This article is about Blue Photon Technology and Workholding Systems LLC and how Post Processing; 3D printing presents challenges in workholding for finish machining.
Written by Mark Kirby AM Business Manager, Renishaw Canada
Metal 3D printing can enable rapid, low cost iterations of new medical devices, since no tooling costs are involved. All devices need testing to uncover problems and develop solutions—allowing the product shape to change “for free” is a powerful advantage with Additive Manufacturing (AM). Other benefits flowing from AM besides enabling more complex geometry are improved accuracy with no component tolerance stack up, and a simplified supply chain with reduced part count.
3D Printed Workholding
Plastic printed jaws are often a good first option, as they are cheap to manufacture—typically in just a few hours on a desktop printer, and can conform to complex geometries (although the design of the jaws can be more time consuming than a simple Boolean subtraction of the component from the plastic). When the design changes after product testing it is easy to print a new set of jaws.
The main disadvantage of plastic jaws is that they will often distort the component as they are tightened. Although the jaws hold the part rigidly for machining, when the component is released from the fixture any machined bores may no longer be perfectly round, and true positions of features will have moved slightly as the component relaxes back into its unloaded shape.
The tracker body was monolithically printed in titanium alloy Ti6-4, stress relieved and then cut from the build plate ready for finish machining of the kinematic mount and the four posts that hold the optical reflector globes.
A plastic set of jaws was designed to clamp the part while leaving the machining areas exposed. Although the plastic jaws clamp the part rigidly, they never clamp the part repeatably, so the exact position of the part must be found using a machine probe and best fitting software such as NC-PerfectPart, from Metrology Software Products Ltd. (MSP). Originally developed for machining of high value aerospace and Formula 1 composite structures, this software is perfectly suited to the challenge of precisely locating an organic-shaped AM part with no obvious datum features.
Points are selected on the component in the CAD environment and the deviations from nominal positions are measured by the probe on the CNC machine. The NC-PerfectPart software then creates a best fit alignment that is a 6-axis coordinate transformation—both translation and rotation. This coordinate shift is automatically recalled into the machine controller before CNC programs are executed.
Problems and Solutions
Unfortunately, the hip tracker component flexed imperceptibly when the plastic jaws were clamped, resulting in true position errors greater than 0.3mm on the machined posts. While the component had been optimized for handling loads during surgery, it had not been designed to resist machining forces.
In order to machine the part accurately it was essential not to bend it with mechanical clamping, but at the same time it was equally important to add rigidity.
The solution was to use Blue Photon’s UV activated adhesive and grippers. By gluing the part onto four gripper posts (that transmit UV light to cure the glue in approximately 90 seconds) the hip tracker was held firmly but still in the free state.
An aluminum block was machined to hold the four gripper posts in the correct positions for the tracker body. Initial machining was successful on the three posts directly bonded to the grippers, but one post was cantilevered above the gripper and vibrated during machining. A plastic support block was printed to hold this post and eliminated this problem. By cradling the part, the support block also allowed for more accurate positioning of the tracker prior to the glue being cured. The glue thickness is optimally around 1mm, and after machining the part and fixture can be separated by simply immersing the assembly in near boiling water for a few minutes, and then peeling apart.
The only disadvantage of using the glue grippers appeared to be the extra work to design and machine the gripper fixture. However, on a subsequent project for an industrial impeller Renishaw used plastic printing to produce the gripper fixture instead of machining. This proved that the manufacture of a robust, custom workholding solution can be reduced to an overnight desktop print.
Refining the Procedure:
Sometimes, the trickiest aspect of developing an effective machining process is figuring out how best to fixture the part. This can be especially challenging for castings and other workpieces that are relatively thin, because those parts are prone to flexing when conventional mechanical clamps (and even vacuum chucks in some cases) are used.
Vibration can also be an issue for parts like these if they aren’t rigidly fixtured, meaning a quality surface finish might be tough to achieve and cutting parameters might have to be dialed back, extending cycle times. Finding a way to effectively fixture complex, contoured parts can be just as difficult.
Ray Bray says Precision Grinding and Manufacturing (PGM) fought these problems in the past. Mr. Bray is a project engineer for the Rochester, New York, contract shop that specializes in complex, short-run, often repeating work for industries including aerospace, automotive, medical, military, optics, photonics and telecommunications.
Bray says PGM has employed a variety of unconventional methods to more effectively secure relatively thin workpieces for machining over the years, going so far as to use clay, lead and sandbags to supplement conventional mechanical clamps in an effort to minimize vibration.
