3D Printed Knob

A common component for 3D printed projects is a knob. I knob replaces a screw with the advantage that it does not need a special tool to use it. A knob can be printed 100% from plastic with a 3D printer. My Prusa printer has a limit to how small I can print threads. Steel also has far superior strength and wear resistance to anything you can print on an inexpensive printer.

My knob design will incorporate a stainless steel screw with a plastic cover. The cover will snugly fit over the screw head with a little dab of epoxy glue to keep in in place.

The first design question is what type of screw should be used. A common design that is readily available is preferred.  I have narrowed it down to a pan head screw, a socket head cap screw or a hex head cap screw. Looking at the screw head designs, it is apparent that the pan head and socket head screws are round and will be dependent on the epoxy to overcome the shear forces when tightening or loosening the knob. For this reason, the hex head screws is the best choice of the three. I have a lot of 1/4″-20 hardware, which is about the right size for the projects I am currently working on. 10 stainless steel 1/4″-20 x 1″ long stainless steel hex head cap screws are available from Amazon.  (link: https://amzn.to/2XzfHnu)

The next question is the material. I have chosen PETG. It is less brittle than PLA, but easier to use than ABS. I purchased a roll of black PETG from Amazon for $24.99. (Link: https://amzn.to/2XtdSIL). I have had issues with PETG sticking to the bed and stringing. When I have a problem with bed adhesion, I slow down the first layers and start with the bed a bit colder than the specs call for.

If you are printing a “benchy”, then shrinkage is not a factor. For the knob, I wanted a tight fit. I modeled a test block in Blender and printed it with different scaling factors to determine the shrinkage.  What I found is that this PETG shrinks about 0.2%.

Here’s how to model the test block. When you load Blender, the default cube is displayed. Most tutorials start off by deleting it. Not this time. Keep it. Change the unit settings to Imperial-Inches. Then go to the Cube properties and make it 1″ x 1″ x 5mm. It may seem strange to use a mix of imperial and metric. Here’s why I do. The printer uses metric. Each layer is metric and the resolution is poor in the Z-axis. If you make a part 1/8″ thick…the slicer will round off to the nearest metric equivalent (depending on the layer settings). The resolution in the XY axis is much better. So why not just do everything in metric? Because my tools measure in inches. I have micrometers and calipers, rulers and gauge blocks. They all are imperial units. It would cost me thousands of dollars to buy metric replacements. For the rest of the world that looks down collective noses at Americans for not using the metric system (and there are billions of you)….how many men have you put on the moon? Get

 

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