A friend recently asked if I could scan and duplicate a decorative finial from an antique table he was trying to restore. The table was supposed to have three identical finials but two of them were missing. He cut the remaining finial off and mailed it to me and then I had to figure out how to copy it. I considered trying to 3D scan it, but it is pretty small and my scanner is optimized for larger objects, so I took a low-tech approach.
I decided to recreate the finial using CAD software, specifically DesignSpark Mechanical (DSM). It is very easy to learn and use, yet very powerful CAD software that produces excellent quality STL files for 3D printing.
Step 1: Start With a Silhouette
I held the finial up in front of a white screen on my computer monitor and took a picture with my phone. I then cropped it and saved it as a .jpg file.
Step 2: Import the Silhouette Into CAD
Open DSM and start up a new design- it will default to the sketch mode- and import the silhouetted image file. DSM places the flat image file on the sketch plane.
Step 3: Add Straight Lines Where Needed
Draw a line straight down the center of the silhouette, one at the bottom, and two short lines at the sides.
Step 4: Use a Spline Curve to Trace the Curvy Bits
Select the spline curve tool and use it to trace the silhouette of the finial. Splines terminate at the ends of the straight lines at the top, right side, and bottom of the image. After selecting the spline curve tool, just click along the edge of the silhouette. The more points you use the smoother the fit will be.
Step 5: Switch to 3D Mode and Turn Your Tracing Into a Solid
Now you will turn the 1/2 silhouette in to a solid by pulling it in a complete circle (360 degrees) around the center line. If you were trying to make a decoration for the corner of a table or box you could just pull it 270 degrees.
Scale the model now to match the height of the original finial. You scale it by using the "Pull" tool (sorry, no image of that move here).
Step 6: Now Make a Dowel Hole at the Top/Bottom of the Finial
You could print the finial with its own mounting post, but it would be pretty weak, it would take a lot longer, and require a LOT of support material. It's better to make a hole in the bottom of the finial so you can glue in a wood dowel to mount it on the table.
You will want to scale the finial before making the dowel hole so that you don't shrink or enlarge that hole. See the previous step.
Step 7: Export the STL File
Turn the finial upright, then export it as an STL file. You have many options to control the resolution of the STL file, I recommend using the preset "fine" resolution.
Step 8: 3D Print the STL File
I printed four of these at the same time using ABS. Notice that when the printer gets to the top of the print, the area on each finial will be very small and will complete printing quickly. If the nozzle goes back to a small area before it has had a chance to cool, you'll end up with a blob. There are a couple ways to prevent the print quality problems that printing tiny layers can cause.
You can print an additional object at least as tall as the finials to give the printer something to do while the tiny finial parts cool between layers, or you can add custom gcode for every layer change that just makes the printer wait for maybe 10-20 seconds to start printing after each layer change. The first option wastes a small amount of filament, the second can be tricky because plastic can drool out of the nozzle while it is waiting and that will put a blob of plastic on the print every time it starts a new layer. I chose the first option and printed a small cylinder along with the finials.
The finial will need support material when it prints, and after it is removed, the underside of overhanging surfaces will be rough- you'll have to do a little sanding. It's OK.
I printed using ABS with 0.2 mm layers and 40% infill for strength, and of course, I used support material. Even though the little knob at the tip of the finial prints solid, it isn't very strong and can break off easily, so be careful.