I am not sure if it is the drawing program, Ventrics 2d of mach3. I draw a circle 4.75 inches in diameter. Save and load into ventrics. create the code and load it into mach 3. now the machine cuts a circle 4.714 inches. I drew the circle with corel draw 11.

Oh yes and i measured my bit and it measures .493

Did you set the tool size in Cut2D to 0.493?
What material are you cutting?
Climb cut, or conventional?
Inside, or Outside?
What kind of machine?

There are about 100 things that can cause what you're seeing.

Tool set to .5 , Melamine, climb cut, inside. 48 by 48 machine

What kind of machine?, not what size is it.

Niot really a model name as it is made by a guy from vancouver.

Have you tried cutting on or outside the line/circle?  You may need to adjust the offset of the tool path to achieve the correct diameter of the finished circle