SMTY means sufficient material to yield. It is not a grade. It is a nesting and cutting method: we water-jet a shape out of CVD sheet about 2 mm larger than the finished blank you actually need.
A full CVD plate is expensive in area you will throw into the scrap bin. If you only need twenty Ø38 mm cores, you should not buy 800 × 1200 mm and learn water-jet kerf on zinc selenide at your own burden rate. You send the diameter (or the square), we nest it, we cut it fat, you receive a cookie of crystal that still has grind and edge stock.
What “2 mm larger” is for
Water-jet leaves a cut surface, not an optical edge. You need stock for:
- edge chip from jet exit (we also spec chip on cores separately)
- your own cylindrical grind or mill
- fixture lands
- the fact that ZnSe chips if you look at it wrong
Circle type, square type, thick type — those shop photos are not three products. They are three nest geometries on the same CVD sheet. Sheet-type SMTY slabs exist when your yield math wants a rectangle, not a round.
SKU language
On the old shop we called water-jet SMTY SMTYSheetWJ. That is still the idea: water-jet, laser-grade or optical-grade sheet, oversized. Tell us finished size, not the size you hope the jet leaves. We add the fat. If you already modeled +2 mm in CAD, say so or you will get +4 mm and a confused incoming-inspection form.