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Workholding plays as important a role as machine selection when it comes to determining the quality of metal products. Yet, centers are usually an afterthought, the last thing people think about when buying a machine tool. Even a state of the art CNC cylindrical grinder will produce poor quality shafts if it’s equipped with bad centers. In any centering application, the accuracy of the parts produced is no better than the accuracy of the centers holding the work piece.
Centers have points that fit into tapered holes drilled into the ends of the work piece. A 60 degree included point angle is standard in the United States. Centers are either “live,” with bearings and a point that rotates, or “dead,” solid pieces of steel that don’t move. A dead center can be mounted in a live tailstock with bearings so that it rotates with the work piece, but it is still considered a dead center.
The shell of a live center typically consists of a tapered shank and a housing containing bearings. The shell is stationary, while a solid spindle rotates on the bearings in conjunction with the part. Alternatively, a solid tapered shank with a rotating housing containing the bearings; such as a pipe nose or bull nose live center.
The tapered shank is a key feature of centers. Morse tapers are the most common locking tapers, followed by Jarno and Brown & Sharp. These tapers are considered self-locking since they fit so firmly into the socket that they lock in place. There is considerable resistance to any force tending to rotate the tool relative to the socket. This fit is so tight that some centers have a thread and nut system to help extract them from the socket.
Dead centers should be manufactured from wear-resistant, through-hardened steel such as 52100. Originally developed for bearings, 52100 is very well suited for centers because of its hardness, strength and wear-resistance. In fact, it offers many of the same properties as tool steel at a much lower cost.
Dead centers can be full centers or half centers. Half dead centers have a large cut out removing most of the point. They’re typically used for additional wheel clearance when grinding small parts whose diameter is less than that of the dead center. The cut away section provides clearance so the grinding wheel can advance to the part without hitting the center. Full centers have the entire point.
Carbide tips can be brazed to the same steel shanks that are used in steel centers. They are also available as full or half dead centers. In high volume operations carbide centers are preferred, due to the high wear resistance of carbide. When holding highly abrasive or very hard materials, carbide centers outperform steel centers.
Similarly, in live centers, the spindle or rotating point should be made from 52100 or a similar material. The bodies or “shells” of live centers should be made from a case-hardened steel to increase tensile strength and protect the mounting taper from wear.
Live centers are available with extended points, which provide maximum tool clearance but sacrifice some rigidity, or with shorter points, which offer rigidity but sacrifice some tool clearance. A wide variety of live centers are available. Generally, look for centers that have been designed specifically for your application. CNC heavy duty centers for CNC lathes, spline rolling centers for spline rollers, super accurate centers for grinding, heavy duty and extra heavy duty models for large parts and bull nose centers for parts with large center holes.
Selecting live centers requires a clear understanding of the application’s needs. Rpm, work piece weight, tool and tailstock pressure, center hole size, and the need for tool clearance should all be considered carefully. Manufacturers’ recommendations for maximum work piece weight and RPM play a key role in center selection. Exceeding limits for a live center is extremely dangerous. When in doubt, consult the center’s manufacturer. In general, leave a comfortable margin for both weight and RPM.
Avoid inexpensive centers with case-hardened points, since they will not withstand repeated regrinding, a key life-extending maintenance procedure. After regrinding, be sure that the new point angle and concentricity remain the same as the original.
The taper must be kept free of nicks, dings and burrs, and both center and socket should be kept clean and free of chips, dirt and grinding debris. Once you notice damage, return the center to its manufacturer for repair. Regrinding dead centers and replacing the bearings and spindle of a live center typically takes one to two weeks. Riten Industries, offers expedited services when turn around is critical.
Dead centers usually need reground once they begin to show visible wear. Changes in surface finish are acceptable, but grooves or wear rings are indications that a dead center will need to be replaced soon. With all centers, be sure to check the center hole in the part. Shallow, off-center or badly formed holes and poorly maintained center drills can all lead to workholding problems. Our technical support department will be happy to discuss your application if you believe that your centers are wearing prematurely. Be proactive and contact Riten Industries with any questions. Our technical support personnel deal with a variety of applications on a daily basis. Take advantage of this wealth of knowledge.