metal work in China

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Machining

Machining is a term used to describe a variety of material removal processes in which a cutting tool removes unwanted material from a workpiece to produce the desired shape. The workpiece is typically cut from a larger piece of stock, which is available in a variety of standard shapes, such as flat sheets, solid bars, hollow tubes, and shaped beams. Machining can also be performed on an existing part, such as a casting or forging.

 

Flat sheet
Flat sheet
Rectangular bar
Rectangular bar
Round tube
Round tube
I-beam
I-beam

 

Parts that are machined from a pre-shaped workpiece are typically cubic or cylindrical in their overall shape, but their individual features may be quite complex. Machining can be used to create a variety of features including holes, slots, pockets, flat surfaces, and even complex surface contours. Also, while machined parts are typically metal, almost all materials can be machined, including metals, plastics, composites, and wood. For these reasons, machining is often considered the most common and versatile of all manufacturing processes.

 

Milled part - Chuck jaw
Milled part – Chuck jaw
Turned part - Stub axle
Turned part – Stub axle

 

As a material removal process, machining is inherently not the most economical choice for a primary manufacturing process. Material, which has been paid for, is cut away and discarded to achieve the final part. Also, despite the low setup and tooling costs, long machining times may be required and therefore be cost prohibitive for large quantities. As a result, machining is most often used for limited quantities as in the fabrication of prototypes or custom tooling for other manufacturing processes. Machining is also very commonly used as a secondary process, where minimal material is removed and the cycle time is short. Due to the high tolerance and surface finishes that machining offers, it is often used to add or refine precision features to an existing part or smooth a surface to a fine finish.

As mentioned above, machining includes a variety of processes that each removes material from an initial workpiece or part. The most common material removal processes, sometimes referred to as conventional or traditional machining, are those that mechanically cut away small chips of material using a sharp tool. Non-conventional machining processes may use chemical or thermal means of removing material. Conventional machining processes are often placed in three categories – single point cutting, multi-point cutting, and abrasive machining. Each process in these categories is uniquely defined by the type of cutting tool used and the general motion of that tool and the workpiece. However, within a given process a variety of operations can be performed, each utilizing a specific type of tool and cutting motion. The machining of a part will typically require a variety of operations that are performed in a carefully planned sequence to create the desired features.

Material removal processes

  • Mechanical
  • Single-point cutting
  • Turning
  • Planing and shaping
  • Multi-point cutting
  • Milling
  • Drilling
  • Broaching
  • Sawing
  • Abrasive machining
  • Grinding
  • Honing
  • Lapping
  • Ultrasonic machining
  • Abrasive jet machining
  • Chemical
  • Chemical machining
  • Electrochemical machining (ECM)
  • Thermal
  • Torch cutting
  • Electrical discharge machining (EDM)
  • High energy beam machining

 

Single point cutting refers to using a cutting tool with a single sharp edge that is used to remove material from the workpiece. The most common single point cutting process is turning, in which the workpiece rotates and the cutting tool feeds into the workpiece, cutting away material. Turning is performed on a lathe or turning machine and produces cylindrical parts that may have external or internal features. Turning operations such as turning, boring, facing, grooving, cut-off (parting), and thread cutting allow for a wide variety of features to be machined, including slots, tapers, threads, flat surfaces, and complex contours. Other single point cutting processes exist that do not require the workpiece to rotate, such as planing and shaping.

 

Turning operation
Turning
Boring operation
Boring
Grooving operation
Grooving
Thread cutting operation
Thread cutting

 

Multi-point cutting refers to using a cutting tool with many sharp teeth that moves against the workpiece to remove material. The two most common multi-point cutting processes are milling and drilling. In both processes, the cutting tool is cylindrical with sharp teeth around its perimeter and rotates at high speeds. In milling, the workpiece is fed into the rotating tool along different paths and depths to create a variety of features. Performed on a milling machine, milling operations such as end milling, chamfer milling, and face milling are used to create slots, chamfers, pockets, flat surfaces, and complex contours. Milling machines can also perform drilling and other hole-making operations as well.

