A manufacturer rejecting your part means one thing: your design cannot be produced as submitted. This happens more often than it should, and almost always due to the same set of preventable mistakes.
Whether you are ordering CNC machined parts, 3D printed components, or sheet metal fabrications, the principles are the same. Here is what goes wrong and how to fix it before you upload your file.
Why Parts Get Rejected
Manufacturers reject parts when the design has features that cannot be physically machined or printed, wall thicknesses are too thin to survive the process, tolerances conflict with process capabilities, files are incomplete or missing dimensions, or the material is incompatible with the required features.
None of these are the manufacturer's fault. And none of them have to happen to you.
Design Checklist: CNC Machined Parts
- Minimum wall thickness: 0.8mm for plastics, 1mm for metals
- Internal corner radii: minimum 1mm (sharp internal corners cannot be milled)
- Hole diameter: minimum 1mm
- Thread depth: no more than 3 times the thread diameter
- Deep pockets: depth should not exceed 6 times the tool diameter
- Avoid undercuts unless 5-axis machining is within budget
- Submit STEP files with a PDF drawing showing tolerances and critical dimensions
Design Checklist: 3D Printed Parts (FDM)
- Minimum wall thickness: 0.8mm
- Avoid overhangs greater than 45 degrees without support structure
- Minimum text size: 10pt raised or recessed
- Clearance for fits: 0.2mm to 0.5mm
- Ensure the model is watertight with no open surfaces or reversed normals
Design Checklist: 3D Printed Parts (SLA)
- Minimum wall thickness: 0.5mm
- Minimum text size: 6pt
- Design hollow sections with drain holes to prevent resin pooling
- Avoid large flat surfaces that may warp during curing
Design Checklist: Sheet Metal Parts
- Minimum hole size: 1.5 times the material thickness
- Minimum bend radius: equal to material thickness
- Slot widths at least equal to material thickness
- Avoid tight bend-to-hole distances that could cause tearing
- Submit DXF files for laser cutting and STEP files for bending operations
File Submission Checklist
- Use standard formats: STEP for CNC and sheet metal, STL or STEP for 3D printing, DXF for laser cutting
- Include a PDF drawing with all critical tolerances and surface finish requirements
- Specify material grade clearly, not just the material family
- Mark reference surfaces or datums for inspection
- Include quantity and expected delivery timeline
How ProtoMandi Prevents Rejections
Every file uploaded to ProtoMandi is reviewed by our engineering team before production begins. If we detect a feature that will cause a rejection or increase cost unnecessarily, we flag it and give you specific feedback.
This DFM review is free and included with every order. It is the difference between getting parts made right the first time and going back and forth with a vendor for weeks.
Upload your file on ProtoMandi and get expert DFM feedback before your order goes to manufacturing.