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Basics of Centerless Grinding: Cutting Tool Engineering

May 6, 2021
Centerless grinding is used for making precision parts such as rods, bars, bolts, and spindles. The process also can be a stand-alone operation, or it may be combined with other metalworking processes to make complicated shapes from intricate patterns.

Centerless grinding is a metalworking process that includes both turning and grinding. It can be used to make cylindrical shapes from flat or rectangular workpieces. The centerless grinder uses a rotating cutting tool called the "center" to grind material away while simultaneously rotating the workpiece around its axis, which causes it to become shorter in length as the material is removed.

Parts made using centerless grinding include machine tooling parts as well as automotive components like crankshafts and connecting rods.

One advantage offered by this technique is the speed at which workpieces can be processed. Because there is no need to change tools, there is the ability to go directly from turning to grinding while rotating the workpiece around its axis. As long as there is enough room on the lathe chuck, both end faces of cylindrical parts can be ground at the same time.

Another advantage is that centerless grinding is versatile, and it’s possible to grind a variety of different shapes while using just one tool. For example, if you need a flat face on your workpiece but also want an angle grinder, then this technique will allow you to achieve both finishes in tandem.

As with any precision machining process, the main considerations when choosing tools are the surface finish required and material tolerance requirements. In general terms, however, there are two basic approaches: use wheels or use form tools, such as abrasive belts that come into contact with the workpiece.

Another consideration to keep in mind is that the material being ground can create challenges not found on a conventional lathe or milling machine, such as chips being drawn into an adjacent workpiece, cross-contamination from one part to another, grinding swarf accumulating around moving parts, and general difficulty maintaining constant power due to variations in load during machining operations. These drawbacks of centerless grinding generally have been overcome by specialized designs that reduce the amount of grinding swarf, reduce the size of chips, and efficiently evacuate them to other areas.

The primary types of centerless grinders include:
-  Single-point grinders
: These are used on a variety of materials and provide the closest tolerance. They also produce minimal swarf but can only be mounted in one position due to their design.

-  Multi-point grinders: These use multiple contact points with different angles that act like many fine cutters, for an improved finish at the cost of an increased potential for generating swarf. A notable benefit is that they can be angled or moved out of the way while operating without the need to change tools, as there are no cutting edges exposed during operation.

-  Fixed orientation: This type of centerless grinder may use wheels made from different materials, such as diamond, carbide, ceramic, aluminum oxide, or zirconia to cut different types of material depth ranges, depending on their hardness level. These hard materials provide high durability but are more expensive than softer ones, like silicon-nitride grinding wheels, which wear out faster but do not corrode easily and cost less per unit volume.

A single-point grinding wheel has two configurations: it's either fixed at 90 degrees relative to its center axis, or it's mounted at an angle of a few degrees. The former version is used in cases where the workpiece needs to be cut straight across by removing metal from either side. For example, when making gears, the wheel has to face perpendicular with respect to axes on both sides so they can mesh and spin smoothly.

The latter configuration is used more often in machining because it allows for the workpiece to be turned around and fed back into the grinding wheel, so that a spiral pattern can be cut on a cylindrical shaft. This ensures concentricity between two parallel walls of an internal borehole.

Wheel-based grinders work use a grinding wheel that is either fixed or adjustable with respect to the part. Ring tools are held in a stationary holder and can be rotated, while handwheels on these tools allow for fine adjustment of their position. They're often used for finer grain sizes than what's possible with wheels because they don't produce as much heat — but this does mean there is less material removed per stroke.

With wheel-based grinders, on the other hand, grinding stones are available in smaller sizes and there's more material removal per stroke, albeit at coarser grain sizes than what can be achieved by ring tools. They also produce considerably more heat, so metalwork will need to be cooled periodically if heavy use continues for long periods of time.

The centrifugal design of the wheel-based grinder incorporates a small radius “cutting head” that rotates in an orbital motion at high speed on its own central axis under power supplied from an external drive motor. When it is properly fitted with cutting tools, this type can produce fairly good surface finishes even when working long lengths of material because the workpiece moves past the rotating spindle rapidly enough for both sides to be ground uniformly.

Ring tool designs use a stationary cutter mounted on one side of a workpiece and a rotating cutter mounted on the other. This type of centerless grinder can produce better surface finishes than wheel-based designs if used with good quality, sharp cutting tools.

The type of workpiece that can be ground by using these techniques includes metal plates as well as welded steel sections, cast iron components, and other similar materials where size constraints are important but an additional finish outside the range offered by conventional finishing technologies is required.

When determining what techniques your job requires, from 3D printing to centerless grinding, there are general considerations to keep in mind. Most often, centerless grinders are used for long, cylindrical workpieces. For this reason, they're a great choice when you need to machine flat surfaces on gear wheels or other parts with parallel walls like shafts and pipe ends. They also can be effective at straightening the edges of round stock by removing metal from both sides.

Centerless grinding also is used to produce special shapes from solid steel plates, and for finishing welds that are too close together or narrow to be done effectively with other techniques. Centerless grinders can work on a variety of materials including hardened steels and cast irons.

Christine Evans is the Director of Product Marketing & Content Strategy at Fictiv, a manufacturing platform for sourcing design and engineering teams, to streamline prototyping and accelerate product development. Contact her at [email protected]

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