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IndustryArena Forum > WoodWorking Machines > Commercial CNC Wood Routers > Chinese Machines > Yet another aluminium sheet cutting thread...
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  1. #1
    Join Date
    May 2013
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    Question Yet another aluminium sheet cutting thread...

    Hi all,

    I know this has been discussed many times, and I have perused many related threads here and elsewhere, but my first attempt at cutting aluminium sheet on my OmioCNC X4-800L (800W spindle) failed miserably and snapped the cutter :tired: I would greatly appreciate a sanity check of my method before trying again as I have a very limited supply of aluminium, cutters and cash...

    The intended result is a small panel, 235x110mm and 1.5mm thick, for mounting a row of eight rocker switches into, which I have modelled using FreeCAD:

    Attachment 498326

    The CNC operation consists of a single "profile" job, since I'm skipping machining the drilled holes for now. For my initial attempt I used a 2mm single flute cutter, 12000 rpm, 0.2mm DoC and 1500mm/min feed rate. Foolishly without lubrication. The aluminium sheet was held down on a piece of MDF only by means of edge clamps. And I failed to notice at the time but the path moved the bit CCW around the cut, which I'm pretty sure is the wrong way for a right-handed cutter. The bit snapped about half way through cutting the first switch hole, and the cut was very rough with lots of gummed up aluminium along the top edges. For my second attempt I intend to make the following changes:

    • Make the paths go clockwise
    • 4mm single flute carbide cutter
    • 18000 rpm
    • 0.5mm DoC
    • 1200mm/min feed (300mm/min vertical)
    • Lubricating with WD40 spray
    • Removing chips with a brush
    • Improve work holding somehow


    I used this formula for calculating the feed rate:

    Chip load x RPM x number of flutes = feed rate
    0.08 x 18000 x 1 = 1440mm/min

    The 0.08mm chip load I got from here. It's on the lower end of their estimate for a 4mm cutter.

    I'm taking it back a notch to 1200mm/min. The aluminium stock I have is the cheap horrible stuff, which I realise may not be possible to mill cleanly, but I want to give it the best possible chance under the circumstances. I did notice increasing noise levels as I made the first attempt, and I suspect the work not being held firmly down may have contributed to the problems. Certainly neglecting to lubricate was a big mistake. I'm hoping that a higher spindle speed and deeper DoC will produce actual chips as opposed to the fine powder I got on my first attempt - though I freely confess that's purely based on numbers I've seen in some other posts and has no theoretical foundation whatsoever.

    Will any of these changes improve my chance of success, or perhaps only make things worse?

  2. #2
    Join Date
    Jan 2016
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    100

    Re: Yet another aluminium sheet cutting thread...

    First you need to address the working holding. A piece that is moving around is going to be a disaster regardless of your speeds and feeds. You could try using CA glue and blue tape, or use something like Scotch Permanent Double Sided Tape (although make sure you can get it off the work piece without wrecking it first). What are you using for a width of cut? I'd start with a width of cut that gives you a real chip and then adjust the depth of cut based on your machines rigidity.

  3. #3
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    May 2013
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    49

    Re: Yet another aluminium sheet cutting thread...

    Thanks @jaguar36, I was going to cut no more than the tool's width, or in this case 4mm. Is there any benefit in making the cuts wider? Going to try the painter's tape and superglue, and will pre-cut the aluminium sheet so that it doesn't cover the screws holding the spoil-board in place, in case I need to take it out of the machine to get the stock off it.

  4. #4
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    Nov 2013
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    4280

    Re: Yet another aluminium sheet cutting thread...

    Hi,
    the best night-and-day improvement I made was flood cooling. Even with air blast and WD40 I would get uneven results, but when I went to flood cooling I get perfect results 99.9% of the time.

    Craig

  5. #5
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    Jul 2018
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    6248

    Re: Yet another aluminium sheet cutting thread...

    Hi Lomax - The biggest improvement I've had with aluminium is using DLC coated tools. Its like teflon and AL doesn't stick to it. I use 0.05mm chipload as a starting spot. Also for cutting the profile learn how to spilt the profile into a LHS and RHS cut. Clamp RHS cut LHS, swap clamps then cut other side. Peter

    DLC - diamond like carbon

  6. #6
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    May 2013
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    Re: Yet another aluminium sheet cutting thread...

    Thanks guys! Unfortunately I don't have any money to spend on cooling or tooling, though I have been dreaming about building a flood cooling system some day. I imagine waterproofing the entire box that my CNC sits in using acrylic sheet and adding a drain to the bottom but I wonder if the Y-axis linear guides and lead screw under the bed may end up getting clogged by swarf? Alternatively the "tank" could be built on top of the machine bed, but that would pose a challenge for my rudimentary G-code skills - I'm sure it would only be a matter of time before I crashed the spindle into one of the walls, flooding my room with coolant...

