The crane operator does not trust it. The lifting plan says 3,850 kilos, but the machine hangs lopsided in the slings and the load cell reads well over 4,300. Work stops, the centre of gravity has to be recalculated, heavier slings have to be arranged. Afterwards the cause turns out to be small: two base plates were still on the template's default material, and a purchased motor sat in the assembly as an empty STEP file with no mass at all.
Transport weight, lifting plan, cost price and quotation all lean on the mass that comes out of your CAD model. If weight and material in your BOM are off, everyone downstream calculates with a wrong number, from the estimator to the haulier. In this article you will see where those deviations come from in Inventor, how to get material and density structurally right, and how to sweep a complete assembly in bulk, helped along by a toolbox like Thundercad.
Where the weight in your model comes from
Inventor calculates the mass of a part in a straightforward way: volume times density. The volume follows from the geometry, the density from the material assigned to the part. The assembly then adds up all the parts. Every weight deviation therefore traces back to one of three things: geometry that is wrong, a wrong or missing material, or a number someone once typed over by hand.
That sounds manageable. But in an assembly of a few hundred parts those error sources run through each other, and none of them shows up on the drawing. The BOM simply displays a total, and that total looks equally trustworthy whether it is right or not. So the trick is not to find that one mistake, but to know the patterns by which mistakes creep in.
The five classic causes of a deviating weight
- Default material from the template. A part that never consciously received a material quietly calculates with the default. In steel construction the difference is often just too small to notice, and that is precisely why it stays.
- A wrongly chosen or wrongly created material. Stainless instead of structural steel, or a custom material where someone entered the wrong density when creating it. Being off by a factor of a thousand, because kilograms per cubic metre got confused with kilograms per cubic decimetre, happens more often than you would think.
- Overridden mass. Someone once entered a value by hand, for example because the model was not finished yet. The model kept growing, the override stayed put and never responds to changes again.
- Empty purchased parts. A downloaded STEP file of a motor or gearbox has precisely the right shape, but no usable material. Without a mass filled in, such a part counts for next to nothing, while in reality dozens of kilos are hanging there.
- Parts that do not count. Items set to reference or phantom, suppressed pattern occurrences and fasteners that only exist as a note on the drawing all fall outside the sum.
If two or three of these look familiar, you immediately know why the total weight of a frame rarely comes out right first time: they are small deviations that sometimes cancel each other by accident and sometimes add up hard.
Where a wrong weight hurts
The most visible place is the site or the dispatch bay. A machine that is hundreds of kilos heavier than the lifting plan claims means waiting for heavier rigging and a discussion about who should have caught it. In transport, weight partly determines the choice of truck and route; a wrong number gets discovered on the weighbridge, at the worst possible moment.
Less visible, but more structural, is the costing side. Anyone who runs kilos of steel through the cost price carries every weight error from the model into the quotation. Suppose, as an assumption, that a recurring frame sits ten percent too light in the list because of two empty purchased parts: that same deviation then lives in every quotation that frame appears in. How to connect model and costing so this data flows through reliably is covered in Faster and more accurate costing with data from your CAD models.
And then there is release itself: a work planner who has been handed a wrong total once will recheck everything afterwards. That costs hours every week that a trustworthy model would save you.
Getting material and density structurally right
The remedy starts with one agreement: material is always picked from the shared library, never typed in loosely as a name or a density. One source prevents three variants of the same steel floating around with slightly different properties. How to set up that library and agree on one name per material is described in Recording materials consistently: one name per material; here the point is that everyone actually draws from it.
Three habits make the difference in daily practice:
- Put a recognisable dummy material in your templates as the default, with a name that shouts that a choice still has to be made. A part that accidentally stays on the default then stands out in every list.
- Only allow overridden mass deliberately. For purchased parts an override is actually good practice: enter the supplier's mass and record in an iProperty that it is a manual value, so nobody mistakes it for a calculated one.
- Update the mass of the assembly before anything goes out the door. A model that was never updated cheerfully shows the weight of three revisions ago.
Want to try these checks on your own assemblies instead of on paper? The toolbox can be tested for a month, no strings attached, right inside your own Inventor environment.
Try 30 days freeSweeping a complete assembly in bulk
Opening parts one by one to check the material is not workable: on an assembly of four hundred items you lose half a day and still miss a few. What you want is the properties of the whole assembly in a single overview. With Assembly iProp Menu from Thundercad you update the iProperties of a complete assembly in bulk: you see in one list what has been filled in per part, filter for the outliers and correct a whole selection at once instead of file by file.
The order that works well in practice: first filter out all parts still on the dummy material and give them a real one, then walk through the purchased parts for a filled-in mass, and finally check the items set to reference or phantom. Three focused passes of a few minutes each, instead of one long unfocused hunt.
Make the check part of your release
It only becomes structural once the check has a fixed moment. The natural moment is release: the BOM is going to work preparation and purchasing anyway, so let the weight ride along in that same round. With Export BOM you push the bill of materials to Excel in your own template in a single action, with material and mass as columns side by side. In Excel you spot the problems at a glance: empty material fields, suspicious zeros, outliers.
Keep this up for two or three projects and you will notice the list of outliers getting shorter each time. Not because the checking got stricter, but because the errors vanish at the source: templates are set up properly, purchased parts get a mass on arrival, and picking material from the library has become a habit.
Frequently asked questions
Why does a part show zero kilos even though it has a material?
Usually the mass was once manually overridden with zero, or the part has no closed volume, for example an imported file consisting only of surfaces. First check whether an override is active and remove it, then check whether the model actually contains a solid volume.
Should I override the mass of purchased parts?
Yes, and do it deliberately. A downloaded supplier model is usually a simplified shape without internals, so the calculated mass says little. Take the value from the supplier documentation and record that it is a manual value, so the override does not silently survive a later change.
How often should I check weight and material?
Tie the check to release, so it cannot be forgotten and costs ten minutes per project at most. If you want to see how such a bulk check with Assembly iProp Menu and Export BOM plays out on your own assemblies, you can try the toolbox free for 30 days.