[OpenSCAD] Engineering Fits and Tolerance

Hugo Jackson hugo at apres.net
Thu Oct 31 16:45:47 EDT 2019



> 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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