So it’s not even July and we’re talking about Christmas.  Not literally of course, we’re talking about an extruded shape that can be used to mate plastic or rubber components to your extrusion, which for designers is every bit as fun as Christmas (that may be a stretch).

The concept is simple, you have a symmetrical shape that resembles a Christmas tree.  One thing I recommend doing is to make sure the “branches” of your tree go past 90 degrees – they’re 120 degrees as shown.  This ensures you have a barbs that will resist pull off of the plastic member.  Additionally, if you’re mating a plastic part on top, design negative grooves with a slight press fit.  Don’t forget to fillet the edges on your plastic part either (not shown here) otherwise you could snap off the legs given the hardness/brittleness of your plastic parts.    Ho x 3.

One thing to consider when designing aluminum extrusions is the required circularity of the part. Circularity, as demonstrated in the picture on the left is the “roundness” of the part – that is how much it approximates a circle. Now, unlike the post on eccentricity where the wall thickness is affected by the movement of the inner torpedo, circularity is affected by inner material stress and gravity as the aluminum is extruded.   As you may guess, the horizontal force tends to push the side walls out.   So, if you’re looking for a perfectly round part you’re going to have to do secondary machining on the required surface (inner or outer) and take into consideration the eccentricity – mill too much and you’ll have a paper thin wall.

For long sections, on the inner wall you’ll have to employ gun boring. This process may add significant cost to your part. If you need high strength on the inner wall, you may also consider press fitting a steel or stainless sleeve – of course you’ll have to bun bore the inner diameter for the correct press fit otherwise it will deform the insert.