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Spherometer for small diameters?

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#1 sarastro

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Posted 09 February 2018 - 06:27 PM

I am going to be grinding some small lenses ranging in diameter of  20mm - 50mm.

Any ideas for a spherometers?

As far as making one I have a drill press but not much else to work with.

 

Thanks,

 

Roger



#2 Pinbout

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Posted 09 February 2018 - 06:45 PM

Brass pipe fitting, use as a ring tool.



#3 BGRE

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Posted 09 February 2018 - 07:04 PM

In principle it can be done.
Drill a hole in the centre of the spherometer disk to take the indicator stem.
Then make a jig that allows the disk to be rotated on a close fitting shaft through this hole.
Clamp it, and drill mounting hole for first ball bearing foot (or for very short radii use shaft mounted balls (standard product).
Unclamp, rotate disk through 120 degrees (not too critical) and drill mounting for next ball foot.
Repeat for last ball foot.

Standard twist drills are too flexible fr accuracy, short stub drills are much stiffer and wander less.
Whilst center drills are often used to make start holes for twist drills the 60 degree cone angle is far from optimum for a standard twist drill.
A spot drill is far better.

Ideally the centre hole would be drilled and precision reamed to size.

For high accuracy the spherometer needs to be calibrated.

How accurate do you need to measure the ROCs of the lenses??

#4 sarastro

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Posted 09 February 2018 - 07:40 PM

I don't think I need super accuracy. I'm just making singlet lenses, not trying to make an achromat or anything like that.



#5 Pinbout

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Posted 09 February 2018 - 07:50 PM

http://eckop.com/opt...ng/spherometer/

 

use a fitting like

 

https://www.homedepo...A3434/301541030


Edited by Pinbout, 09 February 2018 - 07:53 PM.


#6 sarastro

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Posted 09 February 2018 - 07:52 PM

Thanks,

 

I'll check those out.



#7 Pinbout

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Posted 09 February 2018 - 07:54 PM

Your too quick, I changed the type of fitting cause the male end is better cause the point edge of the thread



#8 BGRE

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Posted 09 February 2018 - 08:05 PM

Do those have different effective ring radius for convex and concave surfaces??

If so, calibration using a largish bearing ball only works for convex surfaces.

#9 sarastro

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Posted 09 February 2018 - 08:13 PM

Your too quick, I changed the type of fitting cause the male end is better cause the point edge of the thread

Thanks

I get the idea. I have a Lowe's nearby.



#10 Pinbout

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Posted 09 February 2018 - 08:15 PM

I got that idea from groski. Don’t know the actual fitting he was using but it is the same concept.




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