- Base Curve Mandrel
- Front Curve Mandrel
- How Does It Work?
|The Repeatability of Total Indicated Run-out (TIR) of a Benz precision Steel mandrel during 10 repeated collet loads on a 4-Axis Lathe, using the Benz collet/spindle assembly is show in the above table.|
Precision and productivity have always been key ingredients for the success of custom lens laboratories. Ultra precision contact lenses manufacturing has 3 discrete mechanical steps: base curve machining, blocking and front curve machining. Significant precision is lost as the target button moves from collet to collet. Benz R&D has the technology address two components of collet precision: run-out and position.
Collet run-out occurs because the collet’s center of rotation does not match the lathe spindle’s center rotation. In the Benz type of spindle the mismatch of the collet to its spindle is eliminated by precision lapping the collet cone into the spindle shaft until reaching the desired collet/spindle run-out. A portion of the entering exhaust air from the spindle is also redirected through the collet to prevent swarf from altering the tolerances of the cone and collet. The second component of collet-to-collet precision is repeatability of position. We have solved this by designing:
• a precision dead-length collet and
• a precision steel mandrel
Using this precision position dead-length system and precision dimension blanks, it is no longer necessary to measure the position of the surface before beginning the first lathe pass. This saves time on every lathe machining cycle.
Front surface mandrels are shown in the above figure. There are 8 color coded plastic mandrels available in order to provide optimum blocking of most contact lenses, including high expansion soft lens materials and a full range of RGP lenses. The radius of each of the mandrel is shown in the table below.
|1.||Blank is placed on the base curve mandrel with wax using a
steel locator ring.
|2.||Blank shown on the left is after base curve machining.|
|3.||During the blocking process, the finished base curve is blocked on to a precision plastic mandrel for front surface machining.|
|4.||After blocking process, the semi-finished blank is ready for
front surface machining.
|5.||After front surface machining and polishing, turn the plastic upside down. The plastic mandrel is inserted into the saline solution and as the lenses hydrates; it will expand and detach itself from the wax, eventually falling into the saline solution. Picture shown on the left is a hydration tray available from Benz R&D.|