A motor housing is one of the few parts where the process is decided by the wall rather than by the envelope. The same 60 mm outside diameter can be a deep drawn shell, a formed end cap welded to a tube, or a machined body cut from bar - and those three routes come with different tooling, different wall sections and different dimensional behaviour once the motor is running.
Should a motor housing be stamped, deep drawn or CNC machined?
The process follows the wall, not the outside diameter. A one-piece motor housing with a constant wall, a closed end and a bore that only has to be round is deep drawn: Balford runs steel, stainless, aluminium and brass housings on presses from 25 to 350 t, to a maximum drawn diameter of 250 mm, with drawn walls from about 0.25-1.00 mm in the small sensor-housing range and down to 0.35 mm in thin-wall aluminium. Stamped housings suit end caps, frames and brackets where the wall carries no function, and they run as single-operation or progressive dies. CNC machining is the route when the housing carries a machined bore, step, thread or face that has to hold a tight concentricity band; we machine from bought bar, tube or a stamped blank, and 0.001 mm is quoted only for selected critical machined features confirmed from the drawing. When a drawn housing also needs a machined bore, the usual answer is both routes in one part.
What each route is actually good at
- Deep drawing - a seamless, one-piece shell with a closed end and no weld. This is the route for housings where the wall thickness, the roundness of the bore and the absence of a seam are the function. Deep drawing also forces the wall to thin in a controlled way where the part needs a stepped outside diameter.
- Stamping and forming - blanked and formed parts where the wall is flat: end caps, covers, mounting frames, laminations and brackets. Fast, cheap at volume, and normally run in a single-operation or progressive die.
- CNC machining - the family of features a formed part cannot produce: a bored and honed bore, an internal step, a thread, a sealing face, or a bore-to-face concentricity that a press cannot hold as formed.
Wall section, bore and concentricity: where the three routes separate
A drawn housing carries a wall gradient. Material that started at the blank thickness is thinner at the wall than at the bottom, and the thinning is not uniform around the circumference of an eccentric part. That is normal, not a defect, and it is why a drawn housing drawing should say where the wall is measured and what the minimum is at that point. A machined housing has one nominal wall everywhere and holds it by the stiffness of the setup rather than by tool clearance.
- Roundness as drawn is set by the die and the material, and it is measured before any machining operation. If the drawing only specifies the bore after boring, the drawing has not described the part that leaves the press.
- Concentricity between an outside diameter and a bore is the single feature that most often forces a machined operation: a stamping holds position between two formed features, while a bored bore can be established from a turned outside diameter.
- Magnetic performance depends on the wall and the material, not on the outside diameter alone. Where the housing is part of the magnetic circuit - a solenoid frame, a claw pole, a shading ring - the magnetic route is decided by the grade and the wall, and thin drawn walls change the flux path compared with a machined body.
Where the route flips: volume and tooling
Below a few thousand pieces a year, machining usually wins by default, because it has no tooling to amortise and the geometry can still change. Above that, the calculation turns on how many pieces the tool has to pay for itself over and how tolerant the part is of tool wear. A single-operation stamping die is normally quoted at 15-30 calendar days from a confirmed drawing, and a progressive or transfer die at 30-60 days; CNC machining is quoted at 7-15 days and sheet metal fabrication at 7-30 days. Those lead times are the front end only - the route that is cheaper to launch is not automatically the route that is cheaper over three years of production.
- Count the operations the part needs, not the parts. Two stamping operations with a tapping station are cheaper than a machined part only if the tapping can genuinely run in the die and the thread is not a sealing thread.
- Add the cost of the bore. If the housing needs a bored, honed or threaded bore, that operation exists in both routes unless the press can hold it as drawn - and it usually cannot hold a tolerance band that tight.
- Price the material twice. A drawn housing buys flat strip, a machined housing buys bar and turns most of it into chips. The material line rarely behaves the way buyers expect when the two quotes are opened side by side.
- Check the volume ramp. Tooling is paid once and the unit price falls with volume, so a programme that starts at 2,000 pieces and grows past 50,000 pieces is usually worth a die even if the first order is not.
What the drawing has to state before any of this can be quoted
- Where the wall is measured, and the minimum acceptable wall at that point - not a single nominal thickness.
- Whether the bore is functional as drawn, or whether the drawing expects a machined finish: state the finish and the concentricity band as produced.
- The datum chain. If the functional face is a machined face, say so, and give the machined face as the datum rather than the outside diameter of the shell.
- The material grade and the temper. A drawn housing and a machined housing rarely run the same grade, and the same grade in a different temper forms differently.
- Surface treatment and the process that applies it. Passivation, plating, phosphating, liquid painting and bulk heat treatment are outsourced operations on our side, with their own lead time and their own inspection evidence.
What we do not claim
- We are certified to ISO 9001:2015. Our controls are aligned with IATF 16949 requirements, but we hold no IATF 16949 certificate and do not claim one.
- We do not publish a rejection-rate figure or a zero-late-delivery claim; conformance is recorded per lot and reviewed in the quality routines.
- 0.001 mm is quoted only for selected critical machined features where process, geometry, material and inspection conditions allow, and only when the drawing confirms it.
- Bulk heat treatment, slow-wire EDM, passivation, electropolishing, plating, phosphating and liquid painting are outsourced. Large sheet-metal parts are powder coated in our own booth, small parts are powder coated by a partner on an automatic line.
- Where a housing starts from a casting, a forging, a cold-headed or cold-extruded pre-form, that pre-form is bought in and finished by machining here; we are not a foundry and not a forge.
Key point
Choose the route from the wall and the bore, not from the diameter. Constant wall with a closed end and a bore that only has to be round: deep draw it. Flat wall: stamp it. A machined bore, step, thread or sealing face: machine it, and draw the shell first if one-piece construction is worth the tooling. Then price the two routes over the programme, not over the first order.
Frequently asked questions
Is a drawn motor housing cheaper than a machined one?
At low volume, no - a machined body needs no tooling, so it is normally cheaper below a few thousand pieces a year. At production volume the drawn shell usually wins on material and cycle time, but only if the bore can be held as drawn or the machining allowance stays small. The honest way to compare is to price both routes at the volume you expect in year three, not at the first order.
Can a deep drawn motor housing have a machined bore?
Yes, and it is common. The shell is drawn to near net shape with a small machining allowance on the bore, then the bore, step or thread is machined in a second operation. That keeps the seamless one-piece construction of the drawn shell while giving the functional bore the tolerance a press cannot hold. The drawing has to separate what is formed from what is machined.
What wall thickness can a deep drawn motor housing hold?
It depends on the material and the part size. In the small sensor-housing range we publish drawn walls of about 0.25-1.00 mm, thin-wall aluminium runs down to 0.35 mm, and larger brass or steel housings start around 0.8 mm. A drawn wall is a gradient rather than a single figure, so the drawing has to state the measurement position and the minimum allowed at that position.
Do you press a motor housing in house or buy it as a casting?
The stamped blank is produced in house on our own presses, which run from 25 to 350 t. Where a housing is routed through machining, the near-net pre-form - casting, hot forging, cold heading or cold extrusion - is bought in and finished here; we are not a foundry or a forge. Powder coating is split: large parts run in our own booth, small parts with a partner.