3D Modeling for AutoCAD LT 2000

Autodesk’s AutoCAD LT 2000 contains a great deal of the facilities in the full AutoCAD 2000 for a very much lower price. Mainly, the things that are in AutoCAD 2000 and absent from LT are: raster image support, rendering, Lisp and other programmability. It also lacks the ability to crop x-ref insertions and several of the 3D Modeling tools. It is the last item that is addressed by drcauto’s LT3D package.

The standard AutoCAD LT 2000 has very limited tools for creation and editing of 3D objects although it can fully display 3D models that it finds in DWG files created on the full AutoCAD. Generally, any such 3D objects can be moved and deleted but not otherwise edited.

AutoCAD 2000 (and also R13 or R14) incorporate the ACIS solid Modeling sub-system from Spatial Technology. This is used for the 3D solid Modeling facilities and also for creating true curves and ellipses in 2D or 3D and truly curved, accurate 3D surfaces, which AutoCAD could not do before R13 and ACIS. AutoCAD LT 2000 has a restricted sub-set of the ACIS sub-system, as it is used only for curves and ellipses and displaying 3D objects received from the full AutoCAD. Before R13, AutoCAD used an approximate representation system for curves and curved surfaces. In particular, curved 3D surfaces were defined as an assembly of flat 3D faces with straight-line edges, called facets. The user could specify how coarsely or finely the curved surfaces were to be modeled by setting the number of facets.

The LT3D add-on from drcauto makes use of the AutoCAD ARX custom objects programming system that is built into LT but not normally accessible. It is present because ARX is used for many of the built-in LT facilities. LT3D creates custom ARX objects to add a similar set of 3D Modeling tools to those found in AutoCAD. However, because it cannot have access to the full ACIS solid Modeller, it models curved-surface 3D objects in the same way that pre-R13 versions of AutoCAD did; that is, by faceted surface models. So LT3D does not convert the low cost LT into the same as the full AutoCAD 2000, but adds 3D Modeling facilities about equivalent to those that were in AutoCAD R12, including the Boolean solid Modeling facilities. That is, the method of building solid 3D shapes by combining 3D ‘primitive’ shapes such as blocks, wedges, tori, spheres, cylinders, by Boolean logic operations. LT3D also provides the same set of tools for generating solids by sweeping or lofting 2D profiles around axes or extruding profiles. It retains the terminology and logic of the R12 3D tools. Since it uses custom ARX objects there are some provisos in the usability of DWG files created with LT3D. These are explained below.

The objects that LT3D creates by revolution or extrusion (Revsurf, Tabsurf, Rulesurf etc) are built from standard AutoCAD Polyface Meshes. Its 3D-Mesh command produces a 3D Mesh for consistency with AutoCAD. As these objects are built from standard AutoCAD objects, they can be seen if the DWG file produced with LT3D is opened in standard AutoCAD or LT, although they would then be very hard to edit using ordinary LT tools.

LT3D provides the same set of basic 3D solid primitives and Boolean combination tools as in R12. It creates these objects as custom ARX objects. They embody enough ‘solid’ information to support the Boolean combination operations, but use a faceted surface representation, which inherently involves an approximation of the theoretical curved surface shapes. Hence they are not true solids objects but behave in most respects as though they were.

LT3D’s solids can be exploded into Polyface Meshes, but the process is not reversible, and they naturally lose their solids behavior and Boolean capabilities if they are exploded. The point of doing so would be to enable the objects to be seen in standard AutoCAD or LT without needing any special add-ons.

Because LT3D solid objects are custom ARX objects, they cannot be seen if the DWG file is opened in standard AutoCAD LT or AutoCAD unless the drcauto Custom Object Viewer is also implemented. This viewer module can be downloaded from the drcauto web site. This add-on custom object viewer technology is an Autodesk system designed to overcome the problem of vanishing custom objects (also known as ‘Proxies’ or ‘Zombies’). Autodesk call them ‘object enablers’ and encourage developers who use ARX to create custom objects to provide freely available ‘enabler’ modules. Object Enablers allow the relevant custom objects to be seen properly in standard AutoCAD, but do not allow them to be edited.

Because of all this, any 3D Modeling done using AutoCAD LT 2000 and drcauto’s LT3D should be considered to be usable only on AutoCAD LT 2000 with the LT3D add-on installed. The LT3D objects are quite different from the full AutoCAD’s 3D objects and not fully transferable.

As was the case in AutoCAD R12, there is a great shortage of 3D fillet and blend facilities, which will prevent the system being used for any serious 3D manufactured part design. For that usage you need a proper, full 3D solids mechanical design system such as Autodesk Inventor or Mechanical Desktop – even the current AutoCAD 2000 is not really adequate for that. The range of 3D facilities provided in LT3D would be more suited to Modeling buildings, where fillets, draft angles and surface blends are not needed.

LT3D includes extended keyboard actions. The keys used are the cursor control keys - Home, Delete, Page Up, Page Down, etc, and the arrow keys. They add snap mode overrides, and the arrow keys add panning functionality. The functions of the keys are shown in the table below. These are very useful hot-keys, and have always been a feature of drcauto’s architectural add-on systems for AutoCAD.

LT3D includes some quick viewing commands. These allow setting the viewpoint and UCS with only two key presses. All combinations start with the "v" key, then are combined with a cursor control key to invoke the viewing angle. The key combinations are shown in the table below.