[OpenSCAD] Engineering Fits and Tolerance

Hugo Jackson hugo at apres.net
Tue Oct 29 19:25:13 EDT 2019

This is the wikipedia article on the matter:

https://en.wikipedia.org/wiki/Engineering_fit <https://en.wikipedia.org/wiki/Engineering_fit>

It doesn’t apply to just threads, but to holes and shafts as well, and is easily expanded to include regularly shaped polygons as well (cause like in triangle world a rectangle is just a 4 sides circle :) ) and it’s used to specify the clearance between two mating objects.

The ISO specification uses capital letters to specify the tolerance range for holes, and lower case letters for shafts. Although the system could be used to fudge for over/under extrusion situations, that’s not it’s intended usage. It’s purpose is to remove the ambiguity of expressions like “loose fitting” and “tight fitting” and you often encounter both the ISO and ANSI tolerances in blue prints and machine drawings.

A pointed out in the article, for example, a paired specification like “H7/k6” on a nominal 50 mm diameter would identify a tolerance range of
+0.000 mm to +0.025 mm and a tolerance range of -0.018 mm to +0.002 mm for the shaft.

I find that I can generally hold my most often used FDM 3d printer to a +/- 0.03 range, so while “H8/f7” would be too exacting a specification for my machine, there are tolerance ranges that could be used for most FDM and certainly DLP printing.

> On Oct 29, 2019, at 4:07 PM, adrianv <avm4 at cornell.edu> wrote:
> Well, if it's supposed to be like a screw thread tolerance then the letter
> specifies a "tolerance position", which gives a maximum diameter for the
> bolt shaft.  The number gives the tolerance range below this maximum.  One
> could imagine using this to shrink models to compensate for overextrusion. 
> But this is a specific standard for machine screw threads and the value of
> the tolerance even depends on the screw shaft diameter.   It doesn't apply
> broadly to arbitrary objects.  (It doesn't even apply to nuts, which use
> capital letters.)  
> Even if you could figure out how to define it for arbitrary objects, how
> would OpenSCAD know whether a component needed to be enlarged or shrunk?  I
> don't see that there's any way to do this automatically.  
> nophead wrote
>> Not sure I understand. h7 specifies a tolerance, i.e. a range of possible
>> sizes that can be tested with a limit gauge or a micrometer. How can that
>> be expressed in a model, which always has a definite size?
>> On Tue, 29 Oct 2019 at 22:00, Hugo Jackson <
>> hugo@
>> > wrote:
>>> Has any thought been given to incorporating engineering fits and
>>> tolerances into OpenSCAD? As I understand it, OpenSCAD was originally
>>> conceived for the creation of STL files for a 3D printer and I’ve been
>>> thinking that the base functionality might be improved by a little more
>>> CAM
>>> in the CAD.
> --
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