Machining is the costliest method we have for creating part geometry. When possible, engineers will rely on casting, forging, welding and similar processes and avoid machining wherever they can. However, there are some geometric features that must be machined to ensure the integrity of the component and the final assembly.
Rotating components, like shafts, and the supporting features like bearing housings, are good examples of features that must be machined to ensure integrity of the product. Obviously, creating precise features on a shaft is critical to proper operation. Likewise, the features that support the shaft bearings are critical like those of the shaft.
Bearings are usually pressed into a “seat” creating an interference fit that holds the outer race of the bearing stationary. Bearing seats must be straight, round, sized correctly and have the proper surface finish. If these characteristics are not maintained to the design requirements, the assembly can suffer premature failure.
In most designs, bearings operate in pairs. Alignment of the bearings is a critical factor as misalignment can cause shaft wear, bearing failures, eccentric shaft motion and other problems. Obviously, bearing alignment is dependent on the accuracy and precision of the machined bearing seats.
Machining the seats without repositioning the part is the best solution. In many situations this is not a difficult task and can be accomplished with standard tools. When parts are very large or bearing bores have a large depth-to-diameter ratio things can be difficult.
When the boring tools get long, they can chatter and will droop under their own weight. Ultimately, they are unable to create the de sired geometry. One way to solve this issue is by installing guide bushings that support the cutting tool as it bores the seats. In high-volume settings the guide bushings are installed in the workholding and have little impact to the machining time.
In low-volume settings the cost of complex fixtures with guide bushings is prohibitive. Machinists and engineers must get creative sometimes. It is possible to bore one of the seats, install a guide bushing and use it as a support for the second bore.
Field machinists have a plethora of tools for “line boring,” which is the term used for boring long, straight bores. These devices used in field machining can appear crude compared to a nice boring mill or vertical lathe, but they work well for creating straight, accurate bearing bores when the job can’t be taken to the shop.
Having bearing seats that are well aligned is no good if the seat is the wrong size. Maintaining proper bore diameters ensures the bearing fits in the seat as designed. Using a good boring tool that is set correctly and paired with the proper cutting parameters is the first step to sizing a hole correctly. Find a good cutting tool manufacturer and let them guide you to the best tool for the job. Modern boring tools are phenomenal, and finding a tool for boring tight tolerances has never been easier (or more expensive).
In the modern machine shop, diameter issues are more often a metrology issue than a cutting tool issue. Air gauges are the most accurate way to check bore diameters. Air flows through a small orifice that creates back pressure when the flow is restricted. After being set with a master ring gauge, a low pressure reading indicates a hole that is larger than the master. Conversely, a high pressure reading indicates a diameter that is smaller. Air gauges are expensive and usually reserved for high production or extremely expensive parts.
A high-quality dial bore gauge is sufficient for measuring most bearing seats when the added cost doesn’t justify an air gauge. Like the air gauge, it is always best to set the dial gauge with a gauge ring. More than once, I have seen parts rejected when someone used a micrometer to set a bore gauge. That’s a common, but risky practice.
Sometimes there is a roundness requirement on the drawing that mandates a roundness check, and it is a good practice to check roundness even when there is not a tolerance. Both types of gauges are also good for checking roundness of the bore.
I witnessed a machining line in the automotive industry scrap a lot of parts that an air gauge indicated were good. The bearing seat was machined with a rough and finish-boring operation. The roughing tool was set wrong and did not leave enough stock for the finish tool to correct the clover leaf shape left by the roughing tool. After finishing, the hole was lobed where the finish tool cut in some places but not in others. Machinists could not see the problem because the diameter was about 10" deep in the part. If we had been checking roundness, the problem would have been detected sooner.
Surface finish of the machined seat is less critical to bearing and shaft life than the other elements. This does not mean it can be ignored. Surface finish can affect the fit of the bearing to the seat reducing the amount of interference between the two. If the surface finish is not specified, there are design specifications published by the bearing manufacturers that give recommended surface finishes for bearing seats.
Rotating components can be exposed to severe conditions like high loads and high rpm. While the type and quality of the bearing is the primary engineering consideration, the structures supporting the bearings influence the function and life of components in the system. Machining bearing seats is one of those operations that requires additional care and attention to ensure equipment works as designed.
Related Glossary Terms
- boring
boring
Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.
- bushing
bushing
Cylindrical sleeve, typically made from high-grade tool steel, inserted into a jig fixture to guide cutting tools. There are three main types: renewable, used in liners that in turn are installed in the jig; press-fit, installed directly in the jig for short production runs; and liner (or master), installed permanently in a jig to receive renewable bushing.
- chatter
chatter
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- depth-to-diameter ratio
depth-to-diameter ratio
Ratio of the depth of a hole compared to the diameter of the tool used to make the hole.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- metrology
metrology
Science of measurement; the principles on which precision machining, quality control and inspection are based. See precision machining, measurement.
- micrometer
micrometer
A precision instrument with a spindle moved by a finely threaded screw that is used for measuring thickness and short lengths.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- tolerance
tolerance
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.
Author
Christopher Tate is manufacturing engineering manager at Unified Brands Inc. in Vicksburg, Mississippi.
