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  1. #21
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    Jul 2018
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    6248

    Re: Small Concrete CNC Mill

    Hi Juan - Your machine is very small and will not require high G's or high velocities. High G's require more powerful motors and thats $$$ look at N17 motors as they may do the job. N23 can be quite powerful and they do run quite big machines. In the commercial world you will design the machine then do a value analysis. You will determine the total build budget then apportion the cost of bits vs their functional value. For your machine you maybe over capitalizing on the motors within the project so consider N17's... so do the math to make a performance vs $$$ decision vs just picking some motors.... 0.2g is quite respectable and will change the torque requirement dramatically...Peter

    4000mm/min at 400rpm is a 10mm pitch screw. You probably want a 1605mm pitch or 1204mm pitch for a good force so you won't get to 4000mm/min. Look up the torque curve of the motor.....

    say you use a 1605 screw at 500rpm thats only 2500mm/min maybe 750rpm gets 3750mm/min so maybe there...

  2. #22
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    Re: Small Concrete CNC Mill

    I see. If anything, I want to undercapitalize . I did look at the torque curve of the motors, and I have done so in further analysis.

    I've redone the calculations for an acceleration of 0.2g, 1605 ballscrews, maximum cutting forces of say 50 N (is that reasonable?), ignoring air drag and using NEMA 17 motors such as these:

    https://www.omc-stepperonline.com/nema-17-stepper-motor/3pcs-of-nema-17-bipolar-59ncm-84oz-in-2a-42x48mm-4-wires-w-1m-cable-and-connector.html?mfp=145-holding-torque-ncm%5B46%2C59%2C60%2C65%2C79%5D

    They have 59 Ncm holding torque and can hold about 23 Ncm up to 750 rpm. That is at 24 V, half step. They also have others with 65 Ncm but their torque curve does not go past 525 rpm.
    Also 0.9º motors have less torque at full step than the proposed 1.8 ones at half step.

    According to my numbers they would do nicely, plus I can use something like DRV8825 stepper drivers, which makes everything cheaper.

    For a power supply, according to here https://www.cnczone.com/forums/stepp...ted%20voltage.
    AMPS = SUM(amps required for all motors) X 0.66

    Voltage = Minimum, 10X motor's rated voltage, Maximum, 20X motor's rated voltage
    Motor rated voltage is 2.8 V, 10*2.8V = 28 V, 24 V would be fine and it is what is used in the torque curve.

    Motors are rated at 2A, say I oversize the power supply in case I even want to drive 5:
    5 motors * 2 A * 0.66 = 6.6 A, so the closest available 24 V power supply would be 8.2A

    Juan

  3. #23
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    Nov 2017
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    Re: Small Concrete CNC Mill

    Regarding rails I'm thinking of these https://www.aliexpress.com/item/1005...c00po56al&mp=1 Their 20 mm counterparts from the same store have good reviews and I can't really afford much more.
    Juan

  4. #24
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    Re: Small Concrete CNC Mill

    Good Morning Juan - When I have a moment I'll check the maths. Well done to do the maths. I suggest you try to use a system with higher then 24V, 36V (or higher) will do better. 50N or 5kgf for cutting, will cut foam but not timber and not aluminium. Need 15kgf plus to do hard timbers. Your 5mm pitch screw with a 59Ncm (0.59Nm) motor will produce ~62kgf at stall so that's heaps....

    F(kgf) = 0.59x2xPIx0.9/9.81/0.005 = 62kgf so maybe 30kgf at 500rpm (500rpm is 500x5=2500mm/min and you probably won't heavy cut at more then 1500mm/min, light finishing cuts will be as fast as machine can go) My belt router with the makita I ran at 23m/min finishing cuts.

    As long as your close with the power supply it will be fine on this machine. Switchmode power supplies will only put out what they can and they are hard to blow up. If you look at unregulated power supplies you may find something as well. So break down your calcs to two conditions a) cutting which is close to constant velocity so no acceleration and b) rapids which will need to include accels. But on a small machine to move across a small table does not take much time. So you can check its accel transit and decel to see how long it will take... this is usually called a trapezoidal velocity profile or maybe triangular. Since your making many decisions based on cost downs at least you have the correct basis for making these decisions. From my experience cars and rails are important so maybe that's not the spot to cost down... or if they are really cheap then they can be upgraded into the future... As you say its a learning experience so $$$ matter. maybe since its a timber machine look at supported round rails, they are half the price of square and I'd only use square on the Z. They have a broad base more suitable for screwing to timber... but they are wider so machine gets bigger.... trade offs

    also look at 12mm screws much less inertia...

