Sketchgauge user guide

A first sketch in five minutes

The page opens on an example, a fully dimensioned mounting plate. Use it to see what the status line, the colours and the sheet look like, then start your own with File, New sketch.

1. Choose the Line tool and click three points on the canvas: the first click starts a line, the second ends it and starts the next, and Esc stops. The status line now says Under-constrained and counts the degrees of freedom: three points are six unknowns and nothing holds them.
2. Choose Select, click the first line in the canvas or in the Items tab, open the Constraints tab and press Horizontal. The count drops by one: the two ends of the line must now have the same height.
3. With the same line selected, press Distance. The box shows the length as drawn; type 40 and press Add dimension. The line becomes exactly 40 long and the count drops again.
4. Select the first point and press Fix to pin it where it is. Select the second line and add a Vertical constraint and a Distance of 30. Now every point is determined and the status turns to Fully constrained with zero degrees of freedom: the sketch is green.
5. Open Drawing sheet in the toolbar, give the drawing a title in the Sheet tab and press File, DXF: sheet or SVG: sheet.

Change any dimension value later (in the Constraints tab) and the sketch follows. Drag a free point with the Select tool and the solver moves everything else to keep the rules true.

Drawing a sketch

Choose a tool in the toolbar and click on the canvas: Point, Line (two clicks), Circle (centre, then a point on the circle) or Arc (centre, start, end; the arc runs counter-clockwise from start to end). Select picks items; hold Shift to add to the selection. Drag a point to move it: the solver keeps every constraint true while you drag, and a drag that cannot satisfy them is refused with a message.

Everything can also be typed. The Entities table lists every point with editable x and y, every circle with its radius, and every line and arc with the points it uses, so a sketch can be built and changed without a pointer. Delete removes the selection and anything that depended on it; Undo and Redo cover every change.

A construction item is drawn dashed, is never part of the exported geometry layer unless you ask for it and exists only to carry constraints.

Dimensions and constraints

Select one or two items and the Constraints panel offers what fits them.

A dimension can be made a reference: it is measured and shown in brackets but never changes the sketch. Tangent constraints between a line and an arc that share an end point, or between two arcs that share one, use the exact contact form (the radius is perpendicular to the line there), which keeps them accurate.

Degrees of freedom and status

The status line shows the degrees of freedom (DOF): the number of independent ways the sketch can still change. It is the number of unknowns (two per point, one per circle) minus the rank of the constraint system at the solution.

• Under-constrained: DOF is above zero. Items that can still move are drawn in blue.
• Fully constrained: DOF is zero and nothing is redundant. Items are drawn in green.
• Over-constrained: every constraint can be met, but some repeat what others already enforce. They are marked redundant and listed.
• Conflict: the constraints cannot all be met together. The ones that disagree are marked in red and the geometry is left where it was.

A sketch with only relative constraints can still slide and turn as a whole (three DOF); pin a point with Fix and fix a direction to anchor it. Redundant constraints are harmless to the answer, but they usually mean a dimension should be a reference dimension instead.

Drawing sheet

The Sheet tab places the sketch on a paper size (ISO A4 to A0, ANSI A to E, either orientation) at a preferred scale or at a scale you set. The sheet has a border, a title block (title, drawing number, company, drawn and checked by, material, date, revision and sheet fields) and an optional note.

Dimension text is measured from the drawn geometry, so it can never disagree with it. Dimension lines are placed outside the part and moved clear of each other and of labels automatically; set an offset on a dimension to place it by hand. You can add notes and geometric-tolerance frames (a symbol, a tolerance, an optional diameter sign and material-condition letter and up to three datum letters). The frames are annotations: Sketchgauge draws them but does not evaluate tolerance zones or check them against any drafting standard.

The title block date is text you type; the Today button fills in the current date in Mountain Time.

Files: JSON, DXF and SVG

JSON is the native file: it keeps the sketch, constraints, annotations and sheet settings, and is checked strictly when opened (unknown fields, bad references and out-of-range numbers are refused). DXF import reads LINE, CIRCLE, ARC, POINT and straight polylines (other entities are listed and skipped), merges end points that coincide and ignores the layers Sketchgauge uses for dimensions, notes and the sheet.

