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  1. #1
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    Post CNC project questions

    Hello everyone,
    I have some questions related to my graduation 3-axis CNC machine project. Every answer and helpful comments are welcome. To introduce the project, we are working on making a 3-axis CNC milling machine. We ordered some parts, a 2.2 kW watercooled spindle with a matching inverter. The spindle is regulated from the VFD from min 8000 rpm to 24000 rpm. The dimensions are 80 mm in diameter and 200mm in lenght. 4 bearings in. The frequency is from 0-400 HZ. Also linear bearings,supported rails, ball screw set have been ordered. The working are is 210x297 (A4 paper). The tool should be moving in Y and Z direction while the support table should be moving in the X axis direction. X and Y are 300 mm in lenght and Z 250 mm. Still I'm trying to find all sorts of materials for this project, if you have something to recommend or link it would help a lot.
    So, my questions are:
    1. The goal is to be able to mill steel. But for steel milling the RPM should be at a much lover rate 1000 rpm e.g. So my general idea is to build in a planetary gear reductor or by building in pulleys and a belt. The torque should be higher while the RPM's are being dropped. What are your opinions on this? And, if the RPM's are being reduced more by VFD will the torque be enough to mill steel? Let's say an steel alloy AISI 1020.
    2. How do i calculate heat generated in the spindle motor while on max load. (24000RPMs). I searched on the internet it says a 80W water pump should be enough to cool the spindle, but still I would like to calculate everything. And to simulate that which programm do you recommend?
    3. And do you know any literature that can help? Does anyone have experience in building their own CNC? Any help is welcome.
    Thank you.

  2. #2
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    Apr 2004
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    Re: CNC project questions

    Does this machine have to be able to cut steel? It's easy enough to make a machine to cut wood and soft materials like that. Aluminum is possible if you do a really good job at maximizing rigidity. But steel requires a different sort of frame and an entirely different sort of spindle. The one you've picked out is fine for wood and aluminum, but no - it won't have nearly enough torque when slowed down to 1000 rpm. I suppose you could design an elaborate pulley train to slow it down and multiply torque, but it would probably be a lot easier to start with a different sort of motor that isn't going so fast in the first place.

    Look at examples of milling machines designed to cut steel. They tend to be quite massive, with a spindle that runs in a much lower speed range.
    Andrew Werby
    Website

  3. #3
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    Oct 2004
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    Re: CNC project questions

    I'd see if you could fit a servo gear reduction unit. Don't know about heat build up on it, but they're pretty tough gear reducers, and usually pretty small in size.

  4. #4
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    Re: CNC project questions

    It would be nice if it could mill low strenght steel. With a more powerfull motor that would be possible, that´s true. But the plan is to build a smaller rigid CNC machine that is able to do these tasks.

  5. #5
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    Re: CNC project questions

    I don't think servo reducers are built to handle motors as fast as that spindle; they're designed for a speed range about a tenth of that.

    Since you've already committed yourself to this project and bought all those parts, it looks like you'll have to build it and see how well it works. After graduation, you can use it for whatever it really is best at, if you don't destroy it trying to cut steel (which isn't going to work too well, since you seem to have bought round rails, which aren't rigid enough, and a spindle that goes way too fast for all but the smallest tools). Just try to build it as massively as possible. I'm not sure what facilities you've got for building it, but your choices would be:

    Cast metal - this is what most steel-cutting mills are made of. You can build patterns out of wood and take them to a foundry, or cast them yourself if you've got access to one.

    Epoxy-granite - this makes for a very heavy and stable base; the hardest part is attaching components to it and compensating for any out-of-squareness.

    Welded steel - Use heavy sections of box beam and go slowly on the welding process, tacking it in a few places, letting it cool, and carefully adding metal trying to minimize the buildup of heat, which will warp the straight beams. You can use self-leveling epoxy to create level co-planar areas to mount your rails to. Filling the insides of the beams with something (anything from foam to sand, epoxy-granite, or welded-in vertical sections of steel) will dampen vibrations and make them stiffer.

    Solid granite - While heavy and difficult to work with, granite is massive and very stiff. It's an unconventional material for building CNC machines, but here's a build log from someone who built what looks like a pretty nice machine, based on a granite surface plate: http://www.cnczone.com/forums/epoxy-...54358-cnc.html
    Andrew Werby
    Website

  6. #6
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  7. #7
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  8. #8
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  9. #9
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