Lathe Accuracy | Turning Accuracy

Precision turning

Aronson, Robert B

You can buy lathes with guaranteed accuracies ranging from a few tenths to a few microns. Some years ago precision depended chiefly on the operator's ability. An experienced man who "knew his machine" could do wonders with a good lathe. Today, accuracy is more dependent on the quality of the machine elements and resolution of the control. The trade-off is cost versus precision. The methods of turning a part precisely are well known. The problem is in deciding how much precision you want to pay for.

Conventional lathes have an advantage over milling machines in that only two axes need to be controlled in the cutting operation: Carriage motion (Z axis) and cross feed (X axis). Conventional lathes routinely offer accuracies of a tenth or a tenth and a half--0.0001-0.00015" (0.003-0.004 mm) at stable temperature conditions.

Horizontal and vertical lathes have about the same accuracy, as long as the parts are about the same length. Horizontals begin to lose accuracy on long shafts due to droop.

Higher precision lathes operated to different standards. Dan Luttrell, technical director for Precitech (Keene, NH), a manufacturer of precision lathes suggests that lathes that can achieve tolerances less than 0.00004" (0.001 mm) be consider precision machines while those that achieve tolerances less than 0.000004" (0.0001 mm) should be considered ultra-precise machines.

WHAT'S ACCURACY?

Machine accuracy involves how well the tool point is positioned anywhere in the envelope, and there are three types to consider. The most common is positioning accuracy which is how precisely the carriage and cross slide put the tool tip where you want it. For about 80% of all turning jobs, an accuracy of 0.0005" (0.013 mm) is acceptable and most machines currently offered can easily achieve this.

Dimensional, or part accuracy, concerns how one dimension relates to another and is a second "accuracy." For example, are two diameters on a shaft concentric? The third accuracy consideration is finish.

While accuracy is the ability to position a tool, repeatability is the ability to positoin that tool consistantl over a given time. One lathe manufacturer cautions that sometimes a repeatability figures doesn't give an accurate picture if they involve only part of the motion envelope. For example, if someone turns 100 parts on a 5" (130-mm) diam shaft to an accuracy of 0.0005" (.013 mm) they are talking repeatability, not accuracy. Resolution refers to the smallest distinguishable dimension the machine's sensing systems can detect and is usually four or more times greater than what the machine can deliver.

While accuracy and repeatability specifications give some idea of what a lathe can do, many buyers want to know what a machine can do with a specific part. According to Monarch Machine Tool Co. (Sidney, OH) president, Bob Siewert, "Machine accuracy specifications are not that relevant. Buyers want a guarantee that our machine can give them six-sigma quality with their parts. Often we are required to do one set of parts here at the plant and a second set after the machine is installed."

WHERE ARE THE ERRORS?

There are four major error sources. Displacement caused by thermal expansion is by far the greatest source, sometimes as high as 70% of the total error. According to an old saying, "On a machine tool, everything is a thermometer." Mechanical deviation is around 20%--the errors introduced by wear and tolerances of the various machine components. Forces acting on the part, and tool error and wear, combine to make up the rest.

Designers attack thermal errors in two ways: prevention and compensation. The simplest way to eliminate heat is to apply excess coolant or to have chillers in the coolant circulation system maintain temperature. Next are machine enclosures that are temperature-controlled or entire rooms with temperature conditioning.

In a system developed by Dr. Alex Slocum of MIT, channels made of a special damping material are built into the machine's base and cooling fluid flows through them. The system both damps machine motion and carries off heat.

TECHNOLOGY TRANSFER

A number of government research efforts dedicated to improving turning precision has produced ideas that work under laboratory conditions. The next phase is to find out how "machine specific" these solutions are. Can they be incorporated into a general category of machine tools, and will they produce the same results on the shop floor?