DXF export writes ASCII DXF release 12 (AC1009) with layers GEOMETRY, CONSTRUCTION, DIMENSIONS, ANNOTATIONS, SHEET and TITLE. Dimensions are written as ordinary lines and text ("exploded"), not as DIMENSION entities. Text outside printable ASCII becomes a question mark, except the degree, plus-minus and diameter signs, which use the standard %%d, %%p and %%c sequences. SVG export writes millimetres for a sheet, or true size in the document unit for the sketch alone.

You choose between the sheet (everything on the paper, at the sheet scale) and the sketch only (model units, 1:1). Both are built from the current sketch at the moment you export.

Limitations and limits

Up to 2000 entities and 4000 constraints; coordinates within +-1e9; a group of connected items may have at most 600 unknowns (about 300 points) so a very large sketch must be split into parts that share no constraint.

The solver is Levenberg-Marquardt with exact derivatives. It is accurate to about 1e-11 of the sketch size, which is shown in the status. A solution depends on where the items start: a constraint with two answers (an elbow up or down, a circle on either side of a line) keeps the one nearest the starting position.

The Verify page runs the checks that CI runs: closed-form constructions (equilateral triangle, incircle, circumcircle, tangent lines and circles, law of sines, arc from chord and radius), hand-counted degrees of freedom and constraint status, and export round trips read back by independent minimal DXF and SVG readers. A pass means these cases pass on your browser; it is not a certification of any particular drawing.

Sketchgauge is for education and preliminary work. A drawing must be checked by a qualified professional before it is used for manufacture or construction. The DXF writer follows the public DXF reference and has been checked with our own readers; it has not been opened in every CAD program.

How we verify this tool

The Verify page runs the same case catalogue that the automated tests use, inside the engine worker in your browser: 41 cases with 153 individual comparisons. Each case states its inputs, the independent reference and where the reference comes from. A pass headline appears only after a complete run in which every case passes; a cancelled or failed run never shows a pass.

• Closed-form constructions: the solved coordinates are compared with answers worked out by hand: an equilateral triangle, a 3-4-5 triangle, a triangle from two angles and a side (law of sines), the incircle and circumcircle of a triangle, circles tangent to lines and to each other, a tangent line from an outside point, a two-link arm, an arc from a chord and radius, a quarter arc and a regular hexagon.
• Degrees of freedom and status: the number of unknowns minus independent equations is counted by hand for lines, circles, arcs and rectangles, and the redundant, conflicting, infeasible and degenerate cases must be named as such.
• Export round trips: the DXF and SVG written for the plate and the bracket are read back by minimal DXF and SVG readers that share no code with the writers, and every drawing item must be found exactly once with the same layer and geometry. Paper sizes are re-derived from the A-series rule and from inches.
• Properties: 200 seeded random constraint chains, translations, rotations and scale changes from 1e-9 to 1e8, and a finite-difference rank of the constraint system are checked in the test suite.

The reference values come from plane geometry worked out in the case text and from the public DXF reference and SVG specification for file structure; no third-party data or code is used. A pass shows that these cases pass on your browser; it does not certify any particular drawing.

Troubleshooting

• "The constraints do not allow that position": you dragged a point somewhere the constraints forbid. Release it, or remove or loosen a constraint.
• A conflict is reported after adding a dimension: the new dimension disagrees with others (two different lengths between the same fixed points, or a triangle that cannot close). Delete the constraint marked in red or change its value; Undo also works.
• Over-constrained and redundant: the sketch is correct but one constraint repeats the others. Delete it, or make the dimension a reference dimension.
• A line or arc is drawn in the wrong place after an edit: a constraint with two answers keeps the one nearest the starting position. Drag the item near the answer you want and the solver follows.
• A direction-based constraint is "undefined": a line it uses has collapsed to zero length. Lengthen the line.
• Text in the DXF shows a question mark: DXF release 12 is ASCII only, so other characters cannot be written. The degree, plus-minus and diameter signs are written with the standard control sequences.
• Nothing happens when I draw or edit: the solver runs in a background worker that checks the page is served from simulations4all.com. If it says it cannot reach the site, reconnect; editing, saving and the Guide keep working.

Related tools

Cutnest plans cuts for bars and sheets and exchanges simple rectangle DXF files. Sectionlens computes section properties for shapes you describe, and Gearsmith designs machine elements. The Sketchgauge file formats are open: the JSON file is plain text with a documented schema, and DXF and SVG open in other programs.