Bar pullers don’t offer the extended amount of unattended turning time that bar feeders allow a lathe to enjoy. That said, they do allow shops to either gain some extra machining time overnight or enable one operator to tend multiple machines during the day. Plus, they’re a small fraction of a bar feeder’s cost, they don’t take up any floor space and they don’t need to be wired to the lathe.
A bar puller installs in a lathe’s turret and is used to pull a short length of barstock out of the lathe’s spindle. After the bar puller’s gripper secures the end of the bar, the lathe’s turret retracts along the Z-axis to bring a portion of the bar into the workzone. Once turning and part cutoff are completed on that section of the bar, the turret indexes the bar puller into position to retract the bar once again. This process repeats until the bar is consumed.
The issue with dedicated bar pullers is that they require their own turret position along with all the cutting tools for the various turning operations. Some companies, such as Somma (Waterbury, Connecticut), offer a combination tool that has both a bar puller and cutoff blade. This way, only one turret position is required for the functions of bar pulling and part cutoff. The combination tool also eliminates the need to index between cutoff and pulling, decreasing cycle time.
After turning is completed on the end of the bar, the device’s blade-style tool cuts the completed workpiece from the bar while the bar puller’s jaws simultaneously engage the end of the bar. The puller’s jaws fully grip the bar once part cutoff is completed. The turret then can retract to position the bar for the next turning operations.
The model that Somma offers is adjustable to grip diameters from 0.125 to 3.25 inches. It uses spring-steel fingers that expand over the bar to provide the gripping force. A single screw is used to adjust the jaws for a new job. Each full turn of the screw moves both jaws in or out by 0.1 inch.
When using a bar puller, each new bar should be sized to fit completely within the spindle. During setup, the bar should be positioned an appropriate amount from the chuck so the first part can be turned. The first bar-pulling operation occurs after the first workpiece is created. Programmers must determine the number of parts that can be produced for a given bar length, because the puller can’t signal the machine’s control that the bar has been consumed.