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Thread: Concrete VMC

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  1. #1
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    Concrete VMC

    Hello my name is Stef, I already make some posts on this forum. I have converted an bf25 (g0704) to cnc and used it now for about a year. Right now I want something bigger and more rigid.
    http://www.cnczone.com/forums/bencht...22688-cnc.html

    After a while of gathering parts and looking to different designs. I have start building yesterday right now the x axis is starting to get shape.
    The x axis is made out of steel, the base and column will be made of concrete (uhpc) Travels will be X600 Y300 Z550. It will have an BT30 spindel with a 16 position tool changer on top.
    I will use leadshine closed loop steppers to drive all the axis and use a USBCNC control board to control everything.











    will keep you guys updated.

    stef

  2. #2
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    Re: Concrete VMC

    Sweet!

    Andrew
    Andrew Abken
    www.drewtronics.net - PCB Cutters

  3. #3
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    Re: Concrete VMC

    Is that laser cut steel?

    Looks very nice so far
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  4. #4
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    Re: Concrete VMC

    yes 8mm laser cutted steel, with 60x30mm steel bars on top. Everything now needs to be welded after that it will be stress relived and then all the mounting surfaces will be milled.

    stef

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    Re: Concrete VMC

    Your workmanship is top notch. Any factory would be proud of a product that looks like that.

    Have you already done all of the performance math for speed, acceleration, available cutting force?

    I'm working on a spreadsheet for my own build. To try and determine the best pitch of ballscrew to use for the motors I have, and to account for possible future upgrades, including the effects of ballscrew and motor inertia. I've already been playing with this for rack and pinion. I'm curious if anyone else has done this that I can compare notes with.

  6. #6
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    Re: Concrete VMC

    Great start to the show. Subscribed and getting popcorn
    Mark
    Regards,
    Mark

  7. #7
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    Re: Concrete VMC

    Small update nothing big. Machined the steel strips that will hold the motor and end bearing. Also made two aloy strips that hold everything together so makes it easier to weld.
    There is no math behind this machine, just looking to other machines and looking what feels right. I am still thinking what to do with the motors, using closed loop steppers or using servo's.
    I think closed loop steppers are good enough but with servo's i can get higher speeds if i want. How do you guys think about closed loop steppers vs servo's.


    used a nema38 8nm closed loop stepper from my current cnc to test fit.



    Stef

  8. #8
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    Re: Concrete VMC

    Quote Originally Posted by stef110 View Post
    I think closed loop steppers are good enough but with servo's i can get higher speeds if i want. How do you guys think about closed loop steppers vs servo's.
    It really depends on what you want to accomplish. You have fairly small travels, so what speeds do you want to achieve? You will be limited by the critical speed of your longest screw for maximum speed. It doesn't really matter what kind of motor you have beyond that. What speed do you want?

    Also, acceleration is an issue that will affect your performance, and your machine does not look light. Do you want a speed demon, or what are you happy with? Are there two screws for the bottom tier? I see holes where two screws could go....but that would put those screws out in the open? And you have put alot of work into protecting the top screw from debris, so I don't know what you have planned.......

    What kind of HP will your spindle be capable of? For a machine like this, I assume it's a big one. And I assume that this is for metal work. Your cutting forces could be several hundred pounds if you have a huge spindle cutting metal, and that is not a trivial amount, as it can be for a home made wood router.

    For my own build, I was a little bit surprised to see what kinds of differences there were between 5mm and 10mm pitch. It made a huge difference.

    To answer your question, really, I'd need to know the specifics, such as,

    What is the weight of that big piece you made, including the table, and including the largest work piece you might put on it, and including all of the bits, like the ballscrew and motor? Everything that would be moving back and forth.

    Where is the torque graph for the stepper you want to use, using the appropriate power supply and microstepping? What is the motor inertia for the motor? What are the diameter (s) pitch and length of the screw that is not yet shown but will be used to move the big piece you have shown back and forth?

    What kind of servo are you thinking of, or are you looking for recommendations?

    What are your goals for acceleration (in G's) and top speed (in IPM)?

    What kind of HP spindle are you planning on? Do you know what the cutting forces might be? Easy enough to run some hypothetical numbers through a feeds and speeds calculator to see what the forces are if you know what kind of HP is available.

  9. #9
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    Re: Concrete VMC

    Oh man that is a beast. No steppers for that thing...

  10. #10
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    Re: Concrete VMC

    Thanks NIC for the help.

    Here is a picture off the current design base and column are still "subject to change".



    The weight of the table will be 50Kg think that there will be a max of 100kg on the table so for the X axis there will be 150Kg load.
    For the Y axis there will be 260kg load when there is 100Kg on the table.

