Designing a Drill Shape in Autodesk 123D Design

My 6-year-old asked me to turn the head of the Yakult Man (the red circle in the image) into a drill shape.
Problem: Autodesk 123D Design has no drill tool, or even a helix/spiral tool.

Searching online I found plenty of tutorials for bolt threads, but nothing for drills. So here's the method I worked out myself.

Start with one square. We'll rotate and stack copies of this to build the drill flutes.

The Yakult head is a cone with a base diameter of 35mm and height of 70mm.
But the square's side length isn't 35mm.

The corners of the square correspond to the cone's outer surface, meaning the cone diameter equals the square's diagonal.

Diagonal = side × √2 ≈ side × 1.4, so:
Side = 35mm ÷ 1.4 = 25mm

Draw a 25mm square.

Copy-paste to create a second square. Move it 1mm up the Z axis and rotate it 15°.

This determines the pitch of the drill flutes. 15° isn't mandatory, but a divisor of 90 makes the maths clean.

Scale the upper square down slightly. The amount of scaling determines how quickly the drill tapers.

The cone is 70mm tall, so each 1mm step should shrink by 1/70:
Scale per 1mm = 1 − 1/70 = 0.986

Scale this square to 0.986×.

Repeat: the 3rd square is 2mm up, 30° rotated, scaled to 1 − (1/70 × 2) = 0.972×.

Repeat 5 times total (up to 75° of rotation).
You'll be tempted to go all the way to 90°, but stopping one step before 90° is the key.

Select all 6 squares in order and connect them with the Loft tool.
Going all the way to 90° apparently breaks Loft's auto-detection.

Move the lofted object out of the way, noting the exact distance so you can move it back later.

Add one more square at the 90° position. Select this last square plus the two before it (3 total), and loft them together.

Move the first lofted object back to its original position and Merge the two objects.

If done correctly the two connect seamlessly and the twist becomes one continuous piece.

Continuing 1mm at a time for all 70mm would work but is incredibly tedious, so let's speed things up.

Check the perimeter of the top square: 91.6mm total, so:
Side = 91.6 ÷ 4 = 22.9mm

The base square was 25mm, so the top square's scale factor is:
22.8 ÷ 25 = 0.912×

Copy the object and scale it by 0.912× in the X-Y plane only (not Z). The top of the larger piece will fit exactly onto the bottom of the smaller piece.
Scaling in Z would throw off the pitch.

Since the shape is identical every 90°, no rotation is needed — just stack using the Magnet tool.

Copy → scale → stack, and repeat until it looks like this.

When printing, no support material is needed — in fact, using supports will cause problems.

Here it is printing. Since there's no support material, any part smaller at the bottom than the top needs to be printed upside down, as shown.

With the M3D The Micro, such a small contact area would normally peel off the bed — but notice the yellow paper on the print table.

Check on Amazon
That's the 3M 243J masking tape I mentioned last time.

First time trying it and it's excellent. Even with this tiny contact area, it stays put without a raft.

Better still, after printing you just peel the tape off along with the print — no risk of damaging the piece trying to scrape it off the bed.

Whoever figured out this tape trick deserves some serious respect.