Shell and Fillet: turn a block into a finished part by hand, in the browser, free

Two things separate a printed part that looks designed from one that looks extruded: it is hollow where it should be, and its edges are rounded or chamfered where a hand or a bed touches them. In most CAD programs those are the Shell and Fillet commands, two of the first a course teaches and two of the last a beginner dares to use. In SpeakCAD they are two buttons on the rail, they run in the browser on a phone, and shaping a part with them is free.

Shell: hollow a part to the wall you type

Tap a face, press Shell, type the wall thickness. The part becomes a hollow of that wall, open on the face you tapped (a tray, a cup, an enclosure base) or closed (a lighter solid, a float). That is the whole operation.

What to type:

• 1.6 mm for a case or a tray: four perimeters at a 0.4 mm nozzle, no infill involved, prints fast and clean.
• 2 mm for something that gets dropped, strapped on or screwed to a wall.
• 1.2 mm for a lightweight part or a planter; thinner starts to show light through.

A shell made this way has the same wall on the floor and on the sides. For a planter or a box with a thicker floor, Extrude the inside floor up by the difference afterwards. Anything you Add inside after the Shell, a standoff, a divider, a peg, stands on the inner floor, which is how a bare box becomes an enclosure with its posts in three taps.

Fillet: round or chamfer the edges around a face

Tap a flat face, press Fillet, choose the size and whether you want it rounded or chamfered. The straight edges around that face round over, all of them or only the ones you pick from the chips ("Front 60 mm", "Hole rim 1"). Where a shape stands on the face, a peg or a standoff, the fillet lands at its foot and adds material there, which is exactly where printed pegs snap.

Where it pays:

• Top edges, 1 mm round. The edge your fingers meet. A 1 mm round reads as finished; 0.5 mm is enough on a small part.
• Bottom edge, 0.5 mm chamfer. Against elephant's foot: the first layer's squash spreads outward, and a chamfer takes it in, so the part sits flat and a lid fits over its bottom edge too.
• Hole rims, 0.5 mm chamfer. A screw or a magnet finds a chamfered hole; a square-edged hole chips.
• The foot of a standoff, 1 mm round. Printed posts break at the root; a fillet there is the difference between a case that survives the screwdriver and one that does not.

Round is the other rounding tool on the rail, and it does a different job: the vertical corners of a part, the four edges you see from above a box, rounded or chamfered. Round for the plan view, Fillet for the edges around a face you tapped. Neither picks an arbitrary edge in the middle of a curved surface; that, and a fillet that varies along its length, are AI requests.

A box to an enclosure in nine taps

A case for a 60 × 40 mm board, 20 mm tall inside, done by hand on a phone:

1. Start from a box, 64 × 44 × 22 mm (board plus 2 mm wall each side).
2. Tap the top, Shell, 2 mm, open on the tapped face. You have a tray with a 2 mm floor and walls.
3. Tap the inner floor, Add a cylinder 6 mm across and 4 mm tall, placed by its distances from the inner walls to match the board's first hole; Duplicate it in a Grid to the board's hole spacing. Four standoffs.
4. Tap the top of a standoff, Hole 2.5 mm, for an M3 screw to bite; Duplicate across the four.
5. Tap a short outer wall, Cut a 12 × 7 mm rectangle for the USB port, placed from the floor and the corner.
6. Tap a standoff's top face and Fillet its foot, 1 mm. Tap the top rim of the box, Fillet 1 mm round.
7. Tap the bottom face, Fillet 0.5 mm chamfer.
8. Text on the bottom, engraved, the board's name, 5 mm letters.
9. Measure between two standoffs to check the spacing, then download the STL, or G-code sliced for your printer.

The lid is a second part: a plate 64 × 44 × 2 mm with a 1.6 mm lip that drops inside the box's walls, 0.3 mm smaller than the inside so it fits; Fillet its top edges. The lid is where many people switch to the AI ("a snap-fit lid for this box") because clips on a lid are the one thing the hand tools do not draw.

When to ask the AI instead

The hand tools do the whole case above. They do not do a clip, a hinge, a thread, a curved lid or a fillet along a curved edge; those are a sentence to the AI, with the part you shaped as the starting point, and they spend credits. A new part spends 4 credits on a plan with its first 3 small changes free, a design change 3, a small change 2, a quick edit 1. Everything you did by hand before that stays, and everything after it is still editable by hand. The hand tools page lists all nine with what each is for in printing, and the guides walk through each one.

Why they are free

A hole, a cut, a shell or a fillet is geometry your browser computes; nothing is sent to a model. The AI is what spends credits, because reasoning through a part costs real compute. Why the hand tools are free and the AI is not has the longer answer; the short one is that the tool you reach for twenty times on a part should not have a meter on it.

Shaping a part by hand (a hole, a cut, an added shape, text, rounded corners, a shell, every size typed in millimetres) is free, and so is downloading what you shaped. With the AI, a new part spends 4 credits on a plan with its first 3 small changes free, a design change 3, a small change 2, a quick edit 1; without a plan a new part spends 3. Start with Maker: 50 credits a month for $7.99, cancel anytime.