Magnetic optical punch

[FulmenParticipant @fulmen]

I’m actually quite pleased with myself now. I just made a switchable magnetic optical punch, and it works every bit as good as I expected. And on the first attempt as well. The body is just 3D printed so I might make one in solid metal later. But that will be a vanity project, the printed one is more than good enough for me.

It uses 6 Ø6x3mm neodynium magnets and locks in place with a 30° turn of the central body:

 

The eyepiece isn’t exactly optical quality, but it’s easy to reshape so if anyone has a good trick here I’m all ears.

 

 

[John HaineParticipant @johnhaine32865]

Neat! How does the locking work?

[FulmenParticipant @fulmen]

Like this (viewed from below):

The red parts are mild steel, magnets are blue and in an alternate north/south pattern. The 6 small steel pegs are in the base as seen in the photo. The magnets and the large ring move together. As depicted the magnets are off, if twisted so the pegs align with the magnets they’re on.

[JasonBModerator @jasonb]

How does it perform on non ferrous? I have an O ring in the bottom of mine which stop sit sliding on many different materials.

[FulmenParticipant @fulmen]

I have an o-ring as well to provide friction, the magnets obviously won’t work on non-ferrous.

[bernard towersParticipant @bernardtowers37738]

When I made mine a long while ago the central viewing lens was mushroom shaped so it gathered more light to make the target brighter.

[FulmenParticipant @fulmen]

I did the same, but mainly for ergonomic and esthetic reasons. The stem is 11mm while the head is 16, I don’t know if it makes a difference or not. It works OK but I wouldn’t mind some more light. I have considered adding a light source, but I don’t have a lot of space for that.

[bernard towersParticipant @bernardtowers37738]

the bigger the head the more light gathered

[FulmenParticipant @fulmen]

Idunno about that, I have a big’ol head and I’m still quite dim.

[JasonBModerator @jasonb]

I can see the ring around the edge now that I look closer.

I don’t seem to have a problem of brightness with my commercially made one which only has an 8mm dia stem and 12mm head. Though the height may come into it as yours looks a lot taller than mine which only has a 30mm high body, possibly the straight rays of light hitting the sides rather than being able to get to the bottom or the polished internal bore of mine reflects more light?

[JasonBModerator @jasonb]

[FulmenParticipant @fulmen]

I might have made too big a point out of it, it’s fine as it is. But it does require decent lighting, especially when working against dark layout color. I’ll do some experiments to see if I can improve the light gathering ability.

[Fulmen]

On Fulmen Said:

Like this (viewed from below):

 

The red parts are mild steel, magnets are blue and in an alternate north/south pattern. The 6 small steel pegs are in the base as seen in the photo. The magnets and the large ring move together. As depicted the magnets are off, if twisted so the pegs align with the magnets they’re on.

So a strong detent action when “off” and lower when “on”?  Clever.

[JourneymanParticipant @journeyman]

To help with the light problem, as the main body is 3D printed, you could try printing with a translucent or clear filament. Have you put the design on Thingiverse for others that might want to give it a go?

John

[Dave HalfordParticipant @davehalford22513]

As Bernard alludes to. Isn’t this to do with that weird property plastic has which transmits any light entering it equally to all faces. As used in cinema exit markers where the thinner edges of the letters are quite bright.

Bigger light gatherer surface = a brighter target

[FulmenParticipant @fulmen]

The lens act like an optical fiber, without it it’s impossible to see anything at the surface. Getting light in from the side seems to require a matte surface, but that’s doable.

Haven’t put it on Thingiverse yet, figured I would redesign it to use OTC components first.

[FulmenParticipant @fulmen]

Shaping the lens turns out to be fairly simple using a battery drill, sandpaper and a 3d-printed sanding block. I’m using it up to 400grit, then 800, 1500 and 2500 freehand before polishing with Autosol. Best shape seems to be a simple radius. The length (56mm) puts a lower limit to the radius (25mm), I would say I get around 2x magnification which feels about right. Getting the crosshairs perfect isn’t easy but I still have some extra length to practice on.

Light is perfectly fine. Not perfect but the improved lenses has greatly increased clarity. Roughing the outside of the view piece also seems to help a bit.  Adding a LED would be a cherry on top, but that can wait.

[SillyOldDufferModerator @sillyoldduffer]

Like it!  Bonus points for taking an original design from drawing board to reality.

I’d have expected trouble making the lens.  Sounds straightforward, but was it?

Dave

[JasonBModerator @jasonb]

I just looked mine up and it has 8x magnification. I’ll have a measure later and see if I can get an idea of the radius of the domed top

Regarding “length to practice on”, if that was referring to recutting the cross hairs then beware of parallax the further the end of the clear rod is from the surface.

[FulmenParticipant @fulmen]

It took a couple of tries to get the method right, after that it was surprisingly simple. Just rough it in with a file, then wet sanding using a sanding block with the proper radius.

[FulmenParticipant @fulmen]

What turns out to be the hardest part is to get the crosshairs juuuuust right. The best result so far has been with a height gauge with the lens held in a printed square holder. But it takes a bit of practice getting it right. And every time I get it wrong the lens becomes shorter…

Next up is an automatic center punch. Perhaps a bit of an overkill, but I think it could be a fun side quest. I know how the regular types work, but I’m not entirely happy with the “moving body” as it will have less support at the beginning of the stroke. Can anyone think of an existing mechanism that would be better?

[bernard towersParticipant @bernardtowers37738]

semi auto center punch in ME somewhere in the 40s I believe≥ Made it some years ago and works well. Original is a bit chunky so slimmed it down a bit. will try to find drawings.

[Fulmen]

On Fulmen Said:

I have an o-ring as well to provide friction, the magnets obviously won’t work on non-ferrous.

Pedant alert! It might it work on nickel and cobalt, not that I use a lot of those metals

[John HaineParticipant @johnhaine32865]

An alternative to a punch might be a “spotting drill” – these have precise ground shanks (I have a 10mm dia one) and 90 degree point and are designed for precise hole starting (better than centre drills).  One that was a close fit in the punch body could be twisted by hand to leave a small mark – if you need something bigger then use it to locate a normal centre punch.

On the cross-hairs question, another approach is to scribe a line that is very slightly above the centre line, rotate the body by 90* and scribe another so you have two parallel lines.  Then scribe two more at right angles to give a small square which must automatically be centred.  Then you align the mark with the centre of the square.

[JasonBModerator @jasonb]

You probably want something where you can compress a spring with a plunger and then when you have the point of the punch where you want it press something to release the plunger to strike the back of the point. Think like the pin prick gadgets used to take a small drop of blood for testing.

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