Much to my surprise I have found that low power diode lasers can be usefully employed with machines such as Tweakie for engraving and cutting certain materials using exactly the same vector toolpaths as would otherwise be used for example in engraving and profile cutting. The lasers are quite small in size, cheap to buy, can be fitted directly to the Z axis with the minimal of effort and are extremely effective at what they do.

Please be aware of the potential dangers associated with using lasers, don’t be fooled by their innocent looking appearance, they can bite and with just one mistake they can, in an instant, cause permanent eye damage. Observe all safety requirements and always wear the appropriate safety goggles rated for the wavelength of the laser in use.

A laser, even with a low power rating, is dangerous and must be treated and used with great respect. It can be capable of damaging eyesight therefore suitable eye protection must be worn at all times the laser is powered up. Never use a laser if children or pets, who are unaware of the potential danger, are present.

Here I am using a 150 mW diode laser with a wavelength of 655 nm clamped into a bracket bolted to the front of the Z axis (the part with the red danger label attached). Inside it’s black, aluminium housing is the same type of diode that is used in some high speed DVD re-writers and although it’s 150 mW does not seem like much, when focussed to a spot the size of a pin point it represents a great deal of power indeed.

High density foam 4mm thick is cut too easily, unfortunately the adhesive backing on this piece was not cut and the individual letters were only removable after soaking in some solvent. PVC insulation tape is also easily cut but the fumes are dangerous. Many materials will produce toxic vapours when burned - take adequate care not to inhale these fumes and always provide good ventilation to the workplace.

To drive the laser I am using an adjustable, constant current source which is capable of driving a 2 to 2.5 Volt diode at any current between 5 mA to >500 mA thus catering for most commonly available types. The laser diode used here is being driven at 170 mA which according to the manufacturers specification will produce 150 mW of laser output power (before the lens).

The drive PCB also incorporates a ‘TTL modulation input’ which will allow pulsed operation up to 15kHz and providing the pulse width is small enough and the PRF high enough (check with the diode manufacturers specification) the laser drive current, and subsequent output power, could be increased accordingly (an additional heatsink may also be required for this type of operation). I am, however, operating it in CW mode and as the specification for the modulation input is <0.5 Volt for off and >2 Volt for on, this has meant that I can switch the laser on and off using a signal derived from the Z axis direction pin of the LPT port. Therefore, by disabling any Z axis movement and manually focusing the laser onto the surface of the work I am able to use the same GCode file (vector graphics) albeit with a reduced feedrate that I would otherwise have used for conventional point engraving (+Z moves turn the laser off and –Z moves turn it on again).

Some materials require a slight delay (to allow the laser to bite) between starting a cut and proceeding (a bit like plasma cutting I think) and this delay is easily adjusted, within Mach, by changing the DRO scale factor of the Z axis. The optimum federate, for the material type, has to be found by trial and error and in the above examples 20mm per minute was used as the starting point (slow but sure).

I have added a focusing ring (the clear Perspex part) to the front of the lens to allow for easy adjustment of the focus without the use of special tools and by applying a reduced voltage (approx 2.5 Volts) to the driver PCB the diode will emit a low level light without lasing - this permits the focus to be adjusted visually to the smallest possible spot before starting the work.

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(8) The bracket used to clamp the laser to the Z axis. It is a lot more substantial than it needs to be but it was originally made to clamp an air tool and just modified a bit for the laser.

(9) Bush made from insulating material. All the laser diodes I have come across so far are extremely susceptible to electro static discharge and I have decided to isolate it from the machine frame as a precautionary measure. I am also using a separate PSU to power the diode and it is necessary for the negative side to be connected to the machine frame or ground potential to enable the TTL switching of the diode (on / off) from the LPT port and I don’t want to create any ground loops. Some diodes (not this one) may also have their cathode or anode connected to the casing so this bush serves two purposes.

(10) This is how the laser is finally mounted (in it’s insulated bush) to the Z axis. In photo 1 above it was directly clamped just for trial purposes (I really didn’t expect it to work as well as it does - I learn something new every day).

(11) The modification I made to the lens was to machine a small bush from acrylic and glue it to the front of the assembly (taking care not to get any cyno on the optics) so that the lens ring could be easily rotated to obtain optimum focus.

(12) As an alternative method of switching the beam on and off by using the modulation input, as mentioned above, I have constructed a simple shutter which is driven by a small (90 degree 4 step) stepper motor. With this method of beam control the laser is on continuously (for the duration of the job) and there is less chance of voltage spikes or current surges damaging the diode during the many switching operations of the + / - Z axis commands and although I have not actually destroyed a diode yet, they are extremely fragile and I am perhaps being a little over cautious.

(13) This is the 2 transistor, 2 position driver made to control the shutter stepper motor. It is driven directly from the Z axis direction signal and only requires a switching current of some 25A to move the stepper between either of its two positions (shutter open or closed). The stepper motor power (5 Volts) is derrived from a seperate LM317 voltage regulator. This approach has enabled the laser power supply to be totally isolated from the machine frame and may well be the best way to proceed. I intend to continue using both methods of beam control for a while until I finally decide which is the most suitable for my application.

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(14) Circuit schematic of the stepper driver, simple yet effective.

(15) Stepper driver PCB (as shown in pic 13), isolation routed with a 30 degree engraving point 0.08mm deep.

(16) This is the crankcase of my ageing OS 61 which is currently having a cleanup and new crankshaft bearings fitted. One thing that has always been a pain is cutting the new gaskets - this time I scanned the crankcase flange, converted the raster image to a vector drawing and subsequently into GCode then cut the gaskets using the laser. A perfect fit.

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CNC is only limited by our imagination.

Tweakie.CNC

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