SG90 Mini Delta Pick-and-Place Robot

This is a small delta robot in the FirePick style. It uses parallel kinematics: three SG90 (or MG90S) micro servos hang under a spoked top plate, each on its own cradle. Each servo turns a 50 mm upper arm. Each arm drives a pair of 100 mm parallel link rods down to a triangular effector, so the effector always stays level while it moves in X, Y and Z over the base plate. The effector has a central tool hole and an M3 square pattern for a pen, a small magnet or a micro gripper. It's a good bench project for learning delta inverse kinematics with an Arduino or a PCA9685 board.

The ball joints are fully printed. Each ball is two 12 mm hemispheres clamped on an M3x40 bolt, with the head and nut sunk into the outer halves. One printed spacer on each side of an 8 mm boss puts the two ball centres of each pair exactly 30 mm apart, at both the arm tips and the effector. Each rod has a socket cup at both ends. The rods sit on the outside of the balls, and a small extension spring (or a rubber band) at each end of a rod pair pulls the two cups inward onto the balls. This gives zero-play joints that can pop apart safely in a crash instead of breaking.

Assembly order:

1. Press M3 heat-set inserts into both ends of the three frame columns, then bolt the columns onto the base plate with M3x10 screws through the locating recesses.
2. Push each SG90 through its cradle window and fix the tabs with M2 screws and nuts. Bolt the three cradles under the top plate with M3x10 screws into the captured nuts, but leave them loose in the radial slots for now.
3. Bolt the top plate onto the column tops. Centre each servo to 90° and slide its cradle so all three shafts sit at the same radius (about 45 mm), then tighten.
4. Put each servo horn on its arm, drill 1.5 mm pilot holes through the horn holes, and screw the horn on with its self-tapping screws. Fit the arms level at servo 90°.
5. Build six ball stacks (M3x40 bolt, counterbored hemisphere, plain hemisphere, spacer, boss, spacer, plain hemisphere, nut-pocket hemisphere): three through the arm tips and three through the effector blocks.
6. Set a pair of link rods on each ball pair with the cups facing inward, then hook a spring through the spring holes at each end of the pair.

Tuning notes: The SG90's shaft is offset along the body, and its tab-hole spacing isn't a verified number. Because of that, the cradle uses slotted tab holes and the top plate uses radial slots. Measure the real shaft radius (base radius f ≈ 45 mm) and use it in your IK together with an arm length of 50 mm, a rod length of 100 mm and an effector radius of 20 mm. If a joint pops off during fast moves, use stiffer springs or double up the rubber bands.