# Optical Forums > Ophthalmic Optics >  Lap tool help

## Adnan

Dear all,

I need some help about lap tools kindly answer me

1-Prescription is OD -2.00sph -3.00cyl 90* 
kindly inform me what lap tool will be use if using 4Base 1.498 blank single vision


2-Prescription is OD +2.00sph +3.00cyl 90* 
 what lap tool will be use if using 6Base 1.498 blank single vision


3-Prescription is OD +2.00sph -3.00cyl 90* 
 what lap tool will be use if using 6Base 1.498 blank single vision

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## MikeAurelius

What index for the tools? What center thickness for #2 and #3? What is the sagitta of the base curve for the lens blanks?

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## Adnan

Index of tool is 1.523

dia normal 65 mm

normal sagitta value in 6 Base

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## MikeAurelius

Are you sure of the index of the tooling? Standard (in the US anyway) is 1.530

There is no such thing as a "standard sagitta value" for any base curve.

The lens thicknesses are also required for #2 and #3 in order to properly compensate for lens thickness.

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## Speed

HA HA! I am so sorry. You should probably not be doing this yourself.  This is what it has come to. When you see your friends at the conventions and set around the table drinking beers.  You look at the floor and shake your heads and say, "Man, the optical business sure has changed in the last 20 years".  This is what you are talking about.

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## MikeAurelius

> HA HA! I am so sorry. You should probably not be doing this yourself. This is what it has come to. When you see your friends at the conventions and set around the table drinking beers. You look at the floor and shake your heads and say, "Man, the optical business sure has changed in the last 20 years". This is what you are talking about.


You lost me, speed...

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## Speed

> You lost me, speed...


 I am just wondering if Adnan is getting ready to attempt to manufacture a lens.

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## fjpod

Sounds like he is trying to get somebody to do his homework

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## HarryChiling

> Dear all,
> 
> I need some help about lap tools kindly answer me
> 
> 1-Prescription is OD -2.00sph -3.00cyl 90* 
> kindly inform me what lap tool will be use if using 4Base 1.498 blank single vision
> 
> 
> 2-Prescription is OD +2.00sph +3.00cyl 90* 
> ...





> Index of tool is 1.523
> 
> dia normal 65 mm
> 
> normal sagitta value in 6 Base


#1
Convert 4 base blank from marked curve to material or true curve:

0.53/4 = 0.498/x
0.53*x=0.498*4
x=3.75D

Find the power of the back curve:

B=(-2.00) - [3.75/(1 - (0.002/1.498)*3.75^2)]
B=(-2.00) - 3.82
B=-5.82 

Now convert this curve from the materials index to your tool index:

0.498/5.82 = 0.523/x
0.498x = 5.82*0.523
x=6.11 rounded to the nearest 1/8th diopter 6.125

Find the cylinder meridian:

(-6.125) + (-3.00) = -9.125

So the tool to use for number one would be 6.125 9.125

#2
This question has an additional step, you have to figure out a thickness value to use before you can compute the back curve.  Use the average formula to figure it out then work the question just like the one from above.

Estimated Thickness=(5.00*(32.5)^2)/(2000*(0.498)) + 1mm
ET=5281.25/996 + 1mm
ET=5.302 + 1 = 6.3

I used +5.00 because that is the highest power that corresponds to the power + cyl.  Now you would just work through the problem like before converting your base curve into your material curve and then using the back vertex formula to figure out the back curface power of the lens.  Then converting it back to your tool index.

Since the material you are grinding is 1.498 (CR) then in the US on a minus job I would surface it down to 2mm thickness that's where I can up witht he thickness value for the first question and for the second and third I used the average thickness +1mm which is what I use when it's a plus but you can use 2mm as a edge thickness if you want or whatever figure you'd like there.

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## essegn

Find the power of the back curve:

B=(-2.00) - [3.75/(1 - (0.002/1.498)*3.75^2)]
B=(-2.00) - 3.82
B=-5.82 


How did you get 0.002 ?

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## Rafael

Lens thickness = 2mm = 0.002m

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## HarryChiling

> Lens thickness = 2mm = 0.002m


Thank you yes that's where the 0.002 came from.

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## DragonLensmanWV

Doing a +5.00 on a 6 base?

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## HarryChiling

> Doing a +5.00 on a 6 base?


School work that's why I only worked one out as an example and gave him/her directions for the other questions.

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## essegn

> #1
> Convert 4 base blank from marked curve to material or true curve:
> 
> 0.53/4 = 0.498/x
> 0.53*x=0.498*4
> x=3.75D
> 
> Find the power of the back curve:
> 
> ...


