Entrust Tool & Design Co. Inc., a manufacturer of Unisig and Davis & Thompson machine tools, needed a particular arrangement of components for the cutting mechanism on its new USC50, a 7-axis, 2-spindle deep-hole machining center for heavyduty mold and diemaking. Because this large-capacity machine tool was intended for four-sided machining and deep-hole drilling on workpieces measuring up to 40 in. by 120 in. on a 25-ton CNC rotary table, an extremely high degree of accuracy and rigidity was required, especially at full extension of the tool tip.
The machine had to be highly productive as well as extremely precise, even at the maximum speeds of 394 ipm. And, with a 6.30-in. quill and 19.7-in. travel, an ANSI 85.50 CAT 50 spindle was designed into the machine to handle the substantial torque, so a power drawbar was essential for the machine. Also, a nearly backlash-free worm gear assembly would accommodate the load and provide the stability and repositioning for satisfying the accuracy specification.
After surveying available suppliers, Entrust Tool & Design sought out a longtime associate, Advance Machine& Engineering to assist with these critical components.
For the precision power drawbar, Entrust Tool & Design preferred a standard product that featured bolt-on reliability, through-spindle coolant, steep taper air blast at stand still for blow off, and standard integral sensors for feedback. All the features had to be configurable to customer needs.
Based on previous automotive experience, Entrust Tool & Design selected an OTT-Jakob power drawbar with rotary union and steep taper gripper. This modular ES Series drawbar provided the design flexibility for matching to customer applications and the rigidity for quality part finishes.
The drawbar delivered an optimal connection between the toolholder and the spindle – toolholding tolerances on the USC50 were targeted at 0.0002 in. This, combined with a speed capability of 36,000 rpm, a high pull force, and rotary union options for programmable coolant and lubricant delivery, solidified Entrust Tool & Design’s decision to spec the OTTJakob power drawbar and related components.
Entrust Tool & Design turned to Advance Machine & Engineering again for the worm gear assembly. The decision was made to go with an OTT worm gear design with programmable backlash setting.
Advanced Machine & Engineering, after testing, supplied Class 2 worm gears at no additional charge to the customer, even though a DIN standard Class 3 set of worm gears fulfilled application requirements. Dan Lapp, a vice president at Advanced Machine & Engineering, said that a conventional solution would not work and that a double worm gear set was more applicable in this situation.
He explained that typical machine tool worm gears on the market often have split shafts and are spring-loaded to compensate the stick slip conditions.
In the OTT design, he noted that backlash can be virtually eliminated or programcontrolled as a variable value within one operation cycle. By the positioning of the shaft worm and hollow, the backlash can be changed. The OTT-Matic feature of the OTT worm gear allows a definable minimum or maximum backlash, depending on the process operation, and can be set alternatively while the gear is working.
