Can a CNC shop machine a part from a 2D drawing alone?

Yes, for prismatic parts. Plenty of machinists program 2.5-axis milling and turning straight from a fully dimensioned 2D print. It is when the part has sculpted 3D surfaces that they need a model.

Why do some shops insist on a STEP file?

A 3D model lets them pull geometry straight into CAM without re-typing dimensions, which cuts setup time and removes a source of error. For complex parts it is faster and safer than working from a 2D print.

Do I need a 3D model for CNC, or is a 2D drawing enough?

When a 2D drawing is enough

For a large share of real machining work, a clean 2D drawing is all the shop needs. Prismatic parts, flat plates, brackets, turned shafts, anything made of holes, faces and straight features can be programmed directly from a fully dimensioned print. A 2D drawing also carries the things a bare 3D model does not: tolerances, surface finish, material, and notes. A machinist can quote and cut from that.

This is why drawings have not gone away even though everyone has 3D CAD. More on that in why machine shops still want 2D drawings.

When the shop really wants a 3D model

A 3D model is almost always shared as a STEP file (extension .step or .stp), a neutral CAD format that any shop can open regardless of the software they run. A model earns its keep when the geometry stops being something you can fully describe in flat views:

What you are making	What to send
Flat plate, bracket, simple milled or turned part	2D drawing (PDF + DXF)
Part with a few pockets and holes	2D drawing, 3D model optional but helpful
Sculpted or organic 3D surfaces	3D model (STEP), with a 2D print for tolerances
5-axis work, moulds, dies	3D model required, 2D print for the critical dimensions

The best practice on any non-trivial part is to send both: a STEP file for the geometry and a 2D drawing for the tolerances, finish and notes. The model tells the machine the shape; the drawing tells the machinist what actually matters.

What if you only have a 2D drawing?

If your part is prismatic, a good 2D drawing is genuinely enough to get it made, and a shop that accepts 2D prints will quote it. TechDraw produces exactly that kind of document, a dimensioned 2D drawing exported as DXF, DWG, SVG or PDF, which covers most milling, turning and laser jobs. See whether a machine shop will accept the drawing and which CAD file formats manufacturing uses. If a shop later asks for a STEP file, that is a sign your part has true 3D surfaces and needs a model built in CAD rather than a 2D print.