What format to send
For 3D parts, send a STEP file (.step or .stp). STEP is the universal handshake between CAD systems — every CNC shop on the planet imports it cleanly into their CAM software. It carries solid geometry as B-rep (boundary representation), which means the shop's CAM system can re-derive the toolpaths from a clean model rather than fighting a triangulated mesh.
For 2D-only parts (plate work, sheet metal, laser-routed wood), send a DXF file. DXF is the workhorse format for 2D vector geometry — flat profiles, hole patterns, slot positions. Make sure layers are organised: outline on one layer, holes on another, etch/engrave on a third.
IGES is older and still used in some legacy aerospace and automotive workflows, but for new work in 2026 STEP has effectively replaced it. Don't send a native CAD file (SLDPRT, IPT, F3D, X_T) unless the shop has explicitly asked — most use different software than you and won't be able to open it.
Always include a 2D drawing as a PDF alongside the 3D model. The PDF is where you call out tolerances, surface finish, threads, and any "do not deviate" notes. The 3D model is what the CAM software uses to generate toolpaths; the PDF is what the machinist looks at when they have a question.
Tolerances — the difference between cheap and expensive
The single biggest cost driver after material is tolerance. Most parts don't need tight tolerances on every dimension — but a lot of CAD files arrive with ±0.01mm callouts on features that don't matter, because the engineer was being safe. That safety habit costs real money.
Most Irish CNC shops default to ISO 2768-medium (ISO 2768-m) if you don't specify. That's roughly ±0.1mm to ±0.3mm depending on feature size — fine for the majority of brackets, enclosures, and fixtures. Only call out tighter tolerances where they're functionally required: bearing fits, mating surfaces, locating dowels, sealing faces.
A useful rule of thumb on cost premium for tighter tolerance:
- ±0.1mm — standard, no premium.
- ±0.05mm — slight premium, often the same machine but slower feed and an extra inspection step. ~15-25% increase.
- ±0.02mm — significantly slower, may need a different machine. ~50-100% increase.
- ±0.005mm — grinding territory, specialist work, multi-day inspection. 3-5× the standard cost.
Design-for-Manufacturability (DfM) — the common mistakes
These are the issues Irish shops see again and again. Fixing them before you send saves a quote round-trip:
Sharp internal corners
An end mill is round. It cannot cut a perfectly sharp internal corner — it leaves a radius equal to the tool diameter at minimum. If your CAD file shows sharp 90° internal corners, the shop has to either use a smaller tool (slow, expensive) or EDM the corner separately (very expensive). Always design internal corners with a radius — typically 0.5mm to 3mm. The bigger the radius, the cheaper the cut.
Deep narrow pockets
A pocket that's deeper than 4-5× its narrowest dimension is hard to machine. The tool flexes, chatters, and breaks. Either widen the pocket, reduce the depth, or split the part into a two-piece assembly. This single change can cut 30% off a price quote.
Undercuts without explicit callouts
An undercut (a feature you can't reach from above with a straight tool) requires special tooling — a T-slot cutter, a side-cutting end mill, or a 5-axis machine. They're fine if the shop knows about them, but if the undercut is buried in a feature without a callout in the PDF drawing, the CAM operator might miss it and quote based on simple machining. You'll either get a re-quote or, worse, a wrong part.
Threads — say what you want
Threads can be either machined (single-point cut on a lathe or with a tap), formed, or roll-tapped. Each has a cost. M3 to M16 ISO metric threads are routine and cheap. Imperial threads, fine-pitch, NPT, and threads in soft plastics need a callout. Specify the standard (e.g. "M8x1.25-6H"), the depth, and whether it's through or blind.
Cosmetic surfaces vs functional surfaces
If a face will be visible (logo plate, customer-facing enclosure), say so — the shop will preserve toolpath direction and cleanness on that face. If a face is hidden, don't waste cost on it. Mark surfaces explicitly: "Ra 1.6 µm" for a normal machined finish, "Ra 0.8 µm" for a fine finish, "as-machined OK" for non-cosmetic.
The pre-export checklist
Before you send a CAD file out for quoting, run through this list. It is what an experienced engineer does without thinking, and what saves a junior engineer a painful learning week:
- Model is fully dimensioned in the unit you intend (millimetres, not inches — Irish shops work in metric).
- All features are part of the solid body, not floating sketches or surfaces.
- Holes have specified diameter and depth, callouts in the PDF.
- Threads are called out by standard and depth.
- Internal corners have radii ≥ 0.5mm where possible.
- Tolerances are called out on a 2D PDF drawing — only where they matter, not as a blanket spec.
- Surface finish is specified on cosmetic or sealing faces.
- Material is named on the drawing (alloy, grade — see the materials guide).
- Quantity is stated.
- Coordinate origin is sensible — usually a corner or centre, not somewhere arbitrary.
- STEP file opens cleanly when you re-import it into your own CAD system as a sanity check.
What to send the shop
A complete quote package is three files, named clearly:
partname-rev01.step— the 3D solid model.partname-rev01-drawing.pdf— the 2D drawing with tolerances, threads, finish callouts, material, quantity.partname-rev01-readme.txt(optional) — a short note: lead-time target, end-use context, anything quirky.
Email those three files in one message. Don't link a Dropbox or Drive folder unless the shop asks — a fair few Irish shops still operate behind email firewalls and it's an extra step they don't need.
Once your file is ready
The fastest path to multiple Irish quotes is the cnc.ie quote form — fill it once, we forward to the relevant verified suppliers based on your material and quantity. If you want to skip the form and email shops directly, every entry in the supplier directory links to its own contact page.