    The x axis spindle is a 2005 and is 850mm long
    Y axis spindle will also be 2005 and is around 500mm long

    Here are the specs of the closed loop stepper i want to use right now:
    http://www.leadshine.com/UploadFile/Down/ES-Md_V1.3.pdf I will use model ES-M23480

    Will supply them with 72v and the controllers on current machine are set to a micro step of 1600.

    I was also looking into these kind of servo's DMM | NEW DYN2 AC Servo Drive | AC SERVO DRIVE | AC SERVO MOTOR | ROTARY ENCODER and then the nema34 equivalent. So a 0.75Kw version.

    For the spindle power i not yet decided, I am looking for a big servo motor because i need an indexer to led the spindel stop every time on the same spot because of the tool changer.
    Thinking somewhere around 3Kw will be fine. Right now i have a 1.1KW motor with a 2:1 acceleration on it to a chief 6000rpm with a 3000rpm ac motor. And for aluminium i have enough power.

    I don't have the idea of cutting steels with this new machine. I normally only work with alloy but if possible it will be a nice feature.
    Right now i a chief speeds of 4000mm/m with nema24's 3nm and 1605 spindles faster would be nice but have no idea how fast it needs to be.

    sorry for the messy answer but i hope you right now have a feeling of what i am planning of.

    stef

  11. #11
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    Re: Concrete VMC

    Nice start !

    My advice is servos, absolutely, for the accuracy and acceleration, both very important.
    For any 3D milling work, mostly acceleration is the defining parameter, top speed is pretty unimportant.

    Servos tend to accelerate and run real-world 3-5x faster than steppers, on complex work, ie other than box shapes.

    Fwiw .. I never use the top speed of my servos, and limit the machine speed to about 20-40% of max.
    And I use speed-rate override.
    On a lathe with 18 cm / 40 cm travels, position-to-crash is about 1 sec at top speed.

    Math:
    10 ms to accelerate to top speed- 3000 rpm, = 60 revolutions/sec. (I also use much less than max accceleration. Too Jerky.)
    1500 rpm at screw, 30 r/sec.
    Screw = 4 mm /turn, 120 mm / sec.
    1.5 secs end-end on x, typically about 0.75 secs.
    And about 1 second on typical Z axis positioning, near the chuck.

    Lathe is 12x24" light industrial.

    I suspect You will need

  12. #12
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    Re: Concrete VMC

    Quote Originally Posted by stef110 View Post
    The x axis spindle is a 2005 and is 850mm long
    Y axis spindle will also be 2005 and is around 500mm long
    Are those 15mm bearing end supports? I have a similar set up for supports and mounts, but I'm right now deciding on what pitch screws to buy. I was thinking 25mm OD, but now I think 20mm OD would be better. I'm not sure I can use 15mm ID bearings because of the screw depth? But it looks like you've done it. Are those 15mm ID bearings with the 20mm screw and how did that work out for you?

    Quote Originally Posted by stef110 View Post
    Here are the specs of the closed loop stepper i want to use right now:
    http://www.leadshine.com/UploadFile/Down/ES-Md_V1.3.pdf I will use model ES-M23480
    I used the graph at the bottom of page 8. I didn't bother trying to figure out what they were saying about the holding torque percentage in the software manual. I just used that graph. Interesting, this stepper goes to 2400 RPM.

    Also, how fast do you want to spin your ballscrews at? I limited you to 2878 RPM for a length of 850mm according to the critical speed calculator from Nook. The NCalculators critical speed is much higher than the Nook one, I think Nook takes a percentage of theoretical to be on the safe side. But spinning a ball screw at 2878 RPM seems excessively fast to me already.

    I just did the calculations for the bottom screw that you said is 500mm, pushing 260kg, although I did limit the speed based on the 850mm length.

    So here is what my spreadsheet is saying. Does not include bearing friction. Not liable for errors in math, which may exist. It would be nice if someone else could do some math independently from me to verify or contradict my results. You might need to save the first one and then zoom in on it to see each graph.

    Attachment 364072

    Attachment 364074

    Attachment 364076

    If I had the cash I would use those DMM Nema 34 size 0.75KW on my own build. The graph shows peak output only, not continuous. I don't know what percentage of the time you can run at peak output?

    The 10mm pitch would be better I think. Actually, I think the ideal might be a 2525 driven by a servo with a gear reducer. Of course, your travels are small, so that's overkill. I don't think I can afford that on my own build. For myself, I'm torn between trying to find some 3:1 reducers and using a 2525 with my nema34s or just using 2010 or 2510 with my 15mm bearing end supports.

    Also, this doesn't include limitations in force and speed that may exist in your bearings, etc. I don't know what those are.

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