I have tried to recalculate it again in our rXprogram (from SATISLOH) and the results are:

SPH -2.00 CYL -3.00
Tooling index 1.53
Material index 1.498
Front curve  B:4.00
Diameter 65
Central Thickness 1.7mm

And calculated (1/8 rounded) back curves are:

SPH: -6.375
CYL: -9.625

What is wrong ?

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## Rafael

Esseng you calculation is correct, but harry has calcualted with a 3.75dpt frontcurve.
You have calculatet with 4.00 dpt frontcurve, put in rxpert 3.75 dpt and you will have the same results thats all.
I personaly don´t like dpt values for frontcurves, you have less mistakes if you use radius. So everyone know what you are talking about.

regards
Rafael

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## essegn

I recalculated it again:

I didn't notice that tooling index should be 1.523..
But after recalculation the result is:

SPH +6.0585, CYL +9.2091

Tooling index 1.523
Material index 1.498
Front curve 3.75 (132.8mm)
CT 2mm

What is wrong ?

And one more question about calculation of cylinder meridian: (-6.125) + (-3.00) = -9.125
the way of calculation like above can be used only when the material index match to tooling index, isn't it ?

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## shanbaum

I think RxPert is assuming that the lens front is expressed in material diopters - it figures your 4 base lens is really a 4 base lens.  That is, its surface power is 4, not 3.75.

While labs in the US commonly use 1.53 tooling, and express front curves in 1.53, I don't know of anywhere in the world where the front curve index follows the lap index, besides that one case.  When US labs use 1.6 tooling, they still express front curves in 1.53.  And outside the US, the rule is to express front curves in material index.

Just skip the first step in Harry's calculation (that is, you don't multiply the 4 by 498/530 or 498/523) - just use the 4 in the rest of the formulas where you see 3.75, and you should come up with something close to 6.37 and 9.62.

When working with hard laps (laps having a fixed increment) in indices other than the material index, cross curves get a little complicated; you have to first round to the nearest base, and then add a cyl value modified for index to the rounded base, and then round that.  So, if your tooling index is 1.523, the modified cylinder value is 3 x 523/498 = 3.15.  I think Harry may have gotten that wrong - a 3 diopter cross on a 1.53 lap certainly won't produce a 3 diopter cylinder in CR39 (it would produce around 2.82, that is, 3 x 498/530).

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## HarryChiling

> I recalculated it again:
> 
> I didn't notice that tooling index should be 1.523..
> But after recalculation the result is:
> 
> SPH +6.0585, CYL +9.2091
> 
> Tooling index 1.523
> Material index 1.498
> ...


I did have 2 errors in my post one was converting back to the tooling index I used the 1.523 instead of the 1.53 index, and also you are correct the back curve for both meridians should be computed before converting them back to tool curves instead of just the addition of the cylinder.  I was busy and posted quickly off the top of my head I have attached a excel sheet to show the numbers without any rounding error and their is still a discrepancy, this could be due to the calc program you run having a pad compensation or a number of other things.  I don't know how they are computing their values so I am at a loss to explain the discrepancy.

Book1.xls

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## HarryChiling

> I think RxPert is assuming that the lens front is expressed in material diopters - it figures your 4 base lens is really a 4 base lens.  That is, its surface power is 4, not 3.75.
> 
> While labs in the US commonly use 1.53 tooling, and express front curves in 1.53, I don't know of anywhere in the world where the front curve index follows the lap index, besides that one case.  When US labs use 1.6 tooling, they still express front curves in 1.53.  And outside the US, the rule is to express front curves in material index.
> 
> Just skip the first step in Harry's calculation (that is, you don't multiply the 4 by 498/530 or 498/523) - just use the 4 in the rest of the formulas where you see 3.75, and you should come up with something close to 6.37 and 9.62.
> 
> When working with hard laps (laps having a fixed increment) in indices other than the material index, cross curves get a little complicated; you have to first round to the nearest base, and then add a cyl value modified for index to the rounded base, and then round that.  So, if your tooling index is 1.523, the modified cylinder value is 3 x 523/498 = 3.15.  I think Harry may have gotten that wrong - a 3 diopter cross on a 1.53 lap certainly won't produce a 3 diopter cylinder in CR39 (it would produce around 2.82, that is, 3 x 498/530).


Robert "The Tool Man" Shanbaum,

Right on time, I noticed that error in the cyl and icluded it in the spreadsheet I made up.  I didn't know that the rest of the world didn't express the curves like we do here.

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