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Shop Solutions: Laser Calibration Tightens Tolerances

 

As machine tool builders and their customers learn the connection between tighter tolerances and their potential to reduce costs such as those attributable to scrap, delayed assembly times, too frequent maintenance, and excessive warranties, the need for ensuring accuracy in production is accelerating.

Jobs S.p.A. (Piacenza, Italy) has been manufacturing three-axis and five-axis high-speed standard and linear-motor-driven machine tools since 1980. Two years ago, Jobs replaced its conventional laser calibration equipment with Optodyne's (Compton, CA) patented Laser Doppler Displacement Meter (LDDM)-based calibration equipment. Used in conjunction with a sequential step-diagonal measurement technique, also developed by Optodyne, LDDM calibration equipment enables Jobs to take precision measurements and detect problems before critical production runs. If parts are not ready, assembly and electrical departments may be idled. If parts are not machined to specified tolerances, assembly will take much longer to ensure the final machine can cut to published specifications.

"With the Optodyne volumetric laser calibration equipment, we obtain more complete data with fewer measurements in much less time. This provides us with a better understanding of machine errors, allowing us to correct them and deliver a better machine at a more competitive cost," explains Sandro Foletti, Jobs manufacturing engineer.

The sequential step-diagonal measurement method collects 12 sets of data with the same four diagonal setups. Based on these measurement data, all three displacement errors, six straightness errors, and three squareness errors can be determined. The measured positioning errors can be used to generate a 3-D volumetric compensation table that is uploaded into the control for correcting any positioning errors and improving positioning accuracy.

"The Optodyne system and sequential step-diagonal measurement process require fewer measurements to obtain more data that clearly indicate the machine condition," Foletti explains. "As a result, we better understand such common problems as mounting errors, errors due to temperature, temperature change, and structural problems, and, without increasing assembly time, we are producing a better quality product.

"The sequential step-diagonal measurement method for volumetric calibration requires a maximum of seven measurements from which it is possible to understand the type and the dimensions of the most important errors," says Foletti. "We have validated it as an alternative to such traditional instruments used in assembly as the optical collimator, straight-edges, and granite square."

Optodyne's LDDM technology utilized by Jobs employs the single-beam MCV-500 and dual-beam MCV-2002 to reflect a modulated laser beam off a movable target. The beam is detected and processed for displacement information, which is used to create the lookup table that enables the control to compensate for errors. Setups are quick because an offset for a return beam is not required as with conventional laser equipment. Only two components have to be aligned: the laser head with a single aperture for emitting and receiving the beam, and a flat mirror that acts as a target.

"The dual-beam MCV-2002 allows us to take position and straightness measurements at the same time without changing optics or realigning the laser beam," Foletti says. "The single-beam MCV-500 allows us to take advantage of the sequential stepdiagonal measurement so that 3-D volumetric positioning errors can be measured with a minimum interruption of assembly and, therefore, much lower costs."

The laser and the flat mirror are mounted in the spindle and on the table, and moved along each axis separately and in sequence. The sequence alternates along the X axis, Y axis, and then Z axis, and is repeated until the opposite corner of the diagonal is reached. The diagonal positioning error is collected after each separate movement of all three axes. This technique collects three times the amount of data and allows the displacement error for each separate axis movement to be measured.

The trajectory of the target is not a straight line and the lateral movement is quite large. A conventional interferometer does not tolerate lateral movements this large and cannot take these measurements. However, the LDDM-based laser system uses a flat mirror target, so the laser beam is not displaced when moving parallel to the mirror and the distance from the source does not change. As a result, the measurements are not affected.

To measure the fourth axis (A) and fifth axis (B), the MCV-2002 with rotary calibration system is used. After each rotational movement, the table stops and settles for 3-5 sec, allowing automatic collection of angular data. The setup process is automated by utilizing the motorized programmable rotary table to eliminate the manual return movement. Automatic data collection minimizes operator error and reduces the time it takes to calibrate the rotary motion of a four or five-axis machine.

Setup and alignment are quick because angular measurement is not affected by runout, wobble, or parallelism of the rotary motion. A high degree of accuracy is achieved with this accessory and technique unlike the conventional comparative method, which compares a test device to known inaccuracies with an expensive master rotary calibrator.

A rotational angle of up to +/-10° can be measured. The angular measurement range can be extended to 360°. Windows-based metrology software minimizes and corrects cosine and retroreflector rotational error.

