Stackup problem with small parts?

I've got a small part, an eccentric.  0.375" thick.

I multiply each parameter by 25.4 to get into metric units.

It consists of these layers:

EccentricSheeveThick = 0.078125
EccentricSheeveThick = 0.078125
EccentricGrooveThick = 0.0625
BossLength           = 0.15625

Which when stacked, should equal the total length (the eccentric thickness).
EccentricThick       = 0.375  total

But in the SCAD file, the center cylinder (bore) is taller then the stack up
of the 4 parts, seen on the rendered graphic.

A DXF exported to and then scaled back to inches ( scale 1/25.4) shows each
of the layers as being shorter then specified.  

Stackup?  Or hopefully did I make an error?

Thanks for looking.




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Sharing the source code will help.



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Sure,

Here's the source code for eccentric:


/*
 * Horizontal Steam Engine eccentric  
 *
 * eccentric
 *
 * 03/23/2018 alm
 */

scale = 25.4; // for imperial
$fn = 100;

EccentricSheaveDia = 0.625 * scale;
EccentricSheeveThick = 0.078125 * scale;

EccentricGrooveDia = 0.500 * scale;
EccentricGrooveThick = 0.0625 * scale;

EccentricOffset = 0.09375 * scale;
EccentricBore = 0.28125 * scale;
BossDia = 0.4375 * scale;
BossLength = 0.15625 * scale;

EccentricBossSetscrew = 0.089 * scale;
EccentricSetScrewLen = 0.375 * scale;
EccentricSetScrewOffset = (BossLength + EccentricSheeveThick) / 2.0 * scale;

EccentricThick = 0.375 * scale;

//projection(cut = true)  // project X - Y plane for creating DXF
//  rotate([0, 90, 0])   // flip over if needed

    Eccentric();  // top level function
   
//  ========================================================
module Eccentric()
    {
    translate([0, 0, EccentricSheeveThick / 2.0])  // get 1st sheave bottom
on XY plane to simplify calcs
      {    
      difference()
        {
         union()
           {
        /* Sheeve */
         translate([EccentricOffset, 0, 0])  
           linear_extrude(height = EccentricSheeveThick + 0.2, center =
true)    
             circle(d = EccentricSheaveDia, center = true);
     
        /* Groove */
         translate([EccentricOffset, 0, EccentricSheeveThick])  
           linear_extrude(height = EccentricGrooveThick + 0.1, center =
true)    
             circle(d = EccentricGrooveDia, center = true);
     
        /* Sheeve */
         translate([EccentricOffset, 0, EccentricSheeveThick +
EccentricGrooveThick])  
           linear_extrude(height = EccentricSheeveThick + 0.1, center =
true)    
             circle(d = EccentricSheaveDia, center = true);
       
        /* boss */
         translate([0, 0, (EccentricSheeveThick * 2.0) +
EccentricGrooveThick ])  
           linear_extrude(height = BossLength, center = true)    
             circle(d = BossDia, center = true);
               
//        /* Bore .. goes below as a diff */
//       translate([0, 0, (EccentricThick / 2.0) - (EccentricSheeveThick /
2.0) -.1 ])
//         linear_extrude(height = EccentricThick + .2, center = true)
//           circle(d = EccentricBore, center = true);
           }    
         
                /* set screw   goest here!  */
       translate([BossDia / 2.0, 0, (EccentricSheeveThick * 2.0) +
EccentricGrooveThick + (BossLength / 2.0) ] )
         rotate([0, 90, 0])
           linear_extrude(height = EccentricSetScrewLen, center = true)
             circle(d = EccentricBossSetscrew, center = true);
       
       
//        /* Bore   goes here!  */
//       translate([0, 0, (EccentricThick / 2.0) - (EccentricSheeveThick /
2.0) -.1 ])
//         linear_extrude(height = EccentricThick - .1, center = true)
//           circle(d = EccentricBore, center = true);
      }  
   }
                        /* put in as solid to check */
        /* Bore */
       translate([0, 0, (EccentricThick / 2.0) -.1 ])
         linear_extrude(height = EccentricThick + .2, center = true)
           circle(d = EccentricBore, center = true);
   
  }

//  ========================================================

THANKS!



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Is it the extra 0.2 from:
linear_extrude(height = EccentricThick + .2, center = true)
?



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No, sorry...



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It is all the center=true in the linear_extrude() in the union().
Change them to false, and remove the z translate at the top and it works.
Use "%" at the front of each linear_extrude() to clearly see where each
layer is ATM.



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Thanks, I just tried it!

I re-built and exported a DXF to my CAD program, and in the boss dia and
overall length, I'm spot-on.  A little off on the small thicknesses, 'tho,
and they stack up OK as far as I can see.  The errors are
probably down in the 'noise' of the actual 3D printing, I'm guessing.  

Simpler without the 'true's in the various linear_extrudes anyway.  So why
the errors?  I'd have thought it would have worked anyway.  Is this unique?
I've been using the 'center' quite a bit in my other engine parts.  (I've
yet to print, waiting on a 3D printer) and in most cases, 'center'
simplifies things.  I should check a few parts with a DXF...

