Frame Measurements when ordering un-cut lenses

[Optician99]

To guarantee a large enough blank, should we add a couple of mm to the B and ED when ordering an Uncut lens?
It seems as though our finishing lab sometimes has difficulty cutting out lenses,when we enter the true numbers.
Any thoughts? we have actually always added 2mm to the B to be safe. Also, we have always followed ED= A+2

[King of the Lab]

Minimum Blank Size (MBS) = (GCD – PD) + ED

You add 2 mm to this equation if you would like. Very common practice.

[peyes]

I would say that on minus lenses adding a couple of mm would be ok. Accurate measurements on plus lenses will produce thinner lenses. That’s been my experience.

[Quince]

Results differ by lab. Every great once in awhile, we subtract 1mm from our measurements to get the thinnest lens on a high plus Rx. If adding to yours ensures it will make cut-out, I say do it. I'm sure the lab doesn't like having redos for a consistent problem like that so I would also make sure that the right people are aware of the problem.

Another solution- they might also be willing to give you a tracing unit that can communicate with their system.

[optical24/7]

The ED is measured from geo-center of the shape out to the furthest point then doubled. The A + 2mm won't work on a host of shapes like pilot shapes. A Pallin shape with a 52 A is 59mm on ED as example. (7mm over the A measurement.)

[PRECISIONLAB]

Ask your lab if they can set your uncuts to "NO CRIB" I would continue to provide accurate frame measurements to insure proper grinding (prism for decentration, prism thin on progressives and ideal thickness on plus lenses) and bump the ED 2mm

[AngeHamm]

+1 to the points about plus lenses. You can no crib minus lenses all day long (though it does add wear to your edging wheel), but accurate box measurements are a must for thinning high plus lenses.