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Moldmaker thrives on complexity.

Moldmaker thrives on complexity

Morrison Machine & Tool Co, Rowley, MA, is using a Deckel CNC milling machine with MasterCam CNC programming to machine complex mold cavity surfaces previously milled on a tracer.

The PC-based software supplied by CNC Software Inc, Tolland, CT, generates intricate contours too time-consuming to handle with manual data input. Although time was saved in programming at the CNC console, the job could be set up almost as fast on a manual machine, explains president Bill Morrison. "Because of the large number of calculations for complex surfaces, production of a part would take several weeks," he says.

MasterCam programming uses numerical values entered directly from the designer's geometry, reproducing them within [+ or -] 0.002", compared with the [+ or -] 0.010" to [+ or -] 0.020" accuracy of most duplicating machines.

Morrison Machine produces plastic-injection molds for products from blood analyzers to specialty gaskets and windows with tolerances in the 0.0002" range.

A mold for a bubble-shaped helicopter window measuring about 11" X 11" was challenging in two ways. First, it had arc radii coming in from several different angles; second, it needed a teardrop-shaped flat in the middle to mount an optical lens. From the flat, the window blended into the various helicopter radii.

Programming on MasterCam began by creating a 3D wire-frame model using coordinates collected from a physical model and a layout drawing without dimensions. Measurements of the model were taken off a granite plate. Because the part can be viewed on-screen and rotated, if one point is out of line, you can remeasure it or average several measurements.

Next, the grid-like configuration of points was grouped into a series of parallel splines along the length of the part. The program's "Create Spline Using Points" function (with prompts for key points) automatically finds the rest of the points and generates the splines.

After building the wire-frame model, the flat--an ellipsoid with a larger radius narrowing down to a smaller radius--was created. With that as a reference plane, the radii that connect the flat to the spline were approximated, using MasterCam's Coons-patch function. The boundaries used to define such a surface can be any combination of lines, arcs, and splines.

A filleted-surface function automatically trims and fillets any two complex surfaces with a constant or variable radius fillet. Tool offset was zero because the curves were shallow enough to make the cut with the tip of a ball-nose end mill. The cutter path was 0.0014" along and 0.0015" across the Coons patch, resulting in a very smooth cut, says general foreman Skip Campbell who did the programming.

Next, he entered the tool-path increment (line-segment length), cutter size, spindle speed, feedrate, rapid movements, number of roughing and finishing passes, advance on each pass, and use of coolant.

Finally, the MasterCam program generated the toll path and displayed it on the screen where verifying the cutter path saved a lot of troubleshooting time.

Programming time using MasterCam was about two days, compared with weeks using a manual data-input process. Machining time was one morning, a dramatic reduction from the week and a half it would have taken on a duplicator.

For more information about Morrison Machine & Tool, circle 378. For more information about MasterCam CNC software from CNC Software Inc, circle 379.

PHOTO : MasterCam generated this tool path required to machine the surface of the helicopter window, saving much troubleshooting time.
COPYRIGHT 1992 Nelson Publishing
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Manufacturing Solutions
Publication:Tooling & Production
Date:Jan 1, 1992
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