Short answer: build it as two compartments in one stainless frame — a machine tub that returns flood coolant to a 5.5 gal sump, and a separate controller compartment rated NEMA 4X / IP66 by the manufacturer, ordered at 24 × 20 × 10 in [610 × 508 × 254 mm] so a later 1 hp VFD lifts it only about 11 K. Coolant decides this plan before the mill does. Water-based mist reaches every gap you forgot, and aluminum chips ride the splash into it. The sections below take the decisions in the order you meet them: the split, the rating, the material, the sump, the spare space, the heat, the cable crossings and the order itself.
Why does coolant drive the whole enclosure plan?
Coolant forces the split before anything else: the machine chamber stays open and drained while the electronics sit in their own sealed Type 4X compartment, because one compartment doing both jobs fails at both.
A water-soluble mix is roughly 95% water at the 5% concentration most coolant datasheets set for aluminum. Flood and mist systems both throw it upward as droplets, and a cabinet sharing air with the chamber breathes that mist every cycle. Aluminum swarf in the slurry is conductive, so the failure lands on the electronics, not the plumbing. Keep the controller on its own side with its own gasket, and let the wet side drain. The sealing details for the wet chamber itself live in the mini-mill flood-coolant sealing guide, and the flood-versus-mist choice is worked through in mist, flood or air blast: chip removal.
Which rating does the controller compartment need?
Order the controller compartment rated NEMA 4X / IP66 by the manufacturer; not UL listed — Type 4 covers hose-directed washdown water, and the X adds the corrosion resistance a coolant-soaked shop wants.
NEMA 250 and UL 50E define the types, and the two letters carry different tests. Type 4 covers hose-directed water on a washdown schedule; the X adds corrosion resistance, and in UL 50E's Table 5-1D a Type 4X box takes another 200 hours of salt spray against an AISI 304 specimen. IEC 60529 reads the same box as IP66: dust-tight, plus powerful jets from a 12.5 mm nozzle at 100 L/min. NEC 110.28 sends US installations to the NEMA type, so carry both markings on the drawing. The rating belongs to the controller compartment alone — the machine tub is a drain pan, not a sealed box. Keep the sealed side whole: no drain, no vent, and only IP66-or-better accessories, because the weakest part sets the rating. For humidity in a sealed box, the dew-point approach in the cool-chamber dew point article beats any drilled hole. The full type comparisons sit in the NEMA 4 vs 4X vs IP66 guide.
Which stainless stands up to water-soluble coolant?
Order 304 stainless for a shop running standard water-soluble coolant, and step up to 316L when chlorides, brine or salt air reach the frame — 316L's molybdenum buys the pitting and crevice resistance 304 lacks.
The catalog splits the grades the same way: 304 for indoor washdown and most outdoor sites, 316L for salt air, brine and chemical areas. Chloride stress-corrosion cracking starts near 60 C (140 F) on both grades, and flood coolant on a benchtop mill runs near shop temperature, so that ceiling stays out of reach. The real limit is pitting: chloride deposits left to dry in a gasket crevice pit 304 even at room temperature. Rinse the door channel at cleanup and the 304 tub and box last for years. Pick the door gasket for the same fluid — O-ring handbooks list EPDM from about -50 to 150 C, which suits water-based coolant, while nitrile at -30 to 120 C is the oil-side choice.
How deep should the machine base sump be?
Plan the tub floor 3 in [76 mm] below the table drip edge on a 30 × 14 in [762 × 356 mm] footprint: 30 × 14 × 3 = 1,260 in³, and 1,260 ÷ 231 in³ per US gallon gives about 5.5 gal [20.6 L] of coolant in the pan.
That volume does three jobs: it submerges the pump inlet, gives chips somewhere to settle out, and absorbs the surge when a flood nozzle shuts off mid-cycle. Depth is the cheap variable — each extra inch of floor adds 420 in³, about 1.8 gal, on this footprint. Slope the pan to a single threaded drain fitting at the low corner, screen it so chips stay in the tub, and pipe the return below the fluid surface to cut foam. Keep drain and tank on the wet side of the frame; the door-lip details are in the coolant containment guide, and chip handling options sit in the auger washdown article.
How much spare space does the controller box need?
Leave 25% of the DIN rail spare: three 1.8 in drivers, a 3.2 in supply, a 3.9 in breakout and a 4 in terminal run total 16.5 in, so a 22 in rail in a 24 in wide box leaves 5.5 in — the room a 1 hp VFD upgrade takes.
That puts today's fill at 16.5 ÷ 22 = 75%, mid-band in the 20-30% spare a conversion build wants before the drive lands. The VFD mounts on the backplate beside the rail rather than on it, which is why the order fixes the box at 24 × 20 × 10 in instead of a size matched to today's parts alone. Run 2 × 2 in [50 × 50 mm] wiring duct along the rail; duct fill tables cap wiring at 50% of the duct's interior cross-section, the figure NFPA 79 section 13.5.2 works from, so size the runs for the day the motor leads arrive. Reuse versus replacement has its own answer in the CNC conversion electronics enclosure article, and the case for keeping the drive with the controller is in VFD in the main enclosure or a remote box.
How much heat will the VFD add to a sealed box?
Assume 22 W of drive loss — 1 hp is 746 W × 3% — and the loaded box sheds about 37 W; a 20 × 16 × 8 in stainless box rises 17 K to 42 C at a 25 C shop, 2 K over the 40 C limit drives carry.
