Precision of dimensions? Rounding error?

Hi there,

I’m an occasional user of Shapr3D on iOS and, more recently, on macOS. I really like the direction the Mac version is going in–I have high hopes for the interface redesign that will better adapt the program to Mac-like conventions, especially with regard to keyboard shortcuts and menu items–and hope to continue using it to design prototypes for use in lab/experimental setups for my work.

In the mean time, I have a question about the precision of dimensions in Shapr3D on macOS. When I enter a dimension such as 0.0135", the displayed dimension is 0.013". This behavior seems odd as I’d expect the value to round up instead.

This topic (Precise Dimensions) suggests that the real value is stored even though it is not displayed. However, the displayed value from that post (on iOS) is correctly rounded up, whereas the displayed value on macOS is incorrectly rounded down. I initially thought that the macOS version might simply truncate the number entered after two significant figures (instead of rounding), but a bit of experimenting shows that is not the case: 0.01349" correctly rounds down to 0.013", and 0.01351" correctly rounds up to 0.014". So it seems that the rounding may be occurring incorrectly for the specific case of 0.0135".

Is this a bug? Or have I misunderstood the expected behavior in this scenario?

The rounding rules we use (engineering team) agree with your observations, using the even/odd rule to on average be correct. Under that rubric, 0.0135" would round to 0.014" (odd digit to the left of the five, round up), and 0.0145" round to 0.014" also (even digit to the left of the five, round down). This follows the recommended (but not only) ASTM standard.

Thanks for your reply. That convention is helpful to understand (though since I’m a physicist by training, I’m in the habit of always rounding up regardless).

To be clear, I’m not seeing the behavior you describe in the Mac app. Instead, whether or not the digit to the left of the five is even or odd, the value rounds down. “0.0135” goes to “0.013” and “0.0145” goes to “0.014”.

From a user interface perspective, it would be nice if the exact value that I originally entered (and that is presumably stored in the background, unless I misunderstood that too) were to show up when I edit the dimension. That would at least help the user understand what was originally entered (avoiding loss of information), even when the displayed value is shortened.

More and more, trend seems to be to round up regardless - I’m told by my Quality team that it is because that’s how Excel works, and therefore we are stuck with it. I wonder if the app is just truncating and not rounding - if the significant digits were increased perhaps the next digit would appear to show this is the case.

Snippet from ASTM E29 attached, summarizing the rule. Ideally this could be a choice as well - ceiling function

on a “5” or round per ASTM.