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Press-Fit Construction Tips

About Press-Fit Contruction

Press-fit construction is also known as interference fit or friction fit.: http://en.wikipedia.org/wiki/Interference_fit

This Week's Assignment:

The assignment for this week is to create a press-fit construction kit: http://academy.cba.mit.edu/classes/computer_cutting/index.html

Press-Fit Construction Tips [excerpt, by Shawn Wallace]

Check out the Full Text Here: http://blog.makezine.com/2010/01/14/letters-from-the-fab-academy-part-1/

Fairly precise cuts are necessary in press-fit construction, in the thousandth-of-inch range. Although the laser cutter is a very precise tool, you’ll still find a measurable variance in kerf (i.e. width of material removed) depending on the material, the state of the optics, and the quality of the air exhaust. You’ll also find that off-the-shelf sheet material like plywood usually varies from piece-to-piece by +/- .005 inches or so.

Because you will inevitably have to tweak a press-fit design to match the materials and equipment at hand, your design should be created so that the width of slots and notches can be changed parametrically. It’s also important to chamfer (i.e. cut at a 45 degree angle) the edges of the press-fit notches. Without a proper chamfer, you may not be able to slide the pieces together to get an effective joint.

FabAcademy01img05.jpg

Inkscape is a great open source vector drawing program. When designing for the laser cutter, thin red lines cut and black/grey values are engraved. Note the chamfers…

Full-featured CAD programs, like QCad, make it relatively easy to parameterize parts, but it’s also possible to use Inkscape to make parametric designs using the clone tool. First, follow these steps to determine the base size of a press-fit notch:

  1. With calipers, measure the kerf created by your laser cutter. Cut a square, then measure the inner dimension of the hole and the outer dimension of the piece. Subtract and divide by two: this is the kerf for that material.
  2. Measure the thickness of the material, preferably with calipers.
  3. Use these measurements to draw your best guess for a notch
  4. Duplicate the notch a few times and bracket in .001 inch increments (i.e., make some at w-.002″, w-.001″, w+.001″, w+.002″ etc.).
  5. Cut and test the fit

Once you have a good dimension, create a template layer in Inkscape with your basic notch shapes. When adding a notch to your design, use the Edit->Clone option to create a linked copy of the notch. When faced with modifying your drawing for a new material width, update the dimensions of the notch in the template layer and watch those changes update throughout the document.

More About Using Clones In Inkscape

Inkscape is capable of creating "clones" of objects, which are somewhat similar to Adobe Illustrator's Symbols. When you edit the original, the changes are propagated to all of its clones. Clones can be transformed, but their nodes cannot be edited. Clones can themselves be cloned.

You can use the Edit > Tile clones command to create patterns and arrangements of clones. A clone can be painted if the original has unset fill or stroke. Inkscape's clones are not stored in a palette.

How to Clone / Resize

Other Press-Fit Resources

A General Collection of Links