Quantity Production Techniques

Making metal components consistently

When many metal products are made, their components must be consistent enough to fit together without being individually adjusted. For example, replacement steel brackets need their fixing holes in the correct positions, not merely to look similar.

Quantity-production techniques reduce repeated measuring, speed up operations and help control variation. However, production aids take time and money to prepare. A durable drilling jig may be worthwhile for hundreds of identical brackets, but unnecessary for a single component. Selection depends on the operation, the required accuracy and how often the aid will be used.

Marking out from a reliable reference

Marking out means putting the positions of cuts, holes and bends onto the metal before working it. Cutting or drilling in the wrong place can be difficult to correct, so accurate marking out helps prevent wasted material.

A datum is a fixed reference point, line or surface from which measurements are taken. On a flat metal plate, a straight edge can provide a reference line and a corner can provide a reference point. A flat face can provide a reference surface for checking or setting dimensions.

For a row of holes, measure each hole position from the same datum rather than measuring from one hole to the next. Otherwise, a small error in each measurement can accumulate along the row. The datum itself must therefore be suitable and accurate.

A steel rule measures distances, an engineer’s square establishes right angles, and a scriber scratches fine lines into the metal. Marking blue makes these lines easier to see. Dividers can mark arcs or transfer distances. At a hole centre, a centre punch makes an indentation that helps the drill bit locate correctly rather than slip across the surface.

The marking-out stage is complete when all necessary features are clearly located and their dimensions have been checked against the drawing. Sharp metal edges and scriber points need careful handling. Before drilling, the workpiece must be securely held, with eye protection worn and loose clothing and hair kept away from rotating equipment.

Jigs and fixtures: controlling position and movement

A jig locates the workpiece and guides or controls an operation. A drilling jig can have hardened steel bushes through which the drill passes. Each bracket is located against the same stops and clamped, so the bushes guide holes into consistent positions without marking out every bracket separately.

A bending jig or former controls the position and radius of a bend. A welding jig holds separate parts at the required distance and angle while they are tack welded and then permanently joined.

A fixture holds and locates a workpiece securely but does not guide the cutting tool. For example, a machining fixture can hold a metal component against locating surfaces while a computer-controlled milling machine cuts it. The fixture fixes the workpiece’s position; the machine controls the cutter’s movement. The distinction is the job performed, not simply whether the aid has clamps.

These aids improve repeatability and can reduce handling risks, but they must be rigid, correctly set and securely clamped. A worn bush or misplaced stop can reproduce the same fault across a whole batch. Check the first component before continuing, and inspect the aid during use. Clamping does not remove the need for machine guards or safe operating procedures.

Making an accurate fixture adds preparation time and cost. Its material and construction must withstand repeated clamping and manufacturing forces. A dedicated fixture suits repeated production of the same component, while an adjustable or modular fixture is easier to adapt when the product changes.

Templates, patterns and moulds do different jobs

A template is a reusable guide for repeating an outline, hole position or bend line. Placing the same template on successive pieces of sheet metal is quicker than measuring every feature again. Card may suit occasional use, while metal or another durable material is more suitable for repeated production. A template can wear or move during marking, so it must remain accurate and correctly positioned.

A pattern is a model used to form a cavity in a casting mould. In sand casting, moulding sand is packed around the pattern, which is then removed. The pattern may be slightly larger than the required casting to allow for metal shrinking as it cools. It may also have sloping sides, called a draft angle, to help it withdraw without damaging the cavity.

A mould contains the shaped cavity that receives the molten metal. Once the metal solidifies, it takes the cavity’s shape. The pattern therefore makes the cavity; the mould shapes the casting. In sand casting, the mould is broken away to release the casting, so another sand mould must be prepared for the next component. The pattern can be used repeatedly to make these moulds.

Patterns and moulds allow shapes to be repeated, but their preparation adds cost and time. A damaged or incorrectly sized cavity can produce defective castings, so the cavity must be checked before the mould is closed and metal is poured.

Casting involves molten metal and very hot equipment and requires trained supervision. The route between the furnace and mould must be clear of trip hazards. Suitable protection includes a full face shield, leather gauntlets and an apron, with purpose-made crucible tongs and carriers used for pouring. Tools introduced into the molten metal must be dry and preheated. This does not mean that green-sand moulding sand is dry: it contains controlled moisture to help it hold its shape. The metal must have solidified and cooled sufficiently before the mould is opened and the casting handled safely.

Sub-assemblies and computer-controlled manufacture

A sub-assembly is a group of components joined before being fitted into the complete product. For example, a metal hinge can be assembled and checked as a unit before being attached to a larger product.

Different sub-assemblies can be made separately, allowing work to proceed at the same time and making smaller units easier to handle and test. However, their connecting features must be consistent: an accurately made unit is still unsuitable if it cannot fit the final product. Separate assembly stages also require coordination and handling.

Check both the individual components and the assembled unit. Small dimensional variations in several components can combine and affect the final fit. A functional test checks whether the unit works, rather than relying only on its appearance. For a hinge, this would include checking that it moves as intended before installation.

Computer-aided manufacturing (CAM) uses computer systems to control manufacturing equipment. A computer-controlled machine can repeatedly cut a metal profile or position holes according to programmed instructions. This reduces repeated manual setting-out and is useful when many consistent components are required.

CAM can improve production speed and repeatability, but equipment and programming have costs. Incorrect instructions can produce the same error repeatedly, and worn tools or poor workpiece positioning can still cause faults. A trial component should be checked before committing a batch to production. Computer control does not replace secure workholding, guarding or trained supervision.

Quality control and working within tolerance

Quality control consists of checks and tests to establish whether work meets the specified requirements. For metal products, checks may include the condition of incoming material, cut dimensions, hole positions and depths, and whether completed parts fit together. A drilling depth stop can help control depth, but the resulting feature still needs checking.

Checks during production allow faults to be found before more work is added. For instance, finding an incorrectly positioned hole before welding the bracket into an assembly avoids wasting the rest of the assembly. Periodic sampling saves inspection time, although it may not identify every faulty item.

A tolerance is the permitted variation in a specified dimension. Suppose a drawing specifies a length of $20.0\text{ mm}$ with a tolerance of $\pm 0.2\text{ mm}$. The acceptable limits are $19.8\text{ mm}$ and $20.2\text{ mm}$. A component within those limits meets that dimensional requirement; one outside them does not.

Tolerances matter because real manufacturing processes produce some variation. They must be tight enough for parts to fit and function, but unnecessarily tight tolerances can require more expensive manufacturing and inspection. Measure the relevant feature, compare it with the permitted limits, and investigate any out-of-tolerance result before continuing production.

Efficient cutting reduces waste

Efficient cutting begins before the first cut. Arrange component outlines on standard-sized sheet metal so that they fit closely together. This is called nesting; shapes that tessellate can fit together with very little unused space.

For example, rotating alternate bracket blanks may let their outlines fit more closely than placing every blank in the same orientation. The layout must still allow for the width of material removed by the cut, secure workholding and the required dimensions.

Where the product’s function permits, a small design change may help components fit standard stock sizes more efficiently. Less waste reduces material costs and leaves less scrap to process. However, saving material must not compromise the component’s strength, accuracy or safe manufacture.