[OpenSCAD] Engineering Fits and Tolerance

nop head nop.head at gmail.com
Thu Oct 31 17:00:34 EDT 2019


So basically you are just using the nominal clearance value for the fit,
not the tolerance specification.

I think it would be a bit odd to bake this into OpenSCAD. It can just be
done with a library of functions.

On Thu, 31 Oct 2019 at 20:46, Hugo Jackson <hugo at apres.net> wrote:

>
>
> > On Oct 31, 2019, at 12:44 PM, nop head <nop.head at gmail.com> wrote:
> >
> > But again I think you mix up tolerance and clearance. When a part has an
> ISO tolerance like g12 it is a specification that says the size will be in
> a range. How can you create an STL that represents a range? It can only
> represent a target size.
> >
> > What would the target be?
>
> After I’ve done all the bed levelling and z-level adjustment and extrusion
> width compensation and god knows what other tweaking is required I print
> off a cube. I then use a micrometer to measure the width, depth and height
> of the cube in 9 different places for a total of 27 different readings.
> I enter this information in a spreadsheet and calculate the min and max
> values to determine the amount of compensation needed for each axis. I then
> use the M92 gcode to provide axis compensation aiming to have a centre
> point with equal tolerance on each side… e.g. 15mm +/- .3mm… after having
> done that I reprint the cube to ensure that everything worked as expected
> and then I’m finally ready to start printing.
>
> So the short answer is, the dimensioning of the geometry in the STL file
> is the mid-point of the tolerance range for the specified engineering fit.
> If the tolerance range for a given fit is greater than the tolerance range
> that a printer can produce then I find that more often than not I am
> producing 3D printed parts within the tolerance range of the ISO standard I
> have specified for geometries within the part, and I’m able to produce
> those parts reliably and consistently across a number of different machines.
>
> Of course, no printer bats a thousand, it just takes a little blob to
> throw the tolerance for that face in one print totally out of whack, but
> that in and of itself is not uncommon in regular machining… and if one
> really wants to get picking you can implement go/no-go quality control etc.
>
> I’ve never used or even investigated firms like Shapeways or any of these
> other places that will do your 3d printing for you, but my assumption would
> be that they would be offering clearer guidance for submission than simply,
> “you’ll want to oversize your holes slightly if you intend for a shaft to
> be inserted”.
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