There’s a moment at the end of every design project that I’ve always loved. The model is finished, the drawings are checked, and the CAD files are ready to go out into the world. It’s the point where an idea stops living on a screen and starts becoming something real.
But here’s the thing. Those CAD files are now the only instructions your supplier has. They’ll quote from them, make your parts from them and check the finished parts against them. If something is missing, unclear or out of date, it won’t matter how good the design was. The parts will be wrong.
So let me explain how engineers look after CAD files, which ones suppliers actually need, and the little mistakes that cause the biggest headaches.
Working files and released CAD files
Every engineer has two kinds of file. There are the working files, which change every day while the design develops. Then there are the released files, which have been checked, approved and given a revision letter. Only released files should ever go to a supplier.
It sounds obvious. Yet one of the most common problems in manufacturing is a supplier quoting from a model that was still being worked on. The design moves on, nobody tells them, and the wrong parts turn up.
Native files and STEP
Native files are the original files from the designer’s software. In SolidWorks, which is what I use, those end in .SLDPRT for parts, .SLDASM for assemblies and .SLDDRW for drawings. Because they keep the whole history of how the model was built, they’re the easiest to change later. However, you need the same software to open them.
STEP files are the common language that almost every CAD system understands. They hold the exact shape of each part, without the history. There are a few versions of STEP, and it’s worth knowing which one to ask for. AP203 is the original and carries the shape only. AP214 adds colours and layers. AP242 is the newest and can even carry tolerances inside the model.
If you’re not sure which one your supplier wants, just ask them. And always check the units when exporting. A part drawn in millimetres and opened in inches will arrive 25.4 times too big, and yes, it does happen.
Why the drawing still matters
A 3D model shows the shape. A drawing tells the maker what really matters. I think of it as the conversation between the designer and the person on the shop floor.
In the UK, drawings normally follow BS 8888, which is based on the international ISO standards. A good drawing states whether it uses first or third angle projection, because mixing them up can give you a mirror image of the part. It includes a general tolerance note, often ISO 2768, so the supplier isn’t left guessing on the dimensions that aren’t marked. It shows where measurements should be taken from, the material, the finish and the revision.
One of the habits that separates an experienced engineer from a new one is knowing where not to put tight tolerances. Putting very fine tolerances on every dimension doesn’t make a part better. It just makes it slower to make, harder to inspect and more expensive.
It should also be clear which one is in charge, the model or the drawing. Many suppliers machine from the model and inspect to the drawing, so the two always need to agree.
Sheet metal and the bend allowance
Sheet metal is a lovely example of where the beauty of a design meets the reality of the workshop.
When metal is bent, the outside stretches and the inside squashes. So the flat shape that gets laser cut has to allow for that, using what engineers call a bend allowance or K factor. The right value depends on the material, its thickness, the bend radius and the press brake tooling your fabricator actually has.
If the CAD model uses a different value to the fabricator, the finished part will be the wrong size, even though it looked perfect on screen. The answer is simple. Talk to the fabricator first, agree the bend radius and K factor, and then produce the flat patterns.
Which CAD files your supplier will need
Different suppliers work from different files. This is roughly what each one will ask for.
| Who is making it | Files they usually need |
|---|---|
| Laser or waterjet cutter | DXF profiles, plus a PDF drawing for material and quantity |
| Sheet metal fabricator | DXF flat patterns, STEP model and PDF drawings with bend details |
| Welding and fabrication shop | PDF fabrication drawings with weld symbols and a cut list |
| CNC machine shop | STEP model and a PDF drawing with tolerances |
| Injection mould or casting supplier | STEP model and a PDF drawing showing critical features |
| 3D printing service | 3MF or STL file. 3MF carries units, STL does not |
| Another design engineer | Native CAD files and STEP |
Keeping track of changes
Every released file should carry a revision letter and a note of what changed, and the model and its drawing should always match. Larger companies use dedicated software to manage this. On smaller projects, clear file names, a revision table on each drawing and a simple change log do the job perfectly well. What matters is that anyone, including your supplier, can see at a glance which version is the latest.
Before you hand anything over
Before the files go out, it’s worth agreeing a few things with your designer. Which formats you need, and who will use each one. Who owns the design and the files once they’re paid for, and getting that in writing. Whether you need the native files as well as STEP, especially if someone else may edit the design one day. And how future changes will be recorded.
None of this is complicated. But getting it right is what turns a lovely design into parts that arrive right first time. For me, that’s the whole point. The magic of the idea, made real, and made properly.
If you’d like to read more about the format itself, the STEP standard (ISO 10303) is published by ISO.
Not sure your CAD files are ready?
Send them over and I’ll have a look for the common problems in this guide. You can also read about my CAD design services.
About the author. Ben Sturgeon is a mechanical design engineer with a BEng (Hons) in Mechanical Engineering. His career began in marine engineering, and over 15 years he has designed for agricultural machinery, marine, and electronics and lighting. Read more about Ben, or email him at ben.sturgeon.design@gmail.com.