Typical of thousands of plants in the United States, a plant outside Clarksville, Ark., houses several gear hobbers. Although faithful servants, the 30-plus years of wear and tear have taken their toll. Because of worn gears and bearings, the hobbers lost repeatability. Now, dimensional variations have expanded to near unacceptable limits. Also, production requirements are outpacing the basic hobber designs.
Until recently, each hobber was powered by a single-speed ac motor that — through three sets of gears — powered the spindle rotation, workpiece rotation, and vertical feed, Figure 1. To change the operation of a hobber so it could make a different type of part, an operator had to spend 1 to 2 hr. For each of the three gear sets, he had to look up the needed gear combinations on a complex table, select the individual gears, and install each in its proper spot. Should he make a slight mistake in any of these steps — and doing so was common — the operator spent 15 to 20 minutes just to find the goof and several more minutes to correct it.
Although they are still sturdy and sound, the machines just couldn’t cut it by today’s standards. A new machine with the needed capabilities — including the ability to make a changeover in 5 min or less — costs about $250,000 and would take several years to payoff the investment. Such an investment is often hard to justify.
Because these hobbers are in a plant owned by Baldor Electric Co., which supplies servo systems, Tim Baumann, plant manager, understands the capabilities of the existing machines and servo systems. Therefore, his engineering team investigated the retrofit route to increase both accuracy and productivity.
The team decided to replace the three gear sets with three servo systems controlled by a programmable motion controller, Figure 2. The team selected a Baldor dc servo system consisting of a Smart Motion Control Card (SMCC); three, UM servo controls; three, MSeries brush-type dc servo motors, each with a tach (tachometer-generator) and an encoder; power supply; and an operator interface panel.
The SMCC, which holds the operating parameters for each part in its memory, continuously controls the operation of the three servos. The power supply converts and regulates the output dc for the SMCC and the servo controls. To deliver the proper dc voltage and current to each servo motor, a servo control receives instructions from the SMCC and a tach (speed) feedback signal from a motor. The encoder, also mounted on the motor, provides position feedback signals to the SMCC.
In addition, signals from limit switches establish home and end-of-travel locations to the controller.
Finger strokes replace gear changes
A part number and the part specifications for each part segment are entered via the operator interface, Figure 3 , and stored in the memory of the SMCC. Then, when a part is to be machined, the operator enters the part number. The controller pulls the product specifications for that part from the memory table and directs the machine operation accordingly.
With this system, a hobber changeover that took up to 2 hr, now takes less than 5 min.
No free lunch
As with all advancements, this retrofit did require an investment, but it was only 18% of the cost of a new machine tool with the same capabilities — retrofit was $45,000 vs. $250,000 for a new hobber. This retrofit cost includes more than just the servos. It also covers new bearings, modifications to the vertical feed system, and improvements to the hydraulic system that operates the workpiece collet and tail-stock clamps.