working with tubular goods
A shop feeds the tubing through the machine spindle so it extends from the clamps on the front of the workholding chuck. Any bend or other inconsistency in the tube will result in runout that leads to the machining of unacceptable “black threads” that produce leaky connections.
The Problems with Measuring Concentricity Operators try to compensate for runout by shim- ming the tubing in the chuck, repeatedly using an indicator to measure concentricity. And in addition to wasting time, man- ually placing shims in the machine chuck jaws while working with such large workpieces creates a dangerous environment for the machine operator. Depending on the condi- tion of the tubing and the skill of the machine operator, the shimming process can take a few minutes or much longer. Further, even a lengthy manual shimming process does not guarantee accurate thread production. In any case, the minutes spent shimming add up in high-vol- ume production of energy-related tubing. One Texas shop, for example, processes as much as 300,000 tons (272,000 t) of pipe each year, so the slightest savings in time translates to faster delivery and cost reductions.
Self-contained, automatic pipe-centering, pneumatic-
powered six-jaw front-end chucks provide a solution to threading time, accuracy and safety concerns. LVE air-actu- ated chucks from Röhm Products of America, for example, feature three jaws at the chuck face as well as three center- positioning arms. Behind the spindle, another set of jaws provides additional support and clamping force and helps to center long sections of pipe.
One Texas shop, for example, processes as much as 300,000 tons (272,000 t) of pipe each year, so the slightest savings in time translates to faster delivery and cost reductions.
In operation, rollers feed the tube from the rear of the machine through the spindle to a stop. The chuck jaws behind the spindle clamp the pipe, and the three centering arms in front of the chuck push the pipe end into position for accurate, concentric threading. Then the three compensating jaws at the chuck face clamp the pipe firmly, maintaining the established concentricity. Finally, the three centralizing jaws retract and the threading operation can begin.
Seconds Rather Than Minutes The centering and clamping process takes place in about 10 seconds, compared with the minutes and more consumed in manually shimming and indicating the pipe. Automat- ic operation eliminates the possibility of operator injury or error.
Air actuation of the chuck
LVE chucks feature three jaws at the chuck face as well as three center-positioning arms.
offers advantages as well. The actuating cylinder is incorpo- rated in the chuck body and the arrangement requires no drawtube, permitting use of the full spindle bore diameter. Compressed air acts on a pis- ton that transfers force to the chuck jaws via a wedge mechanism, and non-return
valves maintain the chucking pressure. The pneumatic system is separate from the machine’s hydraulic system and can easily be retrofitted to existing machines. Chucks are available in a series of sizes to suit different pipe diameters. As in the case of many innovative technologies, acceptance of self-centering air chucks has proceeded slowly, despite their clear benefits. When it comes to couplings, part length is less of a consideration in threading operations. Factors that do take precedence are large diameters, heavy weight and a need for precise concentricity and thread matching. A coupling typi- cally has ID threads on two ends, and machining the threads requires two setups. Clamping and unclamping a large, unwieldy part is difficult and time consuming, and part quality can suffer due to variations that occur during the reclamping process.
No Unclamping Required
Indexing chucks with a swiveling center section, such as those from Röhm, enable operators to clamp a part once,
72 — Energy Manufacturing 2015
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