Laser Engraving with Movable Gantry CNC Machine 28

All these while, I had been operating my diy movable gantry cnc machine as a milling machine. I knew that I could easily convert it to become a laser cnc machine, but somehow I had kept postponing it.

Perhaps I was not ready to delve into laser yet because of its potentially hazardous rays which could instantly cause permanent damage to the eyes if not careful. I really wanted to make sure that I was 100% sure how to handle laser before I could use it. I read extensively on the subject, watched many videos, found out about laser modules available for the hobbyist and had actually bought one quite long ago, but never got round to powering it up.

The cnc milling machine that I use follows the path written in the g-code interpreted by LinuxCNC software. If the machine carries a spindle, the latter will just cut away as it moves across the work piece. However, with a laser module, it gets a bit more complicated. The laser must be able to switch “on” when it needs to engrave, and switch “off” when it is not required to. So the g-code must contain movement commands for the machine as well as on-off switching commands for the laser module. Part of the preparation for using laser in my movable gantry cnc machine was also to find out how to control this switching, control the laser power as well as finding suitable software that could generate g-codes for laser engraving. There are basically 2 types of laser engraving – raster and vector. Naturally, finding out and deciding what to use was also part of my fact finding.

Well, finally the time had come for me to take the first step of installing a laser module to my cnc machine.

First came the design for my machine. The spindle for milling would be replaced with a laser module physically. This would not pose a problem at all because I had been doing this all this while.  When I had needed to replace the spindle with a marker pen for pen drawing, I just fabricated a pen holder assembly and attach it to the same Z-axis holder. For the laser engraving function, I would just had to design another holder assembly for it.

The laser engraving function would include the laser printed circuit board, and the laser diode/lens/housing/fan module. I wanted to use the same power supply for the spindle to power the laser module. Because of that, I used a DC-DC Buck converter to convert the 24 VDC supply of my spindle to a 12 VDC supply for the laser module. Since the converter was just a small piece of printed circuit board, I included it at the same laser holder assembly also.

I read that laser engraving would produce a lot of smoke from the burning of the work pieces. This smoke could cause dirt or vapor particles to deposit on the lens thus clouding it. To prevent this, I also tried to enclose the lens and designed some sort of positive air pressure boundary for the lens enclosure. The positive air pressure would come from a small aquarium air pump. This would also be included in the laser holder assembly.

The cnc machine would not be enclosed as I intended to use it as a milling machine also. So I decided to include a shoe piece in the laser holder assembly which I could attach to a flexible exhaust ducting for smoke removal. It would also serve as a light shield to prevent laser rays from coming out into the open. With this laser shoe functioning as a light shield, I felt it would be safe enough to operate without the need to wear protective glasses. At the time of this first test, the shoe piece was not fabricated yet, so the pictures did not show it in use. My upcoming videos will definitely have the laser module operating with a laser shoe.

The distance of the lens from the work piece would have to be determined even at the design stage in order to fit the lens module in place. The lens was movable, but it would be too troublesome to keep adjusting it for focus. So I just found a good position for it in relation with the housing and then fixed it with the set screws that came with the module.

During operation of the cnc machine, the vertical Z-axis could be moved using the LinuxCNC commands on the keyboard of the computer. Therefore the distance of the lens from the work piece was easily adjustable. The laser beam would have a range of distance from the work piece when it would be in focus and would give optimum results – perfectly clear and sharp lines. Out of focus distances would produce faint lines because of less concentrated heat. What I had to determine was the usable range of distance for any work piece and I should be able to get focused rays every time if I kept within its range.

In order to determine the best distances from laser lens to the work piece, I ran the laser at low power against a slanting piece of cardboard. The burnt impression left on the cardboard clearly showed the area with best burnt marks. Outside this area the lines were thin and faint or non existent. At these distances, the laser was not producing focused heat strong enough to burn. By taking note of the distances of the lens from the object when it was strongest during the experiment, I was able to determine the distances that I could use when doing an actual engraving.

The electric power to the laser control board could be turned “on” or “off” by a manual switch. I had used this when turning “on” or “off” my spindle. However, the laser control worked in a slightly different way. The laser would not turn “on” or “off” unless there is a TTL or PWM signal from the software g-code command. However, I also noticed that the laser switched “on” or “off” during boot up of the computer. This had something to do with the change of state of the parallel port pin signals of the computer when booting up. From the standpoint of safety, this could give rise to errors in determining whether the laser was “on” or “off”.

To avoid such errors, I installed a LED as a visual warning indicator for the signal. The PWM signal, partly controlled by software settings, would either be 2 ~ 5 VDC or 0 VDC. Now the LED would light up if the signal was on the high side. When it lighted up, I would be extra careful because the laser could be potentially “on”. I said potentially because it would still not be “on” if the power source for the laser was still not switched on. I did not want to mess around with switching “on” and “off” every so often so I rather have it always “on” and monitor by observing the light at the LED.

My first attempt at laser engraving was carried out without the positive pressure air at the lens and without any other light shield or exhaust suction. During this time, I managed to test it out by just wearing a laser protective eyeglasses. The test run for my first laser engraving was successful.