Learn how to Untrim surfaces in Fusion

Working with an existing complex surface model can be daunting. But what if you could strip it back to its core shape without hours of manual edits? That's where the Untrim command in Fusion shines. It lets you reclaim simplicity for tasks like building molds or forming tools. In this guide, we'll walk through turning a detailed imported model into a clean base form. You'll learn to use Offset Face, Untrim, and Trim together. By the end, you'll handle tough geometry with ease and speed up your workflow.

Section 1: Preparing the Surface for Simplification

Start by grabbing the right faces from your model. In Fusion, head to the Surface tab and pick Offset Face. This tool copies a face without changing the original. Set the offset distance to zero. That way, you get an exact duplicate on a fresh surface body.

Hide the original solid body now. This keeps things focused and avoids confusion as you build.

Section 2: The Core Technique – Unraveling Complex Curvature with Untrim

Now dive into Untrim. Go to the Modify menu, select Untrim, and click your offset face. Fusion removes all the extra bits like holes and tight bends. It pulls the face back to its basic curve. That's the magic—it simplifies without wrecking the shape.

You also have the option to Extend the face. Set it to 10 mm, for example. This stretches the edges out to overlap with nearby faces. Those overlaps help later when you trim everything together. Repeat this on multiple faces to build the shell of the part. Select a few at once, like side panels, and apply Untrim. Keep the extend at 10 mm. Turn on other surfaces to check intersections.

You end up with an surface faces that hug the original curves.

Section 3: Constructing the Envelope with Boundary Surfaces

Some faces throw curveballs. Take the front one in our example. If you untrim it straight, it turns into a full cylinder. We don't want internal faces inside of our model. So, instead of Untrim, we will use Extend instead.

Select the top edge as your reference. In Untrim, enable Extend and drag from that edge. Fusion offers choices: Tangent for smooth joins, Perpendicular, or Natural to follow the curve's flow. Go with Natural—it keeps things true to the surfaces direction.

Now you have extended surfaces that cross each other.

Think of it as sketching boundaries on a map. Each surface marks a limit, and together they outline the territory.

Section 4: Precision Trimming to Refine the Simplified Body

Trimming cleans up the mess. First, understand the tool's flow. In the Surface tab, pick Trim. Select your cutting surface as the Trim Tool—say, a large extended face. Then, choose Surfaces to Remove. Click the parts you want gone. They highlight red, and the tool slices them away.

Start with the top overhangs. Use a side surface as the cutter. Walk around and click each unwanted bit. Do the same for the bottom—select the cutter, pick overlaps, and trim.

For walls that hug curves, it's more targeted. Pick a vertical face as the Trim Tool. Click just one intersecting surface to remove. Repeat around the edges. Each click nips away extras without overcutting.

• Always select the keeper face first as the tool.

• Zoom in for precise picks on curves.

• Undo if a trim goes too far—easy fix.

This step turns loose pieces into a snug fit. Your simplified shape starts to emerge, ready for the final touches.

Section 5: Finalizing the Simplified Model for Manufacturing Use

Surfaces alone won't cut it for making. Use Stitch to join them. In the Surface tab, click on Stich, then draw a box around all your pieces and Fusion welds them into one solid body.

Now you can match key details from the original design. Inspect a fillet on the source model. Our example shows a 6 mm radius. Turn on your new solid, go to Modify > Fillet, and apply 6 mm.

This solid becomes your MDF form base. Lay sheet metal over it, press or stamp, and shape follows. For indentations, use the original as a guide. They act like cutters to refine your form.

Select faces from those dips carefully. Draw a box, but it might grab too much. Switch to Selection Filters in the Select menu. Turn off Select Through. Now your box picks only top faces. Untrim that group to erase holes.

Back in Solid tab, use Split Body. Pick the untrimmed surface as the splitting face. It carves out the indents on your form. Rename bodies for clarity: "MDF Form" and "Removed Piece."

Conclusion: The Power of Surface Regeneration

This Untrim workflow changes how you tackle complex models. Skip deleting features one by one. Instead, regenerate the base shape fast. You save hours and keep the design's intent alive.

Use it for reverse engineering a part. Or build mold bases quickly. It's great for concept sketches too, where details distract. In manufacturing, it speeds up prototyping—like our MDF form for metal bending.