change units and scale from INCH to MM

[armansesar]

Good afternoon all, I know this has been discussed, but I'm currently unable to change the units and scale of a model using the normal process from document settings.

I have opened a model that was designed in mm to design around, however my application options are usually set to Inches. I know its best practice to initially switch the dimensions, but even then, Inventor simply changes the units without updating the scale of the imported model. Meaning 10 inches which I hope would translate to 10mm is now 250mm...etc (units were changed and converted, but scale remains the same). Direct edit is not a solution because I would like all drawings to update as well, not just the final product. 

Inventor forum suggests simply changing the units in document settings and scaling should change automatically, but this is not the case, at least with Inventor 2025.

Also, how can I open a mm model in mm when the settings are set to inches to avoid having to rescale to begin with? Why does inventor open the model and scale up when converting to inches rather than just changing the unit?

I know this is a simple operation in AutoCAD, if drawing is imported in the wrong units for example, but any suggestions on how to do this in Inventor?

thanks in advance.

[pcrawley · Accepted solution]

"however my application options are usually set to Inches" I'm not sure that's the case because there isn't an option in 'Application Options' for units.  I suspect your default templates are set (in Document settings) for Inch - which is why you think its an application setting.

The unit settings in Inventor are per document.  So, Document Settings is where you change them.  An assembly may be inch, but the parts within it may be mm, in, cm or any of the other units of measure available etc.

Inventor won't scale the part if you change document units - so if that's your expectation, you will be a bit disappointed.  If you switch the Document Settings on a part 1in long, it will become 25.4mm.

If you want to scale the model... derive the model into a new assembly and set the scale in the derive dialog box.

[armansesar]

Good to know, my experience with AutoCAD has caused me develop unreal expectations 😄 as you can certainly have drawings scale as you change units automatically. I suppose certain constraints maybe why Autodesk does not offer this for Inventor. Wish they added this as a feature, I've noticed step files always open in the wrong scale if the units don't match and have not found a way to correct this other than manually as you suggested. Good to know that its definitely is not offered at least.

[armansesar]

I felt that there maybe an underlying application setting because step files open in the correct scale and units in some of the other softwares I use, such as my slicer or even fusion 360, figured either that or inventor simply opens the file according to whatever document setting you used most recent, however this does not seem to be the case as I've tried changing the document setting then open a native mm file, and it still opens scaled up.

[pcrawley]

There are some things that are easier to do in AutoCAD.

Regarding the scaling of STEP files - you can set your length import units here:

You can select bodies to import/exclude, filter entities, prefix/suffix body names, and also link the STEP file into Inventor so when the source changes, your Inventor model updates... Can't do that in AutoCAD 😉

[armansesar]

Makes sense, that's a good way to open them from now on, thanks.