If you're planning to send your design for CNC machining, one of the most overlooked but critical steps is preparing the drawing. Not just any drawing—but the right kind of drawing. And here's the thing: it's not about making it look fancy. It's about clarity, communication, and avoiding the kind of ambiguity that leads to delays, re-quotes, or worse—wrong parts.
So how exactly do you create a CNC-ready drawing that your machine shop will appreciate (and act on without a dozen back-and-forth emails)? That’s what we’ll explore here.
There’s a common misconception that a 3D model is all you need. While STEP (.step/.stp) and IGES (.igs/.iges) files are the backbone of 3D CNC workflows, they don’t tell the whole story. CNC machines read geometry, but they don't understand design intent, tolerances, or surface finish—and that’s where a 2D drawing becomes essential.
A proper CNC drawing format usually involves:
A 3D file (preferably STEP)
A 2D technical drawing (PDF or DWG/DXF)
If needed, a BOM or tolerance callout sheet
The 2D drawing acts as the communication layer between your design and manufacturing. It clarifies everything that’s not explicit in a model—from thread types to surface finish to special notes (“do not deburr this edge,” etc.).
At EKINSUN, we recommend submitting both:
👉 CNC Machining File Submission Guidelines
We’ve seen thousands of drawings—from perfect to problematic. The most common issue? Lack of prioritization. Designers often include every dimension, hoping more information is better. But for machinists, too much irrelevant data can slow things down.
What helps most is highlighting critical-to-function dimensions and tolerances. If your hole spacing is crucial for assembly, but the outer profile can vary ±0.5mm, say so clearly. If you need a press fit, mention the fit type (e.g. H7/g6) instead of guessing a tight tolerance.
When in doubt, check our guide on how to calculate hole spacing for machined parts to avoid alignment issues later on.
GD&T (Geometric Dimensioning and Tolerancing) is powerful, especially for complex parts with form controls. But it also adds a layer of abstraction. If your shop understands it, it can reduce ambiguity. If not, it can backfire.
A safe approach? Use GD&T selectively, only where form or orientation truly matters. Elsewhere, stick to linear tolerances and clear notes. Always ask yourself: will this symbol clarify or confuse?
If you’re unsure, our team can help interpret and convert complex GD&T into machinist-friendly drawings—contact us for free feedback before you submit.
Some designers assume if the CAD model has dimensions, that’s enough. But CNC machinists don’t always work directly from the model unless it’s paired with CAM software. Even then, dimensions are used for inspection, setup, and verifying the job post-machining.
A drawing ensures everyone—designer, machinist, quality inspector—is aligned on expectations. It also becomes part of the documentation trail in regulated industries (medical, aerospace, etc.).
So even if your CAD model is perfect, don’t skip the drawing step.
This is where mistakes are common. If your part includes tapped holes, you need to specify:
Hole type (e.g. M6 x 1.0 tapped through)
Depth (if blind)
Whether threads are metric or imperial
Whether thread relief or countersink is required
Likewise, hole diameters should include fit types if the hole will interface with shafts, bearings, or pins. Avoid vague callouts like “tight fit” or “press fit” without definition.
Want to double-check your choice of shaft-hole tolerance? Use our Free ISO Fit Calculator to simulate real fit behavior before committing to a drawing.
Surface finish is one of those details that’s easy to overlook, but critical for aesthetic, functional, or wear-sensitive parts. If it matters—specify it. If not, you're better off leaving it open to avoid extra cost.
For instance:
Ra 3.2 µm is a standard machined finish
Ra 1.6 µm or below starts requiring more time or polishing
Visual surfaces should be noted ("cosmetic face") so shops don’t overlook aesthetics
We wrote more about that in our CNC surface finish guide.
If you’re preparing a drawing for CNC and unsure whether it’s “good enough,” step back and ask:
Can someone unfamiliar with my project understand what matters in this part?
Are the tolerances realistic for the material and process?
Have I separated functional requirements from visual preferences?
Getting it right the first time isn’t about making the perfect drawing. It’s about making a clear and collaborative one.
Written by:
EKINSUN Manufacturing Support Team
Specializing in custom CNC machining, rapid prototyping, and production optimization. We bridge the gap between design and production with one goal—getting your parts made right the first time.
Need help with a tricky drawing or material selection? Request a Quote or start a conversation—we’re here to help.