A Quick Guide for Precision Machining of Metal Parts
Precision machining creates precise shapes and smooth finishes. These functional surfaces help spools and pistons slide in bores, let housings accept rotating parts like shafts and spindles, and ensure accurate mounting and repeatable assembly.
Precision in machining simplifies assembly and enables maintainability. In assembly it eliminates hand finishing otherwise needed to make parts work together, so saving time and money. It also makes it possible to replace parts when they become worn. An appreciation of what’s involved will aid design decisions that optimize part function and cost.
Precision Machining Technology
The principal machining processes are turning, to produce cylindrical shapes, milling, to produce flat surfaces, and drilling, which creates holes. These may be followed by more specialized processes like grinding, honing and tapping. Here’s an overview of each.
In turning the workpiece rotates and is machined by a single point cutting tool brought against the surface. The tool can either be moved parallel to the workpiece axis, in which case it reduces the part diameter, or across the end face, where it reduces the part length.
Cylindrical bar material and similar workpieces are often turned between centers. This refers to them being held on points at each end. It results in high concentricity between regions of different diameters.
The alternative is to hold the workpiece in a chuck. This grips the part between jaws. Chucking is used when the workpiece is non-round – hexagonal bar for example – and particularly when castings need a round feature turning.
Milling is where the workpiece, often a casting, is fixed in place and metal is removed by rotating teeth mounted on a cutter. (Sometimes a fluted cutter is used. This looks like a thick drill with a flat end.)
Milling produces flat surfaces with a distinctive swirling pattern created as the teeth advance. It’s used on parts like housings and covers where flat surfaces are needed for mounting and sealing.
Milling can create pockets in solid material. This can be slow and expensive, so for quantity production it’s almost always more economical to cast the part and mill for the final dimensions and surface finish.
A twist drill is actually a cutter with two teeth. These rotate around a central axis and each cuts material away to form a hole. Flutes running up the sides of the drill extract the metal that has been removed.
Drilling can be performed on dedicated drilling machines. For quantity production, and to reduce variability resulting from moving the workpiece between machines, it’s more common to drill on a milling machine. An example would be the flat face of a pump cover which needs milling flat and then holes adding for fastening screws.
In grinding a rotating abrasive wheel removes small volumes of material. This yields smoother surfaces and more accurate forms: cylinders and shafts are more true and flats are flatter. Grinding is performed on very stiff machines, which results in high accuracy but also makes them expensive.
This grinding-like process is performed on cylinder bores. It removes very little material but improves roundness and leaves a specific cross-hatched surface texture. This holds oil, making it almost essential where pistons or piston rings will slide across the surface.
Tapping is the process of cutting threads into drilled holes. It’s done with a cutter that has a thread around the perimeter to reproduce the form required. Synchronization of feed rate and rotation result in a thread with the required pitch.
For low volume production tapping can be done on a dedicated drilling machine. It’s more commonly performed on a CNC mill or machining center where not removing and re-clamping the workpiece improves accuracy.
External threads, as might be needed on the end of a shaft can be produced by turning or grinding. Grinding produces a smoother surface but needs an abrasive wheel with the thread form cut in.
Delivering Precision Metal Parts
Precision machining provides the fit and finish metal parts need for easy assembly and reliable performance. A precision machine shop will use advanced CNC machine tools to deliver accuracy and speed. Contact us to discuss your precision machining needs.