Guide to Defocused Acrylic Engraving with the Glowforge

https://youtu.be/2B2oMYR1Ov4

In this episode of The Atlanta Maker we take a look at getting higher quality and faster acrylic engravings using defocusing!

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Questions about defocusing when using acrylic pop up quite often in the Glowforge Community and Facebook groups so I wanted to run a few tests to really get an understanding of defocusing and the correct settings to use.

First, lets talk about what Defocusing is and why you’d want to do it.  

Most of us when using our Glowforge rely on the Auto Focus.  When using the auto focus the glowforge shoots a laser down to the material surface which determines the thickness of the material and sets the laser to focus directly on the material surface.

This works great for cuts and scoring acrylic, but it leaves us with a less desirable result when engraving.  

For instance, look at this Dalek button surround I made as my first acrylic project using the default proofgrade settings which took around 23 minutes.  You can see that the engraving is very rough and left a lot of ridges.  It really was an ugly engrave. 

Generally we’d get around this by increasing the Lines per Inch of the engrave, but this can also increase the time it takes to engrave a design significantly.

On this Dalek, if we did an HD engrave with the proofgrade settings it would take an hour and 42 minutes.  But if we use defocusing, we can get a similar result in only 19 minutes and 15 seconds. 

When you defocus the laser, you are setting the focal point of the laser to something a little bit higher than the material you are engraving. 

Defocusing makes the laser beam a bit wider and smooths out the engraving without having to increase the Lines Per Inch and the engrave time.

I designed a test file to run several different focus and LPI settings on both 1/8” and ¼” cast acrylic.  You can download this file here if you’d like to run the same tests on your machine.

The file is created to test three different focus settings along with three different LPI settings.  This results in a nice 3 x 3 grid.

I’ve gone ahead and setup the file in the Glowforge UI.  If you are trying this on your own machine, you’ll have to set each of the items up as the settings do not transfer with the file.

It’s about a 12 minute project, so let’s hit print and get it started.

I think the results are fairly obvious as to why you’d want to defocus when looking at these test cuts.

You can see that the autofocus engravings are pretty bad, but do get better with higher LPI settings, but the engravings on both the .2 and .3 focus settings are significantly better.

So what did we learn from these tests?  Here are my general rules for defocusing when engraving acrylic.

For most engraves you should set the focus height to .2” for 1/8” acrylic and .3” for ¼” acrylic.

You can control the depth of the engraving by changing your LPI.  For instance a 170 LPI defocused engraving is shallower than a 340 LPI defocused engraving.  In our tests the 340 LPI engraving was slightly more than twice the depth of the 170 LPI engraving.

Is there a drawback to using defocusing when engraving acrylic?  Yes, there may be a loss of quality on a finely detailed engraving since the laser is going to be slightly wider.  If you have a finely detailed engrave, you may want to run a test to make sure you like the results.

So to wrap it all up, defocusing can give you better quality engravings in less time for most acrylic projects.

If you have any questions, let us know in the comments below and don’t forget to hit the like button and consider subscribing for more Glowforge content.

See you next time!