This program is obsolete;
SolveSpace
replaces it completely, and is now available as unrestricted
freeware.
* * *
SketchFlat is a twodimensional technical drawing program. It is
designed primarily to generate CAM output data, for manufacturing on
a laser cutter, waterjet machine, vinyl cutter, 3axis mill, or other
machine tool.
Consider the part shown below:
This is a microwave horn, flattened out. I will cut along the solid lines,
and fold along the dashed ones. (I'll make this on a laser cutter, from
very thin metal sheet. I would like to score the fold lines, but lasers
don't do that, so instead I will perforate them. The dashes are not just
cosmetic.)
This will produce a square horn, tapering from 1.000" across at the
narrow end, to 3.500" at the wide end. The narrow end is closed, but
the wide end is open. The length, measured along one of the trapezoidal
faces of the horn, is 8.000". (The distance along the horn's axis is a
bit shorter, of course.) There's a circular hole 0.412" from the back
face, for the feed; this hole is 0.100" diameter.
I might have handcalculated the geometry to produce this template,
and drawn it in any tool, by typing in coordinates. Instead, I drew the
part in SketchFlat:
The dimensioning here is fully parametric: there is a simple mapping
between the specifications of the part and the numbers that I type
in to the drawing. Let us say that I change my mind; the wide end is
currently 3.500" across, but I would rather have 2.000". I will type
in the new dimension:
In order to produce our new horn, many different lines had to move; but
SketchFlat recalculated the geometry automatically. This makes it easy
to make changes. It also makes it easy to draw the part in the first
place, since no handcalculation is required; SketchFlat translates
the specifications into (x, y) coordinates for each line segment in
the sketch.
The program is able to do this because the part is described in terms of
dimensions and other geometric constraints. The magenta
marks describe these constraints. For example, the two inwardfacing
arrows at the far right of the drawing indicate a `symmetry' constraint:
the endpoints of that line segment are symmetric about the dashed blue
reference axis. The H and V characters indicate horizontal and vertical
line segments. The tick marks indicate equallength line segments.
Many different combinations of constraints would describe the geometry
of this part; so as we constrain the part, we have many different
choices available. We will try to choose dimensions that reflect the
design intent; that is, we will try to draw constraints that correspond
naturally and in a simple way to the specifications that we are given.
Feature List
In the example above, we were mostly just drawing line segments, but
many different types of sketch entity are available, including:
 datum lines
 datum points
 line segments
 circles
 arcs of circles
 cubic splines, in Bezier form
 text in a TrueType font
 imported files
A datum is a line, point, or curve that will not appear in the exported
geometry (i.e., it's not a line that we eventually want to cut with the
laser). It's often helpful to draw extra lines and points and curves,
in order to construct some more complex geometric relationship. You can
do this using datum lines and points, or by marking any other entity
(line segment, circle, etc.) as `construction'.
A sketch consists of some combination of these entities. These entities
are drawn on layers. Each layer may be exported directly, or it may be
used in `derived operations', to obtain more complex polygons. If our
part is `a rectangular plate with a circular notch', then it might be
simplest to draw the rectangle on one layer, draw the circle on another,
and then take the Boolean difference. Derived operations include:
 Boolean union and difference
 step and repeat, translating or rotating
 scale
 round sharp corner
 offset edge (cutter radius compensation)
Once a part has been drawn, it may be exported as a DXF, as HPGL, or as
Gcode. All curves are broken down into piecewise linear segments before
output is generated, so the structure of these files is very simple.
Comparison
Versus a typical vector drawing program (Adobe Illustrator, CorelDRAW,
etc.): SketchFlat does not have any graphics features. It makes no
provision to specify with what color a line should be drawn, or to
fill a polygon, or to apply any rastertype effects. (This makes sense,
because these features are not meaningful when you are producing vector
CAM output.) SketchFlat does provide parametric dimensioning, which
drawing programs don't. This is useful when entities in a drawing
should have some exact geometric relationship (e.g., line A is thirty
degrees clockwise from line B), and are not just drawn by eye with
the mouse.
Versus a typical sketchbased 3d CAD program (Pro/E, Rhino, Solidworks,
Autodesk Inventor, etc.): Sketchflat has no 3d features. Sketchflat is
roughly equivalent to any of these program's sketcher (i.e., to the portion
of the program where a twodimensional section is drawn, that will later
be swept or extruded to produce a threedimensional solid). Depending
on the 3d CAD tool, SketchFlat's parametric dimensioning may be more
powerful. SketchFlat has some elementary `derived' operations, to fill
in for operations (like rounding corners) that would typically
be performed on the solid model, and not in the sketch.
