Height Tools vs Touch Probes
I feel like I should preface this article with a mention that I had a natural bias towards height tools despite never using a machine with a touch probe. I personally prefer machines without attachments on the cutting heads because they can move faster. Now I have two machines with the two different focusing mechanisms I'm finally able to write this article from a factual viewpoint.
Why the laser needs focusing
There is a lens in the cutting head of the laser that focuses the wide beam of the laser into a single spot point on the surface of the material. It's at this focal point that the laser is most powerful, allowing you to cut through the material as fast as possible and with the smallest cut width. Everytime you change the material you need to refocus the laser beam so that this point is on the surface of material.
How the laser is focused
There are actually two different ways of focusing the laser beam and they're obvious really. Move the lens to the correct height above the material or move the material to the correct height below the lens.
Most of the mid range laser cutters you find will come with a z axis under the laser bed, the whole work area can move up and down to adjust the height of the material under the laser beam. Most are driven by a motor and have some dedicated buttons on the control panel for the purpose. Some of the low end models have a manual drive and a manual Z axis is a very common upgrade for the super cheap K40 lasers.
The really cheap laser cutters remove the Z axis entirely to save money, if you're lucky the will include a telescoping tube on the laser head to adjust the height of the lens. This telescoping lens holder is actually common on pretty much all laser cutters. For the rest of this discussion I'm going to assume that the machine has a moving Z axis and a fixed lens.
If you start looking at the high end expensive laser cutters or machines that are designed to cut very large sheets of material (and are inherently expensive) they generally include mechanisms to move the laser lens with a motor. Some will automagically track the height of the material that they are cutting to do the focusing for you.
Lightobject is working on a version of this tracking system for the hobby market, it looks very effective and I can see why it would be useful but it still goes against my vision of a clean, junk free cutting head.
Setting the Focus
The focal point of the laser beam is a specific distance away from the laser lens, usually defined in metric, mine is 50.3mm. The simplest method for determining the height is to take a ruler of a known length and place it on the surface of the material, then drive the bed up or down until the ruler lines up against a feature on side of the cutting head. A variation on this is to place a tool directly between the nozzle and material and drive the material up till they meet. I actually dislike these kinds of tools, if the gap is too narrow you have to take the material down before bringing it up again and if you push up instead of down you end up wedging the tool between the nozzle and the material.
The height tool has always been my preferred method, it's simple, straight forward. My machine has a nice big height tool which can be easily reproduced if lost and it has an image of exactly how to use it on the side so there is no ambiguity about which part to line the ruler up against.
The other common way for setting the focal distance is to use a touch probe. In it's simplest form this is a small switch mounted to the laser cutting head, when you press the auto focus button on the controller the laser itself will drive the Z axis upwards towards the lens. The touch probe will make contact with the surface of the material and then the controller will drive the bed back down a set distance until the surface of the material is in focus again. This offset is defined in the controller and would only need to change if you adjust the position of the touch probe with respect to the cutting lens.
The autofocus method sounds pretty straight forward, there's no faffing or fiddling with up/down buttons, you don't have to have your head in the laser cutter to see where the ruler lines up with the material. It's not fool proof though. The touch probe is not in line with the red dot, it will be mounted to one side of the cutting head. Quite often I have seen people line the red dot up with the top corner of their material, push the focus button and not notice that there is no material under the touch probe. In this case the Z axis winds the material up into the cutting nozzle and the probe is not touching anything so it never stops. In other instances I've seen machines without any materials, it auto focuses over the honeycomb, the touch probe fits between the honeycomb so it never registers and the honeycomb is pushed into the head.
Finally, an issue I have discovered from actually using my own machine with a touch probe. The touch probe itself will get dirty, smoke produced from cutting materials coats every surface of the inside of the laser and the probe is no exception, possibly worse because it is so close to the cutting head. When you wind the material up into the touch probe it can leave a sticky mark on the surface of the material. Regular cleaning prevents this build up, but I have to admit that the extraction on my touch probe machine isn't good enough and that regular cleaning means every few cuts which I find pretty tedious.
Touch probes get dirty which can lead to dirty marks on the materials. The 'foolproof' nature promotes complacency leading to machine damage. The added weight on the laser head will reduce the maximum movement speed (if only just a little) and the whole thing will cost £50-£100 to add to the machine in the first place.
Height tools are simple to use, promote an understanding of how the machine works, can carry useful information on the side of them and are virtually free (scrap and laser time). The manual method of Z focusing is needed in either case if you want to try any out of focus engraving tricks.
For me height tools are a clear winner.
If you like this article, check out my beam combiner vs head mounted dots and you may want to consider following me or subscribing to the RSS because there is another article I'm writing that really ties these things together.