Elliptical cribbing tells a generator to crib a lens in an oval, rather than round, shape. This can be beneficial in reducing the thickness at the crib edge on high plus jobs, and reducing problems cutting into the soft tool on modern polishers.
To use elliptical cribbing (this is the ELLH label in VCA terminology), there are several things that must be set up in Rx-Universe.
Setting system defaults
The system wide defaults for ELLH are located in the System Parameters program, under the “Devices” tab.
ELLH thickness breakpoint is used in the surface calculations to determine if elliptical cribbing is required for the job. The minimum value is 0 millimeters, which means thickness will not be checked (ie ELLH value will not be calculated).
Any value between 0.1mm and 5.0mm will trigger thickness checks of the cribbed diameter. Rx-Universe will checking the points of the frame between 30 and 150 degrees and between 210 and 330 degrees. If the edge thickness at the crib diameter at any of these points is less than the "breakpoint" value, then Rx-Universe will reduce the vertical crib by 0.5mm, until the thickness at the crib point is less than the breakpoint thickness, OR the vertical crib (ELLH) is less than the horizontal crib (CRIB) by the "ELLH maximum difference from", OR the ELLH is equal to the CRIB times "ELLH percentage of crib".
Because of the limits on the size of ELLH relative to CRIB, either in mm or as a percentage, it is possible that the edge thickness of the elliptical crib shape will still fall below the "breakpoint" thickness in some cases (very high plus powers along the vertical combined with small B measurements of the frame).
The next 2 settings can be used together or individually.
ELLH maximum difference from crib this specifies how much smaller the ELLH diameter can be than the CRIB diameter. For example, if a value of 5 is entered here, then ELLH can never be smaller than CRIB – 5.
ELLH percentage of crib – this specifies how small the ELLH can be as a percentage of CRIB. For example, if this is set to 80, then the smallest ELLH possible is CRIB * 80%.
If both of the above limits are used, then the largest of the 2 values becomes the acceptable minimum.
Lens Style setting
ELLH can be suppressed by lens style, similar to cribbing. One thing to remember is that if cribbing is suppressed, then the ELLH is automatically suppressed.
Setting up the Device Parameters
The ELLH value on a job can be controlled by VCA device also. The settings in the device parameters screen , under the “Generator” tab, allow for more control on an individual device.
For each device you can set the following:
Suppress ELLH when checked, this will prevent sending any ELLH value to this device. OMICS will send ELLH=?;? instead.
ELLH maximum difference from crib, this value when set operates the same as its matching system parameter, but for this device only.
ELLH percentage of crib, this value when set operates the same as its matching system parameter, but for this device only.
Both the ELLH maximum and ELLH percentage of crib, operate the same as the system default. The largest of the resulting values become the acceptable minimum for the device.
Modifying an ELLH value
The ELLH value can be set by the user only on "Uncut no frame" jobs. This is accomplished via the “F12 Surf opt” button on the order maintenance screen.
Crib and ELLH suppression can be controlled on each job. The ELLH value can also be set on an order in the ‘User ELLH” fields. The User ELLH value will override the device settings (limits) and send the User ELLH to the device.
The fields “Actual ELLH” and “Actual Crib Diameter” contain the actual values that were sent to a generating device; if these values are changed on the generator device, the device will not send back these new values to these fields in Rx-Universe.
The crib and ELLH values can also be viewed in the “Devices fields” screen.