The National Institute for Standards and Technology (NIST) has been working on machine-tool accuracy for some time. In one of their more recent programs, their researchers took a commercial piston turning machine and developed a control program that compensated for thermal and mechanical errors. They have given it back to the manufacturer for "real world" trials. During the tests, the manufacturer will determine the economic feasibility of the design and which features, if any, are transferrable to a commercial product.

One of the problems with this project was the length of time taken to develop error compensation models. An effort is now underway to speed up the characterization of a machine tool to develop the computer models. "Currently complete data acquisition and analysis takes at least several days for each error component," says Alkan Donmez, leader in the sensor systems group of NIST's automated manufacturing engineering lab. "This will have to be reduced to a half day or less to be commercially practical."

In a similar NIST-sponsored project conducted at Saginaw Machine Systems, Inc. (Troy, MI) the goal was to find ways of reducing error on a conventional vertical turning center. In carrying out the program, the first step was to characterize the machine's geometry. Then they measured the effect of thermal changes on the mechanical characteristics over an extended period. Jules Myers, R&D manager, explains, "We found that a large part of the thermal error came from the spindle and ballscrews. The two sets of spindle bearings sometimes heat at different rates causing a skewing effect while the ballscrews and spindles all grow in length with temperature rise." The machine analysis ultimately led to a math model which gives compensation values that can be plugged into the X and Z-axis controls for real-time corrections. Explains Myers, "The program is resident in a separate PC which we interface with the CNC because we can't get into the controller's architecture. Our system interrupts the feedback and 'plugs in' the correction signals on the fly. The separate PC takes the sensor input, compares it with its own program positions, and adds or subtracts data to the control's feedback command. In this way we can use any control and not worry about the need for open architecture. With this system, we are able to get close to 25 millionths (0.0006 mm) axis resolution available."

"Our research efforts are already moving on to the next phase by applying what we learned with the first machine to an ARPA-sponsored program with a major automaker," continues Myers. "We currently can take care of 80% of the thermal and geometric error with this system. Later we will look at fixturing and tool loads. For example, we can measure the stiffness of the fixturing systems and workpiece and make that part of the equation. This will only work, however, if the machine is very stiff and exhibits good repeatability."

NIST is also looking at the influence of fixturing and loads. First, researchers will evaluate process-related errors such as those due to tool load and correlate them with the machine's geometric errors. They then check parts turned on the machine to determine the influence of the tool on the dimensions and surface finish. Both these error sources will then be incorporated into a compensation program.

In another NIST project, a charged-couple device (CCD) checks the tool face during cutting and initiates real-time compensation based on face changes. An outside company is now working to perfect this idea as a marketable instrument.

ULTRA-ACCURATE

The nation's national laboratories, particularly Lawrence Livermore, Los Alamos, and Oak Ridge, have specialized in high-precision turning machines for some time. The work began years ago with attempts to develop high-precision metrology. That technology was later applied to precision turning, particularly turning of material that would be the backing for defense-related systems.

One of the Livermore lathe designs, designated the DTM 3, has a 2-m swing capacity and uses a single-point diamond to create optical components. The second, a vertical axis diamond turning machine with capacity of 1.5 m, built at a cost of $13 million, is billed as the world's most accurate machine tool. It has a volumetric accuracy of 30 nm rms and has turned surfaces with roughness of about 3 nm rms.

There are 14 feedback loops on the machine's two axes with motion measured by laser interferometer operating in a vacuum. Temperature is maintained to a millidegree Fahrenheit. "A major issue is a high-capacity controller with great enough capacity to handle all the inputs and enough bandwidth to work quickly," says Dan Thompson, program leader at the Livermore Lab.

Although the applications suitable for these ultraprecise lathes are limited, the technology they utilize may have wide appeal. "With the high-precision machines made by Livermore and the other laboratories, the rational was, 'We need a precise part, so build a machine that will make it possible,'" says Dr. Ken Blaedel, a member of Livermore's machine tool development group. "On the other hand, industry says, 'We are making a line of machines, what level of accuracy do we have to have to sell these machines?' Now the labs with expertise on precision systems are 'backing off' the technology to see what can be practically applied to an industrial situation."

