The most basic joint is the slot joint, and it’s a staple when designing for CNC cutting. It’s the joint that, in its simple form, just needs just the standard router bit, doesn’t need any other implements like screws or dowels and it’s very easy to assemble. Here are some photos showing the basic idea: This curvature is needed somewhere in the internal corner, but you can arrange them in a few different ways for a slot join: If the piece being slid into the slot carries weight then the last option might not be the best because there is no flat level at the short end. Also if you wanted to hide the dogbone corners when the piece is inserted then you'd use the middle option (which again will only be possible if the material is thick enough) or the last option. All of the other options will cause you to see little semicircular cavities (look at the first photos in this post for an example of this). Some more examples where I've used slot joints: for the couch (using the middle option), and in the shelves inside cabinet (using the last option): I think it works well in those examples but I think there is a risk with slot joints: they can scream “I’m a slot-together CNC-cut object!”. This might be perfectly fine if you want to make that a key aspect of the look and feel of the object. However I wanted to put more effort into the joints as I thought it would pay off in the final look. One thing to keep in mind is that the width of the slot - the width you need to indicate in your CAD drawings - depends on the width of the piece to be slid into it. If you are planning on slotting a piece cut out of 12 mm-thick plywood, then you would think that you would need to draw a slot 12 mm wide. However, materials may not be exactly the thickness that it purports to be. Plywood is often around 1% or 2% thicker or thinner than the labelled thickness, and this is enough to make a slot joint too loose (and therefore wobbly) or too tight to slide in without a very heavy hammer-like implement. My first ever design, a fridge stand, had a slot which did not take the actual thickness into account. My slot was exactly 18 mm and the material was just a couple of tenths of a millimetres thicker. I definitely learned a lesson as I needed a damn heavy hammer to force the pieces together with this slot!
In most cases, I want a slot which is tight but not too tight to need a heavy hammer. The way I have managed this is by asking my CNC machine operator to obtain the material and measure the exact thickness first. I ask them to wait until I have adjusted my CAD file to take the exact thickness into account. Once I send them an updated file, they start milling. This is now a standard part of every job and it works well. No more heavy hammer work for slot joins anymore.
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AuthorI'm Nicolaas, a software engineer with a creative streak. ArchivesCategories
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