Is DesignSpark Mechanical really free to use?

YES, DS Mechanical is free to download & use, no matter where you're from or what your profession is.

You will have full ownership (including Intellectual Property - IP) of any project you create with DesignSpark Mechanical. You may download a copy of the licence agreement, available in PDF here.

We do sell a set of add-on packages that extend the functionality with a complete technical drawing environment and support for 2 industry standard formats: STEP & IGES.

If you have questions about the add-ons, see here: https://www.rs-online.com/designspark/introducing-the-new-designspark-mechanical-add-on-modules

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If your work demands exceed the capabilities including the add-on modules, you can consider the full version offered by the DS Mechanical developer- ANSYS SpaceClaim: 

http://www.spaceclaim.com/en/ANSYS-SpaceClaim-trial-DSM-UI-campaign.aspx 

More info on all their products: https://www.ansys.com/products/3d-design/ansys-spaceclaim 

Note that in Japan only, RS Components directly sell these upgrade packages online for the local market: https://jp.rs-online.com/web/b/ansys/