OVER-THE-COUNTER PRECISION

One of the several techniques applied to economically achieve precision by Precitech is the use of glass linear scales to control motion. The scales are holographically generated with a pitch of 20 microinches (0.0005 mm). Signals from these sensors are interpolated electronically so that the resolution is just under 10 nanometeres or half a microinch. In addition, slides run on fluid-film bearings, both air and oil hydrostatic. "The external pressurization gives us good slide geometry plus smooth motion," says technical director Dan Luttrell. "We use air-bearing spindles which have radial and axial errors of two microinches or less. As we move into hard turning, we may be looking at oil hydrostatic rotary bearings."

Many of the company's lathes are used to make optial elements, lens molds, and small mechanical components. Materials typically turned include nonferrous metals, electroless nickel plating, infrarred crystals, and polymers.

"The tools include single crystal diamond and CNB as well as more conventional materials" says Luttrell. "We prefer to have our machines operate in rooms with central air conditioning controlled to 23degC, +/-2deg. The machines can also be provided with a continuous shower of air controlled to 0.1degC."

Bearings, of course, are a major concern in precision turning. Rank Pneumo, (Keene, NH) another builder of ultra-precision machine tools, also uses air or oil bearing spindles and slides for smooth, accurate motion. "The type and design of bearings selected for each machining system we build relates directly to the stiffness requirements for the applications," says Allen Lake, company sales engineer.

System resolution is also important. The company's Nanoform 600 two-axis CNC lathe utilizes a 1.25 nm (0.00000005") resolution, closed-loop laser feedback system on both slides. A thermal compensation system detects and compensates for thermal drift.

Hardinge Brothers Inc. (Elmira, NY) makes two lines of lathes: "precision" and "Super-Precision." With the precision version the spindle has a runout of 30 millionths (0.00076 mm) TIR. For the Super-Precision version, it's 15 millionths (0.00038).

"We do not make laboratory lathes. They are 10 to 35 hp (7.5-26.1 kW) machines that operate every day and can routinely hold an 8 micron finish," says Robert Agan, company president and CEO, "We make most of the major parts ourselves including the base and spindle. Although we analyze new technologies available to increase accuracy, we don't touch them unless they are cost effective."

The machines have Harcrete polymer composite bases. This is an artificial granite that has a very low coefficient of expansion and low conductivity. In their most precise designs, they offer a surface finish of 8-10 microns using conventional tools. With a single-crystal diamond tool, the finish is 2-3 microns along with a continuous machining accuracy of 0.0002" (0.005 mm) on roundness.

Their designs both isolate heat sources and provide thermal compensation. A mechanical system on the Super-Precision machines measures the thermal migration of the headstock relative to the turret. The operator then introduces an "M" code which causes a positioning compensation. To prevent the compensation motion from disturbing the finish, the command is usually given just before the finishing cut.

Traub-Hermle (Menomonee Falls, WI) also has a temperature compensation system, "We can meet six-sigma specifications with our machines. Our thermal compensation system checks temperatures in the area of the spindle and compensates with offsets. We also use linear guideways which give us much better accuracy than box or dovetail designs," says company president, Harold Welge.

In reviewing the elements that determine accuracy of their lathes, according to Monarch's operations director, Kass Reda, "We use rolling element guideways to eliminate stick slip, as well as cut the need for drive power. The bed is cast iron because welded sections sometimes creep. To monitor slide motion we use optical glass scales with a one micron resolution. Ballscrew accuracy is not a major issue. Many of those available are of high quality, so they are not a major source of error, plus encoders take the ballscrews out of the loop. Our spindle bearings are ball in front and roller in the rear to take the load of the drive chain.

