Motor parts look easy. They aren't.
Every shop can machine a cylinder with a flange. Very few can machine a motor housing that goes together the way the motor designer intended — because the part isn't one tolerance, it's a stack of fits that all have to line up: the stator bore, the bearing seats at both ends, the end cap register, and the shaft features. Get one of them wrong by a few microns and the motor either won't assemble or hums at 6,000 rpm until it dies.
The fits that actually matter
| Feature | Typical callout | Why it matters |
|---|---|---|
| Stator bore | H7 fit (e.g. Ø80 +0.03/0) | Stator presses in without distorting laminations |
| Bearing seats | js6 or k6 (Ø25 ±0.0065) | Preload and clearance on the bearing itself |
| End cap register | 0.02-0.03 mm concentricity | Both bearing centers stay on one axis |
| Rotor shaft journals | ±0.005-0.01 mm, runout ≤0.02 | Vibration and bearing life |
| Face squareness | 0.02-0.05 mm | End caps seat flat, no bending load on bearings |
The one I see missed most often is concentricity between the two bearing seats. A housing can hold every diameter perfectly and still be scrap if the two bearing bores are off-axis by 0.05 mm — the rotor then runs eccentric, and that shows up as vibration and short bearing life. Call it out explicitly: concentricity between both bearing seats, and do it in one setup so the machine cuts both bores from the same datum.
Material: 6061 vs A380 vs 6063
A lot of motor housings start life as A380 die castings — cheap at volume, good thermal conductivity, but porous near the surface and a pain if you need to re-machine sealing faces. For CNC machined housings, 6061-T6 is the workhorse: stable, weldable, anodizes well, and predictable in the machine. 6063 is softer and extrudes beautifully, which makes it a common choice for finned motor bodies — but it's gummy to machine and you pay for it in cycle time. If the motor runs hot and the housing is the heatsink, thermal conductivity and the fin design matter more than the alloy name.
DFM for motor housings: what makes them cheap or expensive
- Keep wall thickness even. A 2.5 mm wall next to a 10 mm boss distorts when you unclamp — and the bore goes oval
- Design sealing faces flat and wide enough for an O-ring groove. Grooves are cheap; lapped surfaces are not
- Put the wire terminal and sensor pockets on one side so the part needs fewer setups
- If it's a prototype of a future die-cast part, say so in the RFQ. The shop can then relax cosmetic callouts and quote machining-only, not casting-plus-machining
- Threaded holes into thin walls: add a boss or use thread inserts. Tapped threads in 2 mm walls pull out in service
What to ask for on the RFQ
Send the stator and bearing specs even if they're not on the drawing — bore fit, bearing class, and expected rotor weight tell the shop which tolerances are functional and which are decoration. And ask for a concentricity report, not just a size report. Any shop can show you 'all diameters within tolerance'. The shop that also proves the two bearing bores are on the same axis is the shop that's built motor parts before.
Frequently Asked Questions
What tolerance does a motor stator bore need?
Typically an H7 fit on the stator outer diameter, so about +0.02 to +0.03 mm for a 60-100 mm bore. The key is controlling ovality and taper, not just the nominal size — CMM roundness checks catch both.
Why do my motor housings vibrate even when everything measures in tolerance?
Almost always concentricity or runout between the two bearing seats. A few hundredths off-axis shows up as eccentric rotor rotation. Ask for a true concentricity callout and a report, not just diameter sizes.
Should I use die casting or CNC machining for motor housings?
Above 5,000-10,000 parts/year, die casting wins on unit cost. Below that, CNC machining from 6061 bar or plate is cheaper, faster, and gives better wall control. Hybrid is common: cast body, machined sealing faces and bores.
Can you hold bearing seats to k6 on a CNC lathe?
Yes. A ±0.0065 mm tolerance on a bearing journal is routine on a quality turning center with CMM verification — as long as the drawing also controls roundness and runout, which affect bearing fit more than nominal size.