GlossaryKind of term: Manufacturing termNear-net shape
What is near-net shape?
A near-net shape is a starting form close to the finished part, such as a forging or casting, with allowance left only on the faces still to be machined.
Near-net shape means the starting material already has most of the finished part's form. Forgings, castings, sintered parts, printed parts and extruded sections are examples. Machining allowance is left on the faces with a requirement the forming process cannot meet; the rest of the surface stays as it leaves the process. The opposite is a block or bar from which you machine the whole part.
What do allowance and first setup of a raw part depend on?
How much allowance is left, and where, depends on the process, the size, the dimensional and form tolerances, the roughness requirement and the skin of the raw part. A casting or forging skin can be hard and carry sand or scale, and the raw part can be out of true; the first cut has to get beneath the skin. A casting can hold porosity that only shows once machined. Raw parts usually have no flat, square references, so the first setup locates on rough datum points, often in a dedicated fixture; the datum faces are machined first and the rest of the work locates from them. For castings, ISO 8062-3 sets grades for dimensional tolerances and machining allowance.
How does near-net shape work in practice?
A near-net shape cuts the volume of material removed and the machining time, and moves cost into the raw part: a die, pattern or mould is a one-off investment, and the raw part has its own lead time. Whether it beats machining from solid depends on quantity, price per raw part, one-off costs, the fixture for the first setup and the risk of scrap from porosity or too little allowance. For one-offs and small batches, block or bar is often the choice.
What is near-net shape often confused with?
Near-net shape is not net shape: a net-shape part leaves the process at final size, as with much injection moulding, and is not machined again. Nor is machining allowance a tolerance: the allowance is the material still to come off, the tolerance is how far the finished size may deviate.
Frequently asked questions
What is the difference between near-net shape and net shape?
A near-net part still carries allowance on the faces with a requirement and is machined there. A net-shape part comes out of casting, pressing or moulding at final size and does not go on a machine again.
How much machining allowance should a casting have?
It follows from the raw-part drawing or the agreement with the foundry, and depends on the casting process, the size and the requirement on the face. Too little allowance on a distorted casting leaves a face that will not clean up.
When should you choose a forging over bar stock?
When the quantity justifies the die cost, when machining from bar would remove a lot of material, or when the grain flow of a forging is a functional requirement. In small batches bar often wins because of the one-off die cost.
How do you fixture a casting for the first operation?
On the rough datum points given on the drawing, often in a dedicated fixture. Machine the datum faces first, then locate the rest of the work from those.