Re[2]: HP-GL for slice data

From: Lee_Humphrey@ccmail.orl.mmc.com
Date: Tue Nov 26 1996 - 17:32:01 EET


     If the RP system is tied to a CAD system then all the people you serve
     would have to use the same system or you would have to have one of
     each. With the .stl, I can and have built parts from 3 different CAD
     systems in a single build. I think the flexibility is dominate trend.
     As far as file size what was huge a couple years ago is normal now.
     Disk size, ram, and transfer rates will continue to increase to handle
     the larger file requirements.
     
     Lee Humphrey

______________________________ Reply Separator _________________________________
Subject: Re: HP-GL for slice data
Author: igibson@hkucc.hku.hk at ORL-SMTP-G
Date: 11/25/96 8:47 PM

Sorry for this long posting, including that already sent, but I felt I
needed to keep the picture.
     
>
>
>On Mon, 25 Nov 1996, Delft Spline Systems wrote:
     
>> How can this language ever represent true (3D) surface data ?
>> The only assumption I can make is that you mean to use slice data
>> to represent the 3D surface in a large number of 2D sections. In contrast
>> to using 3D STL files, when using HPGL the slicing will have to be done
>> in the CAD-software.
>> Regards
>> Lex Lennings.
>
> This is correct. When people speak of using HP-GL as a language for
>communicating to fabricators (or "RP machines"), they are speaking of
>transferring slice data. The CAD program or an intermediate slice program
>would take the responsibility for creating a series of HP-GL contours
>from the 3-D data.
> The current configuration of the Genie fabricator runs on HP-GL. But
>we have a slice program that reads StL and outputs HP-GL.
> HP-GL does have the advantages of being able to represent standard
>curves, such as circles and ellipses, efficiently and without faceting.
>So for these curves, it is better to get the data direct from the CAD
>representation instead of through StL, which will break these curves up
>into little lines. However, I am not aware of a way for HP-GL to
>represent complex curves, e.g. from NURBS surfaces, directly. (If there is
>a way, someone please correct me.) Because of this, I believe we will
>ultimately have to do better than HP-GL.
> Comments?
>
>Best regards,
>Marshall Burns
>marshall@ennex.com
>
>
>
As you all know, I'm a layman, but I am a loud mouthed layman, so here I go:-
     
Am I right in thinking that breaking things up into little lines does not
cause a particular problem provided the lines are small enough? If the stl
file has a minimum triangle size smaller than the resolution of the process
then they are not going to be noticed in the part.
     
However, as the resolution of the current and future rp processes improves,
I feel that the file sizes and amount of number crunching is going to become
ridiculously large.
     
HPGL therefore looks quite attractive from the point of view of cutting out
the middle man. By using this method, slices can be generated directly from
the CAD model without the need for the intermediate stl file. To me this
looks very efficient. A couple of years ago I saw the F&S guys (what
happened to them BTW?) promoting HPGL in this way. They showed a macro that
seemed to slice a very big file in the blink of an eye and it all struck me
as being ingeniously simple.
     
However, I think it means the CAD system should be tied to the rp system
somewhat, which may be undesireable. Variable thickness slicing and dynamic
control of the build height may be problems unless the CAD system is on hand
to provide slice data in real time. Support structures will also have to be
generated using the CAD system. Unless these points are adhered to then
there would be no particular advantage in using HPGL.
     
But doesnt that look interesting, particularly for the small rp systems? A
CAD system directly connected to the rp system, sharing resources, avoiding
the generation of large data files. A direct database of the operating
parameters of the machine inbuilt into the CAD system. Hmmmm.......
     
One other point though. Direct slicing of CAD seems to have gone out of
vogue, largely as a result of the debate from people much more qualified
than I. There are obviously many more issues than I raised here. Perhaps one
of the software gurus can enlighten us?
     
Regards
     
IG
     
Dr Ian Gibson
Dept. Mechanical Engg.
University of Hong Kong
Hong Kong
     
email: igibson@hkucc.hku.hk
www: http://hkumea.hku.hk/acad_igibson.html
tel: +852 2859 7901
fax: +852 2858 5415
     
Did you know that the wingspan of a jumbo jet is longer than the first
flight of the Wright brothers?



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