Experimental performance analysis of an inverse dynamics CNC compensation scheme for high-speed execution of curved toolpaths

2016

Self Cite

A novel high-speed cornering strategy for piecewise-linear motions is proposed, based on the G 2 continuous Pythagorean-hodograph (PH) rounding segments and continuously-variable feedrates described in Part I of this two-part paper. This strategy employs an accelerationlimited approach to feedrate scheduling, including smooth deceleration and acceleration profiles along linear segments entering and leaving the rounded segments. A G-code part program parsing software package has been developed, that automatically identifies toolpath corners and inserts appropriately-sized PH quintic corner rounding segments, with associated feedrate suppression ratios. The method has been tested on a three-axis CNC mill with an openarchitecture controller incorporating real-time interpolators that realize smoothly varying feedrates along the PH corner curves. Tests on a representative selection of toolpaths yield substantial savings (up to 40 %) in execution times, compared to the traditional "full stop" strategy for unmodified sharp corners.

Section: Resultsmentioning

confidence: 99%

Efficient high-speed cornering motions based on continuously-variable feedrates. II. Implementation and performance analysis

Nittler

2016

Self Cite

“…obtained by generalizing (1), (14), (16) to non-constant curvature κ(ξ), yields an accurate indication of chip load variation along the part shape, assuming a constant spindle speed n, depth of cut δ, tool radius d, number of cutting edges m, and feedrate V d along the offset path.…”

Section: Second-order Spindle Speed Modelmentioning

confidence: 99%

“…A key requirement is that the part shape be precisely communicated to the machine controller as a parametric curve r(ξ), to permit exact computation of the part curvature and its derivatives, and the offset tool path -in keeping with the STEP-NC philosophy [3,9,19] of more directly employing precise CAD geometry data in CNC machining, instead of relying upon piecewiselinear/circular G code tool path approximations. This theme has also been pursued in several prior studies [4,8,13,14].…”

Section: Introductionmentioning

confidence: 99%

Suppression of chip load variations by real-time spindle speed modulation

Srinathu

2018

Self Cite

In machining a fixed depth of cut at a constant feedrate to generate a desired curvilinear shape, substantial variations in chip load can occur whenever the smallest concave radius of curvature is comparable to the tool radius. These chip load variations may result in a poor quality of the machined surface or premature tool wear. Conversely, attempting to suppress chip load variations by modulating the feedrate may incur high rates of feed acceleration, that may tax the machine drive systems or induce large contour errors. To address these conflicting influences, the feasibility of minimizing variations in chip load through real-time spindle speed modulation is examined herein. A second-order model is employed to determine the tool angular speeds and accelerations that are required to achieve a specified constant chip load for a given depth of cut along a desired part shape defined by a parametric curve, using a constant feedrate for a tool with a given radius and number of cutting edges. For a spindle driven by a DC motor, the motor voltage variation required to generate the modulated spindle speed is also determined. These analyses facilitate an a priori assessment of the part geometries and process parameters for which spindle speed modulation is a viable approach to the suppression of chip load variations.

“…A detailed treatment of the PH curves may be found in [14], and the references [4,12,16,19,20,34] describe examples of their applications in real-time motion control. We outline only the most essential aspects here.…”

Section: Pythagorean-hodograph Curvesmentioning

confidence: 99%

Feedrate modulation for accurate traversal of trimmed planar offset paths

Srinathu

2018

Self Cite

In milling planar shapes with cylindrical tools, CNC machines must employ tool paths offset from the desired part shape by the tool radius. To prevent gouging the part geometry and to ensure continuous paths, the offset path construction invokes trimming and filling operations at tangent discontinuities of the part shape, and smooth concave regions where the tool radius exceeds the part radius of curvature. A constant feedrate of the tool center along the offset path can result in large chip load variations. Conversely, a variable feedrate that (approximately) achieves a constant chip load can cause large acceleration/deceleration rates at the tool center. Moreover, feedrate reduction is required in the vicinity of the trimmed offset path tangent discontinuities, to ensure accurate execution and to minimize vibrations. A feedrate modulation methodology for trimmed offset paths is proposed, incorporating userspecified parameters to balance the competing demands of minimizing path contour error, chip load variations, and the overall traversal time. The feedrate modulation scheme is compatible with simple real-time interpolator algorithms, and its practical performance is demonstrated by real-time data obtained from several experiments performed on a CNC mill with an open-architecture software controller.