What are the units of - mm or inches?

[scoy]

When I edit the dimensions of the height or width - what unit am I using?

[Jayesh Salvi]

That's a good question and a frequently asked one. 

Short answer

It only matters if you are exporting the model. If you are exporting it, then it matters how the 3D printer or other CAD tool in which you import the model interprets the units.

Long answer

First, units - as expressed in real world units (mm, inches) - will matter only when you export a model made in 3DTin to use it in another CAD program or print using 3D printer or printing services. If you are saving and reusing models inside 3DTin, unit is an abstract notion, which remains uniform throughout 3DTin but doesn't have to have any correlation with real world units.

Let's assume you want to export the model. If the file format you are using allows mapping to real world units, then 3DTin lets you decide how long the 1 unit will be in real world. You will see this option in export dialog, as show in the screenshot.

It's surprising however to note that, only DAE file format supports such mapping. Both STL and OBJ treat unit as abstract entity.

However you may ask: Printing services like i.materialise and Shapeways let you upload an STL file and ask you whether the units are in mm or inches. What am I supposed to say then?

As I said STL and OBJ only have abstract unit. When you tell the printing service that your model is in "mm", it will assume that 1 abstract unit in STL/OBJ file you uploaded corresponds to 1 mm. If you choose "inch", it will map 1 abstract unit to 1 inch. It's a convention, they could map 1 abstract unit to 3 mm, but they follow what's the obvious choice. Moreover it hardly matters eventually, because they give you a sliding control with which you can scale the model. If it turns out that, with the assumption of 1 abstract unit = 1 mm, the overall volume of the model is too small, then you can magnify the model with the sliding scale. This is true with any other CAD software as well. They all have ability to scale the model up or down. 

Right now if you create a cube that is 1 unit long, 1 unit wide and 1 unit deep; save and export it to i.materialise; it will measure 1.5mm x 1.5mm x 1.5mm on their website. We choose 1 abstract unit in 3DTin = 1.5 abstract unit in STL = 1.5 mm in i.materialise, because for typical size models this gives reasonable final volume. But hey, they have a sliding scale control. If you imagined the cube to be a sugar cube, then you could scale it down. If you imagined it to be a paper weight, you could scale it up. 3DTin could have told you 1 unit is 1 cm, but does it matter any more? With your printing service provider you can interpret that 1 unit to be of any length in real world units that you like.

While designing in 3DTin you only need to worry about relative lengths of different parts. For e.g. if my sports car model is to be 10 units tall, how many units should the diameter of the wheel will be? Only when you decide to print the model would you care it it's 10 cm tall or 4 inches tall and you printing software or service will let you decide that then.

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I know that you would have liked a straight answer that 1 unit = 1cm and 3DTin could as well have said so; but at the time of export that promise couldn't have been fulfilled because not all file formats allow mapping to real world units. So instead of misleading, I tried to explain what goes on underneath.

If we can simplify this in future, we will. Till then, hope this helps.

--
Jayesh
Lead Developer, 3DTin

On Fri, Aug 10, 2012 at 3:15 AM, scoy <coy.holl...@gmail.com> wrote:

When I edit the dimensions of the height or width - what unit am I using?

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[MrVPhysics]

Hello, Thank you JYRO for your answer.

However, the download window I see does not have this slider scale, just the file type selection buttons. Has this slider removed since the last post?
If the slider has been removed, how does one go about creating parts to mate with real world pieces...

For example, say I have a bolt hole pattern of specific dimension, how do you recommend I successfully make a mating plate?

Thank you!

On Thursday, August 9, 2012 2:45:44 PM UTC-7, scoy wrote:

When I edit the dimensions of the height or width - what unit am I using?

[Jayesh Salvi]

Hi,

The slider will only appear for DAE file export, because that's the only file format that allows mentioning units.

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Jayesh

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