1.55 Transitions FT28 and fining/polishing question

[snowmonster]

We did a X-Cel 1.55 Transitions FT28 the other day and generated at a 2.2 CT (measured on the block) and then threw the lens onto the finer/polisher on our normal CR39 settings of 1:30/1:30/2:30.

The damn thing came out with a CT of about 1.5 (measured on the block for consistency in measuring).

Is it really possible that this material loses that much material in the fining/polishing phase?

-Steve

[gemstone]

Check thickness before and after fining , on the block to when you need to find out how much stock removal. Also check the timers to see if they are accurate and not sticking. The Transition process does make the lens softer. The tend to warp easier when cut thin. I still don't think you should lose that much stock from the back surface. You may want to test the next few of each material and nail down how much finning allowance you need for each.

[snowmonster]

Good point about the timers, but I know they're not sticking. I was kind of wondering if the 1.55 mid index stuff is just softer, we've never surfaced it until now.

Normally, we get 0.2 mm to 0.3 mm removed during the fining/polishing phase for CR39, polycarb and 1.67. And those thickness changes are all measured on the block to keep things consistent.

-Steve

[braheem24]

1.55 is softer and more brittle.

[DragonLensmanWV]

You might want to reduce your pressures a bit for that. We use that exact lens occasionally ( if I can't get it in Instashades) but really have no problems. We usually only use the 10 base FT-28 because no one else has anything as steep.

[snowmonster]

Can one reduce fining time then as another alternative?

-Steve

[gemstone]

Can one reduce fining time then as another alternative?

-Steve

If you are removing that much stock, sure but only on first fine. Try to remove about 4 points.