Solid Advice about Centers
By Samuel J. Wilkof
Vice President, Stark Industrial Incorporated
North Canton, Ohio
Reprinted from the June 1996 edition of Metlfax
Introduction
Centers are one of the most commonly used work-holding devices. In comparison to other work-holding devices, centers are probably the least expensive, dollar-for-dollar, but most valuable in function performed. Although there are two basic types of centers - live and solid (or dead), this article will focus on solid centers. Choosing the correct center for an application, using it properly in that application, and maintaining the center for future use are all critical points to consider.
Common Taper Systems
Selecting the correct shank size is crucial. While it may seem the easiest part of choosing a center, mistakes do occur. Three of the most commonly used shank types are Morse Tapers, Brown & Sharpe Tapers, and Jarno Tapers. These tapers are referred to as self-holding tapers because they stay in place when seated properly due to the small angle of taper. Generally, no other mechanical method of holding, such as a draw bar, is required.
The Morse Taper system was first introduced by the Morse Twist Drill and Machine Company and was based upon a taper-per-foot of 5/8 inch. In actuality, none of the Morse Tapers are exactly .62500 per foot, but rather range from .59858 to .62565 taper-per-foot. The definition of taper-per-foot is the difference in diameter between two points 12 inches apart measured along the axis of the shank.
Brown & Sharpe Tapers were introduced by the Brown & Sharpe Manufacturing Company and have a nominal taper-per-foot of 1/2 inch. Once again, the tapers are not exact in this specification, but range in size from .49973 to .51612 taper-per-foot.
Jarno Tapers were also introduced by the Brown & Sharpe Manufacturing Company. All Jarno Tapers have a taper-per-foot of .60000 exactly. Further, identifying Jarno Tapers is quite easy. The size of the taper indicates that the gage line diameter (the axial position on a taper where the diameter is equal to the basic large end diameter of the specified taper) is that number of eighths in size. The small end diameter is that number of tenths in size, and the length of the taper is that number of halves. As an example, a #10 Jarno Taper is 10/8 or 1.250 inches in diameter at the large end - 10/10 or 1.000 inch in diameter at the small end - and 10/2 or 5.000 inches in length.
There is another series of self-holding tapers sometimes referred to as the ASA series having a taper-per-foot of .75000 inches. This taper is used routinely, but not nearly to the extent that the other self-holding tapers are used.
Steep tapers are tapers that must have some type of mechanical locking mechanism - a draw bar, retention knob, or locking collar to hold them in place. One of the advantages of steep tapers is their self-releasing ability making removal very easy.
For assistance in determining taper size, the Machinery Handbook is extremely helpful. Gage diameter, small end diameter, length of taper, and taper-per-foot are clearly specified in the handbook. In the case of all tapers, the tolerance on a taper may only be applied in a direction which increases the rate of taper.
Using a Center Properly
Once identified and in hand, the center should be absolutely clean and free from any burr. Before attempting to seat the shank in its mating spindle, the spindle must also be clean and free from burrs. Several spindle wiping devices are available for cleaning the spindle prior to seating the center. The center should be seated in the spindle slowly to avoid nicking either part. In a properly manufactured spindle-center relationship, the spindle should be harder than the shank of the center. Centers are more easily replaced and refurbished than are spindles. When the center is within approximately one inch of being seated, rapidly "slap" the center the remaining distance. This will lock a self-holding taper in place.
Proper headstock and tailstock alignment is important whether they are part of a lathe, grinder, or inspection device. Proper alignment will assist in producing a good finished product because maximum rigidity and conical point contact will have been achieved. Center hole size should be carefully considered keeping workpiece weight in mind. Too small a center hole can cause deflection and even deformity of the center point. If the center hole is too large, the point of the center can "bottom out" in the hole, essentially leaving no support at all.
Tungsten Carbide Tipped versus Solid Steel
Virtually any solid center can be either 52100 steel or tungsten carbide tipped. The mechanical properties of the workpiece should influence the choice of point material. A carbide tipped center is a good choice for hard or abrasive workpieces, or if the rotational speed of the workpiece is high. Just as surface footage is calculated to determine the optimum type of material for a cutting tool, so can surface footage be calculated when selecting a center.
To Calculate Surface Footage When RPM is Known:
(Diameter/12) x 3.141 x RPM = SFPM
For diameter, substitute the largest diameter of the point in contact with the workpiece.
When the surface feet per minute (SFPM) exceeds 100, a carbide tipped center should be strongly considered.
Centers, both steel (manufactured from M-Type HSS steel) and carbide tipped, can be coated with titanium nitride (TiN). Titanium nitride provides a hard surface, well suited to resist the rotational, abrasive conditions under which centers operate.
When to Resharpen or Retip
Too often, centers, just like cutting tools, are pushed beyond reasonable limits. A center point should be examined regularly throughout its use to determine any growing circular wear pattern. Sometimes a ring of discoloration appears on a center. If no surface defect can be felt at the point of discoloration, then it is not necessary to re-point the center. If there is an irregularity in the point surface, the center should be reground.
Both steel and carbide tipped centers can be reground, and assuming a minimum amount of wear has occurred, the cost of regrinding is usually very reasonable. If there is an extensive amount of damage to the point - such as cracking or breaking - the cost of refurbishing will undoubtedly be more significant. The point on carbide tipped centers can be removed, replaced, and reground. When severe wear or breakage occurs to a steel center, hard turning can be used to rough the point of the center so that the actual finish grind time to re-point the center can be minimized.
Conclusions
Whether a center is being used in an old turret lathe or on a state-of-the-art CNC cylindrical grinder, proper fit and point integrity will determine the difference between success and failure. There is no substitute for well-drilled and lapped center holes and no substitute for a high quality, accurately manufactured center. A finished part can be no better than the work-holding and tooling used in its production.