Hobbing helps out: technique improves drill motor performance in search for 'crude'.
Leistritz Corp., of Allendale, NJ, studied the challenges with several well drilling and tooling companies, hoping to find a solution to the machining problems long associated with power section rotors. Excessive tool wear was one of the major challenges. Replacing and indexing inserts was another.
Steerable drill motor
Power section rotors connect directly to the drill bit in directional drilling for oil and gas discovery deep below the earth's surface. The power section is a "positive displacement mud motor" combined with directional technology to steer the drill motor using tilting joints and sophisticated sensors while drilling for oil or gas.
The motor converts the drilling fluid's hydraulic energy into mechanical energy in the form of torque and rotational speed. This conversion is accomplished through the use of a spiral rotor that fits into a stator of similar shape, to form a tight seal. These systems are now capable of drilling within [+ or -]0.15[degrees] to depths to 18,000'.
The rotors within the mud motors are called power section rotors, which are typically five-, seven-, or nine-lobed with smooth spiral profiles in lengths as long as 8m with long leads of up to 40". The rotors fit snugly into an elastomer stator having one additional lobe contour with the same lead, creating a moineau-style progressive cavity positive displacement rotary drive unit.
When the water-based drilling fluid or "mud" is pressurized at the surface and pumped down the hole to the power section, a high-torque, low rpm drive is created to rotate the rock-crushing drill bits.
These rotors have traditionally been milled using a side milling operation to interpolate the profile using CNC control in a connected series of steps. Excessive tool wear, long cycle times, and profile inaccuracies plagued drill tooling manufacturers striving for better drill motor performance.
The milling process produces a snakeskin-like surface that requires many hours of polishing prior to chrome plating. This polishing has been, for the most part, a hand operation. A balance has been needed to weigh shorter cycle times with more scalloping to polish versus a better surface but longer cycle times.
The 24 button inserts in a typical side-milling cutter need replacing or indexing several times during the multiple shift operation of each rotor. This, and continuous measurements by the operator in the middle of a rotor, create unacceptable non-productive time.
Leistritz developed an application-specific solution by combining an existing heavy-duty machine base with a newly designed high performance, multi-axes drive head. This drive head is capable of both side milling and hobbing.
The fundamental idea of hobbing is new to the industry and opens the door for shorter cycle times and much improved profile quality. In effect, these rotors are handled as long gears, but the low number of lobes or teeth creates unique challenges for the hob tooling and machine tool.
A 30hp spindle with high tooth-load hob designs guarantees speed and surface finishes previously unattainable. The 17-4PH stainless steel rotors are machined in a single pass using a large diameter hob (up to [empty set]10") with a face-width of 8".
Key to hob life
Superior hob life is achieved by using continuous diagonal hobshift to utilize the entire hob face during the cut of each rotor. Therefore, a rotor can be hobbed in a single non-interrupted cutting pass with straightness less than 0.002".
A variety of hob materials and coatings have been tested to maximize the hob life and thus reduce the cost per piece. The goal of producing four 315"-long rotors per hob sharpening has been reached and in some cases exceeded.
With 15 sharpenings permissible for each hob, the process has proven to be economical. The long lead of these rotor designs requires special solutions for supporting them while machining.
Leistritz Corp., a machine tool builder most known for whirling machines, uses CNC servo-controlled, self-centering, follower rests both for the raw material in front of the cut and the finished rotor behind the cut to achieve vibration-free cutting.
Manufacturers of mud motors are experiencing hobbing cycle times less than half those of milling while also saving most of the time required to polish. In many cases, polishing is eliminated and the rotors go straight to chrome plating.
The hobbing process also produces a profile exactly to the design specifications within [+ or -]0.001", and the profile is no longer deviated by the polishing process. The better profile tolerances lead to better drill motor performance in both consistency and torque.
Programming of the hobbing process is done at the machine by the operator since only a few critical parameters are needed to determine the necessary rotor criteria.
To reduce the learning curve to manufacturers of these rotors, Leistritz has kept milling as a secondary alternative within the machine tool function. The flexibility of milling allows manufacturers to maintain their known process and add hob tooling as desired for their more commonly produced sizes.
The improved rigidity and spindle speed of the Leistritz machine accommodates accelerated cutting speeds and feeds to reduce milling cycle times by 30-40 percent. When milling, Leistritz has a software package to generate the profile contour and to optimize the feedrate calculations by maintaining a constant chip thickness over the spiroid contour, thus improving cycle times.
In addition, the Leistritz Power Mill machine can use CNC programs of existing equipment to continue side milling for even greater flexibility. The advantage here is the Power Mill machine is capable of 800rpm for future tooling developments such as ceramic cutting tools.
Leistritz Corp., www.rsleads.com/903tp-157
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|Title Annotation:||oil, gas|
|Publication:||Tooling & Production|
|Date:||Mar 1, 2009|
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