Accurate tool registration boosts precision
New technology is permitting the consistent production and application of microtools down to 10µm in diameter and smaller. Tool and die shops are using these tools to create microfeatures in hard and difficult-to-machine materials. One challenge in applying microtools, however, is their registration with respect to the workpiece. To prevent tool breakage and meet tolerances, proper registration must be maintained.
Obviously, the old trick of pinching a piece of paper between the tool and workpiece is not effective in the microworld. These small, fragile tools are difficult to view, even with an optical microscope. As a result, new measurement strategies are required. A combination of workpiece probing and tool setting can provide an effective approach to achieving precise registration.
A Blum laser tool setter mounted on a Microlution 5100-S machine tool. Photo courtesy Microlution.
Applying tool setters
A way to optimize precision in micromachining is to apply tool setters that allow on-machine tool location and size measurement. However, not all tool setters are useful for microapplications.
The challenges of using tool setters in micromachining must be understood within the context of spindle speed requirements. It’s typical to use pressurized air-bearing spindles that operate at speeds from 60,000 to 250,000 rpm. These spindles can produce high error motions due to their relatively low stiffness. Hybrid ball bearing spindles have a stiffness of around 60 newton/µm, while high-speed air bearing spindles can have a stiffness as low as 1 newton/µm. These conditions, combined with misalignment of the tool in the collet, can produce tool runouts from 1µm to 10µm, and these values can vary with spindle speed and tool changes. Measuring the tool diameter at zero speed will not provide useful calibration information. Ideally, the spindle should be operating at full speed during measurement and should not stop before engaging the workpiece.
To address this issue, several companies have introduced noncontact, high-resolution laser tool setters for micromanufacturing (see photo, left). These tool setters can measure with the spindle operating at up to 150,000 rpm. Integral air jets on some tool setters keep cutting tools clean. This can be vital for measurement because chips and dirt particles can be nearly as large as the tool itself. Solvents such as isopropyl alcohol or acetone, combined with an air blast, can effectively remove fluids and particles.
One disadvantage of laser tool setters is they only detect tool diameter and position in one direction. Typically, the tool setter is oriented to measure in a direction that is planar with the structural loop of the machine tool (typically the Y-axis). Due to thermal variations, this direction experiences more displacement, for which the tool setter must compensate.
The entire perimeter of a tool is used during milling, and measuring tool runout in all directions is important. It is a misnomer that once tool runout is measured in one direction it is the same in all directions. This is the case for collet-placement errors, but not spindle rotation errors. Spindles can scribe a multilobed profile as they rotate, and this profile can vary with speed, balance and bearing properties. In micromachining applications, it is advantageous to measure tool runout using multiple tool setter configurations, each oriented to a machine axis.
Workpiece probing for micromachining applications can be a challenge, but careful planning can prevent headaches later in the process. A touch probe placed on a machine axis or in the spindle can register workpiece datums, as in macromachining. Touch probes are commercially available that achieve the precision required for micromachining.
A diagram of machine tool calibration. The position of the tool relative to the workpiece R is determined by adding the position of workpiece relative to the probe R1, the probe relative to the tool setter R2, and the tool setter relative to the tool R3. Illustration courtesy Advanced Machine Technology.
Relying on micromachined features for datum references is not ideal because typical touch probes are a few millimeters in diameter and the probing forces are several newtons. Preparing accurate reference datums on the workpiece in advance can help calibrate workpiece position. These datums may be ground holes or squarely machined edges.
Standard 3-axis machine configurations can create a measurement challenge. The datum references on workpieces generally register a Cartesian reference frame. It is ideal to align the axes of the machine tool to the axes of the workpiece reference frame. In the absence of a rotary stage, the workpiece needs to be “tapped” into alignment with a semirigid tool, such as the handle of a screwdriver. After alignment, the workpiece must be carefully fixed in place with screws or adhesive.
Given the angular precision required for microfeature alignment, this can be a tedious process. Software can be used in the alignment process, but any machine tool inaccuracies will reduce part precision. If the workpiece is mounted to a rotary stage, however, the touch probe can determine the misalignment and adjust the workpiece using the stage.
Pallet systems also can alleviate some of the burden of workpiece alignment. For example, products from System 3R USA Inc., Elk Grove Village, Ill., and Erowa Technology Inc., Arlington Heights, Ill., are available for micromanufacturing (see photo on page 19). For delicate, ultraprecision tooling, our shop uses in-house palletization systems to achieve precision and accuracies below 0.5µm.
Placing the workpieces on pallets and orienting the reference frame to the pallet-locating elements eliminates the need to calibrate each workpiece. In our operation, the pallets are placed on chucks, which are then mounted to the worktable. Subsequent pallet placement will repeat the first pallet’s alignment, assuming the pallet-locating features and reference frame are aligned.
System 3R and Erowa offer calibration targets for defining a standard reference frame. Calibration is required once per machine tool, and the alignment is good for the life of the chuck. As a result, aligning a workpiece on a 3-axis machine requires just one tedious operation.
A Renishaw touch probe and System 3R pallet mounted on a Microlution 5100-S machine tool. Photo courtesy Microlution.
After determining the position of the tool with respect to the tool setter and the position of the workpiece in relation to the workpiece probe, the position of the tool setter with respect to the workpiece probe must be determined. This will close the measurement loop and provide the position of the tool with respect to the workpiece (see diagram above).
The simplest approach is to measure the spherical probe directly with the tool setter. Shining a laser beam at a ruby sphere, which most touch probes incorporate, results in accuracy errors because the ruby is transparent. This results from the light beam continuing to the sensor even when the probe is in the path. Probe manufacturers offer alternative, nontransparent probe materials, such as alumina and carbide, which can avert this problem.
As the tool is traversed from the tool setter to the workpiece, errors accumulate due to geometry and encoder accuracy. The same situation occurs as the workpiece probe is moved from the tool setter to the workpiece. While both of these displacements may be precise, or repeatable, the positional accuracy can vary by several microns.
This accuracy problem can be minimized by placing the machine probe as close to the cutting tool as possible and by placing the tool setter as close to the workpiece as possible. In many instances, machine tool probes can be placed directly in the spindle taper and be operated via infrared or radio signals. This allows wireless communication, which, in turn, permits probe mounting with regular tool changes.
Unfortunately, machine tool probes are not available for the small tapers in many high-speed micromachine spindles. Mounting the probe near the spindle on the machine tool stage, however, can be adequate for most applications, and help compensate for errors.
As micromachinists attempt to effectively apply smaller microtools, these tools must be in registration with the workpiece. Employing measurement strategies designed specifically for micromachining can go a long way toward reaching this goal. µ
About the author: Chris Morgan is chief technology officer for Advanced Machine Technologies LLC, Crestwood, Ky. The company is a tool and die shop for the micromanufacturing field. It performs micromilling, microEDMing and microgrinding; machining of difficult-to-machine materials; and manufactures cutting tools for micromachining. Telephone: (502) 243-0263. E-mail: email@example.com. Web: www.amtech.us.com.