NEMA 23, 34 and 42 stepper mounting dimensions for a plate

Short answer: a NEMA 23 face is a 2.22 in [56.4 mm] square flange with four mounting holes 1.87 in [47.5 mm] apart on a square and 0.18 in [4.6 mm] holes in the motor face. NEMA 34 steps up to a 3.41 in [86.6 mm] flange with 2.76 in [70.1 mm] spacing, and NEMA 42 reaches 4.22 in [107.2 mm] with 3.53 in [89.7 mm] spacing. You lay the four holes out at half that spacing either side of the shaft axis and bore the center to the pilot diameter on the motor datasheet. Check the pattern diagonally: 1.87 × √2 = 2.64 in [67.2 mm] across opposite holes on a 23-frame plate. Divide the frame number by 10 to sort motors into classes, never to size a plate.

What does the NEMA number actually mean?

It is the square face size in tenths of an inch: 23 reads as about 2.3 in [58 mm], 34 as 3.4 in [86 mm], and 42 as 4.2 in [107 mm], and the quotient places a motor in a frame class rather than handing you a dimension to cut.

NEMA stands for the National Electrical Manufacturers Association — Manufacturers, not Manufacturing, the misspelling that lives on older drawings. Its ICS 16 standard covers motion and position control motors, and the mounting faces in the table below come from it. A motor built to that face from any maker drops onto the same plate.

The quotient runs coarse. The tabulated 23-frame flange is 2.22 in [56.4 mm], about 0.08 in [2 mm] under the 2.3 in the division suggests. A pocket cut at 2.3 in misses the register, and a bore scaled from the quotient comes out worse. Use the rule to identify the frame, then leave it behind.

What are the mounting dimensions for NEMA 23, 34 and 42?

Read them from this table: bolt spacing is the center-to-center distance between adjacent holes of the square, the hole diameter is the motor-face hole, and the last column is the diagonal between opposite holes, which is your easiest check with one caliper.

Metric figures are the inch values converted at 25.4 mm per inch. The hole diameter is the size in the motor face, tapped or clear according to its datasheet; the plate always gets a larger clearance hole for the screw. Where the flange matches a row but your measured spacing does not, you are holding a metric-frame or vendor-specific face, so work from its dimensioned drawing from that point on.

How do you lay out the four holes from the shaft axis?

Mark the shaft axis, then put one hole at each corner of a square at half the bolt spacing from it: ±0.935 in [23.8 mm] for a 23, ±1.380 in [35.0 mm] for a 34, and ±1.765 in [44.8 mm] for a 42.

On a square pattern of side s, the holes sit at (±s/2, ±s/2) and the bolt circle through all four runs at s × √2. For a 23-frame: 1.87 × 1.414 = 2.64 in [67.2 mm]. The same arithmetic gives 3.90 in [99.1 mm] for a 34 and 4.99 in [126.9 mm] for a 42. Spacing times root two sets out every square bolt pattern, the geometry behind bolt-circle dimensions on a machined backplate.

Measure the diagonal in preference to the side spacings: one caliper setting reads two holes at once and exposes a skew that a single side measurement hides. For the marking itself, a panel cutout layout template for accurate marking keeps the punch marks from drifting.

What does the frame number leave out?

Everything behind the face: pilot boss diameter and height, shaft diameter and length, body length, connector position, torque and winding all come from the motor datasheet, and two motors in the same frame share only the flange and the four holes.

The pilot is the round register standing proud of the face. It seats in your plate bore and sets the concentricity; the screws only clamp. Bore to the datasheet diameter as a sliding fit — a thousandth or two of clearance still centers the shaft, while an oversize bore lets the motor sit off-center with no adjustment left.

Body length moves the enclosure more than anything else on the drawing: two 23-frame bodies can differ by a factor of two in depth, and the deeper one decides the box behind the plate. Where a datasheet drawing and this table disagree, the datasheet governs, because the drawing you hold is the motor you will bolt in.

How do you machine the plate?

Work in this order — datasheet numbers, axis mark, punch at half the spacing, clearance-drill the screws, bore the pilot concentric with the axis — and trial-fit the motor before anything coats the plate.

You need calipers, a scribe and center punch, a drill press, a boring head or a chucking reamer, and cutting oil if the plate is stainless. Pull power and lock out the machine before you swap a motor, and give the drive's DC bus five minutes to discharge before you touch the leads.

