I've designed a complex model but had system units set to inches, while editing in mm, how can I rescale?

[alexJ682T]

Hello.

Somehow I've changed the system units to inches, and designed a complex model that way, while having edit units in mm. So, when I export model now, it is tiny and unusable. I've changed system units to mm, but exported model is still tiny, I need to re-scale it as I understand. How should I do this to preserve dimensions and make it normal size? 

When I go to measure tool, it shows normal dimensions of the model - 85-69-110 mm (approx). However, exported model dimensions are not 2.54 times smaller, like it would be in case of inch to mm conversion, it is much smaller, with height around 8mm. So how should I find out proper scaling ratio?

[MartinBeh]

System units are the internal numbers 3ds Max uses. Everything else is just UI labeling - which might be used by the exporter to multiply the internal numbers by some value (or to simply tag the exported file with some unit label), depending on the exporter.

If your system units are set to Inches (which is the 3ds Max default) and you created a box with "10mm" on each side, that box will have an internal length of 10/25.4 = 0.39 units inside of 3ds Max.

What export format are you using, and what software are you using to import this?

There is a good chance you can control what should happen with the numbers both during export and during import.

Can you please share a screenshot of the export options and also the import options?

One simple thing to quickly try: turn off any Display Units in 3ds Max (Display Unit Scale = Generic Units), so you directly see (and measure) the internal units; then measure in 3ds Max, and export again. 

[MartinBeh]

PS: Maybe the "Rescale World Units" utility is what you need, with a value of 25.4?

Make sure to only test this on a copy of your .max file... (I personally am always a little suspicious of any scale tricks in 3ds Max)

[alexJ682T]

Thanks everyone for your replies.

I'm exporting to either .OBJ or .STL and importing it into various slicing software for 3D printing or online file viewers. All show same issue - output drawing is tiny. In export options of .OBJ there is "scale" factor, but even setting it to 2 makes object maybe 20 times bigger than it should be.

For the re-scale option, I tried both 2.54 and 25.4 - none give valid dimensions. I know that by step by step approaching I might be able to find proper re-scale value, but I guess, there should be some logical, math friendly way of calculating the re-scale value?

What's interesting, the measure tool shows the proper dimensions, but they are all reduced maybe 10 or 20 times when exporting.

I'm attaching the file below, you can try to check it by yourself.

[MartinBeh]

OBJ and STL formats do not have any notion of units.

These file formats just carry numbers, and it is up to the importer to decide what these numbers are (inches, mm, yards...).

Opening your .max file tells me that your file units are: 1 System Unit = 1 Millimeter. And Display Units are also set to Millimeters so any number you see in the UI labeled "mm" is essentially just the internal system units, without any conversion.

Using the measure tool on Sphere001, I see dimensions of X/Y/Z = 85.7/70/110.3mm.

If I export that to OBJ (with export scale = 1.0) and then reset 3ds Max and import again, I do get a mesh that has exactly these dimensions, measured in system units.

Same is true if I import the OBJ into Meshlab and run the Geometric Measure command: "Mesh Bounding Box Size 85.686600 110.272003 69.952805"

So everything works as expected, nothing wrong with your scene.

You either have some unfortunate export options, or your slicer software does not load the data correctly.

It is possible that your slicing software (which one?) needs the numbers in the OBJ file to be in a different unit, e.g. it thinks the numbers are inches?

If so, scaling down your scene by a factor of 1/25.4 (e.g. in the OBJ export options) should do the trick. Alternatively you should be able to tell your slicer how to interpret the numbers it finds in the OBJ/STL file.

As I already mentioned: For you to be successful, there needs to be a.) proper numbers in 3ds Max, b.) correct export options/behavior, and c.) correct import options/behavior.

PS: The Sphere001 object has non-uniform scaling transformation applied to it, so this will make any further discussion even more complicated.

[MartinBeh]

Here is your mesh, exported to OBJ such that one unit in the OBJ corresponds to one inch.

Maybe your slicer likes that better?

[alexJ682T]

I'm using Chitubox slicer and Anycubic photon slider. Both have same issue.

This is not my first attempt to create something in 3D max and export it to printable object format.

In last 2 years, I've designed and printed maybe 700 objects in 3D Max and had zero issues like that.

I've tried to import that "fixed" model into chitubox and get exactly same error (see attached pic.)

I also tried to downgrade chitubox, install it on another PC - same result. All models, I've exported before, open fine in slicers.

For reference, I'm attaching a 3d object I've created couple of months ago, which exports from 3ds max and imports into slicer just fine. Maybe these files can give some hint what changed?

I've opened that "archie4" in 3ds max, exported it into .stl format, opened in chitubox and it opens in proper size.

[MartinBeh]

Opening your archie4.obj in Meshlab and running Geometric Measures gives me a Mesh Bounding Box Size of 10.0/5.98/18.0 - and I get the same values when importing the OBJ back into 3ds Max. 

How long (in mm) should the longest side of archie4 be? 18mm or 18cm?

Here I have imported both meshes from OBJ, but scaled up the archie4 mesh by a factor of 10 after importing:

Does this look correct to you?

[MartinBeh]

Does the OBJ file attached here load up correctly in your slicer software?

[alexJ682T]

archie4 long side is 18mm, and when imported that OBJ file into slicer, it imports exactly that size.

[alexJ682T]

No, that objection_cm imports exactly 10 times smaller it should be, but good thing is that at least now size is understandable, so I can scale it in slicer.

[MartinBeh]

Some more testing, now with the Ultimaker Cura slicer:

This is your archie4 OBJ mesh and the original objection OBJ export, without any additional scaling:

And this is the same archie4 OBJ mesh and the objection OBJ export scaled by a factor of 0.1 (10x smaller):

[alexJ682T]

Yes, top picture shows correct dimension relations of both objects, thanks for your help.

[MartinBeh]

---

@alexJ682T wrote:

archie4 long side is 18mm, and when imported that OBJ file into slicer, it imports exactly that size.

---

OK, then we know that your slicer wants the OBJ numbers to represent millimeters.

And the objection mesh is supposed to be 110 mm on its longest side, so it will be printed quite a bit larger than archie4, correct?

With that, I believe that the OBJ exported directly from your .max file, without any scaling, is the correct one to use. Attached is my exported data, for reference.

If that one does not work, I strongly believe the issue is with your slicer software. Maybe it has an option to downscale models that are rather large? At least Ultimaker Cura has such an option.

You can always try to import both OBJs into an empty 3ds Max scene, just to see what happens.

PS: On a side note: your archie4 model has its data quite far from the origin, whereas your objection model is nice and close to the origin. Not sure this matters, just thought to mention it.