# What factories need from you: 3D CAD files factories requirements explained

Factories need 3D CAD files factories requirements met before they can quote tooling accurately: native or STEP files with correct scale, complete assemblies, defined tolerances, material callouts, and matching 2D drawings for critical dimensions. Send incomplete files and you get vague quotes, wrong prototypes, and weeks of back-and-forth.

The single most common delay in product development has nothing to do with factory speed. It is the file package the buyer sends. A factory cannot quote a mold, cut steel, or even tell you whether your design is manufacturable until the 3D CAD files factories requirements are satisfied, and most first-time importers have never been told what those requirements are. This article lays them out the way a factory engineer would if they had the time: which formats work, what has to be inside the files, and the mistakes that send your project to the back of the queue.

What file formats satisfy 3D CAD files factories requirements?

The safest format is STEP (Standard for the Exchange of Product Data), usually a .step or .stp file. Nearly every CAD system and every factory in China can open it, and it preserves solid geometry accurately. When in doubt, send STEP. It is the closest thing the industry has to a universal language, and meeting 3D CAD files factories requirements starts with using a format the factory can actually open without conversion damage.

Native files are the second option, and sometimes the better one. If your design was made in SolidWorks, many Chinese factories run SolidWorks too and prefer the native .sldprt/.sldasm files because they keep the feature tree, the design history that lets an engineer modify the part cleanly. The catch is version compatibility: a file saved in the newest release may not open in the factory's older installation. Ask which version they run before sending natives, or send both native and STEP and let them choose. Either way, confirming the format question up front is part of meeting 3D CAD files factories requirements without a wasted round trip.

Formats to avoid: mesh files like STL as the primary design file. STL describes a surface as triangles, which is fine for 3D printing a prototype but useless for cutting a mold, because the factory cannot reliably extract dimensions or modify features from a mesh. PDFs of renderings, screenshots, and hand sketches are not design files at all. A surprising number of buyers send a beautiful rendering and ask for a tooling quote, which is like asking a builder to price a house from a watercolor. The 3D CAD files factories requirements exist precisely because factories need engineering data, not pictures.

What has to be inside the files before a factory can quote?

Complete geometry comes first. Every part of the assembly needs to be modeled, including the parts buyers forget: internal ribs, screw bosses, snap fits, battery compartments, and the small features that make assembly possible. Factories quote from the files, so a missing rib is not a small omission, it is a part they will not tool for, and you will discover the gap when the prototype falls apart in your hands. One of the quiet 3D CAD files factories requirements is simply that the model represents the whole product, not the attractive outer shell, and buyers who internalize that rule avoid an entire category of quoting errors.

Correct scale and units come second. This sounds too basic to mention, and yet wrong-scale files are a weekly occurrence. A part modeled in inches but read as millimeters arrives 25.4 times too large, and the factory wastes days figuring out whether the design is wrong or the units are. Set the units explicitly, model at 1:1, and state the units in your cover message. It takes ten seconds and prevents one of the dumbest delays in the business, which is why units are spelled out in every serious version of 3D CAD files factories requirements.

Defined tolerances come third. A dimension without a tolerance is a suggestion, and the factory will pick whatever tolerance is cheapest to hold, which may not be what your product needs. You do not need to tolerance every feature like an aerospace drawing; you need to flag the critical ones: press-fit diameters, mating surfaces, thread engagements, anything where two parts must fit together reliably. Mark the rest as general tolerance per a standard note. Getting tolerances right is one of the highest-value 3D CAD files factories requirements, because it is where fit problems are prevented rather than discovered.

Material and finish callouts come fourth. The CAD file should name the material for each part (ABS, PC, nylon, aluminum 6061, and so on) and the surface finish (texture, gloss level, color references). A factory quoting "plastic housing" versus "PC with VDI-27 texture" is quoting two different products. If color matters, provide physical color chips or Pantone references by mail, because colors on a screen mean nothing across two monitors on opposite sides of the world. Material callouts are 3D CAD files factories requirements for a simple reason: without them the factory guesses, and factory guesses favor the cheapest option.

Matching 2D drawings come fifth, at least for the critical parts. A 2D drawing with the key dimensions, tolerances, and notes gives the factory engineer a single reference sheet to check the 3D model against, and it becomes the inspection document later. You do not need full drawing packages for every screw; you need them for the parts where a wrong dimension ruins the product.

Why do factories reject CAD files that look fine to the buyer?

The most common rejection reason is unmanufacturable geometry hiding inside a pretty model. Zero-draft walls on a molded part, undercuts with no release strategy, wall thicknesses that vary wildly across a single part, sharp internal corners where the mold cannot form material properly. The model looks complete on screen because screens do not enforce physics. Factory engineers run a manufacturability review (often called DFM) and send back a marked-up report, and each round of that review adds a week or more. The 3D CAD files factories requirements are not bureaucratic; they are the factory's way of making sure the geometry can survive contact with steel and molten plastic.

The second reason is assemblies that do not actually assemble. Parts modeled in isolation often interfere with each other when put together, or leave gaps where there should be contact. Run an interference check in your CAD software before sending anything, because the factory will run one and bill the discovery time back to you in delays. A clean assembly that fits together in software is the baseline expectation behind the 3D CAD files factories requirements, not an achievement, and it is the fastest way to earn a factory's confidence in your files.

The third reason is version chaos. Buyers send "final_v3" on Monday and "final_v3_REAL" on Wednesday, and the factory quotes or even starts work on the wrong one. Every file package should carry a clear revision label, a date, and a short changelog, and only one revision should ever be designated current. When a factory asks which file is correct and you take two days to answer, your project stops for two days. Revision discipline is one of the cheapest 3D CAD files factories requirements to meet and one of the most commonly ignored, which is exactly why it deserves a line in your process.

