Monday, 7 May 2007

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.


  1. As I'm sure you realise, the problem with machining aluminium is stopping it from melting and clogging up the tool. I think industrial methods include using oil or water baths, and straight fluted cutting bits that encourage the swarf to fly out rather than chase round a helical flute. Laser and water-jet cutting are also used.

  2. No I hadn't realised that the tendancy to clogg was due to it melting. I just thought it was because it is soft compared to other metals. Makes sense though, hence the lower drill speeds.

  3. It seems that you discovered peck drilling! For deburring holes like this, you can also use a countersink. It makes for a very nice finish. Also, using a center bit to start the hole puts a nice cone in the material which will guide successive bits.