What is the internal precision for OpenSCAD?  doubles or long doubles?
What's a reasonable limit to the resolution I should expect?  

Thanks again!

<http://forum.openscad.org/file/t2193/EngineAssembly2.png> 



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KM6VV wrote
>  So why
> the errors?  

As you were adding a smidgeon to their height, the center=true dropped their
starting Z position by smidgeon/2, you were not taking that into account
when you placed the next object.

Try this to see (the "%" operator is your friend):

module Eccentric()
    {
    translate([0, 0, 0]) // EccentricSheeveThick / 2.0])  // get 1st sheave
bottom on XY plane to simplify calcs
      {    
      difference()
        {
         union()
           {
        /* Sheeve */
         translate([EccentricOffset, 0, 0])  
           linear_extrude(height = EccentricSheeveThick + 0.2, center =
true)    
             circle(d = EccentricSheaveDia, center = true);
     
        /* Groove */
         translate([EccentricOffset, 0, EccentricSheeveThick])  
           linear_extrude(height = EccentricGrooveThick + 0.1, center =
true)    
             circle(d = EccentricGrooveDia, center = true);
     
        /* Sheeve */
         %translate([EccentricOffset, 0, EccentricSheeveThick +
EccentricGrooveThick])  
           linear_extrude(height = EccentricSheeveThick + 0.1, center =
true)    
             circle(d = EccentricSheaveDia, center = true);
       
        /* boss */
         translate([0, 0, (EccentricSheeveThick * 2.0) +
EccentricGrooveThick ])  
           linear_extrude(height = BossLength, center = true)    
             circle(d = BossDia, center = true);
           }    
                /* set screw   goest here!  */
       translate([BossDia / 2.0, 0, (EccentricSheeveThick * 2.0) +
EccentricGrooveThick + (BossLength / 2.0) ] )
         rotate([0, 90, 0])
           linear_extrude(height = EccentricSetScrewLen, center = true)
             circle(d = EccentricBossSetscrew, center = true);
      }  
   }
                        /* put in as solid to check */
        /* Bore */
       translate([0, 0, (EccentricThick / 2.0)  ])
         linear_extrude(height = EccentricThick, center = true)
           circle(d = EccentricBore, center = true);
   
  }



> What is the internal precision for OpenSCAD?  doubles or long doubles?
> What's a reasonable limit to the resolution I should expect?  

See  the wiki
<https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/General#Values_and_Data_Types>
.
Generally resolution, in itself, is not an issue.
There is a current known problem with tiny numbers, e.g. if your whole space
is < ~10 and using fine details (e.g. 0.02), if you see strange things in
such cases, multiply all you numbers by, say, 1000, then scale the final
result down by 1000.



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I see it:

        /* boss */
         translate([0, 0, (EccentricSheeveThick * 2.5) +

I had 2.0, so I was a half a sheeve (sheave) off.  

I did take out the 'true's in the linear_extrudes, and it looks better
(simpler!):

module Eccentric()
    {
      difference()
        {
         union()
           {
        /* Sheeve */
         translate([EccentricOffset, 0, 0])  
           linear_extrude(height = EccentricSheeveThick + 0.2, center =
false)    
             circle(d = EccentricSheaveDia, center = true);
     
        /* Groove */
         translate([EccentricOffset, 0, EccentricSheeveThick])  
           linear_extrude(height = EccentricGrooveThick + 0.1, center =
false)    
             circle(d = EccentricGrooveDia, center = true);
     
        /* Sheeve */
         translate([EccentricOffset, 0, EccentricSheeveThick +
EccentricGrooveThick])  
           linear_extrude(height = EccentricSheeveThick + 0.1, center =
false)    
             circle(d = EccentricSheaveDia, center = true);
       
        /* boss */
         translate([0, 0, (EccentricSheeveThick * 2.0) +
EccentricGrooveThick ])  
           linear_extrude(height = BossLength, center = false)    
             circle(d = BossDia, center = true);
           }    
         
        /* Bore   goes here!  */
       translate([0, 0, -.1 ])
         linear_extrude(height = EccentricThick + .2, center = false)
           circle(d = EccentricBore, center = true);

       /* set screw   goes here!  */
        translate([BossDia / 2.0, 0, (EccentricSheeveThick * 2.0) +
EccentricGrooveThick + (BossLength / 2.0) ] )
         rotate([0, 90, 0])
           linear_extrude(height = EccentricSetScrewLen, center = true)
             circle(d = EccentricBossSetscrew, center = true);
      }
  }

Thanks for the link, I'll study it.  I had kicked up my 'scale' to 254, but
didn't notice a difference.  I'll keep it in mind.  I have a small 'slide
valve' to model.

I really enjoy modeling parts with OpenSCAD!  A terrific tool, especially
for a 'C' programmer with a little graphics!

Thanks!

<http://forum.openscad.org/file/t2193/Eccentric.png>





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