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:





      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:





      Here is the sketch in solidworks:



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





        • Re: Inaccurate Curve in Solidworks

          Hi Fahd,


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



          • 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:




              Best Regards,

              Ditmars VEINBACHS

              R&D SolidWorks, Modeling, Development Senior Manager