Here is the arithmetic, with both assumptions labeled: 15 W for the present three drivers and supply, and 3% loss for the drive. Wall heat transfer for stainless runs k = 3.7 W/m²K. A 20 × 16 × 8 in [508 × 406 × 203 mm] box exposes 0.577 m² of wall once the mounted back face comes off the 0.784 m² total. ΔT = 37 W ÷ (3.7 W/m²K × 0.577 m²) = 17.3 K, so 25 C ambient lands the inside near 42 C — past the 40 C ambient rating drives and most electronics carry unless the datasheet allows more. The 24 × 20 × 10 in box exposes 0.878 m²: ΔT = 37 ÷ (3.7 × 0.878) = 11.4 K, about 36 C inside. That 4 K of margin is the decision: order the bigger box. Each extra 10 W adds about 3 K in it, so near 50 W total the exchanger question starts — an air-to-air heat exchanger holds the Type 4X seal while moving heat out. Swap the assumed 3% for the drive datasheet's own dissipation figure and redo the two divisions; the 3 K per 10 W shortcut still holds.
How do cables cross from the wet side to the dry side?
Give every conductor its own crossing: a gland rated IP66 and matched to the cable OD — gland datasheets list the range, and a common M20 spans 6-12 mm [0.24-0.47 in] — with a drip loop below it so coolant runs off the cable instead of wicking in.
Shared pass-throughs and unsealed spare holes end the rating, so each stepper, limit-switch and power cable gets its own drawn hole. The crossing patterns that hold a seal are in the motor controller power feedthrough article and the controller cable entry guide; cutout sizing sits in the cutouts and glands guide. Inside the wet chamber, angle a 1.5 mm [1/16 in] polycarbonate shield over each stepper so chips and splash shed off the motor seals, and drip-loop every cable where it leaves the shield. Size the motors before cutting shields: torque and speed first, which is the job of benchtop mill stepper sizing guide.
What goes on the order?
Send one spec: a 304 stainless frame with a 30 × 14 in tub holding 5.5 gal, a 24 × 20 × 10 in Type 4X controller compartment, M20 glands per cable and motor shields — built from one piece, delivered in 10 business days from drawing approval.
| Item | Order it as | Why this number |
|---|---|---|
| Machine tub | 304 stainless, 30 × 14 in [762 × 356 mm], floor 3 in [76 mm] below the drip edge, sloped to one threaded drain | 1,260 in³ ≈ 5.5 gal [20.6 L] of sump for pump submergence and chip settling |
| Controller compartment | 304 stainless, 24 × 20 × 10 in [610 × 508 × 254 mm], manufacturer-rated NEMA 4X / IP66; not UL listed | 37 W sheds at 11 K rise, holding the drive under 40 C at a 25 C shop |
| Rail and duct | One 22 in [559 mm] DIN rail; 2 × 2 in [50 × 50 mm] duct at 50% fill | 16.5 in of parts today; 5.5 in spare for the VFD |
| Cable crossings | One IP66 gland per cable, M20 for 6-12 mm [0.24-0.47 in] OD | The lowest-rated part sets the assembly rating |
| Motor shields | 1.5 mm [1/16 in] polycarbonate over each stepper | Chips and splash off the motor seals |
The frame above fits a mill running flood on aluminum daily. Mist-only duty halves the pan: a 1.5 in floor on the same 30 × 14 in footprint holds 30 × 14 × 1.5 = 630 in³, about 2.7 gal, and at 15 W the electronics rise just 7 K in the smaller 20 × 16 × 8 in box, which comes back as an option. Chlorides, brine or salt air move every wetted surface to 316L. Order through custom stainless enclosures, set every cutout on the drawing, and check the gland positions against the cable runs before you approve it.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 24 × 20 × 10 in | 304 stainless | 24 x 20 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can one frame hold both the mill and the controller safely?
Yes, when a full-height welded partition separates the 5.5 gal wet side from the dry side and each keeps its own sealing: the tub drains to its sump through one fitting, the dry side holds its Type 4X gasket and one gland per cable at 6-12 mm. That partition is what makes the single frame work.
Will water-soluble coolant corrode a 304 stainless enclosure?
At normal washdown duty, no. A standard aluminum mix runs near 5% concentrate and 304 tolerates it; chloride cracking needs about 60 C (140 F), which flood coolant never reaches on a benchtop mill. The risk is dried deposits in gasket crevices, where 304 can pit even at room temperature — rinse the crevices at cleanup, and order 316L when the coolant datasheet lists chlorides or the shop sees brine.
How large a VFD fits the sealed controller compartment?
About 1 hp without active cooling, on these assumptions: 1 hp is 746 W, a 3% loss is 22 W, and the 37 W total lifts the 24 × 20 × 10 in box about 11 K, so a 25 C shop lands near 36 C against the 40 C drive limit. Swap in the datasheet's loss figure when you have it; near 50 W total, plan an air-to-air heat exchanger instead.
Do the stepper motors need enclosures of their own?
No — they live in the wet chamber under a 1.5 mm [1/16 in] polycarbonate shield angled to shed chips and splash, each motor cable sealed at its own gland with a drip loop below. Sizing comes before shielding: pick the steppers by torque and speed first, then cut the shields to fit what you bought.
How fast does a custom two-compartment enclosure arrive?
Delivered in 10 business days, counted from your approval of the drawing for custom work, anywhere in the world from a single piece. The drawing comes back after the order, so confirm the tub drain position and every gland hole on it before you approve — the clock starts at your signature.
