How is a commutator manufactured for a DC motor?

Every turning pass removes some copper, and the design documentation always specifies a wear limit — a minimum bar thickness below which the bar loses the mechanical strength it needs to resist centrifugal force and stay firmly seated in the retaining construction. If a commutator has already been through several repair turnings and the remaining bar thickness is approaching that limit, further turning is unsafe, and the only correct fix is a new set of bars — in practice, a new commutator.

A flashover leaves melted patches on the working surface and leaves the inter-bar insulation charred and partly destroyed; prolonged local overheating from a failed riser joint or stray bearing currents can burn through the insulating sleeve between the stack and its hub. In these cases the damage usually spreads across a large share of the bars at once, a spot repair does not cover the full extent of it, and an attempt to turn the melted surface often shows the damage runs deeper than the allowable margin.

For a common production motor, a commutator can be ordered as a spare part. But for a one-off or purpose-built machine that has gone out of production — an old-type rolling-mill or traction motor, a generator the factory built in-house — no ready-made commutator exists on the market at all, and the lead time for a custom order anywhere would exceed the downtime the equipment can tolerate.

A large share of the industrial fleet still in service consists of machines decades old, domestic or imported, whose manufacturer no longer exists or never supplied spare parts to this market in the first place. For equipment like this, manufacturing a new commutator in-house — from a drawing in the machine’s original nameplate documentation, or by reverse-engineering the existing part — is the only way to keep it running rather than scrapping a whole machine for the want of one assembly.

The starting material is hard-drawn copper bar stock with a wedge-shaped (dovetail) cross-section, in a grade matched to the machine’s duty: grade M1 electrolytic copper for general industrial commutators, or copper alloyed with silver, cadmium or chromium for traction, rolling-mill and crane motors, where a higher softening temperature and resistance to work-hardening under the brush matter. The wedge cross-section — an asymmetric trapezoid — either arrives already rolled to profile, or is machined into a rectangular blank by milling or broaching. The bar is cut into individual blanks of the required length, with a small allowance left for facing the ends, and the number of blanks is set by the number of working sections in the armature winding (or a multiple of it, for some winding schemes).

Sheets of insulating material are die-cut or press-cut to the exact same profile as the bars and to the thickness the drawing calls for. The traditional material is micanite, pressed from flakes of natural mica bonded with shellac or epoxy; new commutators increasingly use reinforced synthetic mica tape or a glass-mica laminate for more consistent, predictable properties. The insulation’s thickness and hardness are chosen so it wears at roughly the rate of the chosen copper grade — that decision is made at this stage, because there is no fixing a poor match once the stack has been assembled.

Bars and insulation sheets are laid up in a circle around a temporary working sleeve or an expanding mandrel, alternating strictly one layer of copper with one layer of insulation until a complete cylindrical ring forms. During stacking, the builder checks that the risers land at an even pitch all the way around and that no bar sits skewed relative to the axis — any deviation left uncorrected here becomes run-out on the working surface after turning, and by then it cannot be fixed without taking the stack apart.

The assembled ring is clamped axially in a hydraulic press at a force read off a gauge and calculated for the specific diameter and bar count. Pressure is built up gradually, often in several stages with a short dwell between them — this lets the stack settle evenly, drives out trapped air between the layers, and seats every bar firmly against its neighboring insulation sheet. Too little pressure leaves the stack loose and prone to further settling in service; too much can deform or crack the brittle mica insulation.

The pressed stack is locked into a permanent construction one of two ways. In a clamped V-ring (shrink-ring) construction, insulated conical rings are fitted onto both ends of the stack, their taper wedging the dovetailed bars inward toward the axis, with the tension held either by a retaining nut torqued to a specified value or by a heated steel band shrink-fitted onto the assembly — this construction stays serviceable, since it can be taken apart and re-clamped during a future repair. In a moulded (resin-cast) or through-bolt construction, the stack is cast in thermosetting resin under heat and pressure in a mould, forming a solid monolith with no metal retaining ring, or is clamped by insulated tie bolts through holes in the bars — a more compact solution common on medium-power production machines, but essentially not repairable if the stack fails internally.

Before the assembly reaches the lathe, it spends time in a thermostatically controlled oven, at a temperature and for a duration set by the insulation’s binder — this cures the shellac or advances the epoxy resin’s cure, and relieves the internal stresses the pressing step left behind. Cooling afterward is done slowly and under control, not in open air — uneven cooling introduces fresh internal stress rather than relieving the old stress. This step gets underrated, but it is what determines whether the commutator holds a stable geometry once the temporary tooling clamps come off for machining: a skipped or shortened bake does not show up immediately — it shows up months into service, as bars slowly working loose and run-out creeping up.

The stabilized drum is mounted in a lathe — on a mandrel, or in a chuck registered off the shaft bore — and the bulk of the metal is removed, taking out the eccentricity that inevitably built up during stacking and pressing. This pass deliberately leaves a machining allowance for the finish pass that follows — that allowance is what lets any remaining error be removed without risking undersize on the final diameter.

A finishing pass, with a fine feed and a sharp tool, brings the surface to the design diameter at the surface finish needed for good brush seating, and keeps the working surface concentric with the shaft bore within the drawing’s tolerance. This is the step where the resulting diameter and run-out are checked with a dial indicator, before moving on to machining the risers and the insulation.

Once the outer surface is true, slots for the armature coil leads are cut or milled into the risers, with the cutter indexed to the bar pitch, and the burrs removed. How precisely this is done directly sets the reliability of the soldered or welded joint made later: a slot cut too narrow will not accept a coil lead without deforming the wire, one cut too wide leaves play that lets the joint work loose under vibration and eventually crack.

Right after turning, the mica between the bars sits flush with the copper — both materials just went through the same cutting pass. A thin slitting saw or a dedicated undercutting tool then removes mica down to the design depth below the copper all the way around, following the curve of the working surface so the groove depth stays even at every point. This keeps the insulation from standing proud in the first months of service, while the surface is still bedding in against the brushes, and builds in the margin needed for future repair turnings — each of which will require the mica to be undercut again.

Undercutting leaves a sharp copper edge along every bar, which chips under the brush within hours of running and scores the working surface of the neighboring bars if left untouched. A fine file or a dedicated tool bevels those edges to a small angle and a shallow depth, keeping a clean groove profile around the mica — the final operation that leaves the commutator ready to be mounted on the armature.