    Keep at it Peter

  5. #25
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    Re: Small Concrete CNC Mill

    Hi Juan - Looked at your math and I haven't checked everything but the logic and most maths are correct. Well done. Now you have the drift you can go forth and design machines. This process picks a design point. It does not fully describe the machine motion. It picks a point up the acceleration curve and checks the motor has enough torque to achieve those conditions. Jerk or jounce has to be considered as well and this smooths (damps or inhibits) the motion out. It takes energy to start and stop things and change their direction so although this maths says you will get to full speed in 0.03 seconds in practice this is unlikely. If your deeply into maths or have access to dynamic FE you can calculate the actual accel time closer... But in terms of this machine design level its adequate for sizing the motors....

    https://en.wikipedia.org/wiki/Jerk_(physics)

    well done keep at it... for completeness check buckling as well.. Peter

    be careful with spring balancing. Springs are not constant force devices. If you use a very long soft spring they can be close to constant but generally not. The idea of balancing the Z axis is to catch it if the power goes down. There is generally enough force in the system to raise and lower the unit on a small machine like this. Ballscrews can be "back driven" ie they do not have enough friction to self stall. I suggest instead of using a spring to consider a motor brake. Its easier to implement and not dear and all spring systems chew up usable Z travel. Unless you use balancing weights. Motor brakes are easy to do and may save a big crash one day!!

  6. #26
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    Re: Small Concrete CNC Mill

    Hi Juan - Designing a balancing system for Z is actually quite an exercise in itself. Highly recommend using a brake. Simpler, no lose of Z travel etc etc. My small router with a plastic nut and 8mm trapezoidal thread will "fall down" under its own weight. Didn't do this when new but over time its worn and now I have to be careful when I turn it off. If I have a small tool in it it will break when it hits something!! Ballscrews will do this at day dot... since then all my machine designs have brakes on z. My first router I used a gas strut but that was hard finding one that was just right. They were not on spec and I had to go to the shop with scales and measure each one until I found the correct one in the shelf... Then it chewed up 45mm of Z travel. By the time you figure brackets, devices etc just get the brake! Peter

  7. #27
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    Re: Small Concrete CNC Mill

    Hi Juan - the driver you are considering can go to 45V I'd go there. Peter

  8. #28
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    Re: Small Concrete CNC Mill

    You have already helped me more than many university teachers. Thank you for your time, I can only hope to give this back in the future.


    I've revised the calculations, separating cutting and rapids as suggested. I've included acceleration while cutting because the tool has to change direction while cutting, say, a pocket, and that means accelerating.
    I've considered max cutting forces of about 200 N. Those would also influence friction in cars in addition to weight, but its almost negligible. I've checked buckling as well, not a problem for 16 mm or 12 mm screws.
    With a cutting feed of say 2000 mm/min and rapid feed of 3750 mm/min, those NEMA 17 motors are fine. They will probably be able to go faster/sustain higher cutting forces, we'll see once the machine is built.


    I knew about Jerk, more from T-shirts "Don't be a d^3x/dt^3" than from any physics clases, where they didn't mention it. Didn't know about snap, crackle and pop (not that they are of any use here). I suppose the CNC controller takes care of the detailed motion profile, and calculating travel time is just to get an idea, so an estimate is fine. Plus, as I said I have a plotter whose velocity and acceleration I can tune to get a feel for it, or even measure the actual time. On another note, I've got that plotter to "print" toolpaths out of Fusion 360's CAM. Not of much use, but interesting to see.


    For the power supply I'll go then with a 36 V switching one. Cheaper and easier than an unregulated one. It can also be adjusted up or down a bit, say to 40 V, better than adjusting a 48 V one down to 45 just to be safe.


    On the Z axis, having a relay put a resistor across the motor windings when there's no power should be easy enough and should hold the axis in place without mechanical brakes, springs or else.


    The motors can move around a 16 mm ballscrew easily, so no problem. Originally I wanted to go with those supported round rails, will have a look, but I think those "cheap" linear rails would be fine, and if I need to upgrade them I'm sure they'll find a new home in another project.


    I'll design a desk/table/somewhere to mount the machine and computer to as well.