You won’t find those types of workholding workarounds being applied there today. Instead, the shop uses an advanced, albeit atypical technology that is particularly effective for fixturing flexible parts.
In short, this technology uses adhesive to temporarily bond a workpiece to numerous cylindrical grippers installed in a fixture plate. Once the adhesive is cured via ultraviolet (UV) light, the workpiece is securely held at a known datum location in an undistorted, freestate condition.
After machining, the adhesive bonds between the grippers and workpiece are easily broken and any excess adhesive is removed from the completed part via a quick, steamcleaning wash.
PGM has a wealth of CNC equipment, including machining centers, turning centers and grinding machines. Milling represents the bulk of the work performed there today, for which the shop uses VMCs with four- and five-axis capability as well as HMCs with pallet pools.
William Hockenberger’s son Todd, PGM’s coporate vice president, explains that because a good portion of that milling work involves thin and complex workpieces, the shop has continuously looked for more effective ways to secure those types of parts for machining.
A few years ago, PGM learned about photo-activated adhesive workholding (PAAW) technology that was developed at Penn State University, which looked to be well-suited for such troublesome parts. PAAW technology was invented by Professor Edward De Meter, who was awarded two patents covering it. In 2012, Professor De Meter and others formed Blue Photon Technology and Workholding Systems LLC to market the technology to industry end users.
PAAW grippers are positioned at various locations, and oftentimes a gripper is used at each hard datum point. The number of grippers used largely depends on the size of the part and its geometry. The threaded grippers install in the top of the fixture plate and require a through-hole to enable the UV light to pass up and through the gripper to cure the adhesive.
- Add adhesive onto pins and install workpiece.
- Insert UV light guide into grippers at the three datum hard points and cure adhesive, which typically takes 30 seconds for each gripper.
- Rotate the table to enable easier access to other grippers and cure adhesive at the remaining locations.
- After machining, use a wrench to turn grippers and break the adhesive bonds between the grippers and workpiece. The workpiece can then be removed. A subsequent steam-cleaning wash might be necessary to remove any cured adhesive remaining on the part, and adhesive might also have to be removed from the tip of the grippers using a metal scale or straightedge.
After machining is completed, a T-handle wrench is used to back off each gripper, twisting the element and shearing the adhesive bond with the workpiece. The workpiece can then be taken off the fixture and a subsequent cleaning operation using a portable steam cleaning device is used to remove any cured adhesive that remains on the workpiece. Adhesive must also be scraped off of the tip of each gripper using a metal scale or straightedge before a subsequent workpiece can be fixtured for machining.
The gaps between the workpiece and grippers (thus, the thickness of the adhesive) can range from 0.010 to 0.125 inch depending on the flatness of the part. The uncured adhesive, which is non-toxic, is sufficiently viscous that it won’t run off grippers regardless of orientation. Axial holding force depends on gripper size and can range from 250 to 800 pounds when using Blue Photon’s BlueGrip S1 adhesive.
Grippers are made from hardened, corrosion-resistant stainless steel and have a black oxide finish. For repeat jobs, PGM will commonly leave the fixtures intact with the grippers still installed and store them for later use. Otherwise, the grippers can be removed and installed in other fixtures created for new jobs. The latter is most often the case for PGM, because new work continuously flows through the shop.
However, it was challenging and time-consuming for operators to mechanically secure either end of the part without causing it to twist about its longitudinal axis.
Operators clamp the middle section of the part as they always have, but use the PAAW system to secure the ends of the part so these ends remain in a free state and there’s no chance of causing the part to twist.
After op. 10 work, the part is flipped and refixtured, and an op. 20 milling operation removes whatever cured adhesive remains from the op. 10 fixturing. Therefore, it’s only necessary to steam clean adhesive left behind from the op. 20 fixturing in this case.
The system will be used to a greater extent for production as well as toolroom work and CMM part inspection as PGM gains more experience with it and experiments with BlueGrip adhesives. Todd Hockenberger says it sometimes serves as a selling point for customers that are either having problems with other part vendors or readily recognize how tough their new design will be to fixture.
The shop has a good chance at winning those types of jobs once the customers understand how the system can be applied to their applications. In addition, it can help shorten product design cycles because fixture design is no longer the challenge it once was when mechanical clamps seemingly were the only option.
"This is big," Mr. Hockenberger says, "because PGM tries to get involved as early as possible in each customer’s new product design cycle to offer design for manufacturability (DFM) advice to minimize production time and cost, and get the product to the market faster."
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