In addition, the MCV-2002 provides automatic temperature compensation, using a platinum sensing element that is accurate to within +/-0.1°. Up to four workpiece temperature sensors can be linked to the automatic temperature compensation unit. Automatic compensation is also provided for such environmental factors as air temperature, barometric pressure, and machine temperature to compensate for thermal growth.

 

Rebuilder Supplies Energy Push

The energy that fuels many furnaces and hot water heaters across the country is carried through an extensive underground network of pipes. Natural gas does not flow on its own through those pipes. It must be pushed, or more specifically, compressed by compressors like those built by the Ariel Corp. (Mount Vernon, OH).

Ariel manufactures a full range of natural gas reciprocating and screw compressors that represent every type of compressor needed to draw gas from the ground, send it to processing plants, and ultimately deliver it to end users. Whether it's a small, 93-kW compressor that pulls gas from a well or a massive 5965-plus kW unit that drives it through transmission lines, Ariel compressors are widely used.

One of the ways Ariel works is through collaborative relationships with partners like Kentucky Rebuild Corp. (Independence, KY). The partnership began in 1995 when Ariel called in KRC to retrofit a four-axis Giddings & Lewis 1.5-m vertical boring mill with a GE Fanuc CNC control system. That successful project led to a number of others, including two Warner & Swasey lathe retrofits, a Lucas boring mill retrofit, a Kearney & Trecker machining center, two Heller crank mill remanufactures, and most recently, a Mazak Slant Turn 80 Lathe retrofitted with a Mazak 640 Fusion control system.

KRC has the ability to rebuild, retrofit, remanufacture, and service a wide range of control systems, such as GE Fanuc, Mazak, Siemens, Mitsubishi, and MachineMate. "Our plant has more than 200 machine tools that must be constantly upgraded for us to maintain the level of quality that we demand," says Ariel maintenance/electrical specialist Craig Thorpe.

An important key to Ariel's work efficiency is the utilization of cell manufacturing, which requires uniform performance within the cell. That's what led to KRC's most recent project. Ariel had purchased two new Mazak Slant Turn 80s for its largepart turn cell. But an older Mazak Slant Turn 80N with a T2 controller was throwing the cell out of balance. "The key to optimum cell production is commonality of performance," explains Ariel facilities specialist Jim Yost. "We couldn't maintain a consistent environment because the older Mazak was unable to keep up with the two new ones."

KRC provided the solution by retrofitting the old T2 controller with Mazak's Fusion 640T CNC control system. Along with retrofitting the controller, KRC performed mechanical work and painted the machine. According to Yost, it "looks and operates just like the new machine that sits next to it, except that we probably saved $300,000-$400,000 by retrofitting rather than buying a new machine."

KRC has worked with Ariel on several levels, including working with its suppliers. For Energy Machine, a company that supplies crankshafts for Ariel compressors and other industrial markets, KRC remanufactured and converted a Heller crank mill to CNC. The crank mill used by Energy Machine was a manual tracer designed to machine large crankshafts with irregularly shaped lobes. Its configuration was similar to a lathe, where the part turned horizontally between centers. However, instead of single-point tooling on a turret, the cutter ring engulfed the part, and included many replaceable inserts that turned in the direction opposite that of the part.

KRC worked with Ariel's IT specialist Ben Staats to design an application algorithm to control the cutter ring in relationship to the part. As a result, the remanufactured design reduced the machining process time from 40 hr machining single-point on the manual lathe to 10 hr with the CNC-controlled machine. The net result is that Energy Machine is able to produce a quality part in less time and with greater savings that can be passed on to customers.

"We're in a service-oriented business," says Yost. "Our continued success requires partners who are also service-oriented. KRC makes that level of service a priority."

IT specialist Staats agrees: "Even when I've had to call them in the middle of the night with programming issues, they've been very good to either spend time walking me through on the phone, or even coming to the plant to help me troubleshoot and solve the problem."

 

Modular Fixturing Tames Welding Jobs

Northern Manufacturing Co. (Oak Harbor, OH) is a 75-person fabrication shop offering cutting, punching, forming, welding, finishing, and assembly of metal parts to customers in power generation, furnace and oven, paper pulp processing, waste water treatment, glass tooling, and farm machinery, among other demanding industries.

