has 0.3 mm of surface variation from roughness or轻微porosity, the machine shop has only 0.7 mm of clean stock to cut — which may not be enough for a stable cut. Specify allowance on the conservative side.
How do I know if my casting needs more allowance for heat treatment distortion?
If the casting will be heat treated after casting but before final machining, account for distortion: add 0.5–1.0 mm additional allowance on surfaces that will be machined after HT. Discuss with your foundry whether HT is performed pre- or post-machining.