Sketches That Enforce Proportion

Sometimes a sketched feature is not intended to produce a particular dimension, so much as to create a chosen proportion.

The most obvious way to enforce proportionality between any two dimensions in SolidWorks is to write an equation. Equations are powerful and useful, and they have their place. But we also teach that geometric logic is more robust and more automatic than numeric logic. There are plenty of times that you want to enforce a design aesthetic, or capture a proportional relation, that would benefit from solving dynamically inside the sketch restraints, instead of through an equation. And this is usually done in SolidWorks by way of dimensioning an ANGLE somewhere in the sketch.

The Tangent of an angle is the proportion of the Rise divided by the Run, so a fixed angle implies a fixed proportionality between the two legs.  The only trick to this method is deciding where in your sketch, the ‘legs’ of the triangle are, waiting to be used.

Consider the sketch below – we have a half-ellipse, with a desired proportion that the height should always be 1/3 the width.

If we set the Height to be the desired value, (1.3333 currently), then we add a construction line that documents that rise/run angle.

Dimension the angle as a DRIVING relation, and you can now set the H dimension to DRIVEN.

Now as you change the Width dimension W, the height will always respond proportionately.  This is especially useful when creating sketches that will used in either a SWEEP or LOFT feature, as this will allow the sketch a degree of elasticity so that it can respond to Guide Curve controls.

More sketching tips & tricks are available on the CAPINC website.


Keith Pedersen is a Principal Engineer at CAPINC, a SolidWorks Value Added Reseller with locations across New England. He is a regular contributor to their CAPINC University blog.

CAPINC was founded on one core principle: Provide the best solutions and services to assist our customers in designing and developing better products. CAPINC provides premier solutions and services in New England to assist our customers in accelerating their design and development process for better mechanical products. Our award winning technical support team is comprised of industry experts with hundreds of years of combined practical experience in mechanical design, design validation and analysis, product data management, and technical communication. We are the award winning 3D solutions partner offering SolidWorks software and training, and the entire Stratasys line of 3D printers and production systems. For more tech tips and blog tutorials check out CAPINC's blog and videos.