One concern with CAD is a fundamental aspect of their architecture: How are curves, arcs and circles handled? All- too-many programs take the expedient route of representing them as as polygons or polylines. Academic paper discussing this G codes for the specification of Pythagorean-hodograph tool paths.
A note on curve approximation: The trivial case with four cubic beziers (one on each quadrant) has very low maximum error, IIRC on the order of 0.1% of the circle radius.
Origin point This is different from most drawing and CAD programs: https://bugs.launchpad.net/inkscape/+bug/170049 as discussed here: http://www.shapeoko.com/forum/viewtopic.php?f=6&t=5102&p=37166#p37166
A note on vector drawing: Vector drawing is like to drafting with rules, triangles, compasses and a French curve, while pixel editing is more akin to colouring in spaces on a grid. Vector drawing uses lines (a straight path between two "nodes" which are x,y coordinates, one is the beginning, the other the ending), regular geometric forms (defined by nodes which denote each point at which lines meet), arcs and circles (defined by the central point of rotation and the radius therefrom) and Bézier curves (a mathematical description of an arbitrary curve which has four "nodes", one on-the-path, two off-the-path and one on-the-path) to allow it to express pretty much any shape or form.
A game to learn about forming such curves: http://bezier.method.ac/#
A note on units: Most CAD programs store drawings in generic units, rather than a specific measure. It is important to know what unit was used when a drawing was created so as to be able to import it at the right size. Forum discussion.
It is also of course, straight-forward to work directly in G-Code, accomplishing Design and CAM in a single step using programs which provide integrated text editing such as NC Plot and NC Corrector (the latter is listed on the Previewing G-Code page. A tutorial is listed on the CAM page.
Note: A number of programs listed on the CAM page include design features for such specialized things as PCBs, Thread and Fret cutting and Gears.
The ShapeOko 3 (and Nomad 883) include a license for Carbide Create a CAD/CAM program developed by Carbide3D (currently in beta).
- Why We Need Professional-Level Open Source CAD
- CAD/CAM Wish List
The Failed Promise of Parametric CAD, a series of articles by Evan Yares:
- From the Beginning
- The Problem is Editing
- The Direct Solution
- Going Horizontal
- A Resilient Modeling Strategy
- Viable Solution
Tracing is a useful option for some projects. Most vector drawing programs have an option for this (one option is to place the image on the desktop or in a window beneath the CAD tool, then make that window transparent), or one can use a specialized tool. One such: http://www.kvec.de/english/index.htm Discussion: https://www.reddit.com/r/CNC/comments/47fqeh/best_raster_img_to_scalable_vector_programs/
|2D drawing (Opensource)||Inkscape||SVG drawing program which can either export to DXF or directly to G-code using plug-ins. Supports pressure-sensitive graphics tablets.|
|2D drawing (Opensource)||Doodle3D||Freehand sketching program for iPad and Mac OS X (source available for other platforms) to extrude and revolve into 3D.|
|2D CAD (Opensource)||LibreCAD||Fork of Qcad. Free and OpenSource CAD (DXF) application, cross platform.
Includes menu option to export to MakerCAM SVG.
|2D CAD (Opensource)||Simple2D CAD/CAM||Windows app to cut simple shapes (pockets, mostly) and simplified gears, includes a geometry mode that allows geometry defined by lines and arcs. It also generates geometry from any OpenType font.|
|2D drawing (Opensource)||Cenon (Mac OS X, NeXT/OPENstep, Linux)||OpenStep CAD/CAM program which has transitioned to opensource.|
|2D drawing (Opensource)||Xfig||G-code enabled Xfig.|
|2D CAD (Opensource)||Dedalo||2d CAD drawing software written in python3.|
|2D CAD (Opensource)||Zcad||Simple CAD program with dxf2000 file format support.|
|2D CAD (Freeware)||OpenS-CAD, a simple 2D CAD application||Windows .exe (87.5KB) and source (225KB).|
|2D CAD (Freeware)||DraftSight||Free CAD software for DWG (and DXF) files by Dassault Systèmes (Windows, Mac OS X, Linux (Fedora and Ubuntu)). http://www.solidworks.com/sw/products/draftsight-getting-started-guide.htm|
|2D CAD (Freeware)||nanoCAD||Free tool for creating, editing, validating, sharing, and managing CAD models.|
|2D CAD (Freeware)||Siemens Free 2D||Free CAD software by Siemens (Windows). textbook available as a PDF.|
|2D CAD:iPad (Freeware)||Sketch It Make It Apple App Store||2D CAD for the iPad.|
- Draftsight (listed above)
- Siemens Free 2D (listed above)
