this happens on preview when i try to subtract objects
but after doing render it looks ok, i keep, have to play with offseting z or the height of object by a fraction of 0.001 or 0.0002 to make it look ok in preview as well..

You need to make sure that the object you are subtracting extends beyond the surface of the object you are subtracting it from, instead of the two surfaces coinciding. Example:

overlap = 0.02;

module tube(outer_radius, inner_radius, height) {
  difference() {
    cylinder(r=outer_radius, h=height);
    translate([0, 0, -overlap]) cylinder(r=inner_radius, h=height+2*overlap);
  }
}

tube(10,8,5);

In this code the cylinder we are subtracting extends by 'overlap' above and below the one we are subtracting it from. I always use a variable called 'overlap' for this so that it is clear in the code what adding these small amounts is for.

Edited 2 time(s). Last edit at 02/18/2018 04:14PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

thanks, couldn't there be a generic method with some form of scale and resize play here?

well for some reason i'm still getting this, and i have no idea what's it's even called, but i can see through the walls..

the following minor changes works for me...

module main(){  
      
     difference(){               
            drawHoles(manArr){                            
                  cylinder(d=holderDiameter,h=8);                                       
            }                                     
             difference(){
                drawHoles(manArr){
                    translate([0,0,-1]) cylinder(d=cellDiameter-1,h=10);       
                 }
             }
     }       
}

h of 10 is just your 8+2 translated down 1mm so 1mm protrudes top and bottom so it draws cleanly

Edited 2 time(s). Last edit at 02/20/2018 12:14AM by Dust.

thanks i was trying to come up with a generic module for this..

thanks i was trying to come up with a generic module for this..that's what the overlap module was for, i started searching and thought it was a bug in openscad