topic Re: Confused by fractional dimension rounding in Inventor Forum

Confused by fractional dimension rounding

tbishop00 — Tue, 27 Jun 2023 16:31:46 GMT

I have a part with a dimension of 11.71873 inches. When I dimension the part on a sheet, the dimension is erroneously rounded to 11 3/4", even when the dimension precision is set to 1/16". It's not until I change the dimension to 11.71862 that the dimension will correctly show 11 11/16".

How does Inventor round dimensions on drawings? I assumed it was simply "round to the nearest 1/16", but that doesn't seem to be the case.

Re: Confused by fractional dimension rounding

kacper.suchomski — Tue, 27 Jun 2023 17:53:11 GMT

In the dimension edit window, check the accuracy and tolerance tab. There are settings for dimension rounding.

Re: Confused by fractional dimension rounding

SBix26 — Tue, 27 Jun 2023 18:25:08 GMT

That's pretty weird-- I tried in 2024.0.1 and got the same results that you did, with the added observation that the Edit Dimension dialog shows the Model Value in fractions to the nearest 1/32" (where did that come from?):

I refined the cutoff a bit further-- it continues to round off to 11-3/4 even at 11.718628, but at 11.718627 it rounds to 11-11/16".

Sam B

Inventor Pro 2024.0.1 | Windows 10 Home 22H2

Re: Confused by fractional dimension rounding

Frederick_Law — Tue, 27 Jun 2023 19:27:13 GMT

There was another thread about rounding, in iLogic.

https://forums.autodesk.com/t5/inventor-ilogic-and-vb-net-forum/ilogic-why-don-t-user-parameters-work-with-arithmetic/m-p/11980478#M152941

https://en.wikipedia.org/wiki/Double-precision_floating-point_format

Part of the problem is base-2 estimation of base-10 number.

Re: Confused by fractional dimension rounding

tbishop00 — Tue, 27 Jun 2023 21:50:13 GMT

I don't think it's a floating point error. I tested this with a simple iLogic rule, and it outputs the correct value for the rounded string. A value of 11.71873" outputs a string of 11-11/16" as expected.

l_feet = Floor(SheetMetal.FlatExtentsLength / 12) l_inch = SheetMetal.FlatExtentsLength Mod 12 l_inchS = RoundToFraction(l_inch, 1/16, RoundingMethod.Round) length = String.Format("{0}'-{1}""", CStr(l_feet), l_inchS)