 

End milling operation
End milling
Face milling operation
Face milling

 

In drilling, the rotating tool is fed vertically into the stationary workpiece to create a hole. A drill press is specifically designed for drilling, but milling machines and turning machines can also perform this process. Drilling operations such as counterboring, countersinking, reaming, andtapping can be used to create recessed holes, high precision holes, and threaded holes. Other multi-point cutting processes exist that do not require the tool to rotate, such as broaching and sawing.

 

Drilling operation
Drilling
Counterboring operation
Counterboring
Reaming operation
Reaming
Tapping operation
Tapping

 

Abrasive machining refers to using a tool formed of tiny abrasive particles to remove material from a workpiece. Abrasive machining is considered a mechanical process like milling or turning because each particle cuts into the workpiece removing a small chip of material. While typically used to improve the surface finish of a part, abrasive machining can still be used to shape a workpiece and form features. The most common abrasive machining process is grinding, in which the cutting tool is abrasive grains bonded into a wheel that rotates against the workpiece. Grinding may be performed on a surface grinding machine which feeds the workpiece into the cutting tool, or a cylindrical grinding machine which rotates the workpiece as the cutting tool feeds into it. Other abrasive machining processes use particles in other ways, such as attached to a soft material or suspended in a liquid. Such processes include honing, lapping, ultrasonic machining, and abrasive jet machining.

Shell mold casting

Shell mold casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.

Shell mold casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.

The shell mold casting process consists of the following steps:

  1. Pattern creation – A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.
  2. Mold creation – First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.
  3. Mold assembly – The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material.
  4. Pouring – The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.
  5. Cooling – After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.
  6. Casting removal – After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold.

shell-mold-casting-small

Shell Mold Casting

Capabilities

Typical Feasible
Shapes: Thin-walled: Complex
Solid: Cylindrical
Solid: Cubic
Solid: Complex
Flat
Thin-walled: Cylindrical
Thin-walled: Cubic
Part size: Weight: 0.5 oz – 220 lb
Materials: Metals
Alloy Steel
Carbon Steel
Cast Iron
Stainless Steel
Aluminum
Copper
Nickel
Surface finish – Ra: 50 – 300 μin 32 – 500 μin
Tolerance: ± 0.015 in. ± 0.006 in.
Max wall thickness: 0.06 – 2.0 in. 0.06 – 2.0 in.
Quantity: 1000 – 1000000 100 – 1000000
Lead time: Weeks Days
Advantages: Can form complex shapes and fine details
Very good surface finish
High production rate
Low labor cost
Low tooling cost
Little scrap generated
Disadvantages: High equipment cost
Applications: Cylinder heads, connecting rods

Sand Casting

Sand casting, the most widely used casting process, utilizes expendable sand molds to form complex metal parts that can be made of nearly any alloy. Because the sand mold must be destroyed in order to remove the part, called the casting, sand casting typically has a low production rate. The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern. The sand mold separates along a parting line and the solidified casting can be removed. The steps in this process are described in greater detail in the next section.

sand-casting-mold
Sand casting overview

Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases.

Capabilities

Typical Feasible
Shapes: Thin-walled: Complex
Solid: Cylindrical
Solid: Cubic
Solid: Complex
Flat
Thin-walled: Cylindrical
Thin-walled: Cubic
Part size: Weight: 1 oz – 450 ton
Materials: Metals
Alloy Steel
Carbon Steel
Cast Iron
Stainless Steel
Aluminum
Copper
Magnesium
Nickel
Lead
Tin
Titanium
Zinc
Surface finish – Ra: 300 – 600 μin 125 – 2000 μin
Tolerance: ± 0.03 in. ± 0.015 in.
Max wall thickness: 0.125 – 5 in. 0.09 – 40 in.
Quantity: 1 – 1000 1 – 1000000
Lead time: Days Hours
Advantages: Can produce very large parts
Can form complex shapes
Many material options
Low tooling and equipment cost
Scrap can be recycled
Short lead time possible
Disadvantages: Poor material strength
High porosity possible
Poor surface finish and tolerance
Seondary machining often required
Low production rate
High labor cost
Applications: Engine blocks and manifolds, machine bases, gears, pulleys
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