CUTTING TOOLS

Tool technology continues to lead that of machine tools in some applications. New variations of tool material, size, and geometry are being introduced at regular intervals. One example is the Jet Cut insert from Iscar Metals, Inc. (Mansfield, TX) which allows coolant flow though the insert at the cutting face for improved accuracy. The system can tap into the machine tool's coolant system and provide flow at 30 psi (0.2 MPa) or have a separate high-pressure supply at 1500 psi (10.2 MPA) or more. With this type of lubrication, fluid is between the cutting face and part. It penetrates the heat barrier and gets to the workpiece where it will do the most good. According to Ken Johnson, product manager for Iscar, "Through-tool-cooling increases tool life at least twice. The cooling helps maintain the cutting edge and because there is a more uniform face, the cut is more accurate. Repeatability is also improved with more parts per offset. Because heat is generated in a smaller area, the metal breaks with fewer curls and chips are more manageable."

Most machine tool manufacturers are looking at high-pressure coolant. Some systems report operating pressures of 55,000 psi. At these pressures, containment is a consideration. In addition, the fluid must be carefully filtered so sludge does not block the insert hole.

According to John Israelsson, product manager for turning, Sandvik Coromant Co. (Fairlawn, NJ), modular turning is another way to reduce tool-induced errors. "It offers more precision because you set the tool against a reference before you put it in the lathe. This presetting offers repeatability of 80 millionths (0.002 mm) and precision to one tenth, while a conventional insert will give you a few thousandths," he explains.

Matching the tool to the material is also as major factor in controlling accuracy. "For example carbides work at 45 R sup c , and for them you need CBN and ceramic tools, " says Israelsson.

ULTIMATE CUTTER

In ultraprecise turning, the single-crystal diamond is the optimum cutting tool for high-precision turning of nonferrous material. It's hard and has a smooth face that introduces no scratches to the surface being cut. The tool is made from a natural diamond that is precisely shaped so that the diamond presents the best cutting surface. Some of the natural diamond's market has been taken by lower cost polycrystalline diamonds, but these materials are bonded, with a lot of small stones held together with a binder, so the tool face is not as smooth as a real diamond's.

It's a paradox that most of the pioneering work on diamond turning was done in our national laboratories for various defense programs. Now, a major commercial use of diamond a turning is heads for VCRs, which is done almost exclusively by the Japanese.

PRECISION BEHIND THE PRECISION

Accuracy of any machine tool depends on the attention given the manufacture of individual components. When Russell T. Gilman decided to make a major commitment to high-quality spindles, they built a 13,500 ft sup 2 temperature-controlled addition. Spindle components are manufactured in this area which has a cleanliness equivalent to a class 100,000 clean room. Within this area is a separate certified class 10,000 clean room where spindle assembly takes place. Temperature is controlled to +/-2degF (1.1degC). Equipment includes precision turning, grinding, and boring equipment. A key machine is the Swiss-made SIP AFZ (Elmsford, NY) precision vertical coordinate boring and milling machine which has a spindle that can develop 10-15 hp (7.5-11 kW) and speeds to 6000 rpm. Axis speed ranges from 0.004 to 400 ipm (0.1-10,000 mm/min). Resolution of the measuring system is 0.00001" (0.00025 mm) and positioning accuracy over the entire travel of the three axes is 0.00006".

As a result, according to Lothar Kinscher vice president of manufacturing, "Our spindles, which were formerly offered with a runout of 0.0002 to 0.0003" [0.005-0.008 mm], are now routinely made to a runout of 0.0001" [0.003 mm]. Where customer specifications require, we can also produce a surface finish of 25 microinches."

INFORMATION SOURCES

SME has many opportunities to broaden your understanding of turning. The most basic resource is the Tool and Manufacturing Engineers Handbook series, particularly vol. I, Machining. Consider also a new video course, "Turning Center Programming and Operations" which is concerned with CNC turning centers. Call Customer Service at 1-800-733-4SME, 8 am-8 pm Eastern time, Monday through Friday.