How to Make a File for CNC Machine Work
A machinist reads your file, not your intent. This guide shows how to make a file for CNC machine work in six steps: choosing the CAD export, setting tolerances, writing the 2D drawing, naming the finish, and packaging everything before upload. Written for design engineers and buyers who need a clean first-pass quote.
In this article
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Key takeaways
What the shop needs before you make a file for CNC machine work
A CNC quote is only as good as the geometry behind it. When a programmer opens your file, four things get checked in the first minutes: is the model a closed solid, are the units correct, is there a drawing that pins down the tolerances, and is the material and finish stated. If any of those are missing, the quote comes back with questions instead of a price.
The model is your digital master. Every dimension, curve and surface starts there, so a shelled body with three open faces will stall the job before anyone looks at a tolerance. Run a geometry check in your CAD system and heal gaps before export. This one habit removes most back-and-forth.
The 2D drawing carries the contract. A 3D model shows shape; it does not say which face is the datum, which hole is a press fit, or which corner must stay sharp. Put datums, GD&T frames, thread callouts and surface finish on the drawing, and reference the model for everything else.
Keep the whole package to three or four files: the STEP solid, the PDF drawing, the native CAD file, and a short notes sheet. More files mean more chances that a programmer quotes revision B while the shop cuts revision C.
Pick the CAD format the CAM system can read
STEP (AP203 or AP214) is the default. It transfers solid bodies, assemblies and units cleanly into most CAM packages. AP214 is the safer pick when you have colors, layers or assembly structure. Export in millimeters unless the shop asks otherwise, and state the unit in the file name.
Parasolid (.x_t) is the second choice and often the better one for complex organic surfaces, thin walls or parts that started in NX or SolidWorks. IGES still appears in aerospace and legacy workflows, but it moves surfaces rather than solids, so small gaps can appear at fillets and blends. If you must send IGES, run a stitch check after import.
STL is for 3D printing and rough visualization, not for machining. It is a triangle mesh with no units and no smooth surface definition. A curved boss that looks fine on screen may come out faceted, and a Ø8 H7 bore cannot be trusted from an STL. Avoid it for any feature with a tolerance.
Native files such as SLDPRT, CATPart or IPT are useful as a backup when a feature fails to import, and we can open them. Do not send them as the only file, because version mismatches are common. A STEP export sits one version ahead of trouble.
Set tolerances you can actually inspect
Default block tolerances are convenient but dangerous. A drawing that says ±0.1 mm everywhere tells the programmer nothing about which feature matters. Call out tight tolerances only where function demands it, and leave the rest at general tolerance. That keeps cycle time and inspection cost down.
GreatLight holds ±0.005 mm (±0.0002 in) on critical features, and that number is a process limit, not a target for every dimension. A bearing bore or a sealing face may need it. A mounting flange hole rarely does. When everything is tight, the shop has to slow the machine, add semi-finish passes and inspect more, which shows up in the price.
Surface finish follows the same logic. As-machined faces land around Ra 1.6–3.2 μm, a good general finish is Ra 0.8–1.6 μm, and a fine finish is Ra 0.2–0.8 μm on aluminum. Specify the finish per face, not for the whole part. A single Ra 0.4 μm callout on a deep pocket can require a different tool and a longer cycle.
Write the tolerance on the feature that touches another part. If a shaft must press into a housing, give the housing bore an H7 and the shaft an h6 and let the shop do the rest. That is faster to read than a stack of plus-minus numbers.
Five file errors that delay a quote
Open or overlapping geometry is the first. Two solids touching at a face, a zero-thickness wall, or a surface that never closes will stop CAM from generating a toolpath. Run a solid check and fix it before export, not after the shop emails you.
Scale and unit errors are the second. A part drawn in inches and exported as millimeters comes out 25.4 times too large. State the unit in the file name and in the drawing title block, and check the bounding box against a known dimension.
Missing thread and fit data is the third. A hole modeled at Ø5.0 mm with a note reading M6 × 1.0 is not the same as a modeled thread. Give the tap drill size or the pitch, and mark the thread depth. The same applies to counterbores, countersinks and keyways.
Undercuts and tool reach are the fourth. A pocket 40 mm deep with a 6 mm corner radius may need a long, thin tool that deflects. If the corner must be sharp, say so and accept EDM or a smaller tool. If it can be rounded, say that too.
The fifth is a drawing that disagrees with the model. When a dimension on the PDF conflicts with the STEP solid, a good shop stops and asks. A rushed shop picks one. Keep the model authoritative and mark the drawing as 'do not scale'.
