I've Wasted Over $4,000 on Prototypes: A 12-Point Pre-Submission Checklist for Xometry CNC & Injection Molding Orders

Who This Checklist Is For (And Why I Created It)

If you're uploading a file to Xometry for a CNC part, an injection molding tool, or a 3D printing run—and you've got that nagging feeling you might have missed something—this is for you.

I've been handling on-demand manufacturing orders for about 6 years now. I wish I could tell you I was good at it from the start. I wasn't. In my first year (2018), I made a classic mistake: I uploaded a .STEP file generated from a complex assembly, not a single body part. The quoting engine accepted it, but the part that came back was... well, it was a single body from the assembly, not the assembled part I'd pictured. That was a $320 order, straight to the trash.

The 12-point checklist I created after my third significant mistake has saved us an estimated $8,000 in potential rework. Here it is, in the order I run through it before every submission.

The 12-Point Checklist (Run This Before You Click 'Submit')

Step 1: Is Your File a Single, Watertight Solid Body?

This is the number one rookie error. Xometry's quoting engine needs a single solid body, not an assembly. If you send an assembly file, it might quote on just one component, or reject the file entirely. I still kick myself for that first $320 mistake.

Check: Open your file in your CAD software. Can you click on the part and see it highlight as one solid object? If it highlights multiple items, you've got an assembly. Merge it into a single body.

Step 2: Check Your Units (Twice)

Honestly, I'm not sure why this is still a problem in 2025, but it is. A part designed in inches, submitted as millimeters? Your 10-inch part becomes a 10mm part. I don't have hard data on industry-wide rejection rates for this error, but based on our team's experience, my sense is that it accounts for about 5-7% of re-requests for clarification.

Check: Before you export, confirm the units in your CAD file. Xometry defaults to millimeters, but it handles inches perfectly—you just need to tell them in the upload form.

Step 3: Add Threads as Cosmetic Features (Don't Model Them In)

This one took me a while to learn. If you model a physical helical thread in your CAD file for a CNC part, the CAM software might try to cut that complex path. That's expensive and often unnecessary. For most applications, you just need to call out the thread in your drawing (e.g., "M6 x 1.0") and let Xometry's machine shop tap it in post-processing.

Check: Are there any complex features—threads, internal gears, fine knurling—that are better defined by a 2D drawing note than a 3D model?

Step 4: Define Your GD&T on a 2D Drawing (PDF)

Here's the thing: the 3D model defines the shape. The 2D drawing defines the tolerances. If you upload only a .STEP file and it passes the geometry checks, Xometry will use their standard tolerances. For CNC machining, that's typically ±0.005 inches (or ±0.13 mm). If you need tighter tolerances on a critical bore—say, ±0.001 inches—you won't get it unless you put it in a drawing.

I once ordered 50 CNC brackets and didn't include a drawing. The critical hole fit was 'loose.' It worked, but it wasn't great. That was my lesson.

Check: Have you created a 2D PDF drawing with all critical dimensions and tolerance callouts?

Step 5: Check for Wall Thickness (Injection Molding Specific)

If you're using Xometry's injection molding service, this is a big one. Variable wall thickness is the enemy of good injection molding. Thick sections cool slowly, leading to sink marks and warpage. Aim for consistent wall thicknesses. A good rule of thumb: keep it under 4mm for most engineering plastics, and try to keep variations under 25%.

Check: Use your CAD software's 'Thickness Analysis' tool. Are there thick sections or abrupt changes? If so, add ribs to support the wall rather than thickening it.

Step 6: Design for the Draft Angle (Injection Molding)

Straight vertical walls are fine for CNC machining. For injection molding? Not so much. The part needs a slight taper—a draft angle—to release from the mold.

Check: Aim for at least 1 degree of draft per side. For textured surfaces, you might need 3 degrees or more. Xometry's instant quoting analysis will flag if your part is likely to stick in the mold, but it's better to fix it in your design up front.

Step 7: Mind the Minimum Feature Size (For 3D Printing & CNC)

This varies by process. For Xometry's 3D printing services (like SLS or MJF), a hole smaller than 0.5mm in diameter likely won't print open. For CNC, an internal corner with a radius smaller than your chosen end mill's radius is physically impossible to cut. The machine will leave a 'dogbone' or 'T-bone' fillet.

Check: What's the minimum feature size for your chosen process? Look it up in Xometry's design guidelines. For internal corners in CNC, always specify a radius. A sharp corner looks impossible to manufacture, but it's actually possible with a tool change—it's just very expensive.

Step 8: Double-Check Your 3D Printing Orientation

A lot of people ignore this when they just upload a file. The orientation of your part on the print bed determines where support structures go, which surface quality you get on which face, and how strong the part is in different directions.

Check: In your quote request, specify the orientation. Want a smooth bottom surface? Print it face-down on the bed. Need strength along a particular axis? Align that axis with the printer's Z-axis (which is usually the weakest due to layer adhesion).

Step 9: Confirm Your Thread Size & Pitch (A Step Many Forget)

Look, I'm not saying budget options are always bad. I'm saying they're riskier when it comes to fasteners. You need to be exact. If you need a 1/4-20 UNC thread, specifying "1/4 inch thread" is not enough. There's 1/4-20 UNC and 1/4-28 UNF. They are different.

Check: In your drawing, clearly state Thread Size, Class of Fit (e.g., 2B for internal), and the depth.

Step 10: Add a 'No Burrs' or 'Break Sharp Edges' Note

This is a tiny thing that makes a huge difference in feel. A sharp edge on a machined part can be a literal hazard. By default, most shops will break sharp edges, but they might not break them to your standard.

Check: Add a note on your drawing like: "Break all sharp edges, 0.1mm max chamfer." It costs nothing in the quote—it's just a setup note—but it saves you from filing down parts when they arrive.

Step 11: Review Your 'Instant Quoting Engine' Outputs

Xometry's instant quoting engine is powerful, but it's a first pass. I've seen it quote a price for a part made in stainless steel when I'd selected ABS plastic. Don't blindly trust the output. Real talk: the price is based on geometry analysis, material density, and complexity. If it looks too cheap for a complex 5-axis part, it might be quoting it as a simple 3-axis part.

Check: Does the quoted price make sense for the complexity of your part? If not, contact Xometry's sales team for a manual review.

Step 12: Plan for 'Low Volume Production' with a Pilot

If you're using Xometry for low volume production (say, 50 to 1,000 parts), don't order everything at once. Order 5 to 10 first. This is the single most expensive lesson I learned. The perfect prototype is not the perfect production part.

Check: On your first order for a new part, use the 'Rapid' or 'Standard' lead time and order a small batch. Inspect them carefully. If they're good, place the full volume order. If they're not, you've only wasted a few hundred dollars, not a few thousand.

What to Do If You Miss Something

You will still make mistakes. It's fine. The real question is: can you catch them before metal is cut or resin is cured?

One thing I'd recommend: if you get a part back and it's wrong, don't just re-order. Analyze it. Ask yourself: was the error in my file, my drawing, or my material selection? Document it. Tracking this in a simple spreadsheet has been our team's secret weapon for hitting quality targets on subsequent orders.

And for those asking about other tools—like dust collection for injection molding machines or whether to press the brake when starting a car—that's a topic for another day. For now, focus on getting your file and drawing right before you hit submit.