|3D modeling (Opensource)||K-3D||K-3D is free-as-in-freedom 3D modeling and animation software. It combines flexible plugins with a visualization pipeline architecture, making K-3D a versatile and powerful tool for artists. Available for Windows, Mac OS X and Linux.|
|3D modeling (Opensource)||BRL-CAD||BRL-CAD is a cross-platform open source solid modeling system that includes interactive geometry editing, high-performance ray-tracing for rendering and geometric analysis, image and signal-processing tools, a system performance analysis benchmark suite, libraries for robust geometric representation, with more than 20 years of active development. Discussion of shortcomings.|
|3D modeling (Opensource)||Art of Illusion||Free, open source 3D modeling and rendering studio. Features include subdivision surface based modeling tools, skeleton based animation, and a graphical language for designing procedural textures and materials. Default design software for the RepRap project.|
|3D modeling (Opensource)||MeshLab||Open source, portable, and extensible system for the processing and editing of unstructured 3D triangular meshes.|
|3D modeling (Opensource)||HeeksCAD||Together with HeeksCAM comprises HeeksCNC. Youtube: HeeksCAD Unofficial Tutorial Part 1:Introduction General overview of GUI for HeeksCAD|
|3D modeling (Opensource)||FreeCAD||FreeCAD is a general purpose feature-based, parametric 3D modeler for CAD, MCAD, CAx, CAE and PLM, aimed directly at mechanical engineering and product design but also fits a wider range of uses in engineering, such as architecture or other engineering specialties. It is 100% Open Source and extremely modular, allowing for very advanced extension and customization. See also the Python-query-scripting language CadQUERY and Cnc25D CAD Library.
A variety of macros are available, i.e., http://www.freecadweb.org/wiki/index.php?title=Macro_FCGear 
See also the graphical version Ulticreator, an interactive front-end for CoffeeSCAD.
See also http://www.loria.fr/~slefebvr/icesl/ which adds slicing for 3D printing.
|3D parametric modeling (Opensource)||ImplicitCAD||Similar to OpenSCAD (above), but written in Haskell and using an internal geometric representation, as opposed to the polygon mesh used by the former|
|3D parametric modeling (Opensource)||NaroCAD||3D parametric modeling CAD application for Windows, based on OpenCascade.|
|3D modeling (Opensource)||Blender||Free, OpenSource and crossplatform 3D program. Allows modelisation, shading, rendering, animation, interactive 3D, compositing. Not a CAD software though it can be used for any kind of modeling. See Blender CAM for a tool to export files for EMC. If importing from Illustrator, see this forum discussion for a note on how to handle units. Tutorials.|
|3D modeling (Opensource)||Wings3D||Free, OpenSource and crossplatform 3D modeler. Wings 3D is an advanced subdivision modeler that is both powerful and easy to use.|
|3D modeling (Opensource)||Shapesmith||Parametric, Open-Source 3D Modeling in your browser, can do basic primitives and addition/subtraction operations and export to STL.|
|3D modeling (Opensource)||SolveSpace||Parametric 3D CAD program for Windows. See also its 2D predecessor, SketchFlat.
1.4Mb executable that doesn't have to be installed.
Does basic 3D operations as well as 2D. A good free option for basic modelling, as well as for education (the basics are the same as any full featured parametric CAD system). Outputs in .svg. .stl output, should be pretty well suited to simple modeling for 3DP.
Features include: Constraints and assemblies
|3D modeling (Opensource)||POVray||"The Persistence of Vision Raytracer (POVray) is a high-quality, Free Software tool for creating stunning three-dimensional graphics".|
|2D drafting/3D modeling (Opensource)||lignumCAD||Tool for designing furniture.|
|2D drafting/3D modeling (Opensource)||Salome||Generic platform for Pre- and Post-Processing for numerical simulation. Can be used as standalone application for generation of CAD model, its preparation for numerical calculations and post-processing of the calculation results.|
|3D modeling (Freeware)||Caligari Truespace||Commercial program released to freeware.|
|3D modeling (Freeware)||DesignSpark Mechanical||Freeware software for Windows distributed by RS Components International. Free version of SpaceClaim 2012.|
|3D modeling (Freeware)||FormZ Free||Freeware software for Windows 7/8/8.1 and Mac OS X 10.7 or later.|
|3D modeling (Freeware/Windows)||Microsoft: 3D Builder||Create, print (List of supported printers) and scan (requires Kinect v2 sensor) functionality.|
|3D modeling (opensource Linux/Mac)||Antimony||Computer-aided design (CAD) tool inspired by LISP, based on: framework for tracking information flow through directed acyclic graphs, geometry engine for doing CSG and includes a standard library of shapes and transforms.
