+ stock Machining allowance / stock

How to verify

Classic layout on a surface plate with a height gauge, or probing on the machine itself by taking points on the faces to be machined and comparing them with the model. On large, complex parts, a 3D scan with best-fit alignment is the quick way to see whether the allowance is distributed well on all faces at once. For spot checks, an ultrasonic thickness gauge lets you verify how much material there is between the outer face and an internal cavity before risking machining.

Workshop guideline values

General

3–6 mm per face on medium sand castings, and more on large or steel castings.

Accuracy

1.5–3 mm on castings with a tight tolerance grade or metal patterns.

High precision

0.5–1.5 mm in high-precision processes, where machining only provides the finish.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Confusing machining allowance with tolerance: allowance is material that must be there, and if it is missing the part cannot reach size no matter how much margin the dimension has.
• Checking it only on the first face and discovering halfway through machining that another one is short.
• Programming the first pass too light and making the tool cut inside the scale, which is what breaks the most edges.