1. Copy five numbers off the motor datasheet: bolt spacing, pilot diameter, pilot height, screw thread, body length. The table above carries the spacing for 23, 34 and 42.
2. Mark the shaft axis on the plate and punch the four holes at (±s/2, ±s/2) — ±0.935 in [23.8 mm] each way for a 23.
3. Pilot-drill each mark at about 2.5 mm [0.098 in] and check both diagonals while the holes are small and still easy to move.
4. Open the holes to screw clearance for the datasheet thread: 5.5 mm [0.217 in] for M5, 6.6 mm [0.260 in] for M6 (ISO 273, medium series).
5. Bore the center to the datasheet pilot diameter in steps, finishing with the boring head; this hole sets concentricity, so it gets the slow, careful pass.
6. Deburr both faces, trial-fit the motor, then coat. Powder coat adds roughly 0.003–0.004 in [0.08–0.10 mm] per face and closes a sliding-fit bore.

How do you check the finished plate?

Measure across opposite hole centers and compare with the bolt circle — 2.64 in [67.2 mm] for a 23, 3.90 in [99.1 mm] for a 34, 4.99 in [126.9 mm] for a 42 — with both diagonals matching inside 0.01 in [0.25 mm].

• Both diagonals equal inside that band: one long and one short means a skewed square.
• Adjacent spacings equal to s on both axes, so the screws enter square to the flange.
• The pilot seats by hand, fully home, with no rocking on the bore.
• Turn the shaft by hand before the screws go in, tighten in a cross pattern, then turn it again; new drag after tightening means the bore runs off the axis.

Worked example — a 23-frame plate. You punch at ±0.935 in [23.8 mm], drill 5.5 mm [0.217 in] clearance holes, and measure 2.66 in [67.6 mm] across one diagonal against 2.64 in [67.2 mm] across the other. The arithmetic: 1.87 × 1.414 = 2.64 in, so the long diagonal runs 0.02 in [0.5 mm] over, about 0.010 in [0.25 mm] at one corner. A 5.5 mm hole on an M5 screw leaves 0.25 mm [0.010 in] of radial play, so that corner binds. The pilot still centers the motor, so open the drifted hole to 6.0 mm [0.236 in] and the screws swing through; with two corners long, re-mark the square and start again.

Where does the motor mount in an enclosure build?

On the backplate inside the box, beside the drive that runs it — a 23-frame flange covers about 2.5 in [64 mm] square of plate — while a motor bolted through the enclosure wall puts a bare pilot bore in a sealed panel, and the box then seals only as well as that joint.

Backplate material and mounting decides what the motor bolts to, and the backplate and DIN rail layout guide keeps the drive's rail clear of the motor body.

Heat is the second reason for keeping it inside. A stepper holds full winding current at standstill, so its losses land in the box even with the axis parked. Stepper motor controller enclosure layout and selection covers the spacing between drive and motor, and a sealed box that runs warm takes a closed-loop heat exchanger for a drive enclosure sized to the load.

Where the shaft must pass through the wall, seal the pass-through as its own component, because the weakest part sets the rating — what a panel keeps after you cut it and mount devices walks through that trade. Our enclosures are rated Type 4X / IP66 by the manufacturer, not UL listed.

We can cut the pattern before anything ships: the cutout designer lays out the four holes and the pilot bore on any face, custom stainless enclosures with factory cutouts start from 304 and 316L bodies, and custom enclosures in your material and size set the rest. Custom work is delivered in 10 business days, counted from your approval of the drawing.

Which mistakes ruin a motor plate?

Three account for most scrapped plates: machining from the quotient instead of the 2.22 in [56.4 mm] flange, drilling the plate to the motor's 0.18 in [4.6 mm] face size instead of a clearance size, and boring the pilot oversize so the screws, not the bore, position the motor.

• Coating the plate before the trial fit: the finish changes a bore the motor has already proven, so fit first, coat second.
• Trusting a metric-face motor to match a NEMA row: measure the 1.87, 2.76 or 3.53 in spacing before you cut.
• Skipping the turn-before-and-after check: misalignment you never felt goes straight into the coupling and the load bearings.

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.

Questions