|3D modeling (opensource)||Dilay||https://github.com/abau/dilay --- https://www.reddit.com/r/programming/comments/46130v/dilay_a_free_3d_sculpting_application/|
|3D modeling (Chrome app)||Design Something Google App Store||3D modeling application running on Chrome and Firefox. Pre-defined shapes, operations include extrude and sweep.|
Android app for Mattel's Thingmaker: https://play.google.com/store/apps/details?id=com.mattel.thingmakerdesign
Interesting post on efficiency: http://community.carbide3d.com/t/how-to-make-your-stl-not-take-3-days-to-calculate-tool-paths-for/169
Some may find OnShape sufficiently free to consider it: https://www.onshape.com/
|PCB (opensource)||KiCad||Electronic design automation (EDA) software suite for designing schematics of electronic circuits and printed circuit boards (PCB).|
|PCB (opensource)||gEDA||Full GPL'd suite and toolkit of Electronic Design Automation tools.|
|PCB (opensource)||QS Cad||Tcl/Tk EDA Package. Schematic capture through to PCB layout. Has Part editor as well as editor for Schematic and PCB Decals.|
|PCB (opensource)||Stripboard Designer 1.0||Open source (BSD) app written in freeware Visual Basic 2008 Express for Wintel machines. See also Stripes, a GPL Java version.|
|PCB (opensource)||DIY Layout Creator (DIYLC)||GPL Java software for creating stripboard and universal board designs.|
http://darwinne.github.io/FidoCadJ/download.html --- (Java) a free graphical editor for (more than) electronics
|3D modeling --- Surfboards||BoardCAD||Opensource CAD/CAM-system (written in Java) for surfboards.|
|3D modeling --- furniture||SketchChair||Opensource CAD/CAM-system for flat-pack furniture. Forums/troubleshooting|
|2D CAM --- Jigsaw puzzle piece cutter (Online - Silverlight)||CNC Puzzle Maker||Program for cutting out puzzle shapes w/ tabs.|
|3D CNC --- foam cutting for model airplanes||Jedicut CNC||Program for cutting out wings and fuselages.|
|3D aircraft modeling||OpenVSP||Vehicle sketch pad. NASA opensource parametric geometry. Described as suited for general use.|
|2D CNC --- sails||Sailcut CAD||Sail design and plotting software.|
http://rattlecad.sourceforge.net/ --- Bicycle framebuilding CAD app
- https://gitlab.com/damien.andre/openmeca --- https://www.reddit.com/r/cad/comments/44kzi3/a_free_software_to_model_mechanical_systems/
- http://blog.rectorsquid.com/linkage-mechanism-designer-and-simulator/ 
Other design software
3D CG to 3D Printable 3D model converter for Windows PC --- CGto3DPrint is a free software that converts 3D CG(Computer Graphics) models to 3D printable manifold 3D models using MeshLab for surface mesh to voxel conversion and Blender for baking color textures. Converted 3D models are automatically exported from Blender to STL file for single color 3D Printing or manually exported to PLY file for full color 3D Printing. 
Redrawing may be a better option: https://www.reddit.com/r/cad/comments/4mmqno/im_trying_to_reverse_engineer_a_component_to_draw/
FreeLSS --- laser scanning program for the Raspberry Pi. It allows a Raspberry Pi to function as the core to a complete turn table laser scanning system. Files for printing a 3D scanner: http://www.murobo.com/printables.html
Photo measuring app
- Partometer --- http://www.vistechprojects.com/app/partometer
Additional programs which may merit investigation
- https://opendesignengine.net/projects/mtk/wiki --- Mach 30's Mathematics Tool Kit, unified engineering analysis and documentation tool
- http://dyama.github.io/siren/ --- simple CAD viewer for Microsoft Windows.
- https://github.com/christopherpoole/cadmesh --- A CAD file interface for GEANT4
- http://sourceforge.net/projects/elmerfem/ --- Elmer finite element software Open source finite element software for multiphysical problems
- http://www.lemkesoft.de/en/products/cadintosh/ ---- venerable Mac OS app from Germany, updated for OS X.