Since its founding 55 years ago, the company has built a reputation for know-how that goes beyond that typically expected of a fab shop. In 1992, for example, Northern developed the patented Versagage metrology tool that is widely used in metal fabrication. As a fabricator, the company has become known as a go-to source for fabrication of complex shapes and sizes, mostly in difficult stainless alloys—a capability that wins Northern jobs for everything from free-form sculptures to components requiring tight tolerances over very large dimensions. For welding workpieces in relatively low quantities, the majority of Northern's jobs, the company has adopted Demmeler modular welding fixturing systems from Bluco Corp. (Naperville, IL).

Quintin Smith, Northern's CEO and sales manager, first saw the Demmeler system at a trade show while exhibiting the Versagage product. He could see that adding a modular fixturing system for welding in his facility was a natural next step. The first purchase was a D16 system for a new robot welder purchased in 1997. Today, Northern has a total of six D28 systems and tables operating in all areas of the plant.

The Demmeler system is based on a five-sided high-tensile-strength steel table with a grid of accurately located 28-mm bores on 100-mm centers, a pattern of grid lines across the top, and a scale etched on all four edges to aid setups. Flatness of the table is 0.10 mm overall and the bores are located +/-0.03-mm hole-to-hole and +/-0.05 mm overall. System angles and blocks can be attached to the sides of the table to act as outriggers for parts that are larger than the table surface.

Fixture elements are precision engineered to match to the table's hole and grid pattern for quick setup, repeatable performance, and easy removal. Structural pieces have slots to locate around irregular assemblies that are between the hole grid pattern. Positioning and clamping bolts attach the fixture angles, workpiece positioners, and other elements to the worktables or to each other. The hardened clamping bolts provide up to 3 tons (27 kN) of clamping force, and withstand up to 25 tons (222 kN) of shear.

Northern is using the Demmeler modular fixturing in an application involving the large-scale laser welding of a "box assembly," a large component critical to the operation of paper-processing machines. The nozzle-like part is 20' (6.1-m) long and made of stainless. Its large area and complex surfaces make it difficult, if not impossible, to form from a single sheet of stainless, so several pieces need to be joined.

Northern utilizes one of its two Prima Rapido CNC five-axis laser cutters/welders to weld the long joints. The Rapido laser is of a moving bridge type on which the workpiece remains stationary while the bridge-plus-laser head moves to generate all five axis motions.

The laser is well-suited to welding a joint this long. Chad Geretz, process engineer, explains: "Northern obtains high strength joints with minimal distortion due to heating, a small heat affected zone (HAZ), a narrow weld profile with good appearance, and considerably faster weld rates than traditional welding, but all of this would not be obtainable without precision fixturing." The various subcomponents making up the finished box assembly are assembled prior to reaching the Rapido laser machine, which performs final welding operations.

To accommodate the overall length of the part, Northern has replaced the table that came with the laser with a 5 x 10' (1.5 x 3-m) Demmeler table. The part is longer than the travel on the laser, so one section is welded, and then the part is indexed to a new position to complete the welding operation.

The laser operator uses the Demmeler horizontal clamps to ensure that all joints are properly aligned and secured. Angles mounted to the sides of the table extend the surface to support and locate larger parts. "When you're CNC laser welding, it's absolutely critical to precisely locate the joint. The laser welding head moves to an absolute position, and the parts better be exactly there," Geretz says. "Tackling this job would not have been as fast and precise without the Demmeler system."

Another job requiring special setup that benefits from Demmeler modular fixturing is fabrication of a stainless steel frame for an egg packaging machine. Northern found it could improve assembly efficiency through use of a "slots and tabs" method to speed positioning of loose frame pieces prior to welding. The same operation that laser cuts the frame pieces also cuts the slots and tabs. While this method speeds initial positioning of parts, the entire assembly still needs to be precisely aligned prior to welding. This is accomplished on Northern's 1000 x 2000 mm Demmeler table with a D28 modular fixturing system.

Because the egg machine frame is used in a food service application, Northern does everything it can to eliminate the possibility of carbon transfer during fabrication. This can occur if the stainless steel parts are scratched by the carbon steel of the fixturing, which can cause rust to form on the scratched stainless during its service life. And, of course, rusting is unacceptable in food-grade applications.

To prevent the stainless parts from coming into contact with the mild steel modular fixturing components, Northern carefully covers the Demmeler table with paper, and all clamping surfaces and angle faces are fully taped or covered with paper.

 

This article was first published in the December 2005 edition of Manufacturing Engineering magazine. 


Published Date : 12/1/2005

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