Optical center punch formula

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Stone

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I use an optical punch as an aid when doing metalwork (see attached picture). It is one of my favorite gadgets! I broke the optical rod when I knocked the device of the table. I use it a lot and thought this is a good time for me to finally make my own improved version that does not require I swop the optical rob with the punch each time.

I have a short length of 10mm Perspex rob that I used to make the optical rob from. It worked well but the focus is completely out. I used a 13 mm radius in my attempt, which is the same as the original one I purchased; however, my viewing rod is shorter and would require a different radius to focus correctly.

I was wondering if anyone knows the formula to calculate the focal length so that I cut the curved end to create a focal point at exactly the other end of the rod, where I have the cross hairs marked? I spent a few hours online but can’t seem to find this equation.

The refractive index of the Perspex should be about 1.495.

I tried another optical rod with radius 11 and it was much better.


my required dimensions.jpg


They are quite time intensive to make so I do not want to have to make many at different radii to try find the correct length, also I am running out of Perspex rod. I live in a remote area so driving to buy more Perspex rod is a 200km round trip.



If anyone has any knowledge in this area, I would appreciate your help.
 

ShopShoe

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Stone,

I remember reading someone's post on how to make one a few years ago. I did a quick google search and got many hits. Too many to read and select one to give you a link.

Try googling "optical center punch diy" I think you'll find what you need.

BTW, I think the story I remember used a coddington lens as the magnifier.

Good luck, and please post your finished project if it turns out OK.

--ShopShoe
 

Stone

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Thanks for the reply ShopShoe.

I did do an extensive search on optical centre punch as you suggest. All show how they cut the radius (e.g. radius cutter, cnc lathe, sharp template, etc), but not one shared how they calculated the correct optical length. Most just use 13 mm, which is what most commercial ones are. Mine, however, is much shorter and 13mm is completely out of focus.

I found a few formulae, but they produced a radius of 72 mm, which is completely wrong.

Hopefully someone has some good optics info to share
 

ron4vt

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Stone, greetings!

I’ve been dealing with fine mechanics throughout my life and could accomplish some unusual feats such as building Super 8 projectors, repaired optical equipment thus manufacturing small prisms and this tiny “steam locomotive”:
As far as I could move along all these things and facing your request, you can consider your punch press cylindrical lens as having its focal center half the distance of the radius. Please refer to the attached image. Perspex (polymethylmethacrylate) is the best choice ever and can be easily be polished on the lathe, with any automobile fine polishing paste, with very fine results. Best to you, Ron
 

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LorenOtto

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Ron, Thank you for sharing your video and sketch. Your layout shows your commitment to every detail. To depicting with accuracy the life of a small village on a railroad line.
 

ron4vt

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Loren, thank you for your comments. My first reply raised interest on also machining an Optical Center Punch, something which is very useful in every shop so I wish to change some basic thus necessary measurements. I hope it helps. Regards, Ron
real radius.png
 

Stone

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Most grateful for the info ron4vt.

I will cut one of 81mm radius in the morning and give feed back on how it went.
 

ron4vt

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Looking forward to hearing very good news from you, Stone !
For better transparency ( as I have always done), you gently apply 360 grit sandpaper first, on the surface of the recently machined acrylic part while the lathe is still rotating, then 400 grit and 600, all soaked in water and dish detergent. Clean the surface twice with a soft cloth and then apply fine automotive paint polishing paste for a few minutes. The result is astounding. Ron
 

Stone

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I tried to experiment today on cutting a 81 mm radius without any luck.

How does one go about this?

I do not have a ball turner. 81 mm is big even for most ball turners.
 

Mosey

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I have two of these, Fowler or Flexbar, not sure of the brand. Both in excellent condition and for sale for any reasonable amount so you don't have to make anything, just put them to work.
I closed my shop.
 

ron4vt

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Wow ! That's the best opportunity EVER ! Stone, go for it and enjoy !!
 

SirJohn

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Here is an optical center punch that you can purchase at Lee Valley Tools for $64.95. Seems like the answer if you need a punch.
 

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arvidj

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Here is an optical center punch that you can purchase at Lee Valley Tools for $64.95. Seems like the answer if you need a punch.
I have the Lee Valley optical center finder. Wasn’t that much when I bought it, but its great. Helps for accurately finding centers, especially with older eyes.
Went to their site and it looks like it is no longer available ... https://www.leevalley.com/en-us/shop/tools/hand-tools/marking-and-measuring/marking-tools/45502-veritas-optical-center-punch?item=05N5901 ...
 

cds4byu

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Amazon has half a dozen different optical center punches available for various prices.

Carl
 

Scott_M

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I played with this today. And can confirm that these numbers work. ( end radius = 2X Focal length )
I used some round numbers just to make it all easy, I made the element 2.5" OAL and cut the radius @ 5" on a CNC lathe. I just did a quick polish 600- 1000- white scotch-brite and a jewelry polishing cloth. I scratched a cross on the bottom with the height gauge.
It seems to work rather well :)

The big radius end
OCP1.JPG


The bottom end
OCP2.JPG


The mark
OCP3.JPG


It was really hard to get a good shot with the camera
OCP4.JPG



Scott
 

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