Cutting Tool Engineering
March 2012 / Volume 64 / Issue 3

In-depth reliability

By CTE Staff

Machine Tech CNC, Edmonton, Alberta, machines downhole-well-equipment parts for the oil and gas industry. They include components for multistage hydraulic fracturing equipment, directional drilling motors and hydro- mechanical jar assemblies to free jammed drill strings. Workpiece materials range from chromium-molybdenum steels to 13 chrome stainless and high-strength 4330-V steels. Production volumes extend from single parts to more than 500 pieces, and lead times vary from 1 to 2 days to 2 months.

Many of the components are long cylinders with bored IDs. According to Machine Tech President Bob Johnston, bores can be as long as 48 " with diameters typically from 3 " to 5 ".

The large length-to-diameter ratios are challenging to machine. The shop employed a 4 "-dia. bar with damping features but it was reliable only to a 10:1 ratio. “With a 4 "-dia. bar, you could only bore about 38 " deep and then try and back bore from the other side,” Johnston said. “You have to maintain the tolerances on length, and it becomes very critical.” And for bores smaller than 4¼ " in diameter, the company couldn’t do many of them. “There was no room for the bar to fit,” he said.

To improve boring, the shop sought assistance from Jim Cordoviz, Alberta team leader for Sandvik Coromant Co. He suggested applying the Fair Lawn, N.J., company’s 80mm-dia. (3.149 ") Silent Tools carbide-reinforced boring bars tooled with CoroTurn SL quick-change cutting heads. A Silent Tools boring bar contains a heavy tuning body suspended between two rubber bushes; inertia of the body absorbs vibration generated in the cutting process. The damping is factory-set and requires no adjustment. “There are no tuning screws,” Cordoviz said. “I call it a plug-and-play tool.”

The bar’s carbide reinforcement reportedly makes a smaller-diameter bar more rigid than a larger-diameter tool. Johnston noted that, in most cases, the bar eliminates the need to machine a long bore from both ends. “While a 4 "-dia., 10:1 bar extends 40 ", a 14:1 Silent Tools 80mm-dia. bar extends 42 ",” he said.

The extra length enables most bores to be a machined from one end. “It makes a huge difference,” Johnston said. “We actually exceed the guaranteed capacity of the bar, run them out to 48 ", and they don’t vibrate.”

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All images courtesy of Machine Tech CNC

To machine precise bores with lengths more than 14 times diameter, Machine Tech CNC employs a Silent Tools dampened carbide-reinforced boring bar from Sandvik Coromant. The bar contains a heavy tuning body suspended between two rubber bushes; inertia of the body absorbs vibration generated in the cutting process.

Obviously, a smaller-diameter bar will fit in a smaller bore. “With the 80mm-dia. bar, I’ve machined a bore as small as 3.25 " in diameter. We modify the tool’s adjustable head slightly to get even a bigger range of travel,” Johnston said.

Repeatability improves as well. Johnston pointed out that larger boring bars, such as the 4 "-dia. tools the shop previously applied, can impede the flow of cutting swarf from the bore. The result is a poor finish and occasional gouges in the bore where the swarf has pushed the bar, he added. Because the 80mm-dia. bar is 20 percent smaller than a 4 "-dia. bar, there’s 20 percent more room for swarf. “It upgrades the repeatability and produces less scrap,” Johnston said.

According to Johnston, the bars’ CoroTurn SL quick-change cutting heads provide flexibility because they permit threading, grooving and back boring, and the heads can be machined to hold an application-specific toolholder.

Cordoviz said the cost of Silent Tools bars can be six to eight times that of conventional bars, but Sandvik Coromant had such confidence in the tools’ benefits that “We gave him a 100 percent performance guarantee. If the tool was not able to do the job, we would take it back.”

Johnston purchased three boring bars and noted that the return on investment was 4 months.

To maximize the capabilities of the Silent Tools boring bars, Machine Tech worked with machine tool builder Mazak to add features to a Powermaster CNC turning center, including a special holder for the bars, increased coolant capacity and high-pressure coolant pumps. Johnston said the arrangement reduced cycle time 40 percent in operations where the damped bars are applied.

Reduced manufacturing costs resulting from use of the Silent Tools bars have enabled Machine Tech to win bids for additional work. Regarding a boring job that required a 32-HRA finish and a 0.005 " total tolerance, Johnston said, “everybody else in town bores the parts and sends them out for honing. We bore them to size and finish in one operation. We virtually get all the work because our price point is so much better.”

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