STEP file BOM export: how to get a bill of materials from an assembly online
A supplier emails you a STEP file for a 60-part welded assembly and asks for a quote by tomorrow. Before you can price anything, you need a bill of materials — every component, how many of each, and a rough mass for costing the raw stock. The old way: open the model in heavyweight CAD, expand the assembly tree by hand, and retype each part into a spreadsheet. An hour gone, and one mistyped quantity throws the whole quote off.
There is a faster path. With a browser-based STEP file viewer you can open the assembly, let it read the structure, and export a clean BOM to CSV in a few minutes. This guide walks through what a STEP file actually carries, how a STEP file BOM export works, and where the common traps are.
What is a STEP file BOM, and what does the file actually contain?
STEP (ISO 10303, usually the AP242 or AP214 protocol) is a neutral CAD exchange format. Unlike a native SolidWorks or CATIA file, a STEP file is readable by any compliant viewer, which is why suppliers and customers trade them daily. An assembly STEP file stores each solid body, the assembly hierarchy, and — when the exporter includes it — product structure metadata like instance names and part identifiers.
A bill of materials (BOM) is the list that falls out of that structure: every unique part, the quantity of each, and usually a mass or material note for costing. The key word is unique. A bracket used eight times should appear as one BOM line with quantity 8, not eight separate rows. Getting that roll-up right by hand is exactly where manual BOM building goes wrong.
How to export a BOM from a STEP file in the browser
Step 1: Open the STEP or IGES file
Go to cadnexa.com, sign in with Google, and open the 3D Viewer. Drag your .step, .stp, .iges or .igs file in. The model renders in the browser — no desktop CAD licence, no install, and the file stays on your machine rather than uploading to a server. Rotate, pan and zoom to confirm you have the right assembly.
Step 2: Read the assembly tree
The viewer parses the product structure and lists every component in the model tree. Each node is a part instance. Use the exploded view to separate stacked components visually — this is the quickest way to catch a part that is buried inside a weldment and easy to miss when counting by eye.
Step 3: Let the BOM roll up quantities
CadNexa's auto-BOM groups identical instances into a single line and counts them, so a flange that appears six times reads as one row, Qty 6. For each line you get the part name, quantity, and an estimated mass. The mass estimate uses the solid volume from the geometry multiplied by a material density — change the assigned material (mild steel, aluminium 6061, SS 304, brass) and the weight column updates. For Indian shops, the auto-BOM can apply local material rates to give a first-pass material cost.
Step 4: Export to CSV
Click export and the BOM downloads as a CSV you can open in Excel or Google Sheets. From there it drops straight into your costing template, RFQ, or ERP import. Columns typically include item number, part name, quantity, estimated mass, and material — the fields a buyer or estimator actually needs.
Export a BOM from your next STEP file — free
Open a STEP or IGES assembly in the browser and download a clean CSV bill of materials in minutes.
Open the 3D Viewer — Free →What a good BOM export should include
| Column | Why it matters |
|---|---|
| Item / line number | Stable reference for quotes and POs |
| Part name or identifier | Traceability back to the model node |
| Quantity | The whole point — identical parts rolled up correctly |
| Estimated mass | Raw-stock costing and freight estimates |
| Material | Drives density, price, and process choice |
Notice what a STEP file usually cannot give you on its own: tolerances, surface finish, and the inspection characteristics on the 2D drawing. Geometry alone is not the full manufacturing definition. That is why a STEP BOM is the starting point for a quote, not the inspection plan.
Common mistakes when building a BOM from a STEP file
- Counting instances as unique parts. Eight identical bolts becoming eight BOM lines inflates your part count and confuses the buyer. Always roll up by geometry.
- Trusting an IGES tree. Many IGES exports flatten the assembly into one body. If the viewer shows a single node for a clearly multi-part assembly, ask for a STEP file instead.
- Assuming the mass is exact. The estimated mass is volume times an assumed density. It is excellent for quoting, but verify the material before you cut steel — a part modelled as aluminium but quoted as SS 304 is a costing error of roughly 3x in density.
- Skipping fasteners and hardware. Some suppliers model bolts, others leave them off and note them. Check whether standard hardware is in the model before you assume the BOM is complete.
- Forgetting the drawing. The STEP file gives you the parts list; the inspection requirements still live on the PDF drawing.
From BOM to inspection: the next step
Once you have the parts list, the quality side begins. The 2D drawing that accompanies the STEP file carries the dimensions and tolerances you have to inspect. CadNexa's auto-ballooning reads a PDF drawing with Smart Detect and Box+Balloon OCR, numbers every dimension, and builds an inspection sheet you can export to CSV — the same workflow covered in converting a drawing into an inspection sheet. You can start that directly from the auto-balloon tool.
So the full path from a customer file to a quote and a first article looks like this: open the STEP file and export the BOM for costing, then balloon the PDF drawing for the inspection plan. For a wider tour of every tool, the complete CadNexa guide and the learning center walk through each one.
Why a browser-based viewer for BOM export?
- No install and no per-seat CAD licence — open it on any laptop, including a buyer's machine that has no CAD at all
- Your STEP files stay on your computer; nothing is uploaded to a server
- Works on Windows, Mac, Linux and Chromebook
- Auto-BOM with quantity roll-up and weight estimates, exportable to CSV
- Free tier to start; paid plans from ₹399/month for heavier use
Stop retyping assembly trees into spreadsheets
Let the viewer read the structure and hand you a costed BOM you can quote from.
Try the STEP File Viewer — Free →Frequently asked questions
Can I export a BOM from a STEP file for free?
Yes. CadNexa's 3D Viewer opens STEP and IGES assemblies in the browser and builds an auto-BOM you can export to CSV. The free tier covers occasional exports; paid plans from ₹399/month suit teams running many quotes a month.
What is the difference between a STEP file and a BOM?
A STEP file is the 3D geometry and assembly structure of a part or assembly. A BOM (bill of materials) is the list extracted from that structure — every unique component, its quantity, and usually a mass and material for costing. The viewer derives the BOM from the STEP assembly tree.
Will a STEP file give me part weights?
The geometry gives you each solid's volume. Multiply by the material density and you get mass. CadNexa estimates weight automatically once you assign a material per part, so the BOM carries an estimated mass column. Confirm the material before final costing, since density varies widely between aluminium, steel and brass.
Why does my IGES file not show a parts list?
IGES is surface-based and many exporters flatten an assembly into a single body, so there is no hierarchy to roll up into a BOM. For a reliable parts count, request a STEP file (ideally AP242), which preserves the assembly structure.
Does the BOM include tolerances and surface finish?
No. A STEP file carries geometry, not the full inspection definition. Tolerances, surface finish and GD&T live on the 2D drawing. Use the BOM for costing and the ballooned PDF drawing for the inspection plan.