New Approaches to Making Parts for the Oil Patch

Rod Zimmerman of cutting tool manufacturer Iscar Metals lives in a pleasant green zone in a Fort Worth suburb. Yet within a half mile of his home, an oil company has sunk a vertical hole 7,500′ (2,286 m) deep, from which it has splayed nine lateral lines, each going about half a mile. It’s pumping fracking fluid into these lines at 9,000 psi to extract the natural gas. It’s as good an example as any of the ubiquity of the fracking revolution and the amazing technology involved.

TACKLING THE FRACKING BLOCK

As alluded to above, hydraulic fracturing (more commonly called fracking) recovers gas and oil from shale formations by drilling into the earth and injecting a mixture of water, sand and chemicals into the rock at extremely high pressure. Generating that pressure requires giant pumps, a key component of which is the “fluid end” or “fracking block” and its valves, pistons, and liners.

The high pressure and abrasiveness of the slurry pumping through these blocks cause rapid wear and cracking. And since oil field operators need to maintain high pressure, they’re forced to replace the fluid ends every few days, according to Paul Best, a product specialist at Allied Machine, Dover, Ohio. Best said fluid ends are machined from solid blocks of steel and are generally about 3′ tall, 4′ wide and 2-3′ (0.61-0.91 m) thick. With complex contours, numerous channels, three to five through-holes (each up to 8″ [203.2 mm] in diameter), and three to five intersecting cross-holes, fluid ends require huge amounts of milling, drilling, and threading.

Zimmerman, vice president of sales, West Zone for Iscar, said fluid ends have traditionally been made from 4000 series alloy steel, but to combat wear, users are trying “more exotic alloys like 17-4 PH stainless-steel material, super duplex stainless and 15-5 stainless.” Deluca estimated that roughly half the fluid ends he sees now are a variant of a pre-hardened stainless steel and the proportion continues to grow.

Gayle Vollmer, engineering services for Hartwig in Houston (Okuma’s dealer in the area) said there’s also a move to use harder materials for the parts of a fracking block that are most susceptible to wear, and an attempt to redesign fracking blocks so that more components are easily replaceable. But he surmised that current demand for high production has put such research “on the back burner.”

Schooley said that while boring mills typically have larger tables for even bigger parts, horizontals like Toyoda’s FH1250SX are almost four times faster. That’s due in part to a gear-driven 6,000 rpm spindle (optionally 8,000 rpm or a 15,000 rpm high-torque spindle) and a rapid feed rate of 1,654 ipm (42 m/min). The machine is built on a Meehanite cast iron base for maximum rigidity with cylindrical roller guides for increased speed, while the dual ballscrew drive on the Y and Z axes increases machining stability during heavy cutting.

The FH1250SW also has a W-axis quill for deep drilling and boring operations. He added that in recent years “a number of machine tool builders have opted for a built-in U-axis head” or a W axis, which ride parallel to the X axis or Z axis respectively. This makes it possible to change the center of rotation for a fixed turning tool, a useful feature when cutting large bores or other large contours. That’s because not only do fluid ends have large bores, the bores are often “bottle-shaped” inside, so they can’t simply be drilled. For machines that don’t have a built-in U axis, users can add a “feed-out” head that performs the same function, D’Andrea being one such supplier.

There are disadvantages to mounting a large head onto a rotating spindle, including possible part interference and incompatibility with through-spindle coolant delivery. The aftermarket option also needs its own external drive and it may not be possible to automatically switch out the rig as you would with other tool changes.

But Schooley said that interference is not an issue on frack block applications if the machine’s Z-axis travel is long enough to keep the add-on head from extending past the gauge line. With 1,850 mm of Z-axis travel and 200 mm from spindle nose to table center dead-band, the 1250 machine tackles frack blocks without interference. He added that Toyoda has further developed auto-change capability for its contouring heads to “help our customers stay ahead of the curve in an industry as volatile as oil.”

In Schooley’s experience, the modular boring tools available from Innovative Tooling Solutions and others come with through-spindle coolant as a standard feature and many shops making frack blocks use a dedicated horizontal boring mill with a contouring head to finish the bores. Toyoda also offers quill-spindle horizontal machining centers with 1,560 ft-lb of torque for heavy-duty operations. Schooley said the 4,000 rpm quill spindle combines the reach and strength needed in boring and deep drilling, allowing workpieces previously requiring two operations to be completed on a single machine. The machine’s rotary 360,000-position pallet provides maximum flexibility for heavy-duty machining on a variety of workpieces.

Okuma America Inc., Charlotte, N.C., has developed a programming option called Turn-Cut that enables a horizontal machining center to mimic a lathe in cutting circular and angular features on large, unbalanced parts without using a U-head. As Ted Winkle, Okuma’s Houston tech center coordinator explained, “Instead of rotating the part, which would be difficult or impossible for some huge parts, you rotate the tool around the part. And instead of a U-head moving the tool out from the center of the rotational axis, Turn-Cut interpolates those axes in X and Y while it’s also feeding a single-point turning tool in Z.”