How to make a file for CNC machine work, step by step
Follow the order. Skipping step 2 is the most common cause of a re-quote.
- 11. Clean the modelRemove construction geometry, extra bodies, hidden sketches and unused components. Check that the part is one closed solid and that wall thickness is at least 0.8 mm in aluminum, 1.0 mm in steel. Heal gaps and re-check.
- 22. Fix units and orientationSet the document to millimeters. Orient the part so the main machining face points up along Z, and place the origin at a datum corner or a center of a primary bore. Confirm the bounding box, then check that it fits a 4,000 mm maximum processing envelope if the part is large.
- 33. Export STEP AP214Export as STEP AP214 with solids, not surfaces. Name the file with part number, revision and unit, for example 4471-B-01_rev-C_mm.step. Open the export once in a viewer to confirm that no faces were dropped.
- 44. Build the 2D drawingShow three views plus any section needed to see internal bores. Add the three datum features, GD&T frames for position and flatness, thread callouts such as M6 × 1.0 depth 12, and finish symbols per face. Mark critical dimensions in a box.
- 55. Write the notes sheetOne page: material grade (6061-T6, 304, 17-4PH), finish (clear anodize, electroless nickel, bead blast), quantity, and any assembly note. List the tolerances that are not on the drawing. Note whether edges must be broken or left sharp.
- 66. Check, zip, uploadConfirm that the STEP, PDF and native file all carry the same revision letter. Zip them with the notes sheet, or upload directly to the quote page. Encrypted upload is fine; an NDA is available on request before files move.
- 77. Review the DFM reportWe return a quotation and free DFM analysis within 12 hours. Read the flagged features: thin walls, deep pockets, sharp internal corners, tolerances below ±0.005 mm. Answer the questions in one reply and production can start within 24 hours.
CAD format decision table
Match the format to the feature and the downstream process.
| Format | Best for | Watch out for | Send when |
|---|---|---|---|
| STEP AP214 | Solid models, most CNC work | Large assemblies can hide small bodies | Default for milling and turning |
| STEP AP203 | Simple single solids | No color or layer data | Legacy CAM systems |
| Parasolid .x_t | Organic surfaces, thin walls | Version mismatch between CAD seats | Part started in NX or SolidWorks |
| IGES | Legacy aerospace, surface data | Open edges at fillets and blends | Customer requires it |
| STL | 3D printing, visual checks | No units, faceted curves | Never for toleranced features |
| Native CAD | Backup for failed imports | Version and license issues | Alongside STEP, not alone |
The short version
Send one clean STEP solid, one drawing that marks the datums and fits, and a notes sheet with material and finish. That combination quotes in hours instead of days.
Questions engineers ask before upload
Do I need a 2D drawing if I send a 3D model?
Yes, for anything with a tolerance, a thread or a finish. The solid defines shape; the drawing defines what gets measured and accepted.
If the part is a simple bracket with no critical fits, a model plus a notes sheet is often enough. We will tell you within the 12-hour DFM review if something is missing.
Can you work from a scan or a hand sketch?
A scan can be reverse-engineered into a solid, and a dimensioned sketch can be modeled. Both add engineering time before cutting.
Send the scan as STL or point cloud plus a few known dimensions and the function of the part. We will quote modeling separately from machining.
What tolerance can you hold on a typical part?
We hold ±0.005 mm (±0.0002 in) on critical features, with 100% inspection before shipment. General dimensions are usually held at ±0.05 to ±0.1 mm.
Tolerance depends on material, feature size and setup count. A deep bore in titanium behaves differently from a shallow slot in 6061-T6.
How tight can surface finish be?
As-machined is Ra 1.6–3.2 μm, a high finish is Ra 0.8–1.6 μm, and a fine finish reaches Ra 0.2–0.8 μm.
Fine finishes need the right material and a stable setup. On soft aluminum a polished face is routine; on 316 stainless it costs more cycle time.
Are my files kept confidential?
Uploads are secure and confidential, and we can sign an NDA before files are transferred.
Our information security management follows ISO 27001:2022, and access to customer data is limited to the engineering and programming staff on the job.
How many parts can I order from one file?
No minimum order quantity. The same file runs as a single prototype or a 10,000+ part production run.
For volumes above a few hundred, send the file early. We can flag design changes that reduce cycle time before tooling and fixtures are made.
Upload your file and get a DFM review
Send the STEP, drawing and notes in one package. We return a quotation and free DFM analysis within 12 hours, and production can start within 24 hours.
12-hour quote±0.005 mm100% inspectionNDA on request
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