Versus a typical 2d CAD program (AutoCAD, etc.): SketchFlat does
not have the hierarchy and structured document features that most of these
programs offer. It is therefore unsuitable for drawings containing a large
number of
entities. SketchFlat does not have features to indicate how lines are
displayed (thickness, color), which makes it unsuitable for producing
humanreadable output. SketchFlat does provide complex parametric
dimensioning. I've seen parametric features or addons for some 2d CAD
software, but only in basic form (only the `acyclic dependency
graph' kind; refer to my description of SketchFlat's internals to see
what this means).
Once a part has been drawn, SketchFlat's `offset curve' tool may be used
for basic cutter radius compensation. This might in some cases make it
possible to live without separate CAM software.
Sample Sketches
An `easy' demonstration: Napoleon's theorem. (It's easy because you can
solve the sketch while considering at most two equations at a time.)
A `harder' demonstration: Peaucellier's linkage for straightline
motion. (It's harder because the solver must consider many equations
simultaneously.) As the highlighted link rotates around the origin, the
rightmost point moves along the green vertical line. The right endpoint
of the highlighted link may be dragged with the mouse.
(SketchFlat can solve arbitrary planar linkages. A sketch with only
distance and pointcoincident constraints is a linkage; the distances
are the lengths of the links, and the coincident points are the pivots.
The solver can quickly determine (a) whether the linkage can be assembled
at all, (b) how many degrees of freedom the finished linkage will have, and (c)
the positions of all of the links, as one or more of the links is dragged. Linear
slides are also possible, in the form of pointonline constraints.)
A `hard' demonstration: A twelvesided regular polygon. I drew a twelvesided polygon. I then used
equallength constraints to force all its edge lengths to be equal,
and constrained all its vertices to lie on a circle. This requires the
simultaneous solution of 22 unknowns.
A Euro (currency) symbol, in terms of its
official
geometric definition:
Internals
I've used Pro/E extensively, so SketchFlat is to a certain extent inspired
by Pro/E's sketcher. Like Pro/E, I use a numerical method to solve many
nonlinear equations simultaneously. This permits me to accept a wide
variety of constraints; SketchFlat will readily solve many high school
geometry problems. I've seen other commercial tools that can't do this.
SketchFlat is built around a trivial symbolic algebra library. The geometry
of entities—like lines or splines or circles—is described in
terms of those symbolic variables. For each constraint, SketchFlat
writes a symbolic equation. `Easy' constraints are solved by symbolic
forwardsubstitution on the system of equations, to reduce the number
of unknowns.
The system of constraint equations is then partitioned into the smallest
consistent subsystems we can find, and each subsystem is solved using a
multivariable Newton's method. If the system is underconstrained (i.e.,
if we can find an infinite number of solutions that still satisfy our
constraints), then we will automatically make assumptions, until we have
restricted ourselves to a single solution. If the system is inconsistent
(i.e., if we can prove that no solution satisfying the constraints exists),
then we will report this, and indicate which constraints should be
removed to bring the sketch back to consistency.
A longer description of SketchFlat's internals is available:
Limitations, and Future Enhancements
This is the first release of SketchFlat. I don't know of any actual
bugs, but I expect that several areas will later be improved:
 The automatic assumptions are not as good as I would like. This is
subjective, and userspecified dragging behaviour may be forced with a
`draggable' constraint.
 The numerical solver: better special cases, improved numerical
stability, faster partitioning, faster solution of subsystems, etc.
 The derived operations would benefit from better computational
geometry. A lot of things are O(n^{2}) that could be
O(n log n).
 The derived operations are in general an afterthought, and should
be improved.
 Portions of the TrueType font spec are not yet implemented. Some
glyphs, including all composite glyphs, will not display correctly.
 I need to write a manual.
Other defects may exist; please contact me if
you find one.
Download
The program is written for Win32. I develop under XP. I expect but have
not tested that it will work under 95 and later. There is a single .exe
file to download:
There is no installation program; just save it somewhere and run it.
There's no manual, but I have a
long tutorial.
The source code is also available for download.
This program may be distributed and modified under the terms of the
GPL Version 3.
Oct 2007, Cambridge MA
