Get on a roll with centerless grinding

Author Christopher Tate
Published
May 02,2022 - 11:30am

Centerless grinding was developed in the early 1900s to improve the manufacture of rollers for roller bearings used in bicycles. Today, the process is used by automotive manufacturers, medical device manufacturers and small job shops to produce a countless variety of components.

The operation has a combination of qualities not found with any other machine tool. The process is simple in appearance, exceptionally productive, very repeatable and capable of holding tight tolerances.

Centerless grinding looks deceptively easy. The grinder is constructed with a grinding wheel mounted in a stationary housing on one side of the machine with a work rest and regulating wheel, also known as a reg wheel, opposite the grinding wheel. The reg wheel and tool rest are mounted on a two-slide arrangement in which the work rest is mounted to the lower slide and the reg wheel is mounted to the upper slide. This arrangement allows the reg wheel to move relative to the work rest, which is primarily for setting the height of the workpiece relative to the grinding wheel. The lower slide carries the work rest and reg wheel to the grinding wheel and controls the depth of cut and diameter of the workpiece.

Although the arrangement of these primary components is simple, the effect is significant. Centerless grinding is not subject to errors caused by a part that is out of round or bent. Because the work rest is positioned 90 degrees to the wheels, grinding plane geometric imperfections do not produce irregular motion in the cut plane. This relationship makes centerless grinding the most efficient process for creating very round parts.

Supporting the entire working area with the rest prevents deflection that would occur in other kinds of grinding operations. Therefore, centerless grinding is ideal for parts that are long and slender. Because lengthy, flimsy workpieces are manufactured easily with centerless grinding, the medical industry extensively uses the process to make things like catheters, which have very tight diameter tolerances and would be impossible to create on other types of machine tools. Since the work is well supported and the depth of cut can be as little as a few millionths of an inch, centerless grinding is perfect for thin-walled tubular parts or really brittle workpieces. An extreme example is companies that produce glass tubes with diameter tolerances in the 0.00254 mm (0.0001") range using centerless grinders.

A centerless grinder is constructed with a grinding wheel mounted in a stationary housing on one side of the machine with a work rest and regulating wheel opposite the grinding wheel.
A centerless grinder is constructed with a grinding wheel mounted in a stationary housing on one side of the machine with a work rest and regulating wheel opposite the grinding wheel. Image courtesy of Cutting Tool Engineering

Slide travel, wheel arrangements and the overall size of a centerless grinder limit the maximum working diameter of a workpiece. However, the unique configuration of the wheels and rest allows parts to be fed through the machine from entry to exit without interruption, aka through-feed grinding. So there is no limit to the length of a workpiece. The most common type of centerless grinding is through-feed grinding. Reg wheels are dressed to a slightly concave, tapered shape, which pulls the workpiece through the machine. Therefore, the entire grinding process happens without movement of the slideways. The part enters on one side and is fed across the grinding wheel and exits on the other side. Cutting speed (feed rate) and size are controlled by the shape of the reg wheel.

When set up for through-feed grinding, a centerless grinder requires very little adjustment. Once dialed in, through-feed grinding is exceptionally repeatable, and a well-developed through-grinding process can produce large quantities of close-tolerance parts with no interruption. With a good setup, the only time that the process is interrupted is for wheel dressing. Like all grinding operations, the wheel will wear and need dressing to restore the shape and expose new abrasive grains. Since dressing changes the diameter of the wheel, slide adjustments are required to maintain the diameter of the workpiece. Wheel life on a centerless grinder is typically much greater than other grinding operations, so dressing and subsequent adjustments are infrequent.

Because the process is so stable, centerless grinding is automated easily. High-volume manufacturers outfit the machines with robots, pick-and-place automation and vibratory feeders, eliminating the need for a person to load the machine. It is also common to find automatic gauging attached to the machines, which creates a feedback loop with the control allowing in-process dimensional adjustments and wheel dressing. Automation combined with the very stable grinding process forms an extremely productive system.

Centerless grinding has many obvious applications for high-volume manufacturing or situations in which materials are difficult to machine. The process also has applications at low-volume shops.

At a previous employer, we were making parts from stainless steel tubes, and the 50.8 mm (2") dia. needed to be reduced by 0.8128 mm (0.032") because we could not buy metric tubes. Our tubes had to be 50 mm (1.968"). We were trying to turn the parts, which were 1,219.2 mm (48") long, and struggled with chatter. The result was an ugly finish that required a lot of time polishing to correct. We eventually began sending the raw material to a shop that specialized in centerless grinding and decreased the diameter of the tubes, eliminating our turning operations.

In another case, we were making shafts that were simple cylinders. There were no features on the outer diameter except for a few milled flats. It was the same diameter from end to end. After machining, the shaft was hardened to 62 HRC, and the final finish required grinding. We tried several ways of completing the work on our OD grinder but never could eliminate the mismatched lines and taper caused by grinding a cylinder in multiple setups. As you probably can guess, this was an ideal application for centerless grinding. As in the other example, we sent the shafts to the grind shop, which saved money for us. Later, like with the stainless steel tubes, we started sending the raw material to the grind shop and eliminated the turning operations for this part as well.

Centerless grinding is efficient and perfect for high-volume manufacturing. The process is stable, accurate, economical and many times the only method that can do the job. Although its roots are in high production, centerless grinding can benefit low-volume manufacturers too thanks to the same attributes. 

Related Glossary Terms

  • abrasive

    abrasive

    Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.

  • centerless grinding

    centerless grinding

    Grinding operation in which the workpiece rests on a knife-edge support, rotates through contact with a regulating or feed wheel and is ground by a grinding wheel. This method allows grinding long, thin parts without steady rests; also lessens taper problems. Opposite of cylindrical grinding. See cylindrical grinding; grinding.

  • 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.

  • cutting speed

    cutting speed

    Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

  • depth of cut

    depth of cut

    Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.

  • dressing

    dressing

    Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.

  • grinding

    grinding

    Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

  • grinding wheel

    grinding wheel

    Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.

  • outer diameter ( OD)

    outer diameter ( OD)

    Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.

  • outer diameter ( OD)2

    outer diameter ( OD)

    Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.

  • polishing

    polishing

    Abrasive process that improves surface finish and blends contours. Abrasive particles attached to a flexible backing abrade the workpiece.

  • turning

    turning

    Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.