Tweakie.CNC - Making stuff - Routing and Profile cutting

It was at this point that I first discovered Lithophanes and once seen, I had to be able to make them, they are really impressive. Yet another piece of software was required. I started with 'Bmp2Din' and later tried the trial version of 'Bmp2Cnc', both produced good results. This Handsome fella is a striking example of my fascination with lithophanes.

Another software is 'CamBam', this is capable of producing the depth maps necessary for lithophanes and just about anything else I would want to make. The down side is that I am finding it difficult to learn how to use it properly. According to its authors, 'CamBam' is still under development but it already has a large following with very active website, forums and people willing to help with problems. One such person is 'Yohudi' aka Danny at cnc4free.org. CanBam is one of the few programs I have tried which handle the ‘cutter radius offset’ function without programming extra ‘lead in’ and ‘lead out’ paths. The accuracy of profile work (assuming that the cutter diameter is specified correctly) is truly outstanding. 10 out of 10 for CamBam.

Initially there are decisions to be made which once you have decided are probably difficult to reverse. Stepper motors or servo motors, there are many arguments for each but I have chosen stepper motors. Operating language, there are many of these, I have chosen G-Code. Both of these decisions were based solely on popularity amongst 'hobby CNC builders' and the availability or otherwise of the software & hardware.

Tweakie has been constructed using bipolar stepper motors as the driving force for each of the three axis. These stepper motors require a dedicated form of control electronics (which, fortunately, is readily available and is not expensive) this consists of one or more discreet p.c.b's. which require minimal wiring to get the whole thing working. An additional requirement is of course a PC. All that is required of this driving computer is to convert the source G-Code to Step and Direction pulses for each of the 3 axis and the software 'Mach3' takes care of this task perfectly. I originally tried a low priced laptop without much success as it appears that Mach3 derives its timing pulses directly from the microprocessor and if the processor has a high overhead (carrying out tasks in the background) it multi-tasks. This affects the timing pulses which become erratic, upsetting the speed and reliability of the stepper motor positioning. An old and very basic PC solved the problem, I can run the steppers at high speed and no steps or pulses are lost. Artsoft (owners of Mach3) do provide helpful tips on reducing background tasks and suggest ways of improving performance - the bottom line, of course, is that they provide the software on a 'try before you buy' basis. I wish all software was available like this.

Now that Tweakie is operational, design software is required to create the G-Code in order to make things. There are of course many good (and free) programs out there and my first trial was with 'G-Simple'. This program let me design and profile cut the CNC cube. It was quite easy to get started by following the examples given and I had produced a perfect G-Code for my cube within a couple of hours. I was, and I still am, absolutely amazed at the accuracy to which these parts were cut. They were a little undersize (probably due to non-concentricity of the Jacobs chuck I am using) but each part was as identical to each other as I could measure with verneer callipers. To be fully proficient at using G-Simple, well, this would take some time.

For vinyl cutting, different design software is required but a fortunate thing is that pen plotters have been around for a long while now and there is a lot of freely available programs which produce HPGL Code (Hewlett-Packard Graphic Language) which has become the industry standard for plotters and guess what, HPGL is easily translated to G-Code. My first venture was downloading ready made HPGL graphic files (Company Logos), converting them to G-Code using 'Target2GCode' then feeding the result into 'Mach3'. Success first time ?, not quite, the vinyl cutter I had made and fitted to Tweakie needed setting up but I was soon making lovely decals. To create original artwork I used a trial (now sadly expired) version of 'SignGo' which again outputs HPGL which I converted to G-Code as before. Now I had to buy lots of different coloured vinyl (sticky backed plastic) and the low tack application tape and I was making signs.

Wood routing. Tweakie was designed to take a 27000 rpm Bosch router but this was so incredibly noisy I tried the 3000 rpm, 3 phase motor I had used for profile cutting and with a 6mm, single flute milling cutter this routed softwoods, mdf and hardwoods perfectly (obviously the feed rates had to be reduced because of the lower rpm but the results were really good). I should say at this time I had a lot of failures along the way, not getting the cutting depth correct, starting at the wrong point on the work, getting the scale factor wrong, not zeroing the axis properly etc. etc. etc. and my photos only show the good stuff, not all the junk that went into the bin.

Because I am using preloaded ballscrews for the X and Y axis drives there is no backlash here and profile work is extremely accurate. My Z axis is at present a trapezoid leadscrew with a bronze nut and this has about 0.25mm backlash but the overall weight of this axis takes up the slack in one direction with no problems. This is only true because I machine soft items such as wood and plastic - for a machine that cuts metal this would not do and some form of backlash removal would be necessary. I have made future plans to rebuild the Z axis using another preloaded ballscrew but with so many projects on the go at once this will have to wait at least for a while.

The experienced PC which I am using to drive Tweakie has just decided to fail for the second time this year. Again it is the ATX power supply. There is no problems replacing this item but I am wondering if the workshop environment with its wide temperature swings is what's causing the failures. If I get another failure here then some redesigning of the system will no doubt be necessary.

E-Stop and Mach3.

The software polled 'E-Stop' function implemented by Mach3 should (from experience) always be connected by a 'Push to Make' switch and the input signal configuration set to 'Active Low'. Although the software allows for 'Push to Break' and an ‘Active High’ configuration this can, under some circumstances, particularly when running long programs cause unwanted triggering of the E-Stop. The problem may be caused by signal glitches from the parallel port or by the way in which Mach3 operates, either way this becomes unimportant if a 'Push to Make' switch is used. Although any unwanted triggering, can in itself be overcome by increasing the 'Input Debounce Interval' this in turn can cause further undesirable effects. For example, if the debounce interval is set at say 500mS or greater and the E-Stop switch is of the 'non-latching' type then in an emergency a quick push of the switch can be missed by Mach3, in other words the switch must be held for the length of time set by the debounce interval for the E-Stop to be recognised.

Ideally then, the E-Stop switch should be of the 'Latching - Push to Make' type. Commercial stop switches usually have a 'latching' action and these should be used in preference to the standard push button.

E-Stop should not, of course, be relied upon for your personal safety or protection and as I have mentioned elsewhere, the only safe stop function instantly disconnects the incoming power supply to all functions of the machine. The machine itself should also be designed to operate in a ‘Fail Safe’ manner so that there are no undesirable consequences upon disconnection of the power.

Use a glove or piece of rag when handling sharp cutting tools. Not only will this prevent cuts to your hands it will also prevent the finger prints, which cause corrosion, from damaging the fine cutting edges. Engraving bits are especially susceptible to this sort of damage.

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Tweakie.CNC