How do you prepare a file package factories accept on the first send?

Build the package as a checklist and run it the same way every time. Start with the 3D files: native format plus STEP export for every part, assembled in the correct positions. Add the 2D drawings for critical parts with dimensions, tolerances, materials, and finishes noted. Add a bill of materials listing every part, its material, its finish, and its quantity per assembly. Add color and texture references. Then write a short cover document: product name, revision, units, target quantities, and the three or four things you most want the factory to check. This cover document is what turns a pile of files into a quotable project, and it is the part of the 3D CAD files factories requirements that buyers most often skip, which is a shame because it is also the easiest to write.

Before sending, do your own pre-flight. Open the STEP files yourself in a fresh session (not the native assembly you have been staring at for weeks) and check that everything is there and at the right scale. Run the interference check. Confirm the units. This ten-minute routine catches the majority of the errors that trigger factory rejections, because most rejections are for things the buyer could have seen themselves. Think of it as the final item on your personal 3D CAD files factories requirements checklist, the one that verifies all the others.

Send the package to two or three factories, not ten. Each serious factory spends engineering time reviewing your files, and blasting the package to a dozen shops burns goodwill and returns a dozen inconsistent quotes based on different assumptions. A short list of vetted factories gets you comparable quotes you can actually evaluate. Sourcing Ally runs this kind of supplier sourcing with proper file handling as standard practice, making sure factories receive complete packages and that the quotes come back on comparable assumptions, with translation support when the technical discussion gets detailed.

Key takeaways

  • STEP files plus native CAD files are the standard package; STL meshes and renderings are not design files and cannot be used for tooling quotes. Format choice is the first of the 3D CAD files factories requirements for a reason: nothing downstream works if the factory cannot open your files cleanly.
  • Complete geometry, correct 1:1 scale with stated units, defined tolerances on critical dimensions, material and finish callouts, and 2D drawings for key parts make up the core 3D CAD files factories requirements.
  • Most factory rejections are for unmanufacturable geometry, parts that interfere in assembly, or revision chaos, all of which a buyer-side pre-flight catches.
  • Tolerances only need to be explicit on the dimensions that matter; mark the rest with a general tolerance note.
  • Send a short cover document with every package: product name, revision, units, quantities, and what you want checked.
  • Quote with two or three vetted factories on identical file packages instead of blasting incomplete files to a dozen shops.

FAQ

### What is the best file format to send to a Chinese factory?

STEP (.step/.stp) is the safest universal choice because virtually every CAD system opens it and it preserves solid geometry. If you know the factory uses the same CAD software you do, send the native files as well so their engineers keep the editable feature tree. Confirm version compatibility first. Always export and check the STEP yourself before sending, since export settings occasionally drop features or flip units. Format selection sounds trivial, but it is the 3D CAD files factories requirements item that causes the most avoidable first-round failures.

### Do factories need 2D drawings if I already have 3D CAD files?

For the critical parts, yes. The 3D model defines the geometry, but the 2D drawing is where tolerances, materials, finishes, and inspection dimensions live in one readable sheet. It also becomes the quality document the factory inspects against later. You do not need drawings for every minor component, but any part where a wrong dimension breaks the product deserves one. This is a standard part of 3D CAD files factories requirements, not an optional extra, and skipping it is one of the most common reasons first quotes come back wrong.

### How detailed should tolerances be in files sent to a factory?

Detailed on the dimensions that control fit and function, general everywhere else. Identify the press fits, mating surfaces, and thread engagements and give those real tolerance values; mark remaining dimensions with a general tolerance note. Over-tolerancing everything drives up cost because the factory has to hold precision it does not need, while under-tolerancing the critical few guarantees assembly problems. If you are unsure which dimensions are critical, say so in the cover document and ask the factory's engineers to flag them during DFM review.

### What is a DFM review and when does it happen?

DFM (design for manufacturability) review is the factory engineers' analysis of your files for problems like zero-draft walls, undercuts, uneven wall thickness, and assembly interferences. It typically happens after you send the file package and before tooling is quoted or started. Expect at least one round of DFM feedback on any original design; it is a normal part of the process. The review goes faster when your files already meet the 3D CAD files factories requirements, because the engineers spend their time on real design issues instead of file problems.

### Can a factory work from just a prototype or a competitor's product?

Some factories will reverse-engineer from a physical sample, and it happens often, but it introduces legal and quality risks you should understand before choosing that path. Reverse-engineering a competitor's product can infringe their intellectual property, so run a patent search before manufacturing in China if the design is not yours. Quality also suffers: measurements taken from a sample accumulate error, and you end up with a copy of a copy. Proper CAD files you own are always the better starting point.

Conclusion: good files are the cheapest schedule insurance you can buy

Meeting the 3D CAD files factories requirements is unglamorous work, and that is exactly why it gets skipped. But every hour spent on complete geometry, correct units, real tolerances, and a clear cover document pays back in weeks of avoided delay: faster quotes, cleaner DFM reviews, prototypes that fit together the first time. Factories move quickly for buyers whose files are ready and slowly for buyers whose files need rescuing. If you take one habit from this article, make it the pre-flight: open your own STEP exports, run the interference check, confirm the units, and only then hit send. The 3D CAD files factories requirements are not a hurdle the factory invented to slow you down; they are the minimum information any manufacturer needs to build your product right, and the buyers who treat them that way get better quotes, faster prototypes, and fewer surprises.