    Improved work-holding seems to be where I can expect the most improvement for the least cost, so I'll start there, but I'm curious to hear what you think about the other parameters?

  7. #7
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    Re: Yet another aluminium sheet cutting thread...

    Hi Lomax - your going to need a new bit sometime and a worn bit has no chance with aluminium so get DLC bits when the time comes or when you start tearing your hair out... . They don't need lub so then you sidestep the flood issue. Peter

  8. #8
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    Re: Yet another aluminium sheet cutting thread...

    Thinking about it, I might design a test object in FreeCAD that only consists of a number of parallel slots, each performed with a different combination of DoC, feed & spindle speeds. That would allow me to empirically compare a whole bunch of settings - provided I don't break the tool...

  9. #9
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    Jul 2018
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    Re: Yet another aluminium sheet cutting thread...

    Hi Lomax - Don't be haphazard with your test. A good starting chipload is 0.05mm. 0.025mm is more like a finishing cut (if your machine and tool runout is good enough to cut 25um)... Thats all you need then its best to use a 1F tool this solves the runout problem and the feed speed problem. A bunch of random feeds/speeds is asking for trouble. It all comes back to your machine rigidity and can it do the cut? Once you get a good cut then stick to it... DOC is a function of machine power and rigidity so start at say 0.5mm DOC, pick 0.05mm CL then 0.1mm chipload if successful then get deeper. But always understand and work with your CL thats the secret to good cuts... Peter

    runout is a problem as if you have too much, one tooth is cutting and one is rubbing (with a 2F tool) the rubbing tooth gums up and your history. With a 1F tooth your always cutting even with some runout...

  10. #10
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    May 2013
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    49

    Re: Yet another aluminium sheet cutting thread...

    I'm not sure "random" is an entirely fair description of what I had in mind. But I could cut a series of parallel slots gradually increasing DoC and chip-load, and see which comes out the least messy. Saying that, I don't believe the relationship between spindle speed and feed speed is completely linear, so it would be interesting to explore that parameter space first while keeping the chip-load relatively constant. For example, these four combinations all have a chip-load of 0.05mm, but may not produce identical results:

    12,000rpm 600mm/min
    15,000rpm 750mm/min
    18,000rpm 900mm/min
    21,000rpm 1050mm/min


    Perhaps repeat these with 0.5mm and 0.2mm DoC, then pick the best of them and run another test of that one with different feed speeds, to get chip-loads between 0.02 and 0.1. What could possibly go wrong :devious:

    Interesting that DLC bits don't need lubrication - I'll put some on my wish list...

  11. #11
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    Re: Yet another aluminium sheet cutting thread...

    OK so the next thing to put in the mix is surface speed. The faster the surface speed the more the friction, the more heat and then you gum up the edge. Have a look at FS wizard it spits out various info for you. 4mm tool at 10000rpm is 125m/min which is good. FS says 10000rpm 587mm/min at Fz=0.029mm chipload for soft aluminium. So you could run at 20k and 1200mm/min at 250m/min Vc and it would be good. <300mm/min surface is good. Peter

    FS says this requires 30W with 1.6kgf cutting load so even small routers can do this.... this is HSS with 2F tool. tool suppliers have charts with max surface speeds in them...

  12. #12
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    Re: Yet another aluminium sheet cutting thread...

    Hi,

    but I wonder if the Y-axis linear guides and lead screw under the bed may end up getting clogged by swarf?
    Do you not suppose that the Y axis linear guides and leadscrew gets bombarded by swarf as it is? Coolant is not going to make that any worse, nor really likely to do any harm either.

    I bought a cheap 8l coolant tank and pump eleven years ago and I use it daily. You are right you need a big tray to sit your machine in so you can catch all the coolant and have it drain back into
    the coolant tank via a filter. For quite some time (with my first mini-mill) I used a really big plastic bag and slipped it over the mill like a condom to keep the coolant in check, and so it drained into the tray underneath.
    The plastic bag was clear so you could still see the job being done....it worked mostly. My new mill has an enclosure and a big tray underneath...its a joy to use.

    One of the improvements that I want to do over the next year or so is to redo the tank to 100l, have a filter that you can pull out to clean and a high pressure pump, possibly with a filter in the overpressure return to the
    tank. I have a 1hp three phase motor and VFD already in hand for the upgrade. I want really good flood cooling, not disimilar to Piotr's machine. The good thing about flood cooling is that it washes the chips out of the
    cut zone....and that is the key....if you can do that then cooling and lubrication is just icing on the top.

