My Choice

After researching a bit I found there were several variations on the K40 that were being sold. The main variations that I cared about were

1) Analogue or Digital power setting
2) Air assist
3) Moshi-Draw / Laser-Draw / Corel-Laser
4) Location of device (UK or China)
5) Cost

1) The Analogue or Digital power setting is the method of setting the laser's power level. The laser can be adjusted from 0% (not firing at all), to 100% (40w of laser power). The Analogue version has a knob and a readout meter (misleadingly measures in milliamps). The Digital version has a set of buttons marked +/- 0.1%, 1% and 10%. The readout shows values from 00.0 to 99.9. They are essentially the same thing, but I wanted a digital readout for no real reason. I've since found out that there are real differences under the hood that some people think matter. The input to the power supply unit that delivers the high voltage that the laser needs has a power level input pin. You can vary this pin from 0 volts to 5 volts to get the intend power level. The Analogue version literally does this, it varies the power to this pin to give the required power. The Digital version however doesn't do this. Instead it uses PWM. PWM stands for Pulse Width Modulation. Simply put it means in any given time frame (in this lasers case 50 microsecond slices) the controller will put 5v on the pin for X amount of time and 0v on the pin for Y amount of time, X+Y being equal to the timeslot length of 50 microseconds. This means that having a very short 5v or "ON" length and a long 0v or "OFF" length will fire the laser for a short amount of time in the timeslot. Repeat this hundreds of times a second and you get lots of short pulses.

If you want high power the ON pulse will be long and the OFF pulse short, giving over time an average or more ON than OFF. At full power the on pulse is the full length of the timeslot, effectively giving always on. Doing it this way means the laser is never at half power for example, just at full power for half the time. The results in the actual cut are the same but some argue that running the laser at full power is bad for it and it wears out quicker, and using PWM means it is on full power, even if only in small bursts, all of the time. Others argue that the first arguments are unfounded and the laser life is rated as (time used * power level), which is essentially the same with both techniques. I've yet to find a definitive answer on this.

2) Air assist is a technique used to increase cutting depth and reduce smoke and heat damage to the material being cut. Air assist simply blows air directly at the point where the laser is cutting, cooling the material a little and blowing away smoke and debris caused by the cut. This increases the cut depth as the smoke and debris can get in the way of the laser beam and reduce its power.

3) Moshi-draw is the original software that came with the first K40 models. It is still shipped with some today. It is not nice software and barely allows you to use your laser cutter. It should be avoided at all costs. Early revisions didn't even let you cut, only engrave. Also, the versions I've seen being supplied are XP compatible only. How many people still have an XP PC? Laser-draw is the newer release which handles cutting and engraving, runs on Windows 7 and is at least usable. Preferably you want Corel-Laser. This is a copy of Corel-Draw (a half decent-ish vector art package) with a plugin for the laser cutter. Unfortunately because the machine is locked to a supplied protection dongle, you can't just use any software with it. you must use the supplied software. Many people use third party software to design their cuts / engraving and then import a save file into Corel-Laser or Laser-Draw.

4) I felt the location of the device was very important. There was very little chance that I would get away without paying import duty, which I know can fluctuate by exchange rate and service charge of the courier. And I assumed (wrongly) that if I found a UK supplier that they would have at least checked the machine for major damage before shipping it on to me. I also assumed (wrongly) that I could deal with a local seller in case of any problems.

5) I was prepared to pay between £500 and £600, but obviously I wanted it as cheap as possible with the features I needed. In the end the choice was half made for me. A batch of cutters came on eBay for £350 including delivery. They were from a UK supplier. They had a digital power setting. They had Laser-Draw and Corel-Laser but unfortunately didn't have Air Assist. However at £350 I didn't want to miss out. So I pressed buy one lunch time and began worrying.

Since this post, some sellers are as low as £320 including delivery.