Ben Sturgeon, Author at Ben Sturgeon Design

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 If something is missing, unclear or out of date, it won’t matter how good the design was.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.

Talk to Ben about your files

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.

Irwin clamp corner adaptor parts laid out on a white surface

Ask most people what product development will cost and they’ll think about the engineer’s rate. It’s a fair question. But after more than 15 years of designing products, I can tell you it’s rarely the thing that decides the final bill.

What really decides product development cost is the choices made at the very beginning. The material. The way it will be made. How many parts there are. How accurate each one needs to be. Get those right early and everything that follows is smoother, quicker and cheaper. Get them wrong and you’ll pay for it later, usually when it hurts most.

So rather than talk about prices, I want to walk you through how an engineer actually thinks about cost.

Where product development cost is really decided

When I started out in marine engineering, a young 20 something just out of university, I learnt this lesson quickly. A change on a sketch takes five minutes. The same change once the moulds are made, the parts are cut and the boat is half built? That’s a very different conversation.

It’s the same with any product. Every decision you make at the design stage locks in part of the cost of every single unit you’ll ever make. That’s why a good engineer will want to slow down at the start, ask a lot of questions and look at more than one idea before drawing anything in detail. It can feel like you’re not making progress. In reality, it’s where the biggest savings are made.

It all starts with a good brief

I always say the cheapest tool in product development is a clear brief. It doesn’t need to be long. It just needs to answer a few honest questions.

What does the product need to do, and where will it live? What does each one need to cost to make for the idea to work? Do you expect to sell ten a year, or ten thousand? Which features are essential, and which would just be nice to have? Will it need to meet any safety standards or carry a UKCA or CE mark?

That question about volume is the one most people forget, and it changes everything. The right design for 50 units is very often the wrong design for 50,000. Knowing it early saves a lot of redesign.

Why engineers work in stages

Good product development doesn’t happen in one long push. It moves in steps, with a pause to check at the end of each one. Engineers call these stage gates.

First, we agree what the product must achieve. Then we look at two or three ways of doing it and pick the best. Only then does the detailed design begin, followed by prototypes and testing, and finally the drawings and files that go to the manufacturer.

At each step, the design is checked against the brief before more money is spent. On more complex products, I’ll also run through what could go wrong and how badly, which engineers call an FMEA. It sounds formal, but really it’s just asking “what if?” before the customer does.

Working this way protects your budget. You never pay for detailed drawings of an idea that hasn’t been agreed.

Designing around the way it will be made

This is the part of the job I enjoy most, because it’s where the magic of an idea meets the reality of a workshop.

Every manufacturing process has its own rules. Sheet metal wants sensible bend radii and holes kept away from the bends. Welded fabrications need room for the welder to reach and a plan to stop the heat pulling things out of shape. Machined parts get expensive with deep pockets and lots of set ups. Injection moulded parts need draft angles and even walls, and every undercut adds to the cost of the tool.

3D printing is wonderful for prototypes and small numbers. However, the processes with expensive tooling, like moulding and casting, are the ones that give you the lowest cost per part when you’re making thousands. Choosing the right one for your volumes is one of the most valuable decisions in the whole project.

Prototypes with a purpose

Prototypes are where budgets most often run away, and it’s usually because a prototype is trying to answer too many questions at once.

I like to give every prototype one job. Does it look and feel right? Does the mechanism work? Can it actually be made by the process we’ve chosen? Each of those is a different prototype, and most products need more than one round. Planning for that from the start is far better than hoping the first one is perfect. It rarely is, and that’s absolutely fine. That’s what prototypes are for.

Keeping product development cost predictable

Engineering work is usually priced either by the hour or as a fixed price. Hourly suits early, exploratory work. A fixed price suits a clearly defined job. Lots of projects use both.

Whichever way you go, two simple habits keep things on track. Write down exactly what’s included before work starts. And agree any change in writing, with what it means for time and cost, before anyone acts on it. Projects rarely go over budget because of one big change. It’s nearly always lots of small ones that nobody wrote down.

If there’s one thing I’d like you to take away, it’s this. The rate on the invoice matters far less than the decisions made in the first few weeks. Get those right, and your idea has every chance of becoming a product that works, looks great and makes sense to manufacture.

Have an idea you’d like to talk through?

Send me your sketches, photos or notes and I’ll tell you honestly where you are and what the next step should be. You can also read about my product development services or see some real projects.

Talk to Ben about your idea

Author: Ben Sturgeon

CAD Files Explained. What Engineers Send to Suppliers and Why?