- http://www.allycad.com/homeversion.asp --- UK
- http://www.cadvance.com/65form.htm ---Free CADVANCE 6.5 for Windows, re-release of CADVANCE Version 6.5, a CAD software system designed for Windows 3.1, originally released in 1995 now available to anyone free of charge.
- http://milkshape3d.com/ --- low-polygon modeler initially designed for Half-Life.
- http://sourceforge.net/projects/technecadcam/ 
- GtkCAD --- (abandoned) modular, full featured, modern CAD/CAE/CAM system for GNU/Linux.
- JCad --- 2D CAD application written completely in Java. AutoCAD DXF native format.
- oCADis --- 2D CAD system for Linux. Basic drawing objects: line, arc, circle and more. Supports HPGL (export), DXF (import and export), PostScript output.
- ThanCAD --- 2dimensional cad, with raster inclusion capability and emphasis in engineering. Written in Python using the wxPython library.
- PythonCAD --- R38
- AnyCAD 1.4 Free Version --- Chinese CAD program available in English
- K3DSurf --- Program to visualize and manipulate Mathematical models in three, four, five and six dimensions. Supports Parametric equations and Isosurfaces.
- Frusta --- Tcl toolkit to manipulate data for a 3d-printer
- HYCAD 5.33 --- Free (abandoned? Last Updated: December 5th, 2007) CAD app for Windows from China by JiangJiang (developer homepage English link is 404, main site times out). Simplified AutoCAD interface, mouse gestures.
- LiteCAD --- 2D CAD program.
- A9 CAD
- Replath --- a program that can plot various 2D file formats either directly to a RepRap or to a GCode file.
- eCabinet --- CAD for custom cabinet makers (requires membership/registration)
- gCAD3D --- 3D - CADCAM - software for MS-Windows and Linux (available in German and English)
- Misfit Model 3D --- an OpenGL-based 3D model editor. It supports multi-level undo, skeletal animations, texturing mapping, scripting, command-line batch processing, and a plugin system for adding new features.
- CADemia-Community --- free Java CAD software for true-to-scale technical drawings and documents. Also available in German
- Fandango: scripted 3d CAD --- formerly elcad, a python scripted 3d CAD application.
- AR-CAD - freeCAD and StCAD --- not the FreeCAD above, free 3D CAD Software with Motion Simulation Capabilities.
- The Piglet Drawing Editor --- 2-D engineering drawing editor that supports hierarchical object description, inspired by Hewlett-Packard's proprietary HP PIGLET (Personal Interactive Graphic Layout EdiTor).
- SagCAD --- Japanese CAD program
- AnyCAD --- .net Windows program / programming library
- GNU CNC CAM --- Tool for converting DXF (CAD) Files to G-Code (RS-274) NC-Files.
- Varkon --- parametric 2D/3D CAD, modeling and application development tool for engineering, design, architecture, etc. does 2D drafting, 2D and 3D modeling, sculptured surface modeling, and animations. Includes a 6-minute tutorial.
- CNCSimple ---3D modeling C# source code for Visual Studio Express 2010 at .
- Minos3D --- Windows program to avoid wireframe views.
- MonoWorks --- a full-featured, Open Source 3D Computer Aided Design (CAD) suite that includes object-oriented math and modeling libraries, as well as interactive viewing and modeling applications.
- AutoQ3D --- Opensource (GPL) version is AutoQ3D Community available for Windows here. Mac, iPhone/iPad, Android, Windows and Linux versions are available for AutoQ3D CAD.
- Anim8or --- Windows 3D modelling tool able to import and modify .3DS files
- netfabb Studio Basic --- Free application for Linux, Mac OS X or Windows for mesh edit, repair and analysis
- Wildcat CAD --- A modern 3D solid modeling kernel and associated applications for CAD and all types of related engineering. BSD license, last up-dated Oct 2008.
- CollabCAD --- 3D CAD/CAM Software system for collaborative design & development of Industrial Designs.
- pyturn a simple python code generator for cnc lathe programs
- 3DCrafter --- Real-time 3D modeling and animation tool that incorporates an intuitive drag-and-drop approach to 3D modeling. Free, Plus and Pro versions.
Bézier drawing programs
- http://al.chemy.org/download/ (opensource, cross-platform)
- https://inkscape.org/ --- listed above
- https://krita.org/ --- also has painting / pixel image features
Mac OS X
- http://www.cenon.info/frame_gb.html --- listed above, CAM module is a commercial option
- http://www.nchsoftware.com/drawpad/index.html (Windows)
- http://www.serif.com/free-graphic-design-software/ (Windows)