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  1. #11
    Ok,I'm going to attempt it...only because I am eventually going to build the CNC in Allum but that will be sometime as I'm aquiring the gear to make my own Allum melting furnace with a view to casting the parts(can'twait to do this,then its party time.
    Slow in aquiring the parts though,only got the propane bottle,3 kilos of Allum,Prpane Gas Torch,Welders helmet.

    Need an Apron,Gloves and crickters leg pads and a crucible. :heehee:

  2. #12
    I would look to get some large pieces of Ally to melt as the smaller the pieces the more oxide you will produce.
    Ideally you need a reducing atmosphere for ally not easy with propane as you need a lot of air to get the temperature.
    I would like to try but not enough garden(boss has too many plants)

    peter

  3. #13
    OK,I'm going to attempt it...only because I am eventually going to build the CNC in Allum but that will be sometime as I'm aquiring the gear to make my
    Good on you. I wasn't trying to discourage you, just offering some options. If the usage and design can be matched then I don't see why It wont work. I just hope you arnt expecting this to mill the Ali castings you will produce

    Start a build log stating exactly what size cutting area, type of spindle, motors and material to be cut and the materials you have already got.

    I'm thinking something like A4, A3 max with a high speed dremmel running on metric studding for cutting foam blanks,pcbs etc......

    Exactly, the sturdiness is not there if the machine is vibrating :). With perspex you will have them both: lack of sturdiness (this means vibrations), and cracking on vibrations - perfect match :).
    I would tend to disagree with that :naughty:(not like me I know) it is a stiff machine that will vibrate, less sturdy machines will move and damp the vibration. The vibrations are caused by the machine in motion or the spindle and cutting tool, so if gentle accel , decel and a smooth spindle are used then it should be OK no?

  4. Quote Originally Posted by Ross77 View Post
    ...it is a stiff machine that will vibrate, less sturdy machines will move and damp the vibration. The vibrations are caused by the machine in motion or the spindle and cutting tool, so if gentle accel , decel and a smooth spindle are used then it should be OK no?
    Hi Ross77,

    My understanding is like this: the lower the stiffness (I mean more elastic), the lower is the resonance frequency for a given assembly. Rising the resonance frequency over the mechanical frequencies generated by the movements and the spindle makes the assembly less prone to vibrations - that's why everybody is trying to make the machines sturdier for a given mass.

    It's true, you can absorb the vibrations with an absorbent material - like rubber, or, in this case, perspex :). It's true, as well, that you can reduce the accelerations (accel & decel) and the feed rate until a given structure will not vibrate - if you are lucky enough to not obtain resonance... but, is this practical?

    Now, I am sure that George's machine will work somehow, but in my opinion, the pieces or perspex he has would work much better as material for some nice products after the machine is ready :).

    Just my 2p.

    Florin

    P.S. Absorbing the vibrations in perspex I believe is exactly what's cracking it, and it starts where the micro-fracture points are induced by mechanical processing. It happened to me before...

  5. #15
    Sorry Ross, I'm going to disagree with your disagreement :exclaim:

    I think that a stiffer machine will vibrate less than a weaker machine. Why? Well the first explanation is Hooke's law:
    force = stiffness x displacement
    or
    displacement = force / stiffness

    For a given force (from the cutter), a higher stiffness gives a lower displacement.

    But we are also talking about dynamic stiffness and need to consider resonances. Undamped natural frequency = SQRT (stiffness / mass). A stiffer system will have a higher natural frequency, and in turn these have lower amplitudes. I believe this is to do with higher frequencies having more energy in the wave. Since you are putting the same energy into the system (from the cutter), the higher frequencies must vibrate at a lower amplitude to keep the 'area under the graph' the same in both cases.

    Higher frequencies can be confused with 'more vibration' because to the touch they will tend to give more of a tingle as the frequency increases, but if you were to measure the displacement it would be lower. Run a sine sweep through a loudspeaker and watch the cone. Low frequencies you will see it move - at high frequencies it will virtually be stationary. I know there are problems with this analogy but I couldn't think of a better one!

    In addition, a stiffer system, with a higher natural frequency, would be more unlikely to be driven at that natural frequency by a cyclic input (e.g ballscrew rotation), and less likely to go into resonance. Again the displacement is less.

    When you talk about damping, this comes from 2 sources:
    Inherent damping in the material (e.g wood is well damped, aluminium has some, and steel has less)
    Mechanical damping at the interfaces (e.g. microscopic bolted joint movement)

    You can apply external damping to panels (e.g. damping sheet) which this turns the vibration energy into heat, but this isn't really applicable for structures.

    Damping only has an effect on the amplitude of the resonances. It has absolutely no effect elsewhere i.e. away from resonances. So a less sturdy machine has no way of 'damping' the vibration soley from being 'less sturdy'.

    Another way to reduce vibration is to add mass. Although this lowers the natural frequency, because of Newton's Law:
    force = mass x acceleration
    or
    acceleration = force / mass

    Addition of mass, reduces acceleration during vibration.

    How do I know all this stuff? Well, my day job is a Noise and Vibration Engineer and we have to measure this sort of thing, and solve problems relating to it.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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  7. #16
    Huuummmm. can of worms well and truley opened about time someone shut me up......

    I'll have a wee think on re-phasing my logic and reply later, I've not a nice new x y table in the workshop that is begging to be taken apart.....

  8. #17
    Please don't shut up, you have alot to offer and have helped alot of people out! Sometimes your keyboard is faster than your mind in your eagerness to help, that's all. I think you know this stuff really, just need time to mull it over. Have fun with the X Y table (that ebay one?)
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  9. #18
    Sometimes your keyboard is faster than your mind in your eagerness to help, that's all
    :rofl:If you knew how long it takes me write a post then my mind must be slower than a snail on national go slow day

    I certainly dont know everything (but I'm keen to fill in the gaps) I just try to look at things from a practical point of veiw as well as pages of theory and calcs.

    My logic was that vibration can only be transmited through a stiff material. If you take a steel bar and tap one end then vibration is transmited to the other end, but if you took a similar bar of say rubber or wood then much less vibration will be transmitted.

    Therefore only a stiff structure can vibrate, No?

    Anyway its getting late, Any good books you can recommed for me brush up on this subject?

  10. #19
    I would look to get some large pieces of Ally to melt as the smaller the pieces the more oxide you will produce.
    Ideally you need a reducing atmosphere for ally not easy with propane as you need a lot of air to get the temperature.
    I would like to try but not enough garden(boss has too many plants)

    peter
    Hi Peter

    I don't think you have looked into this as much as I have?

    You melt down all the small stuff together and mould decent sized? ingots out of them ready fo melting the components to be made.
    The ideal Aluminium is the type thats already been casted before ie cylinder heads or any large casted component as this will be prime material.

    As for Propane torch,there is lots of source out there in constructing a torch head to give maximum heat flame within the homebrew furnace,believe me its so simple.

    Follow this guys series of making his own CNC,everything is home made/built/casted,such a pity he didn't have the brains to protect himself for safty when casting though.
    http://www.youtube.com/watch?v=DXFDs...eature=related

  11. #20
    I have done casting before but under lab conditions, reducing atmosphere but it still remains that evry time you melt Aluminium the surface of the pieces are scrap as the oxide will never melt.
    You only have to see the prices paid for ally turnings, not worth a lot at all.
    Each time you melt you produce more oxide scrap and are just wasting energy.
    If you want to play then go ahead but you could end up paying more in propane than the excercise is worth!

    Peter

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