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.

Heavy metalwork

Just spent all evening making a 150mm cut across a 6mm sheet of aluminium. Not fun I can tell you. I used a jig saw and got through three HSS blades. After the first blade I tried using paraffin as a lubricant and it made a big improvement. Holding the saw for about an hour and a half on and off was horrendous because of the the vibration. It was all the more annoying because I paid over the odds on eBay to get a piece exactly the right size but the description changed during the auction. There was even a line already marked where I had to cut it.



Ironically, after finding it so hard to saw, I managed to clean up the edges on my bench sander demonstrating how soft aluminium is. I probably needed a saw with coarser teeth. Perhaps I should have tried a wood blade but I think that would have been too coarse.



Drilling next: I hope that goes a bit better. I will definitely use paraffin again. I am not sure where I got that tip from, it might have been my Dad.

PS: Found a better way of blogging my pictures so now you can click on them for a higher res version. If anybody wants to see a better version of a previously posted image let me know and I will upgrade it.

More trouble at' mill

Well things did not quite go to plan. The first problem was that most of my milling bits have ends that are smaller than the top of their shafts so they cannot go very deep. The only exception is a 1/8th inch end mill that came as part of my PCB routing set. It is intended for routing PCB board outlines. The problem was that my first drill did not have a collet big enough to take it. The Minicraft replacement has a three jaw chuck which will. So plan B, I fitted the Minicraft drill into the bottom MDF mount intended for my other drill. Rather than tackle the big block of hard plastic I thought I would try it out on a sheet of scrap polystyrene first. I stuck this down with double sided sticky tape.



I started with a feed rate of 4mm per second. This was far too high as it stalled the drill. I dropped down to 1mm per second and then to 0.5mm. At this point it seemed to be able to handle the cutting but it kept snatching horribly. I dropped down to 0.1mm per second which took ages but it still snatched. I only got as far as the first corner before I aborted. You can see that after the first 15mm it no longer goes right through the plastic. This is because I did not tighten the chuck enough and the bit slipped.

I might be wrong but I think the snatching is due to the set up not being rigid enough. I had already identified that as a weak point and the new motor mount was aimed at improving it. The first drill was supported at both ends but this one is too short so it needs a much stiffer mount, which is what I was trying to make! Perhaps the end mill bit is not suitable for styrene, or perhaps styrene is not very machinable, or perhaps the RPM is too high, or too low. Can you tell I am a bit out of my depth here?

One thing I can do to improve stiffness is to replace the 2mm aluminium plate with the 6mm slab I already bought for the job. I was putting that off until I got the new mount so as not to have to drill two sets of holes in it.

Another thing I might try is to make a top mount out of PolyMorph.