Paraknob

This is a control knob for attachment to a control spindle.  It carries a pointer on an outer skirt.  The back of the knob is recessed to allow for a thin control fixing nut so that the skirt can be very close to the panel.  The knob is fixed to the spindle with a 3mm diameter screw.  The design has a slot that will take a standard 3mm hexagonal nut and the fixing screw hole will accept a 3mm cap headed screw with a hexagonal socket for an allen key.  The screw length required is dependent on the diameter of the knob, never more than than the body radius nor longer than 8mm of threaded length.

The knob is printed in two parts which fit closely together.  The knob body is best sliced (I used Prusa Slicer) using variable layer height, the dome being set to 0.04mm layers for a smooth finish.  I used a standard 0.1mm detail print layer profile but adjusted the minimum layer height to 0.04mm, then applied variable layer height automatically with maximum quality and maximum radius.  My print time on the Prusa i3Mk3S+ is 3 hours 48 minutes for a 2 inch diameter knob 26 mm high.

I suggest that the skirt is printed with a colour change set at 10 layers down from the top.  I printed in Black PETG and changed to White PETG at that point.  The skirt is surrounded by a 'sacrificial' ring.  After the printer restarts from the colour change I noted that any nozzle 'drip' after the change made a mess, so the ring is there to be printed to first and absorbs any 'accidental' extrusion, leaving the skirt with a clean change.  This prints in 49 minutes.

The two parts of the knob fit closely together and can be superglued when required.  As a bonus the relationship of the pointer to the control spindle can be set anywhere, especially useful when there is a flat on the spindle for the fixing screw to bear.  The two part print means that both parts can be printed flat and so no supports are required.  The position of the pointer is always along the X axis as X movement (on my bedslinger) is more accurate than Y.

At present I have published the OpenSCAD file, two STL files for body and skirt, and the two G-Code files [AT YOUR OWN RISK!!] which were suitable for my printer and the (Prusament) filament I used, YMMV.

I created the knob using OpenSCAD and have arranged for customization to fully parameterised settings, so that a complete range of different dimension knobs become possible.  By loading the .scad file into OpenSCAD and selecting the Customizer window on the top menu you will have the following controls:
1. which part or parts are rendered.  You can see the complete knob or can show the Body or Skirt separately (or seperated).
2. The diameter of the knob body (in millimetres) from 18mm to 80mm.  Note that at the small end of the range the fixing nut may be very close to the spindle and the fixing may need to be a grub screw as there is no room for the cap head of a standard screw.  Also for small knobs there may not be enough space behind the skirt for the panel nut.
3. The height of the knob overall (including the skirt).  For larger knobs it is a good idea to make this taller than the default of 26mm for best positioning of the fixing screw hole.
4. The diameter of the spindle to be fitted, from 1/8 inch up to 1/2 inch.  Big spindles in small knobs are not a good idea.
Under Directives you may also set the number of facets to be created for curved surfaces.  Lower numbers are OK for speed during test rendering but I suggest at least 100 for the final render.

All other dimensions are calculated to give a sensible appearance to the finished item.  This includes the number and depth of the finger grips (which also affects the position of the fixing screw hole).

Once you have selected your dimensions and reviewed your model you should choose to render the body alone, then finalise using F6 before exporting as an STL file.  The layer settings are all determined in the slicer.  Similarly you should choose to render the skirt alone (with its sacrificial ring), finalise and export to its own STL file.  The colour change to highlight the pointer on the skirt can be set, I suggest, 10 layers down from the top, in your slicer.  Print each part separately.

The OpenSCAD file Paraknob.scad has been compressed for brevity and all comments and unnecessary whitespace removed.  Edit at your own risk!

Paraknob © 2024 by Steve Sims aka Polymath is licensed under CC BY-NC-SA 4.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/4.0/  Please take note of the relevant licensing.  Enjoy your personal use of this design.

Polymath, December 2024.