Dust bustin'

I decided to tackle dust extraction as my first attempt at milling plastic made a right mess as you can see below in the video. The problem with taking a very fine cut is that you get very fine chips. These defy gravity and even stick to the underside of the things, presumably due to static.

My first idea was to enclose the tool in a pipe which reaches down to just above the workpiece. A vacuum feed then needs to come in from the side. I bought a small vacuum cleaner for the bargain price of £17.42.



It seems a bit ridiculous to use 1300W to remove dust from a machine that takes less than 5W to make it but I can't think of a better solution. I might be able to reduce the power with a motor controller if I find I have suction to spare. If not I can always put the air conditioning on to waste even more power.

I tried using a reducer and a small diameter pipe but the reduction in suction was too great so I decided to keep the full bore right to the workpiece.



I started with a small waste pipe T-junction and cut most of it away to leave the shape I wanted.



I then used HydraRaptor to mill a ring to reduce the top pipe diameter to the size of the drill shaft. This will also be the means of attaching it to the underside of the bottom motor mount. I practiced first on a piece of scrap 2.5mm polystyrene before making the final piece in 5mm ABS. You can see that it was a good idea to practise on the scrap first as I made two really stupid mistakes. The first one was to cut the outer circle first! The second one was to offset by just over the diameter of the ring to do a second one. I forgot to add the diameter of the tool! The ring on the right is the finished article. I was able to increase the feed rate to 20mm / second, sticking with the 0.1mm cut depth.



Here is the finished article :-



This is a bit of a landmark as it is the first useful item that HydraRaptor has made. Not very RepRap as it is a subtractive rather than additive process, but it is a part of itself.

While HydraRaptor was making the ring I held the vacuum cleaner nozzle close to the work piece. It was very effective at removing the dust even though there was only a very poor seal. It has the advantage that it does not need to move with the z-axis. It only needs to be suspended in a fixed position just above the work piece. I decide to abandon the piece I had just made and go for a bracket that can hold the nozzle at a fixed but adjustable height. I made this contraption out of a bracket that was formally used to hang a microwave oven on a wall.



This method works well for plastic but I may have to revert back to plan A when milling copper as the chips are a lot heavier.

BTW, I came across a much better plastic identification flow chart, than I mentioned last time, in the RepRap forums: www.texloc.com/ztextonly/clplasticid.htm.

The next thing I intend to tackle is a tool height sensor as it is a pain getting Z = 0 to be exactly where the tool meets the sacrificial base material each time a tool is changed.

Successful milling

The new metalwork did the trick. It completely solved the snatching problem I had previously. I managed to mill this test shape out of 2.5mm polystyrene, at least that is what I thought it was. I found a site: www.tempatron.co.uk/weld_rods.htm which describes how to identify plastic by setting fire to it and the closest match is ABS, so it may in fact be ABS.


As you can see it has nice clean edges and it measures 34.07mm by 60.02mm by my cheap electronic calipers so is pretty accurate.

Getting the feed rate and cut depth right took a few attempts. The problem was that the plastic kept melting as you can see here :-



I found that with a cut depth of 0.5mm I could only feed at about 0.2mm per second to prevent melting. Reducing the cut depth to 0.1mm allowed me to increase the feed to 10mm per second which is ten times more productive. I probably could have pushed it further but I am not particularly interested in making anything from this material. Here is a video of the test :-



I now have a small but highly accurate CNC milling machine. A few improvements are definitely needed :-

1. Dust extraction!
2. Tool height detector
3. Spindle motor control and stall detect

Stiffening up

Having severely underestimated how stiff a milling machine needs to be several times, I decided to make a solid job with my next attempt. Two days of metal (and plastic) work later here it is :-



The back plate is 6mm aluminium sheet strengthened up with a couple of 8mm by 20mm aluminium rails. The bottom mount is 13mm thick aluminium and the top mount is the mystery metallic plastic composit I mentioned earlier. I am still none the wiser as to what it is but I can say that it is light, very rigid and machines very nicely. Drilling makes perfect holes with no burr. The only thing I don't like about it is that it gives off a very fine dust. I expect that goes hand in hand with machining well.

The end result is solid as a rock. I think the only weak point now is the MDF frame but as I only want to do light milling I hope to get a way with it.

I have improved my metalwork techniques since the previous attempt. It is actually more difficult that one would imagine to drill accurate holes. Here is the method I use :-

1. Make a template in Visio with the outline of the work piece and cross hairs where the holes should be.
   
   
   
   


2. Print it out 1:1 and cut round the outline.


3. Place it over the work piece and centre punch where the cross hairs are with an automatic punch.


4. Drill pilot holes at 1.5mm, using a drill press.


5. Open the holes out 2 or 3mm at a time by using successively larger drills until the target size is reached.


6. De burr with a larger drill.

When drilling aluminium you need to keep the drill speed low, especially with larger holes. A drill with continuously variable speed is handy.

If the going seems to suddenly get tougher, for no apparent reason, then it generally means the bit has got clogged. Back out, switch off and pick the swarf out of the flutes. Using paraffin to lubricate helps to prevent this.

The next thing is to try milling again although I no longer need the drill mount I was trying to make earlier as I have had to make it by hand.