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Bevelling a cylinder creates a void
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Re: Bevelling a cylinder creates a void
 
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Re: Bevelling a cylinder creates a void
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Re: Bevelling a cylinder creates a void
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Re: Bevelling a cylinder creates a void
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Re: Bevelling a cylinder creates a void
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In reply to this post by adrianv
Convexity is to tell the renderer how 'deep' it has to look when
determining which faces are 'inside' and 'outside'. OpenCSG does some smart things by ignoring faces depending on the CSG
operation and whether the face is in or out. If it doesn't look deep enough it will either drop faces or show some
which shouldn't be there. Take this bearing with threaded holes, cut to see the insides:
If you look from the right you have 8 transitions. If you look from above the threaded holes, 'lots' as you drill through
the threads. Nophead, this is the one with self-intersections and degenerate faces that had preview performance problems. My conjecture is the algorithm used to determine the surface gets confused with such faults, causing
it to do more processing. This is from a description of Goldfeather: "There are important constraints on surfaces used
for parity test- ing. Parity logic depends on no surface being interior
or exterior to the volume it represents. Also, all boundaries of the
volume should be covered by surface. This means that the surface
should be closed, and should not contain any holes caused by an
incomplete descrip- tion, such as missing polygons. Additionally, the
surface must not self-intersect, forming folds or loops." Link. The 'new' algorithm described (which I think OpenCSG non-goldfeather is
using) is more complex, with more opportunity for issues with bad geometry. > -----Original Message----- > From: Discuss [mailto:[hidden email]] On Behalf Of adrianv > Sent: Sun, 17 Jan 2021 08:25 > To: [hidden email] > Subject: Re: [OpenSCAD] Bevelling a cylinder creates a void > > With lower convexity some bits are missing...but the missing bits aren't > "pixels" they are larger regions. I don't think any conclusion can be drawn > about how many pixels are drawn. It seems more like differing computations > are done to determine what to draw, and with lower convexity the result is > that the wrong stuff is drawn. > > > nophead wrote > > No idea but with a lower convexity some bits are missing, so I assume > > filling them in involves more drawing and that is why it is slower. > > > > On Sat, 16 Jan 2021 at 20:25, adrianv < > > > avm4@ > > > > wrote: > > > >> Why would higher convexity cause it to draw more pixels? Where would > >> these > >> extra pixels come from? > >> > >> > >> nophead wrote > >> > Presumably it has to draw more pixels. > >> > > >> > On Sat, 16 Jan 2021 at 18:59, jon < > >> > >> > jon@ > >> > >> > > wrote: > >> > > >> >> Excellent experiment! Thank you! > >> >> > >> >> I wonder why it gets jerky if the preview time is always the same > >> >> > >> >> On 1/16/2021 1:55 PM, adrianv wrote: > >> >> > I did a little experimentation using the beveled cylinder as a test > >> >> case. It > >> >> > appears to display correctly with convexity 2 but wrong with > >> convexity > >> >> 1. I > >> >> > don't notice any difference in performance with convexity 1 vs > >> >> convexity > >> >> 10. > >> >> > With convexity 100 it's still hard to identify a difference. Maybe > >> a > >> >> very > >> >> > slight degradation? With convexity 1000 the model rotates jerkily > >> and > >> >> it is > >> >> > hard to control. > >> >> > > >> >> > I increased complexity by making a 10x10 array of beveled cylinders, > >> so > >> >> 100 > >> >> > objects. In this case a difference is very clear. With convexity 1 > >> it > >> >> > rotates very responsively (with incorrect display). With 2 it is > >> also > >> >> > responsive. With 10 there is a very noticeable lag with jerky > >> >> > rotation---it's hard to rotate the model. With 100 it's unusable. > >> >> > > >> >> > Note that the time to produce the preview is the same regardless of > >> the > >> >> > convexity value. > >> >> > > >> >> > >> >> _______________________________________________ > >> >> OpenSCAD mailing list > >> >> > >> > >> > Discuss@.openscad > >> > >> >> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org > >> >> > >> > > >> > _______________________________________________ > >> > OpenSCAD mailing list > >> > >> > Discuss@.openscad > >> > >> > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org > >> > >> > >> > >> > >> > >> -- > >> Sent from: http://forum.openscad.org/ > >> > >> _______________________________________________ > >> OpenSCAD mailing list > >> > > > Discuss@.openscad > > >> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org > >> > > > > _______________________________________________ > > OpenSCAD mailing list > > > Discuss@.openscad > > > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org > > > > > > -- > Sent from: http://forum.openscad.org/ > > _______________________________________________ > OpenSCAD mailing list > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org _______________________________________________ OpenSCAD mailing list [hidden email] http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org |
Re: Bevelling a cylinder creates a void
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Re: Bevelling a cylinder creates a void
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Leaving out the detail, but coming more from a 3 axis machining
perspective, my approach would be different. The first thing to do
would be to drill the hole right through the centre of the
cylinder. The drill would start from outside the cylinder (cutting
air) and go past the far end of the cylinder. A few reasons for
that- most drills have a pointed end, and even if using a slot
drill, or boring bar/whatever, if the depth of the drilling is
exactly the same as to the end of the material, is there going to
be a thin web at the bottom, or not? The same reason that an exact
diameter pin will not fit into an exact diameter hole, unless
there is some external influence. In order to get the exact bevel,
with a perfect finish, then the piece would be set at an angle,
and would be milled flat. There would most likely be hand
finishing/polishing whatever required. There are slight problems
to be solved in that approach concerned with holding the cylinder
at the correct angle. If the top surface is not level, (by more than one layer height) then the Hilbert curve top layer will show that. Another aspect, is that unless one spends maybe ten times the price you think that you need to, you generally expect too much from what you've got, unless you can compensate with patience - usual practical problems. 
-----Original Message----- From: Discuss [[hidden email]] On Behalf Of Luc Le Blanc Sent: Sun, 17 Jan 2021 03:35 To: [hidden email] Subject: [OpenSCAD] Bevelling a cylinder creates a void I'm trying to bevel a drilled cylinder, but I when I do that, the top of the cylinder disappears into a (green) hole. I attached a screenshot. The 3D printer-owning friend I sent my STL to says there's a strange gap at the top. $fn=100; difference() { difference() { cylinder(h=39, d=13.69); // bevel translate([-6.845,-15,18]) { rotate(a=[0,-56.89,0]) { cube(30); } } } // bolt hole cylinder(h=39, d=4); } Of course, if I remove the bevel, the top of the cylinder is normal (yellow). Is this a bug? Is there a better way to bevel a cylinder? -- Luc Le Blanc _______________________________________________ OpenSCAD mailing list [hidden email] http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org |
Re: Bevelling a cylinder creates a void
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Re: Bevelling a cylinder creates a void
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