[OpenSCAD] Engineering Fits and Tolerance

nop head nop.head at gmail.com
Wed Oct 30 05:22:49 EDT 2019

My experience is totally different. The only setting I change is I measure
the filament diameter when I start a new spool. I always model in
actually dimensions. I use polyholes for round holes. If I want an
interference fit I make the dimensions the same and they always press in,
with varying degrees of force.. If I want a loose fit I leave 0.2mm gap.
Since most of my holes are clearance holes using the standards for screws
they always work because the standard is quite generous. For example 3.4mm
for an M3 clearance hole.

If I want a super accurate hole, for example a pulley bore that is supposed
to be H7 I print it slightly undersized and ream it with an H7 reamer. That
actually makes the finished hole too small because of spring back in the
plastic. But because the plastic has some spring it make a very nice snug
fit on a motor shaft. It is is very had to even measure holes in plastic
because of this.

On Wed, 30 Oct 2019 at 04:57, Hugo Jackson <hugo at apres.net> wrote:

> > On Oct 29, 2019, at 8:40 PM, shadowwynd <shadowwynd at gmail.com> wrote:
> >
> > If you are printing a bulbasaur, slop in the printer doesn't make much
> > difference.
> I know what you mean, and let’s face it, the world can always use another
> bulbassaur if it’s cute! :)
> >
> > If you are building parts that have to work with other parts - there is a
> > dizzying array of variables.  Filament type / brand / color, humidity,
> flow
> > rate, print/bed temperature, layer height, Z-Offset, edge curl/elephant
> > foot, slicing algorithm, and even the infill percentage and support
> > structure can subtly alter the print.  There are days when I marvel that
> any
> > of this FDM stuff works *at all*.
> Indeed… but the machines ARE getting better… more reliable, more
> repeatability and as I mentioned
> With the advent of affordable DLP machines, we’re starting to move to some
> impressive precision.
> But as mentioned in this thread I think there’s a clear division of
> responsibility of the machine to print
> objects to the specifications it is given. All the factors you mention are
> relevant to getting a machine to
> print a part, but the goal is ultimately to get as close to the design
> specifications as possible, is it not?
> If I specify I want a part that’s 30 mm wide +/- .05mm then that IS what I
> want… if my machine isn’t capable of
> accomplishing that then I need to either find a machine that can produce
> those kind of tolerances or rethink my
> design.
> >
> > I can easily get 1mm+ difference between prints if I start changing
> > …
> > easily tweak a hole diameter by 0.2mm and it can apply to every hole in
> my
> > model at the same time.
> > diam = 10; // Ideal Diameter
> > thickness = 1;  //Height
> >
> > // Fudge Factors - Printing on MakerGear M2, 0.2mm layer height, brown
> > UltiMachine PLA
> > diam_e = 0.1;
> > thickness_e = 0.04;
> >
> > /*  // Fudge Factors - Printing on Makerbot, 0.3mm layer height, Hatchbox
> > white PLA
> > diam_e = 0.0;
> > thickness_e = 0.2;
> > */
> >
> > //Create Model
> > cylinder (d=diam+diam_e, h=thickness+thickness_e);
> >
> Personal experience shows me that I can get dimensioning of +/- .03 on a
> MakerGear M2, Prusa i3 Mk3 and a Monoprice mini-select and an XYZ DaVinci
> 1.0 Pro, for my
> larger formbot I can get around +/- .07 tolerance. But that involves a lot
> of time intensive measurement and tuning, but as I’ve identified, I’m not
> talking about Engineering Fits to address untuned printers, I’m just
> talking about incorporating tolerance dimensioning at a low level in
> OpenSCAD objects.
> I confess that when I’m doing development I actually use a second
> dimensioning assist that is all about compensating for an untuned printer,
> I call it $slop. But when a project is finished $slop gets set to 0 (or
> [0,0,0] actually… one for each vertex) and I work with the printer to get
> it to printer objects of the dimensions I’ve specified rather than
> having created a model that will only print accurately on the untuned
> printer I have sitting in front of me.
> As I say, engineering fit is NOT about adjusting an untuned printer, it’s
> just about the clearances we want our interconnecting parts to have.
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