Small Series-Wound Electric Motor with Laminated Field and 12-Slot Ring Armature (Robertson, Boy Mechanic 1913)

"The field frame of the motor, Fig. 1, is composed of wrought sheet iron."

Mark a template 1/16 in larger than Fig. 1, cut enough sheet-iron plates to make 3/4 in, drill four rivet holes and rivet them. Mark the bore with dividers set at 1/8 in, drill 1/4 in holes round it and file to shape. Bore to 2-3/4 in on a lathe; mark the field core at 3-3/8 in and the outside at 4-1/2 in and finish to the marks.

Drill and tap two 3/8 in holes 3-5/8 in apart for the bearing studs (Fig. 2) and 1/4 in holes for feet bent from 1/8 in brass or iron. Make the bearing supports from 1/8 in brass with a 5/8 in hole and a 5/8 in brass rod soldered in and drilled for the shaft. Fit them after the armature is made: wrap paper round the armature, set it in the field, slip the bearings on and solder them, then check it turns freely.

Cut two 1/8 in wrought-iron outer plates and enough No. 24 sheet iron to make over 3/4 in (Fig. 5), drill six 1/8 in holes and rivet. Anneal at cherry red and cool in ashes. Bore to 1-11/16 in, fit a brass spider turned from 3/4 in brass on the machine-steel shaft with a setscrew, solder, turn the outside and face it to 3/4 in. File 12 slots 1/4 in deep and 7/16 in wide.

Turn a piece of 3/4 in i.d. brass pipe to 3/4 in long, chamfer both ends at 60 deg, divide into 12, drill and tap a 1/8 in hole in each segment for a brass pin, turn the pins to 1-1/4 in outside and slit them for the wires. Saw it into 12 bars, fit them on turned fibre ends with 12 mica strips between, clamp with the nut and true up.

Make the brush holder of fibre (Fig. 8) and studs of 5/16 in brass rod (Fig. 9). Roll brass or copper wire gauze and flatten it to 1/8 x 1/4 in, solder one end, and fit the holder on the bearing with a setscrew.

Insulate the field cores with 1/64 in fibre and end washers 1-1/8 x 1-1/2 in (softened in warm water, then glued). Wind each side with four layers (about 50 ft) of No. 18 double-cotton-covered wire, A to B and C to B, and join the ends at B.

Line the ring and spider with 1/16 in fibre and the slots with 1/64 in fibre channels. Wind each slot with about 12 ft of No. 21 wire, 40 turns in four layers of 10, shellacking each layer, leaving 2 in ends. Join each coil's start to the next coil's finish and solder the joints to the commutator pins.

Screw the motor to a wood base 8 x 6 x 1 in with two terminals and a switch. Wire it in series: field end A to a terminal, field end C to one brush stud, the other brush through the switch to the other terminal.

From W. A. Robertson in **The Boy Mechanic, Book 1** (Popular Mechanics, 1913), pp. 345-347 (checked on the archive.org scan boymechanic0000unse_n5a8). Sizes not in the text are in his Figs. 1-9; enamelled wire for cotton-covered and the low-voltage advice are ours.