Producing Precision Holes and Bores
Used for fasteners and alignment pins, for lubrication and as part of the product function, holes are common in many precision machined components. Depending on their purpose holes may need tight tolerances, accurate placement, and smooth or textured surface finishes. Alternatively, a hole may need to be nothing more than a cavity or void in the part. This overview of hole production techniques will help part designers understand what to consider.
Casting vs. Machining
For quantity production, the least expensive way of producing a hole is usually to cast it. This entails placing a core in the mold cavity to occupy space that would otherwise be filled with metal. Once the metal has solidified the cast part is released and the core is removed.
Casting holes with cores has some limitations. The surface finish will be no better than that of the core, metal shrinkage will cause some dimensional variation, and there may be some variation in core placement. For parts like valve bodies and housings, cores are a good way of reducing the amount of metal used, but some machining will usually be needed.
Holes in solid metal parts are produced by drilling. This may be followed by secondary processes, depending on the function of the hole. Cast parts will often need through-holes with clearance for fasteners while blind holes are typically tapped (threaded.)
The twist drill used to machine holes has two cutting edges on opposite sides of a cone. Each skives material away from the bottom of the hole, pushing it out of the hole.
Small diameter holes (1/16th) or less, can be hard to locate precisely because the drill may bend and wander. This is particularly likely when the hole is deep and the drill protrudes significantly from the chuck or tool holder. Wandering is minimized by creating an indentation on the surface to capture the drill tip.
Deep holes, where the depth is more than five times the diameter, pose two challenges. The drill can bend as it moves through the workpiece, creating a non-straight hole, and the flutes can clog with chips. This generates frictional heat which can seize and break the drill. When a drawing calls for deep hole drilling a precision machine shop will typically use a specialized process called “gun drilling.”
Boring is a form of turning, (the operation performed on a lathe,) used to machine internal bores. Like turning, it’s a single cutting point technique, and bore size and depth are constrained by the need to insert the cutting tool. Boring is often used to create accurately machined surfaces on cast holes. Housings often need boring to create surfaces for installing seals or bearings.
Holes cut by drilling often have poor cylindricity and rough surfaces. For clearance holes or shallow blind holes that will be tapped this isn’t a problem. If however, the hole needs to be of a precise size, shape, and finish, as is the case if a dowel pin is to be inserted, it will need reaming.
Reaming is performed with a tool the size and shape of a drill bit that cuts along the sides. By shaving away slivers of material it improves cylindricity and surface finish.
Holes that have been bored, and shallow holes where the diameter exceeds 1”, may be finished by internal grinding. Similar to boring, a small diameter grinding wheel is moved into the bore where it rotates at high speed to remove metal and improve size and shape. Valve bodies may require internal grinding to create smooth surfaces and maintain the tight tolerances needed to operate.
Cylinder bores in engines and elsewhere often need a specific, oil-retaining surface finish to provide a seal. This is produced by honing. In honing abrasive blocks are moved up and down against the walls of the bore while simultaneously rotating. This results in a diamond-like pattern of microscopic grooves that retain a lubricating film.