3 Replies Latest reply on Mar 4, 2013 11:54 AM by Ditmars Veinbachs

    Inaccurate Curve in Solidworks

    Fahd Motlak

      Dear Solidworks geeks,

       

      I have drawn a curve in a 2D sketch with this function:

       

      y(x)=50*sin(2*pi*x/550)

      2.jpg

       

      and i have drawn a line tangent to the curve at the point x=550 to get the slope the curve at this point, i found the angle of the slope is 29.73 deg.

       

      While taking the derivative of this sine equation to get the slope at point x=550 gives the angle 32.72 deg, the first derivative equation is:

       

      y'(x)=pi/5.5*cos(2pi)

      3.jpg

       

      Here is the sketch in solidworks:

      Untitled.jpg

       

      do any one knows what is the problem here, is solidwokrs accurate enough to find the slope angle? or Not!! ?

       

      regards,

       

      Fahd

        • Re: Inaccurate Curve in Solidworks

          Hi Fahd,

           

          Would  you mind please posting  your SW  part with the curve in it - thanks

           

          Mark

          • Re: Inaccurate Curve in Solidworks
            Roland Schwarz

            I've had issues w/ equation-driven curves not quite adding up when checking slopes (first derivatives).  In my case, it was using an equation-driven curve to create an involute gear tooth profile.

             

            I just accepted the, as I assumed it was due to interpolation in the curve.  Such is the nature of curves and surfaces in CAD when one departs from simple arcs and lines.

            • Re: Inaccurate Curve in Solidworks
              Ditmars Veinbachs

              Hi Fahd

              You had me going on this one for a moment!

              The equation for the derivative is correct:

              Dy/dx = Pi/5.5 * Cos(2Pi)

               

              Since Cos(2Pi) =1

              The slope of the curve (derivative) at this point is Pi/5.5 or 0.5711.

               

              Reviewing the value, one of my colleagues pointed out that….You’re not done yet!

              I was an engineering student in the days before computers ruled the world. Slope approximates the angle of the line in radians for SMALL angles ( < 5 Degrees ). To properly calculate the angle, you must take the ArcTan of the slope.

               

              That calculation will result in 29.73 degrees.

               

              Here is a link to a website that explains the calculation:

              http://www.mathopenref.com/coordslope.html

               

               

              Best Regards,

              Ditmars VEINBACHS

              R&D SolidWorks, Modeling, Development Senior Manager