    Juan
    Attached Files Attached Files

  9. #29
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    Re: Small Concrete CNC Mill

    Hi Juan - Yes there is a centripetal accel around a corner but the motion controller whilst cutting is maintaining a constant velocity. The centipedal accel is not reflected to the motor but to the tool and frame. To understand this you will need to do a free body diagram of the tool whilst cutting. But due to various things this is usually neglected at hobby level. Very fast commercial machines consider jerk. The toolpath is calculated forward, then backward then forward in an iterative manner to minimise jerk. In a valve train in your car if it uses camshafts jerk is very important a this is what makes the cam follower bounce. Then digging a little deeper toolpaths are actually linear, they are thousands of very small lines. So an arc is made up of say 0.001mm lines. Each one has a jerk at the start and its end, if the tool could perfectly replicate the toolpath. But it can't as your mechanical parts have play and elasticity. This damps and smudges the toolpath into a apparent continuous cut vs the contiguous toolpath... keep at it, Peter

    going back to a statement - although I said your timber machine will cut aluminium it won't cut like a mill. You will have to be patient and figure out various things to do this. And on the arm 2 pieces of ply thick at the bearings with a good size metal washer or plate. I siggest a single think al plate.... for under the bolts

  10. #30
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    Re: Small Concrete CNC Mill

    incorrect entry

  11. #31
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    Nov 2017
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    32

    Re: Small Concrete CNC Mill

    I see. Well, at any rate those motors are good enough for this application, even better if they don't have to withstand acceleration forces while cutting, I'll probably order them soon.Will read more on jerk, seems an interesting subject.


    I know it won't cut like a mill. It is not a mill, more like a router in a mill form-factor. I'll go through a lot of wood before attempting any aluminium that's for sure.


    I'll make the suggested changes. Thanks. Will get the plywood soon, and begin cutting out the pieces.
    Juan

  12. #32
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    Nov 2017
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    32

    Re: Small Concrete CNC Mill

    Ordered motors, power supply and other bits and pieces. Ordered the rails too. Kind of blew the budget, but par for the course, things cost what they cost. Here are all the wood pieces cut out with a jigsaw. I'm using the router which will become the spindle to finish the pieces up to the marked lines. Edges where the rails will mount to will be trimmed in place when the frame is partly assembled.
    Juan

  13. #33
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    Re: Small Concrete CNC Mill

    Hi Juan - very exciting, look fwd to the build. What class or type of plywood did you get? Peter

  14. #34
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    Nov 2017
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    Re: Small Concrete CNC Mill

    The plywood I got is not that great, what they had at the local home improvement store in the appropiate size. It's poplar, easy to work with but kind of soft. Now that I look at the spec sheet it is 4-5 GPa... I'll make sure I coat important surfaces with epoxy and use aluminum plate under the bolts. On the other hand it wasn't that expensive and the leftover board will become the table the machine will mount on.
    Juan

  15. #35
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    Nov 2017
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    32

    Re: Small Concrete CNC Mill

    All the wood pieces have been trimmed to size. The router has been great, and now I have a bag of sawdust. A couple mishaps, but nothing major and it also builds "character" as they say. I'll glue the "subasemblies" together, but maybe the base and column will remain separate until the rails arrive. Will be working on the electronics and the spindle mount.
    Attached is a mockup. It may be a small machine, but not too much by my standards.
    Juan

  16. #36
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    Re: Small Concrete CNC Mill

    Hi Juan - Looking great. I see you make model airplanes? Thats the use for the CNC? Keep the sawdust, good filler to make the next concrete machine lighter.. Peter

  17. #37
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    Re: Small Concrete CNC Mill

    Quote Originally Posted by peteeng View Post
    Hi Juan - Looking great. I see you make model airplanes? Thats the use for the CNC? Keep the sawdust, good filler to make the next concrete machine lighter.. Peter

    I have that model plane halfway assembled. I turned to model submarines, less catastrophic when things go wrong (somewhat) . This is some of my work:https://youtu.be/4K4q5dHAo6c
    Will be using it for that and other projects that come up. Maybe making some PCBs as well.
    Juan

  18. #38
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    Re: Small Concrete CNC Mill

    Hi Juan - You maybe interested in this. Your modelling looks to be very good. Keep Making Peter
    Attached Files Attached Files

  19. #39
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    Re: Small Concrete CNC Mill

    Thanks, will have a read.
    Juan

  20. #40
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    Re: Small Concrete CNC Mill

    Interesting article. I'm sure I'll learn more on the subject on my automation classes.

    Here is a simple workbench that will hold the machine and associated electronics/computer.
    Juan

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