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Raymond Hope wrote:

*>
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*> Hi all;
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*>
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*> My name is Ray Hope. I've been working on slicing procedures for layered
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*> manufacturing, and am using layers with sloping boundary surfaces to
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*> eliminate the stair case effect. I have recently been working on adaptive
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*> slicing, and have come across some cases that can cause problems. So I
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*> thought I should call on the greater collective knowledge of the group and
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*> see if we can come up with some ideas.
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*>
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*> Note I am obtaining the definition of parts from B-spline surfaces. Layer
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*> error is approximated from the radius of curvature and angle of the
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*> surface normal.
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*>
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*> Problem 1.
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*>
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*> Joins between two or more surfaces, and vertices can cause the error
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*> approximation to give incorrect results. Previously published work (by
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*> others) has tried to solve this problem by slicing the part so that the
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*> surface joins coincide with layer joins. However this can only work if the
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*> surface joins are in the same plane as the layers. In many cases where a
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*> part is defined by two intersecting surfaces, the intersection curve is not
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*> in the layer plane. So what do we do?
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*>
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Hello Ray and others interested in this thread,

I have done research in the area of adaptive slicing and generating

layers with ruled edges from BREP geometry. I also calculate the

error from the radius of curvature and angle of the surface normal.

The problem of generating layers with ruled edges is that you use

2 consecutive, parallel contours to generate a ruled surface in between.

So you need to determine how to connect these contours, how to find

a correct starting point on both contours and what the parametrisation

is

along these contours.

(following is an excerpt of a paper I wrote :)

This is possible by first determining 'segments' in your CAD model.

A very simple and basic way of doing that is to analyze your

CAD model. You have to carry out a surface and curve analysis

to find parting planes to create segments which can be

processed individually.

At sharp edges of the geometric model the assumption that a

local approximation by a circle with radius $R$ can be

made is no longer valid.

Therefore the object must be divided into distinct segments

in such a way that each block can be approximated separately

with a set of layers. Branching and finding of corresponding

contours will cause problems to reconstruct a surface between

successive contours.

Therefore the criteria for the segmentation algorithm using

the BREP model are extrema in the

z-direction of edge-curves between adjacent surfaces and extrema of the

surfaces. This algorithm will find possible problem areas in the

vertical (slicing) direction that can cause the above mentioned

problems.

Furthermore vertices of the geometric model (begin- and endpoints of the

edge-curves) indicating possible problem areas are found.

Inspection of edge-curves takes place by means of sampling to find

extrema in

the z-direction. Inspection of surfaces takes place by means of sampling

to

find horizontal sections. These sections are extrema, i.e. (local)

minima and

maxima in the z-direction of the surface. The list of segmentation

planes is

sorted according to the z-coordinates and the portion of an object

between two

successive segmentation planes is referred to as a 'segment'. The

resulting

segments can be processed without further problems.

When you have a segment, you can slice this segment (adaptively) and

connect 2

adjacent successive contours easily becasue all contour parts from one

contour have a corresponding part in the other contour, because they

are originating from the same surface.

*> Problem 2.
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*>
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*> The method mentioned above to predict the error, produces a predicted
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*> error much less than the true value near inflection points. This is
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*> because at an inflection point the curvature becomes zero, and is also very
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*> small nearby. So how should we predict error near inflection points?
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*>
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You can perform a curvature analysis to find these problem areas.

(another excerpt:)

An other approach is to use a two-way approach to ensure that regions

of higher curvature are not missed. First in the upward direction a

new layer thickness is calculated. Secondly in the downward direction

at the new proposed

sliceheight a check-calculation is made. This check-calculation is made

to

ensure that the proposed layer thickness does not result in exceeding

the

user-specified $\delta$ in the approximation. This

check is necessary because of a possible changing curvature in

the upward direction of the CAD model.

I hope I have helped you with my comments.

Best regards,

Peter de Jager

Delft University - Industrial Design Engineering

p.j.dejager@io.tudelft.nl

*> I look forward to hearing some interesting suggestion from all of you.
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*>
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*> Thanks
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*> Ray Hope
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*> Department of Mechanical Engineering
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*> The University of Queensland
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*> Brisbane QLD 4072
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*> AUSTRALIA
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*>
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*> Ph +61 7 3365 4179
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*> Home +61 7 3217 7410
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*>
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*> Facsimile +61 7 3365 4799
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*> Email: hope@mech.uq.edu.au
*

**Next message:**Daniel J. Mickish: "DuPont Company Announces new "Somos 6120" Epoxy Resin for Solid State UV Laser Systems"**Previous message:**M. Burns (marshall@ennex.com): "Re: Criteria for RP expert system/selector program"**In reply to:**Raymond Hope: "Adaptive slicing problems."**Next in thread:**Raymond Hope: "Re: Adaptive slicing problems."**Reply:**Raymond Hope: "Re: Adaptive slicing problems."**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

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