Winkle thinks Turn-Cut will play a greater role in the oil and gas industry for parts such as fracking blocks because “as bores get larger in diameter, it takes a lot of torque to run a solid tool to make a hole that size. Having turn-cut capability, as opposed to a U-head and all the expensive tooling associated with that, is a benefit.”

On the other hand, Turn-Cut isn’t cheap because it requires additional software for specialized acceleration, deceleration, and synchronization, plus glass scales and ballscrew cooling. But you’ll get better precision for a variety of parts with these features and it increases the likelihood of being able to finish parts in one setup.

Schooley countered that the technique requires a lot of repetitive motion in the same area of the ballscrews and guideways (the smaller the diameter, the more this would occur) and this might decrease machine life.

Then again, Winkle said, unless you use this capability frequently on the same size parts, you wouldn’t have excessive motion in the same area. And if you don’t need the capability frequently, you wouldn’t be able to justify the large expense of a U-axis, so Turn-Cut would be a more logical option. Like any machine tool feature, there are trade-offs.

HORSEPOWER HEAVEN

For Makino Inc., Mason, Ohio, the “go-to” platforms for parts like fracking blocks are its T1 and T2 five-axis horizontal machining centers. David Ward, product marketing manager, said both machines have direct-drive spindles that are “ideal for the challenging stainless and high-alloy steels used in this industry.”

The T1 has an HSK100 12,000 rpm spindle with 143 hp and 740 ft-lb duty rated torque. The larger T2 spindle utilizes an HSK125 interface for radial stiffness. The 4,000 rpm T2 spindle delivers 200 HP and 1,107 ft-lb of duty rated power and torque.

Ward added that both spindles maintain their maximum torque values all the way to 1,000 rpm. “By carrying the maximum torque values out to this point, it ensures that the spindle will be able to take advantage of new advances in cutting tool technology, like high-temperature insert coatings.”

Winkle observed that the oil patch is dominated by CAT 50 machines but agreed that the face contact provided by the HSK and BIG Plus interfaces would be an advantage “if you have a long overhang. But since most of the machining forces are axial, it doesn’t make much difference which” spindle type you choose. As long as it’s beefy. “Approaching a frack block with an ISO 40 spindle machine would be an exercise in futility,” he said.

Sources agreed that high-pressure coolant is essential for heat and chip removal in many oil and gas applications, or as Iscar’s Zimmerman put it, “almost regardless of the application. When working deep in a bore on a fluid end, it’s critical to flush the chips out and difficult to do so. A coolant-fed milling tool is by far the way to go on such applications.”

So, if the machine didn’t come with this capability, users are adding it with units from suppliers such as ChipBLASTER. “I wouldn’t even consider buying a milling machine today without through-the-spindle, high-pressure coolant,” said Zimmerman. “It’s a game changer for both milling and drilling.” The only exception is a situation in which you can’t direct the coolant to the cutting edge. In that case coolant “sometimes works against you and it’s better to machine dry to avoid thermal cracking of the inserts,” he said.

Deluca said machine tool builders have responded to this demand such that he now commonly finds machines with an OEM-supplied pump delivering 1,000 psi at 20 gpm. Ward said Makino’s T1 delivers 26 gpm at 1,000 psi while the T2 supplies this same pressure at 53 gpm.

CUTTING TOOL ADVANCES

While you might love to have a high horsepower monster for your fracking blocks, Allied Machine’s Deluca said they’ve increasingly faced the opposite challenge: the need to drill large holes with machines focused on milling where spindle speeds are higher but horsepower maxes out at 40 to 50.

“The companies making these parts are usually small to medium in size and typically not dedicated to making frack blocks,” he said. “And many people are leery of focusing on this market and spending half a million dollars on a machine for an industry that runs like a roller coaster. So we had
to develop a tool that can cut a 4″-diameter hole at much higher spindle speeds and much lower feed rates than our traditional drilling products.”

The challenge in drilling fracking blocks goes beyond the size of the required holes. “Interrupted cuts are inherent to fluid end block manufacturing since the cross-holes are drilled perpendicular to existing holes,” said Allied Machine’s Best. “Because of the resulting shape of the intersecting holes, the outside edges of the drill will continue to be engaged in the cut, while the inside edge will not. This can destabilize the drill, which can destroy the tool and damage the integrity of the fracking block.”

Also, if the customer is economizing by using material that hasn’t been heat-treated, the drill might encounter significant variations. Deluca of Allied said ordinarily that means an operator must constantly monitor the process or risk catastrophic drill failure within seconds of hitting a pocket in the casting, which might wreck a part you’ve put hours into. Shops also experience material variances from block to block, making it difficult to choose the perfect drill and operating parameters.

