I've had a wonderful LG stove with induction cooktop and convection oven for a couple years. Everything about it is great, except the control knobs. The knobs are chrome plated plastic with black text stenciled on. The chrome plating reflects light in such a way that the thin black text is difficult to read in almost any light. The advantage of using a chrome finish is that it's easy to clean, but being able to read the text on the knobs is kind of important, too.
The front of my LG stove with the original, hard-to-read knobs.
I recently got a Bambu Lab H2C 3D printer so I decided to try to print some new knobs for the stove with easier to read text using the multicolor capability of the printer. I thought it would be nice to use contrasting colors for the bulk of the knob and the text, and even to raise the text above the knob surface. And of course, I will want to be able to put the knobs in the dishwasher once in a while, so printing with PLA is not an option.
As you can see, it is pretty hard to read the thin, black text on the chrome knobs. This is true under almost any lighting situation. Also note the positions of the burner maps on the stove panel. If you study it hard enough, you tell which knob goes with which burner!
Designing New Knobs
I started by making some measurements of the existing knobs, and the control shafts on the stove. The shafts are round with a single flat surface on one side, commonly called a "D" shaft. It will be critical to get the knobs to fit tightly on the shafts, but not so tightly that the knobs can't be installed and removed without breaking the controls. The original knobs have steel inserts lining the holes and providing spring pressure against the shaft.
There are multiple ways to approach the design, and naturally, I started with a bad way. I attempted to print the knobs to fit on the D shafts on the stove and just couldn't get the consistency I wanted. They were either too tight or too loose.
The underside of one of the knobs. You can see the large steel weight used to make the knobs feel less cheesy, and the thin steel insert that grabs the control shaft inside the hole. I was unable to extract that insert. Note the white plastic around the shaft hole and apparently under the steel disc.
Some failed test prints used when designing the new knobs. Don't try to print knobs that directly fit on the D shafts of the controls. It doesn't work.
I took another look at the original knobs and found that beneath the steel weight used to make the plastic knobs feel less cheesy, there is a white plastic insert glued into each knob. The glue they used was a sort of soft and rubbery, probably polyurethane. After a little prying with a screwdriver, the glue bond broke and the insert popped right out.
The insert is glued inside the knob shell with some sort of soft white glue.
The insert and steel weight extracted from the knob. I modeled this in just enough detail to design a knob to fit around it.
I put the insert assembly on the stove to check the clearance between it and the surface of the stove. In order to turn a burner on, you have to push in the knob 2 mm and then rotate it. I measured the clearance between the steel disc and the stove surface with the insert pushed in to figure out how far my knob design could extend toward the stove surface.
I went to work on modeling the insert in just enough detail to design a knob to fit around it. I printed a single color prototype and it fit perfectly on the first attempt.
The inside of the successful prototype knob with a cut-out designed to fit the insert and steel weight.
Single color prototype knob on the stove. I designed it so it sits 4mm closer to the stove control surface than the original knobs - it looks like it was specifically designed for this stove that way.
Multicolor Painting
I decided I wanted to use white text on a black background for maximum readability. Then I added some purple/blue color for the body of the knobs, and finally a little green, as you'll see below.
Coloring the print is done using a painting tool in the slicer. The knob is going to have white text against a black background. You can either start with a black knob and paint every surface of the text white (very tedious!), or you can start with a white knob and add the black color around the text- much easier!
Here's how you turn a CAD model into a multicolor print in Bambu Studio:
Bring the part into Bambu Studio as a white object...
Flip it over... That white tower is there because I have the smooth time-lapse feature enabled. It wouldn't be there otherwise.
Select the paint bucket and black color...
Mouse over the area to be painted black- note that the edge detection finds the outer edges of the text and the edges of the polygon.
Click to paint it black. Now you still have to fill in the holes in the text, such as the center of the letter "O", and the corners of the polygon, but that's a lot faster and easier than coloring the text itself.
When you start with a white knob and paint the black areas, the inside of the knob will be mostly white.
Start with a black part and paint the text white, and you'll get a mostly black interior.
Coloring the bulk of the knob- positioning the mouse pointer highlights the boundaries.
One click of the mouse fills in the color.
Roll the mouse pointer over the target area and the boundaries get highlighted.
One click and the color is applied. Repeat 11 more times going around the knob.
More Details
There are three different kinds of knobs on this stove. There's one for the oven, that has a LOT of text, one for the warming burner, and four identical knobs for the induction burners. The oven knob was the most challenging- I had to fit a lot of text on it but the printer managed to produce easily readable text even with just 3 mm high characters.
The inserts weren't just glued into the original knob shells. They have concavities that mate with retention features inside the shells. There's also an index notch in the insert so that it can only go into the knob in the correct orientation (the flat side of the control shaft is down while the "off" text is up when the knob is on the stove).
Concavities used to retain the inserts in the outer shells.
Retention bumps that snap into the concavities in the inserts. I'm sure these have a name, but I don't know what it is. On the left side you can also see the indexing bump that prevents the insert from being installed in the wrong orientation.
I made a test print with added retention bumps and an indexing bump in the interior of the knob design. The inserts with attached steel weight snap in tightly and don't wiggle at all. They can be easily popped out with a screw driver so the printed covers can be cleaned in the dishwasher.
While I was at it, I also reduced the bulk of plastic on the interior of the knobs so they'd print a little faster and use a little less filament (instead of the bulk of the interior printing, it has to print larger tree supports).
CAD render of the interior of my knob design. You can see two of the four circular bumps that fit the concavities in the inserts. Toward the right you can see the index bump that ensures the knob fits on the insert in the correct orientation. The walls are about 3 mm thick - more than strong enough for the job that the knobs need to do.
Finally, I added burner maps to the front surfaces of the knobs so it's easy to tell which knob controls which burner. I marked the target burners with green filament which matches the green sparklies in the purple/blue filament.
Printing
This is a situation where printing all the knobs at once saves a lot of time and reduces wasted filament. There are 292 filament changes and only one prime tower (about 20g of filament) for the full set, which requires about 21 hours to print - a little over 3 hours per knob. If you print the knobs individually, the printer performs 292 filament changes and prints a 20g prime tower for each knob. So it takes about 5.5 hours to print a single knob.
The knobs are printed in 0.2 mm layers, with tree supports, 4 perimeters, 3 mm top and bottom thickness. The knobs print solid- there's no sparse infill. The filaments were Bambu Lab Black PETG-HF, some random white and transparent green PETG filaments I am still trying to use up, and the sparkly stuff is Stronghero 3D Mirror Chrome PETG in Wizard Voodoo Purple/Blue that I first opened in October of 2023. My hot girlfriend pointed out that it's a nice match for the blue porcelain interior of the oven.
Sliced set of LG stove knobs in Bambu Studio. The full set uses about 420g of PETG filament - about $10 worth. The 292 filament changes add about 2.5 hours to the total print time of 21 hours.
The Results
I printed one knob first as a test of the fit and finish, was satisfied with the result, so I printed the remaining five on one plate:
The remaining five knobs took about 18 hours to print. The four burner control knobs go into the slicer as identical parts. The green color that makes them unique gets added in the slicer.
The printer has cameras built in and it's easy to make a time-lapse video:
And here they are on the stove. The purple/blue filament looks different depending on the light source and viewing angles. It's also full of green sparkles!
It's so much easier to read the knobs and to identify the burners being controlled.
