Live centers are used to hold or support a workpiece in a lathe or other machine tools, often between the headstock and tailstock. Live centers revolve with the work. Dead centers do not.
The following criteria should be used to determine which of the available live centers is best for the application at hand. First, the taper of the machine should be determined. Most lathes have Morse taper, while grinders often use Jarno and Brown or Sharpe tapers. Once the taper is determined, the weight of the workpieces should be figured into the equation. Many machine tools have a listed “load rating,” which, along with the anticipated cutting force can help to determine this. Additional specifications include the thrust load needed to keep the workpiece seated on the live center (generally this value is 2/3 of the workpiece weight), and the point style that is best for the given application.
Live centers are available with a wide degree of points, which are the portions of the center, which pierce or hold the work piece. Common point types include standard points, bull nose points, long points, pipe points, and tracer points. Standard points have a constant point angle and have no special attributes such special tips or hardening. Bull nose points have flat faces, which allow the work piece to sit at least halfway up the point. Long points have a small straight diameter extension near the point that allows for better tool clearance. Pipe points are specialty holders used with tubing, piping or other thin walled pieces. Tracer points have a standard angle tip which then changes to a shallower angle on the remaining portion of the point. For instance, the tip angle is about 60 degrees and the rest of point angle is 30 degrees.
The final criterion for live centers is the operating speed, bearing in mind that specialized tips and features for live centers are available for high-speed and hard material applications. Some of these tips and features include carbide tips, high-speed engagement centers, super accurate centers, chucking centers, and changeable or replaceable centers.