Focus, Out of Focus

[Bob WrightNC]

Sorry for the play on the other thread, but I've got a question about focus on glass that I know you guys can answer. I'm looking at etching glasses and working through whether I really need a rotary or not. The rack star guy's main business is doing glasses and he says he doesn't use rotary with any of them, just shifts the focus about half way between the outside edge and the center(at least that's what I thought I heard him say). If you do this on a standard 16 ounce glass without any additional angles, how wide can you make your logo and still keep a good looking etch? Thoughts? Thanks in advance!

[Dave Sheldrake]

On a 100mm diameter surface using a standard 50mm (2 inch) focal length lens there will be a total width of 26.59mm within the focal depth (1.8mm) of the lens.(ie: the lowest point will be within 1.8mm of the highest point)

cheers

Dave

[Bob WrightNC]

Thanks Dave. But physics/geometry 40 years ago has left me a bit rusty. Just for a simple guy, how wide could you make the design before you would see bad distortion and what lens would you recommend. Thanks.

[Dave Sheldrake]

Gimme a few minutes Bob I'll run up a drawing for the different focal length lens's, once you go outside of the flat surface you will get distortion, so long as you stay within the focal depth of the lens's it shouldn't look too bad (within sensible limits)

You may need to stretch or compress the image though if you need something very wide or the rapid change in angular position will start to make things look funky.

For basic glass beer mugs etc I'd probably go with just a normal 2 inch lens but I'll draw it up so it makes a bit more sense

cheers

Dave

[Bert Kemp]

I thought I read somewhere in here that focusing on the lowest part would make for a better etching.Or did I misread.?

[Bob WrightNC]

Thanks for the help Dave!!

[Scott Shepherd]

I thought I read somewhere in here that focusing on the lowest part would make for a better etching.

Better etching is achieved from putting the glass in the sandblast cabinet

[Dave Sheldrake]

Bert,

The *focal point* (the bit where the beam is at it smallest) has a point 1.4x that diameter either side, that is the depth of field.By focussing at one extreme or the other you lose half the depth of field.

cheers

Dave

[Dave Sheldrake]

Okayyyyyy here we go...

dofcircular.jpg

On each of the different circles the RED line is 1.8mm from the topmost point of the circle. 1.8mm is the rough depth of field of a 50mm (2 inch) lens with a standard incident beam (input laser beam from the tube)

On each circle of different diameters there is a point where the RED line touches each side, the distance between these two points is the width that the beam will be in focus for each diameter.

For a 25mm Diameter the beam will be within it's depth of field for a width of 12.92mm

For a 50mm Diameter the beam will be within it's depth of field for a width of 18.63mm

For a 65mm Diameter the beam will be within it's depth of field for a width of 21.33mm

For a 100mm Diameter the beam will be within it's depth of field for a width of 26.59mm

So long as you are within this engraving width there won't be too much evidence of stretching or distortion (there will be some, nature of the beast I'm afraid) but it's unlikely to be over noticeable.

For different focal length lens's there will be a different depth of field so simply swap the RED lines distance from the top of the circle to that of the depth of field of the different lens then measure the distance between the two points where the RED line meets the circles circumference.

It's possible to get better spot sizes and longer depths of field by swapping out lens's and adding beam expanders (the size of the spot is controlled by the input beam and the focal length of the lens in some part) but that can be a little too much work for a few glasses or simple items. On a bigger run(500+ maybe) there could be benefits but beam expanders are costly ($150+ for a basic version) so the cost can be prohibitive given their purpose.

hope that helps

Cheers

dave

[Bob WrightNC]

Thanks Dave! That helps.

[Bert Kemp]

Thanks Dave

[Mark Sipes]

Why on earth do you want to limit the size of you logo to only a postage stamp sized surface area. Most of the wedding glasses, golf awards, coffee cups, etc have text that wraps around the glassware up to 3 inches on the circumference. A rotary is a given if you want to grow that area of the business.