Milling good parts from bad geometry

Aug. 19, 2004
Accurate and efficient machining usually depends on three things: Position of the workpiece, shape of the stock at the start of the operation, and the shape to be produced.

Machine tools can pick up a touch probe to take measurements of stock shape and its location. The information can then adjust toolpaths for a more accurate part. The function makes adaptive machining possible.

PS-Fixture software can read the points from a touch probe and determine stock position. Adaptive machining routines then adjust the toolpaths to fit the material position.

Adaptive Machining, a recent concept in NC work, means its still possible to produce accurate parts when only two of the three conditions are met. "Adaptive machining instructs a machine tool to pick up a touch probe, take several measurements, and adjust the toolpaths to fit the needed part," says Steven Hobbs, CAM development director with Delcam, Birmingham, England ( "Measurements from the touch probe close the loop on most of the CAD/CAM process chain."

In a nutshell, the program works like this: When a stock position is uncertain, it's often easier to adjust the toolpaths to the position of the stock than it is to move the stock into a required location. "In most machining operations, stock is placed on the machine tool in a known position and orientation," says Hobbs. But aligning stock for a large part is a laborious, error-prone, and time consuming. Software from the developer called PS-Fixture makes the adjustment. The inspection software generates a probe path off line and fixturing software guides a touch probe around the part.

The fixturing software allows probing on the machine tool as an alternative to using machining fixtures. A best-fit calculation determines part location on the machine table. The position is transmitted to the CNC as a datum shift/rotate function to align the toolpath to its physical part.

The part is then machined with the new alignment. Setups this way are faster because it is easier to realign the toolpath than the stock. It also eliminates errors from incorrect setups and the need for accurate fixtures. "The end result is shorter machining times and lower costs," says Hobbs.

The problem of an unknown stock shape arises when machining near-net shapes, such as castings and forgings, and when repairing worn or damaged tooling. "In this case, adaptive machining (AM) first probes the stock to be machined and uses fitting algorithms to position the final part shape within stock with an even distribution of material to be removed," says Hobbs. This positioning method ensures even stock removal from around the part while avoiding overmachining and undermachining some areas.

And lastly, the unknown part shape comes when a component is deformed in service, such as turbine blades distorted by heat. "Any repair has to match actual surfaces instead of the nominal CAD model," says Hobbs. "In other cases without CAD data, repairs may need only blend existing surfaces without creating complete new models."

Then using a geometry-morphing capability in CAD software would make the adjustment. Reverse-engineering tasks are examples. Morphing software allows altering existing CAD models to match the physical part. Toolpaths for the correct shape can then be recalculated.

Hobbs cautions that implementing an AM program is a bit more complex and process specific than conventional programming. Although most AM functions are based on common capability, getting such a program up and running takes some hand holding and custom software for particular applications.

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