Sunday, 13 May 2007

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:

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.

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