Reverse Engineering: How I Got an OBJ File from a Client and Turned It into an Editable CAD Model.

Last week, a client sent me an OBJ file. At first glance, it looked like a 3D model. In reality, it was a raw cast of reality: a triangle mesh full of holes, irregularities, and chaos. And my job was to turn it into a proper, editable CAD model that the client could drop into an assembly and modify freely.

Step 1: An OBJ File Isn't a Model, It's an Imprint

The client sent me the file after scanning. It often happens that you don't participate in the scanning process; you just get the raw data. And believe me, it's always raw.

An OBJ is a triangle mesh that looks like a model, but it isn't one. It has millions of triangles, holes, jagged edges, and local distortions. You can look at it, but you can't edit it the way a CAD designer is used to.

At this point, some people tell the client: "That's what the scan looks like, it is what it is." This is where I start working.

Step 2: Stitching Holes and Straightening Geometry

A scan mesh always has holes. The scanner doesn't see everything. Undercuts, deep recesses, sharp edges at bad angles. This uneven struggle can take quite a while ;D

(Top left corner) Scanned part with visible mesh errors. (Bottom left corner) Corrected geometry of the detail. (Right side) Display with mesh orientation highlighted – this helps in finding holes in the model.

Closing holes is tedious work. It's not just about plugging an opening, but logically filling in the missing fragment. You often have to make decisions: does what's visible around it allow you to faithfully reconstruct the original, or is it better to introduce symmetry because the scan was mistaken in that area?

Controlling fillets and locally sharpening edges allows you to bring out the proper shape of the part.

Then there's geometry correction. Moving points, smoothing curves, removing artifacts. It's like editing a photo that turned out noisy. Every correction affects whether the final model will be suitable for production.

To simplify the final version of the model mesh, it's worth giving up edge loops in flat areas and areas less critical to the overall shape at an earlier stage.

For the flat key, the most attention went into the transitions between the shank and the head, and into precisely reproducing the teeth. Every tenth of a millimeter counts there.

Step 3: Triangle Reduction – Because Less is More

A raw mesh can have hundreds of thousands, sometimes even millions, of triangles. In theory, the more, the more accurate. In practice, your computer chokes, and you wait several minutes for each operation.

It was clear that the file was very complex and difficult to edit further, also due to its size. The number of triangles was reduced from nearly 2 million to 27,000 while preserving the model's geometry.

Reduction (decimation) is a process that reduces the number of triangles while preserving key details. As a result, the model becomes lightweight, responsive, and visually still looks like the original.

On the left, the model after 3D scan optimization. On the right, a further stage of work on the detail with highlighted edges resulting from the original geometry's shape.

After this operation, the mesh is ready for the actual conversion.

Step 4: Conversion to CAD – The Moment the Model Comes to Life

And here we get to the heart of the matter.

A triangle mesh, even after cleaning, is still just a collection of pixels in space. You can rotate it, zoom in, but you can't change the radius of an arc, you can't pull a dimension, you can't drop it into an assembly and check if it fits with other parts.

That's why the key step is conversion to a CAD format.

On the left (top left corner) you can see the part mesh exported directly into the CAD environment. Editing the model made it possible to achieve a fully editable part compatible with the CAD environment (bottom left corner). In the next step, I added a cutout with a fillet in the key body, according to the client's specifications.

After this step, every surface becomes editable. Arcs are parametric, dimensions definable. The model is fully readable by software like SolidWorks or Fusion 360.

The client doesn't get a frozen cast of reality. They get a tool that they can:

• drop into a larger assembly,

• modify without needing to go back to the scan,

• prepare for production.

This changes the perspective from "I have a scan" to "I have a model I can continue designing with."

What's Key?

Every radius, every transition, every asymmetry in the case of some scanned parts is something you can't easily model from scratch. Someone once made those decisions so that the part (in this case, the key) would function, feel good in the hand, and not break during use.

Reverse engineering isn't just about recreating geometry. It's also about reading intent. In the case of a key, it's not so obvious, but with architectural details of historical value, fighting for every imperfection is important and lends authenticity. That's why it's so crucial for the part to come out exactly as in the original.

https://video.wixstatic.com/video/4f1c42_14674441683a450c8c84aa27eb0ea4a5/1080p/mp4/file.mp4

Why This Matters

If you're designing something from scratch, that's fine, it's a different path. But if you want to use an existing product, adapt it to a new application, or integrate it into a larger system, without a proper CAD model, you're stuck.

A scan gives you a starting point. But only conversion to an editable format gives you control.

Practical Advice

If you're processing such files yourself:

1. Expect the OBJ to be dirty. That's normal. Don't stress about it, just fix it systematically.

   
   

2. Close holes at the beginning. Otherwise, they'll come back to haunt you in later stages.

   
   

3. Reduce triangles without overdoing it. Find a balance between file weight and detail.

   
   

4. Invest time in CAD conversion. This is where the model gains practical value. Without this, you just have a nice visualization.

If you made it to the end…

This post isn't about scanning. It's about what happens afterward. About how a raw scan file turns into a part that you can continue designing with.

If you have a scan file and are wondering what's next, or if you want to chat about how I approach conversion, feel free to reach out.

Write anytime.

P.S. The key was in the client's assembly within three days of me receiving the file. It worked without any modifications. And that's exactly what matters.