Allied has met these challenges with a “next-generation” tool called the APX. It features a tougher carbide substrate and a proprietary coating to withstand the shock of hitting a pocket. “You may chip the tool and you’ll get a squeal,” Deluca said, “but you’ll still be able to finish the hole.”

He added that Allied addressed the difficulties of cross-hole drilling by adding a wiper to the insert to improve stability, plus wear pads to provide additional stability when the pilot exits into empty space or a cross-hole. “We also recommend lightening the feed and maintaining the same speed when drilling angled exit holes so the tool doesn’t want to push off or walk. It’s usually not necessary when drilling the main line or the cross holes but there’s only so much engineering you can put into a tool to compensate for the extreme interrupted cut of the angled holes.”

For milling, Zimmerman said Iscar’s HeliDo H600 double-sided, six-corner insert works extremely well for this type of material. “We also offer the Mill 4 Feed, a square, single-sided insert with a more positive geometry for a softer cut that requires less horsepower.”

His main message is that since Iscar has long been committed to the oil and gas industry and hires only experienced machinists, it can develop the optimal process for a customer and choose from a wide variety of tools. “We have many styles of feed mills and can optimize a feed mill based on the application, material, and machine, considering its rigidity, horsepower, and torque.”

MULTI-TASKING AND AUTOMATION

It’s likely that 9,500 lb fracking blocks don’t lead one to think “automation,” but Makino and other builders offer solutions that enable one operator to run multiple machines. For frack blocks, that generally means a rotating pallet changer that switches between a raw casting and a finished part in under a minute. After that, the operator has plenty of time to remove the finished piece.

With cycle times of a few minutes, fracking gun tubes are at the other end of the spectrum. And since they’re a “one-time use item,” Vollmer said, “the production rates are astronomical. Thousands are made every day.”

As a result, the part is ideal for automation, and Hartwig and Okuma have worked together to offer several options. These include robots for load/unload and bar-feed type systems like those used for small parts on Swiss-style turning machines, except that here they handle diameters from 2-4″. The fastest approach feeds the stock into a twin-spindle machine, machining one end and then feeding the part directly into the sub-spindle for work on the other end.

Vollmer said the parts require turning, threading and some milling, ideally using live tools for the latter, a feature Winkle referred to as “one of the biggest changes in the industry in recent years.” Fracking and directional drilling are again a big reason for that transition.

However, for many of the largely cylindrical parts needed in today’s “down-hole” operations, the ideal lathe has more than live tooling and a turret or two. It has a swiveling head and multi-axis milling capability like Okuma’s MULTUS mill-turn series.

“The parts needed for both drilling and the completion phase are much more complex than with traditional vertical wells,” explained Winkle. “You have to be able to open and close valves to change zones of production. Parts have hydraulic plumbing through them, as well as electronic monitoring gear.”

This requires cross holes and intricate milled features on the OD and ID, so the machines need five-axis positioning and nine or so axes of machine movement to cut a part on one end and switch spindles to work the other.

Threads are another challenge. Winkle said there are literally hundreds of types and the tolerances are tight: “Plus 1.5 thou, minus nothing.” That’s because in oil and gas, threads don’t just hold parts together, “they have to seal perfectly and endure enormous tensile stress from the gigantic string of parts going miles deep into the hole,” said Winkle. “And they have to deal with thermal expansion and shock loading. This sealing and load-bearing component doesn’t exist in any other industry.”

Cutting these threads requires a rigid machine and a setup that resists chattering. Kyle Downs, director of project management and marketing for SMW Autoblok, Wheeling, Illinois, said this is all complicated by the fact that “incoming material is very inconsistent. Automating the centering of hooked, bent, and/or oblong pipe can be achieved with two scenarios, either a 3+3 auto-centering and compensating chuck, or a compensating chuck and a tailstock-mounted centering solution, which is typically also a chuck.”

Finally, the addition of laser metal deposition capability to metal-removal machines adds new possibilities, as exemplified by Okuma’s new LASER EX series. If, for example, you had a part with just a few protruding features, you could add those features to the main body, rather than cutting the features out from larger stock. That’s potentially an enormous savings in machining time.

Winkle added that you can also fuse dissimilar materials, combining, say, Inconel and stainless steel, which is “never possible with purely subtractive technology. The joint is as strong or stronger than if it were machined from solid,” he said.

That has immediate potential in oil and gas for adding the wear pads needed on drill assemblies or adding the side fins used to keep pipe from scraping the ID of a well casing as it moves through a bend in a directional well. Okuma also offers the ability to case harden a zone of material up to about 50 Rockwell using the same laser with a different aperture.