View Full Version : Depth of Focus question

mel davis
08-27-2010, 3:05 PM
A friend of mine who rebuilds Jeeps as a hobby asked if I could engrave on aluminum. Research revealed Cermark so we agreed to split the cost of an aerosol can , Great so far! He left two aluminum dashboard knobs on my workshop door, opened them and horror!! they have a slight curvature on the end to be engraved. My question for my Epilog Summit is how much curvature can be handled prior to the beam being unacceptably out of focus? As a photographer I would stop down and focus 1/3 of the way into my subject but this of course is projecting light the other way.
Any help would be appreciated.
Mel Davis

John Noell
08-27-2010, 3:45 PM
... how much curvature can be handled prior to the beam being unacceptably out of focus?In my limited experience, it depends on the material and what YOU consider satisfactory. When the beam is less focused, you need more power/slower speed to cut through (or heat the cermark enough). I cut and engrave a lot of curved surfaces - so I got a slightly longer focal length lens to get more depth of field. The old 2" worked, but to get enough power to cut at one distance resulted in too much (burning!) for the exact focus level.

Dan Hintz
08-27-2010, 5:08 PM

How much power do you have to work with (put it in your sig so we won't have to ask again)?

If you're using a standard 2" lens, you would normally have about 1/4" of depth to work with. However, Cermark requires a lot of power, and at those extremes the power will drop off too much. I've never attempted a curved surface with Cermark, but I imagine you won't get more than +/- 1/16" focus from the center plane before you're either too low in power to make a solid mark or too high in power to prevent burning it off.

Dee Gallo
08-27-2010, 6:33 PM
I think Dan is right - I was doing an aluminum piece which had a small 1/8" lip on one side. I forgot to account for it and the result was a weak engraving there. Once I leveled it out, everything was fine. So my vote goes to 1/16" before you lose useful focus.

:) dee

mel davis
08-27-2010, 8:19 PM

Thanks for the suggestions, I have added my signature information (I think). I guess the best way to find out is to go to the workshop and do some tests. Watch this space. I have been reading the posts for some time, now hope to be more of a contributor having overcome the digital shyness.

Richard Rumancik
08-27-2010, 10:38 PM
Mel, regarding the question about depth of field - the depth of field for a particular lens depends on the focal length of the lens (longer FL = larger depth of field) and it depends on the incoming beam diameter to the focal lens (which you probably do not know). So actually calculating it might be impossible. But in reality, the term "depth of field" is not really a scientifically derived value. The beam diameter will increase as you defocus in or out of the material; it seems that by convention the accepted "depth of focus" is the distance between the two points where the beam diameter grows by about 40% (i.e. the area doubles.)

But if you start with an in-focus beam of .005" diameter and it grows 40% to .007" because of curvature, will the Cermark still work? The definition won't help you decide even if you could calculate the DOF exactly for a lens. As you stated, you will just need to try it and see what provides acceptable results.

The growth of the beam from .005 to .007 may not sound like much, but in actual fact the energy per unit area will go to about half. (.005**2/.007**2) That would be quite a variation in effective energy.

Levelling the part is the first approach because it reduces the out-of-focus error by half. If the part is really curved like a stainless flask, I recall one published solution: the flask was lasered once close to the higher point of curvature and again at a lower point of curvature. (Re-lasering over the good text/graphic won't hurt it, but re-lasering at the different focus will improve the poor fading areas). Of course this doubles the job time and might not be a solution for you, or for all situations. But in some cases it could work (I have not tried it myself.)

If you are interested in the math for depth of field check out:
http://www.parallax-tech.com/faq.htm (http://www.parallax-tech.com/faq.htm)

Joe Pelonio
08-27-2010, 10:46 PM
I have etched tapered glasses as big as a 1.5" area with a focus compromise, and no deterioration of image, but as others mentioned cermark is another thing entirely. Krylon makes a paint that looks like colored anodized aluminum,
spray them with that, let dry and engrave and you'll have a better result than
trying to do cermark.

Mike Null
08-28-2010, 6:17 AM
I just engraved a company logo on a set of pan lid handles using Cermark. There was a slight curvature, not more than 1/16" and it worked very well.

mel davis
08-28-2010, 4:01 PM
Here is my first effort, the slight curvature turned out not to be an issue. I focused the laser just below the highest point. The picture is a macro image, to the naked eye it looks visually fine but I would appreciate any comments on what appears some granularity in the engraved lines? The image is .5" across on a 1" knob.

Settings Power 100 Speed 3

Thanks again for all your help. :)

Dan Hintz
08-28-2010, 7:03 PM

It looks like the Cermark is chipped around the edges. This could be due to the brushed finish of the knob (doubtful), but it's more likely your settings aren't dialed in perfectly. A speed of 3 seems a tad slow... I'd try something around 6(ish). That speed assumes a resolution of around 500 dpi.

mel davis
08-28-2010, 10:41 PM
Thanks Dan I will try that and the setting was 600 dpi for the last one.