Friday, 8 July 2011

Another Thursday, another partial success

The touch-sensitive posts last week were leading up to making some cool stuff but we've been waylayed with other stuff and beset by problems. Not least of all, getting our CNC machine to cut at a reasonable speed.

Another night at BuildBrighton and I arrived early to get set up and get cracking on with things. In the last three previous weeks, I'd had problems with the PC running Mach3 (the milling software that actually controls the CNC) and ended up having to do a fresh Windows install each time, just to get the PC working!
This week I was determined I wasn't going to have to install Windows yet again. The PC, however, had other ideas. After plugging everything in and switching on at the wall, the HP Compaq base unit gave a little cough, popped, fizzed, at started to smell really bad. On further inspection, the entire power supply had frazzled. Not only did I need a fresh install of Windows, I needed a fresh PC!

Despite this little hiccup, Robot Steve came to the rescue with a little tiny base unit that was already pre-installed with Mach3, from when he ran his own home-made CNC Machine. Which was great. Until I tried to import some dxf drawings. That's when Mach3 started moaning about invalid parameters, unit scale lengths being all wrong, tool paths went haywire - basically nothing worked as it should!

It took nearly three hours to get the CNC reproducing the 0.1" pitch grid that we successfully got working last week - but eventually we were up and running!

I installed a routing bit into the Dremel clone that was used as a router, got a simple dxf drawing to convert to gcode (and draw at the correct scale) and set it cutting.

Things went well (except for the horrendous noise and thick smoke billowing out of the MDF) until about half-way around the shape, when the Dremel stalled and the CNC machine stopped moving, despite the PC still sending movement commands.
It seems that when trying to cut some 6mm MDF in a single pass, the router bit was biting into the wood and being pulled down, out of the collet. The end result being that the cut was getting deeper and deeper - eventually stalling the machine once it hit the metal cutting bed. Ooooops!

Undeterred, I set about cutting a new piece of MDF - specifically adding some feed-rate commands to slow the machine down on the sharper corners, and setting the cutting height to about half the wood's thickness (the idea being to cut the shape out in two passes). This worked to a degree - except that on the sharper corners, the smoke really did billow out from the cutting head - and on the straight bits, the router jarred and bounced around. In the end I had to go for multiple, shallow cuts, to get the shape to cut out at all.

the banding on the edge of the piece was caused by setting the router to different depths and making multiple passes

The idea behind using a router rather than laser cutting was speed - the router was supposed to cut through the wood in a single pass, whereas the laser cutter would take three or four passes to cut the shape out. However, it seems that laser cutting 6mm (or even 9mm) MDF over three or four passes might actually be quicker than cutting the same shapes out with the CNC machine (which after testing tonight, seems to take three or four passes itself to cut through relatively dense material).

It might be that the Dremel router isn't powerful enough.
Or maybe I tried forcing the cutting head to quickly through the wood.
Or maybe not quickly enough.

In short, I don't know why, but routing MDF to cut out shapes was only partially successful. IT worked, and was repeatable (the cutting head went to exactly the same start place and followed exactly the same line every time). But given that it took so long to get a single shape out of the machine, it might not be the quickest way to produce shapes from MDF.

Different coloured bands caused by running the router at different speeds to determine the best cut. Darker bands caused by running the router very quickly, moving the CNC machine slowly: it created lots of smoke!

But on a brighter note, after getting it to cut some wood, I decided to try a drilling pattern, in preparation for next time; I'm hoping to pre-cut some copper boards before applying a PCB design with press-n-peel and etching. If this is half as successful as tonight, it will save hours when making circuit boards for the up-and-coming projects on this blog!