Finishing Operations
Finishing operations are manufacturing processes carried out after primary machining to improve the dimensional accuracy, surface finish, and functional properties of a component. These processes remove small amounts of material and often correct minor geometric errors left by earlier operations. Common finishing operations include honing, lapping, reaming, broaching, and tapping.
Honing
Principle: Honing uses an abrasive stone or stick mounted on a mandrel that rotates and reciprocates inside a bore to remove small amounts of material and improve surface finish and geometry.
Key features
- Combines rotary and reciprocating motion.
- Produces a characteristic cross‑hatch pattern on the surface.
- Removes minute amounts of material (microns per pass).
Applications
Finishing of engine cylinders, hydraulic cylinders, and precision bores where high roundness and surface finish are required.
Advantages
- Improves surface finish to Ra ≈ 0.1–0.8 μm.
- Corrects minor bore geometry errors (taper, out‑of‑round).
- Enhances oil retention due to cross‑hatch pattern.
Lapping
Principle: Lapping is a precision finishing process in which a workpiece is rubbed against a lap (a soft iron, copper, or lead tool) charged with fine abrasive slurry, under controlled pressure and relative motion.
Key features
- Material removal is extremely small and controlled.
- Produces very high dimensional accuracy and surface finish.
- Can be one‑sided or two‑sided (double‑sided lapping).
Applications
Finishing of gauge blocks, optical lenses, sealing surfaces, and precision flat surfaces.
Advantages
- Achieves surface finish Ra ≈ 0.01–0.1 μm.
- Removes subsurface damage from previous machining.
- Can produce flatness within a fraction of a micron.
Reaming
Principle: Reaming uses a multi‑edge cutting tool (reamer) to slightly enlarge and finish a previously drilled or bored hole to a precise diameter and smooth surface.
Key features
- Removes a small allowance (typically 0.2–0.5 mm on diameter).
- Reamer has straight or helical flutes for chip removal.
- Cutting speed is lower than drilling; feed is higher.
Applications
Finishing holes for dowel pins, bearings, and other precision fits.
Advantages
- Improves hole tolerance to IT7–IT9 range.
- Surface finish Ra ≈ 0.4–3.2 μm.
- Quick and economical for high‑accuracy holes.
Broaching
Principle: Broaching uses a multi‑tooth cutting tool (broach) with progressively larger teeth to remove material in a single pass. The tool is pushed or pulled through the workpiece.
Key features
- Each tooth removes a small layer; successive teeth deepen the cut.
- Can produce complex internal or external profiles in one stroke.
- Requires a broaching machine or suitable press.
Applications
Cutting keyways, splines, internal gears, and non‑circular holes.
Advantages
- High production rate with consistent accuracy.
- Excellent surface finish (Ra ≈ 0.8–3.2 μm).
- Can produce complex shapes in one operation.
Tapping
Principle: Tapping uses a tool called a tap to cut internal threads in a pre‑drilled hole. The tap has cutting edges and flutes to form and clear the threads.
Key features
- Performed by hand (tap wrench) or machine (tapping attachment).
- Requires correct tap drill size to ensure proper thread depth.
- Lubrication is important to reduce friction and prevent breakage.
Applications
Producing threaded holes for bolts, screws, and studs in metal, plastic, or composite parts.
Advantages
- Quick method for creating internal threads.
- Available in standard and special thread forms.
- Can be used on a wide range of materials.