how to process slab off

[OPTIDONN]

I was just wondering how to process slab off. I know how to calculate it and our software does to. We have and old 108 ct coburn generator. I was wondering how you physically go about doing it? Do I need special equipment.

[Darryl Meister]

I'll provide the basic gist of it, but you'd be better off referring to a good surfacing manual, like Brooks's Understanding Lens Surfacing:

1) Mark a straight horizontal line across the front of the lens blank at the height of the segment ledge for reference. Surface the back curve with the correct amount of base up slab-off prism for the near region and a center thickness based upon this prism (fine and polish normally).

2) Wrap (masking) tape around the edge at the back of the lens blank in order to create a "reservoir," which you will then fill with a shallow layer of an appropriate epoxy resin (a quick-cure, two-part epoxy may work but there are probably better resins available for the job). You may also want to spray on a clear precoat before applying the epoxy.

3) Generate a second back curve, without the slab-off prism, to roughly the original center thickness (from step 1). The slab-line should be approaching the bifocal segment from the top of the lens blank. Take a few more passes until the line is within a few millimeters of the segment. The exact distance required will depend upon the amount of slab-off prism.

4) First fine the lens blank until the slab line is within 0.25 mm or so from the bifocal segment. You can apply pressure to the distance portion in order to straighten the slab-line out some if needed. Second fine the lens blank, ensuring that the slab-line reaches the bifocal segment, and then polish normally. Carefully remove any remaining epoxy after deblocking the lens.

It's pretty straightforward to grind a slab-off. However, it takes a great deal of experience and skill to grind one perfectly in a single try with a straight and crisp slab-line at the right height and with no waves.

[Jacqui]

Learned how to do them on a hand polisher. Now I get them done on a freeform generator.

[Darryl Meister]

Now I get them done on a freeform generator

Messy slab-lines though... ;)

[Jacqui]

Splits the prism between the lenses, hardly noticeable.

[OPTIDONN]

Thanks Darryl! I have the book at home. I guess it might help if I actually looked at it!:hammer: