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.

On the right path

Having tried a cheap mini drill as a router and found it lacking I ordered a cheap laminate trimmer to try instead. While I was waiting for it to arrive I remembered that I had inherited a Minicraft drill from my aunt.



To my surprise this seems quite promising. The bearing is solid and it revs to 20000 RPM without too much noise. It also has the advantage that it is small and light so is suited to a multi-headed machine. I decided to see if I could use the first drill to mill a mount for the new one out of a block of hard plastic. I designed a simple two part clamp using Visio.



I represented the outline of the parts using a couple of Python lists.

bottom_clamp = [Line(0,34), Line(13,34), Arc(30,34,47,34), Line(60,34), Line(60,0), Line(0,0)]
top_clamp = [Line(0,26), Line(60,26), Line(60,0), Line(47,0), Arc(30,0,13,0), Line(0,0)]

Line and Arc are simple classes with a do method which instructs HydraRaptor to move along the segment. E.g.

class Line:
   def __init__(self,x,y):
       self.end = (x,y)

   def do(self,hydra):
       hydra.feedto_xy(self.end)

It was then trivial to get HydraRaptor to draw the outlines with a pen using a for loop :-

for seg in outline:
   seg.do(hydra)

However in order to be able to mill it out I need to create a tool path which is offset by the radius of the bit. This is a little harder than it sounds. It is easy enough to offset a line at right angles to its direction but then the ends no longer meet. A new intersection point has to be calculated and a different calculation is required for each possible pair of segment types. I decided to prove to myself that I could still do O-level maths and worked out the equations from scratch. A quick Google afterwards verified that they were correct before coding them up. Here is the "line to line" case http://local.wasp.uwa.edu.au/~pbourke/geometry/lineline2d/ and here is "line to arc" http://local.wasp.uwa.edu.au/~pbourke/geometry/sphereline/. The trick is to represent lines with a pair of parametric equations rather than simply y = a +bx to avoid having special cases for verticals.

Having done that I got HydraRaptor to draw the tool paths with the pen. Here is the result :-



The next step it to actually mill it. I am not sure how I am going to hold the plastic block down yet. I think I will try double sided tape and if that fails contact glue.