Modern Machine Shop

DEC 2018

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BETTER PRODUCTION Fixturing MODERN MACHINE SHOP 85 CI SAAM 3D printers feature a number of machined components that require CMM inspec- tion. Mr. Pershken uses an entry-level CMM that has to be programed by manually moving the probe to identify the datums so that the machine can determine the part axes and execute the program correctly. With conventional off-the-shelf fixturing, it takes time and a fair bit of skill to set up a part on the table and then to perform the manual hits with the probe. 3D Printer Parts, 3D-Printed Fixtures While working with one fairly complex machined part, Mr. Pershken had the idea to 3D print fix- tures instead of using off-the-shelf items and a pegboard. That part (pictured on the facing page) is an aluminum bearing block that goes into the 3D printer gantry. Two slots hold the idler bear- ings around which the belt is routed. The compo- nents like this that make up the 3D printer frame are the most critical to measure correctly because they most directly affect how accurate the final printer will be, he says. On this bearing block, the tightest tolerance is a roughly 30-micron window for the pin that supports the bearing. In inspection, the bearing block needed eight hits in each axis for a total of 24 manual hits. "One day I was just sick of putting the manual datums in again," Mr. Pershken says. This frustration led him to create plastic fixtures using an CI SAAM 3D printer instead. To design this fixture for 3D printing, Mr. Per- shken says he "hacked" the injection-mold envi- ronment of a CAD program to create the positive half of a mold as a mount for the part. To make the mold design work as a fixture, he then removed some material and added features such as a stan- dard base that would fit on the CMM table. The 3D-printed fixture is designed to allow parts to drop in place without clamps or screws. To hold the bearing block, Mr. Pershken made use of one of its features: a long slot, visible in the photo on the facing page. The mold-design soft- ware generated a solid protrusion to fill this slot, but Mr. Pershken split it into four pieces. When 3D-printed in polylactic acid (PLA) thermoplas- tic, this feature has enough flex to hold the part securely in place without marring it. As a result, the fixture only obscures one noncritical face of the bearing block, enabling Mr. Pershken to mea- sure the remaining five faces with one fixturing setup. Setting up the 3D-printed fixture simply required calibrating the CMM probe on the ref- erence sphere, mounting a bearing block on the fixture, and then measuring all three planes man- ually to give the machine a reference point and direction. Subsequent parts can then be placed and measured with the press of a button. Parts placed on a 3D-printed fixture are not going to be in exactly the same position every time, but they do not need to be. "As long as it's close enough and the CMM gets the part datums correctly, it'll run just the same," Mr. Pershken says. "I can just load them up, hit 'go' and do some other work while it's running." Saved Effort and Reduced Cost With cycle times of just a few minutes (it takes only one to two minutes for the CMM to measure the bearing block, for instance), a 3D-printed fix- ture is "not a huge time saver," Mr. Pershken says, "But it is a huge effort saver." Fixturing the parts conventionally would involve the use of a fixture kit. A basic option may cost $1,000, and this would not be a drop-in- place solution. "You'll have to introduce a clamp- ing force somewhere," Mr. Pershken says, which runs the risk of damaging the part. "It's going to require somewhat more of a skilled operator and little more time in the setup." Mr. Pershken now has a collection of about 10 3D-printed fixtures, some of which have been in use for as long as two years without needing replacement. Because the fixtures essentially get checked automatically with each CMM cycle, Mr. Pershken tests them periodically but has not found a need to worry much about loss of accu- racy. "I haven't noticed any slop in them or any bad measurements," he says. "I haven't had to replace one yet." Many of the machined parts for the CI SAAM printer are fairly simple cylinders and plates that would not benefit much from a custom 3D-printed fixture. However, for those that have an intermediate to complicated geometry and require high accuracy, such as parts for the 3D printer frame, the 3D-printed fixtures simplify the CMM process significantly. As CI begins to produce more of its printer parts through machining and sheet-metal forming, Mr. Per- shken plans to continue 3D-printing fixtures to support CMM inspection. | Cincinnati Inc. | 513-367-7100 | e-ci.com | NVBots | 857-529-6397 | nvbots.com

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