    Anyone who tells you that chips get hot because of friction does not know their physics. The tool exerts enough pressure on a very small area of material than the material exceeds its yield strength and deforms.
    The chip deforms until its ductility is exhausted and the material reaches its max tensile strength whereon the chip breaks. That process is how a chip forms and it requires a transfer of energy as the tool 'does work'
    on the chip. No doubt friction also plays a part, but the vast majority of the heat is the 'work done' on the chip. When I get home I find a reference for you to look up....but is describes how a chip is formed
    and the energy (in Joules) that goes into each cubic mm of chips for a range of materials.

    Craig

  13. #13
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    Re: Yet another aluminium sheet cutting thread...

    Hi Craig et al - They (chips) get hot because of internal friction as its deformed and some surface friction. If you get a wire or paper clip and flex it you will feel it gets hot. Thats the internal grain structure rubbing against each other just like rubbing your hands together to get warm. From the colour of the oxide you can see they get very hot around 600-800C so there's lots of internal friction going on. It all comes from the tool contact hence the spindle motor... Peter

  14. #14
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    Re: Yet another aluminium sheet cutting thread...

    Hi,
    there is work done on the chip that is to say there is a force that operates over a distance, and that constitutes work or energy transfer.

    I understand you could say that its 'internal friction' and that the heat is a result of friction over atoms/molecules sliding over each other, but no amount of lubrication is ever going to get inside the
    chip to reduce that friction. I prefer the notion of temperature rise as a result of work done. The temperature rise is the work done times the volumetric heat capacity assuming we can agree on
    thermodynamics???

    I'll post the reference later tonight, the book was written by Paul deGarmo, a professor in mechanical engineering. Good read.

    Craig

  15. #15
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    Re: Yet another aluminium sheet cutting thread...

    Of course lub is not going to get into the chip... Its not a thermodynamic issue. It can be modeled using thermodynamics to figure out the heat generated but; Using a volumetric heat capacity inferes the heat is coming from an external source. Its not, its the internal friction due to deformation creating the heat and this must equal the work done to create the chip (minus a bit that transfers to the environment). If you grab a hot chip and put it into a bomb calorimeter you can measure the heat loss of the chip. This gives you the work input to the chip (plus a bit) so you then can figure out the forces involved to deform the chip... If that's of interest to someone. Just get the right size spindle for the job Peter

  16. #16
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    Re: Yet another aluminium sheet cutting thread...

    Hi,

    Its not a thermodynamic issue. It can be modeled using thermodynamics to figure out the heat generated but; Using a volumetric heat capacity inferes the heat is coming from an external source. Its not, its the internal friction due to deformation creating the heat and this must equal the work done to create the chip (minus a bit that transfers to the environment).
    I disagree, of course its thermodynamics.
    The first law is the change in internal energy = the heat transfer minus the work done .

    Attachment 498386

    In our case we assume that the chip is adiabatic, ie no heat transfer into or out of the chip, a reasonable approximation, and the work done on the chip, as opposed to the equation where work is done by the chip.
    So the change in internal energy = the work done to the chip (by the tool, which is of course the integral of the force applied by the tool by the distance over which that force is applied).


    The second law is that in an isentropic system the temperature rise is equal to the work done (internal energy rise) divided by the heat capacity, and volumetric heat capacity seems the logical choice.
    If you want to be particular about it you should integrate over both the constant volume path AND the constant pressure path, but usually you choose one (constant volume OR constant pressure) as the
    closest approximation to the actual integral path.

    Attachment 498390

    Craig

  17. #17
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    Re: Yet another aluminium sheet cutting thread...

    Lots of words Craig - but what your saying is that if I push a block of steel across my kitchen table the work done by my finger will heat up the steel. Not so, its the deformation that heats it up. I worked a press once that squashed steel and the parts came out hot... Peter

  18. #18
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    May 2013
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    49

    Re: Yet another aluminium sheet cutting thread...

    Just a quick update: Since I already have shop air I've decided to buy a mist coolant set-up and install this before trying again. It's very little money and from what I've read it could make a huge difference. The kit looks like this:

    Click image for larger version. 

Name:	YS-BPV-3000.jpg 
Views:	0 
Size:	150.8 KB 
ID:	498396

  19. #19
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    Re: Yet another aluminium sheet cutting thread...

    Hi peteeng,
    exactly, its the deformation that causes the heating, not surface to surface friction. That deformation requires that work be done on the chip and that work shows up as a temperature rise.

    Craig

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