# Best Homemade Tools >  Optical centre punch

## knoba

Link source: https://knoba.wordpress.com/2015/07/...-centre-punch/

CAD file: https://www.dropbox.com/s/2qhmuuqipj...02015.dxf?dl=0

Thanks Ken!

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gunsgt1863 (Oct 30, 2017),

high-side (Jun 11, 2020),

HobieDave (Mar 28, 2020),

janders1957 (Jul 28, 2018),

jere (Aug 12, 2015),

johncg (Nov 25, 2022),

Jon (Aug 11, 2015),

kbalch (Aug 10, 2015),

mwmkravchenko (Jun 10, 2020),

oldcaptainrusty (Oct 29, 2017),

olderdan (Oct 29, 2017),

Paul Jones (Aug 8, 2015),

PJs (Aug 11, 2015),

tonyfoale (Oct 29, 2017),

will52100 (Jun 9, 2020),

zarembak (Jul 26, 2018)

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## DIYer

Nice tool, knoba!  :Thumbs Up:

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Altair (Aug 8, 2015),

knoba (Aug 8, 2015),

mwmkravchenko (Jun 10, 2020)

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## knoba

Thanks DIYer!

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## Altair

Pretty thing to look at and look into. Great idea on the crosshairs. Spot on!

Al

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PJs (Aug 11, 2015)

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## Captainleeward

wow thats a nice punch eyeglass as good as mine. :O)

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## knoba

Thanks guys'!

Mornin' Cap!

There's nothing shoddy about your punch, mine just looks fancy in a round tobacco tin...  :Wink:

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PJs (Aug 11, 2015)

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## Captainleeward

i'm gonna get a tin toooo...:O)

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PJs (Aug 12, 2015)

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## kbalch

Thanks knoba! I've added your Optical Center Punch to our Measuring and Marking category, as well as to your builder page: knoba's Homemade Tools. Your receipt:












Optical Center Punch
 by knoba

tags: punch, centering

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## PJs

Knoba, Like I said I just love this thing and will be making one soon but realized you had a 17.8885mm spherical radius for your optics...how on earth did you determine that let a lone make it? I'll have to hunt for a tin though!  :Lol:

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## Okapi

A really good and unexpensive idea !

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## knoba

> Knoba, Like I said I just love this thing and will be making one soon but realized you had a 17.8885mm spherical radius for your optics...how on earth did you determine that let a lone make it? I'll have to hunt for a tin though!



I'm not that smart! 

The radius of the optic was set to allow for some manual adjustment, later by trial and error, on the lathe. The actual radius ended up somewhere near 9/16th. The optic was gradually altered until it appeared to give a good magnification without too much distortion. Only the top surface of the curvature of the optic needs modifying as needs require.

Sometimes I get stuck knowing where to start with a project (like with my welding turn table). My answer, to myself, is just to start it, anywhere, somewhere, the answers will define the questions that need to be asked.

Thank you for your compliments!

Let's not forget the pink packing foam. It was specially imported from China...  :Wink: 

**Have updated the cad file and changed the optic to a 14mm radius.

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Paul Jones (Aug 12, 2015),

PJs (Aug 12, 2015)

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## knoba

> A really good and unexpensive idea !



Thank you, Okapi, nice compliments, much appreciated!

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## PJs

Thanks Knoba, I get it about "just start" and that it would be a hunt and peck for optimum clarity and magnification...especially with trifocals.  :Lol:  Don't have a ball turner for my lathe yet but pretty good with a graver. Thanks for a good starting radius and no worries about the foam...have a bunch! Need to look at your turntable again...but have to say, you are one Prolific builder!!

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knoba (Aug 13, 2015)

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## Okapi

The simpliest you make, the most efficient it is and the more reliability you have, polycarbonate magnifying glass are the best unexpensive friend too for helping us after 50 years old

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Captainleeward (Aug 17, 2015),

knoba (Aug 13, 2015)

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## kbalch

Hi knoba,

Very impressive device! Your Optical Center Punch is the 'Tool of the Week'!

You'll be receiving one of our official HomemadeTools.net T-shirts:

 
 
Just let me have (via PM) your details (size, color choice, and mailing address) and we'll get things processed directly.

Congrats!

Ken

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## kbalch

This thread has been moved to the Must Read subforum. Congrats (and thanks) to knoba for making such a valuable contribution!

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## PJs

Congrats Knoba! Simple and Elegant always a winner in my book!

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## knoba

Wow!

Thanks for the award & thanks for the compliments, guys!

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Captainleeward (Aug 17, 2015),

Paul Jones (Aug 16, 2015),

PJs (Aug 15, 2015)

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## kbalch

Hi knoba,

Don't forget to PM me your info and shirt selections (size & color)! We'd like to get it right out to you.

Ken

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## tonyfoale

Nice one..

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## olderdan

Hi knoba
I found your optical punch design on your website before joining HMT. I copied it right away and it has been my go to marking punch ever since. I have used a HSS punch and all acrylic for the rest and made two optics, one with crosshairs and one with a bullseye (this is my favourite) I even copied the storage tin. Many thanks for the idea and everyone should make one, it never misses the mark.



Well deserved award
Regards 
Olderdan

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Jon (Oct 29, 2017),

PJs (Jul 28, 2018)

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## Christophe Mineau

Agree with olderdan, but can somone explain how to polish the acrylic lens ?

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olderdan (Oct 30, 2017)

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## olderdan

> Agree with olderdan, but can somone explain how to polish the acrylic lens ?



Acrylic is one of the easiest materials to polish, after removing any machining marks with 1200 grit wet & dry any metal polish (or jewellers rouge if you have any) hand worked will quickly do the job.

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## Christophe Mineau

OK thanks, I think I had bad experience in the past, maybe I was too fast on the polishing disk, will try by hand next time.
Thanks !

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olderdan (Oct 30, 2017)

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## Toolmaker51

In the trade, these are known as bombsights.
My optical center punch is commercially made, it's a long time 'friend'. I have no clue about the optic radius, actual magnification or focal length, but it must be relatively high.
I'm near-sighted yet cannot use it without eyeglasses, that would make the focal length at least 18". It's not crosshairs but a .003 dot, slightly wider than a fine scribe line. It will locate centers within .002 all day.
There is no reason to suspect knoba's is any less accurate, therefore a worthwhile project.

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PJs (Apr 4, 2018)

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## Okapi

> OK thanks, I think I had bad experience in the past, maybe I was too fast on the polishing disk, will try by hand next time.
> Thanks !



Hi Christophe,
You can found polishing paste for watch synthetic glasses which goes for cleaning and polishing all optical sights or protecting glasses in the workshop, the result is spectacular only with a clean cloth and some human oil… ;-)

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PJs (Apr 4, 2018),

Toolmaker51 (Apr 3, 2018)

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