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  1. #11
    How long is the cable on the parallel port.

    A TTL signal might get you a reliable 10 feet without handshaking at that speed, if you were lucky

  2. I think its a fact that microstepping, where you are actually stopping at the microsteps (i.e. using them for resolution) definitely is less torque, simply because the motor cannot hold that position accurately against load. In a dynamic situation however, lots of factors come into play. For example, if the axis is in constant motion the only torque required is to overcome friction and cutting loads, there is little torque required to provide acceleration. Unless the motor is being operated close to its torque limit (at that speed/volt/current combo) then microstepping should have little impact. The general rule I have used is in the spreadsheet is that the motor should provide 3x required dynamic torque at the maximum speed.

    Large motors have high inductance so the torque drops off very fast with speed - the corner speed of those motors is 240rpm. I don't know how big your axis are, but I'm guessing its going to be around 1 - 1.2m? With your 10mm pitch screws 2.5m/min = 250rpm, so that is close to optimal (and 1/8 stepping = 6664steps/sec) and it looks OK at cutting speeds, but its very marginal at 10m/min rapids and that is where you may have lost steps (=27000steps/sec). You need to reduce rapids to 7m/min but it should be OK at 1/4 or 1/8 stepping.

  3. #13
    Thanks again, i will reduce the rapid to 5m/min and the working speed up to 2,2 m/min as top speed, setting the microsteps to 1/8.


  4. #14
    I have a couple of questions about your motor calculation spread sheet. The motor inertia is input in gm-cm^2 (B32). The rotor inertia given in I27 must be Kg-m^2 since the total inertia is the sum of the screw inertia, load inertia and motor inertia, the others are in Kg-m^2. I27 = G35 which is a lookup to convert units of the motor inertia.
    The problem is 1000 gm = 1 Kg and 100 cm = 1 m so gm-cm^2 should be divided by 1000*100^2 = 1*E7 not 1*E8 as in G35. The motor inertia in the sum is 10 times too small.
    I cannot find the time used for the acceleration torque (I34). The equation is G32*I28. G32 is labeled as V but it is really the number of pulses/second of the stepper based upon a 200 pulse/revolution motor and the screw speed (which is the Max. linear speed divided by the screw pitch). From looking at the equations, the running torque is small compared to the acceleration torque unless there is a lot of friction in the system. I have not found any references as to what range of acceleration is needed for a CNC. Obviously, when a stepper system is tuned it will limit the acceleration so positioning steps are not dropped due to the loads. Too small motor torque will result in a very slow CNC due to acceleration limitations. At some point a larger motor torque results in small performance gains for the cost increases. Are there any guidelines for this? Thanks.

  5. #15

    I am building a gantry robot and for the Y-axis I am using a 1610 ballscrew (16mm diameter, 10 pitch, 1300mm long). I estimated that the mass is 36Kg and for the speed I planned to have 1800mm/min.

    Using these calculations I concluded that the total torque is 0.23Nm. From the Excel the estimated torque resulted to 0.67Nm.

    Now I have the following questions:

    1) Can I use there calculations for a ball-screw system?

    2) What is the calculation to determine the estimated torque (0.67Nm) from the total torque (0.23Nm)?

    3) Is 0.67Nm enough for such mass? I did some research for similar projects and they use a much higher torque stepper motor.

    All kind of help will be appreciated and sorry for my bad English.

    Thanks in advance.

  6. #16
    I'm trying to use the spreadsheet to check some parameters of my planned new router. This has thrown up a few questions:
    1. Acceleration seems to be the largest contribution to torque required, but I can't find where load acceleration is built in. It must be in one of the formulae somewhere but I can't manage to unpick the formulae in the hidden columns to find it.
    2. The spreadsheet recommends a motor torque based on an assumed safety margin of 3; if the safety margin isn't high enough with my intended motor, I presume that I could restrict the machine to a lower acceleration? But given that I don't know what acceleration is assumed, it's difficult to see what's happening. I do see that the acceleration torque number is linked to cutting speed.
    3. For a twin ballscrew X axis, I assume that I can use the spreadsheet using actual ballscrew dimensions (single ballscrew) to give critical speed, but then use a double-length ballscrew in the spreadsheet to do the torque, etc, calculation.
    4. If I go to a twin-motor setup, can I simply double the nominal motor torque? Or is it easier to halve the gantry mass and do the sums for a single motor/ballscrew?
    To put some reality into this exercise, what would be sensible numbers to use for cutting speeds and accelerations? Bit of a "how long's a piece of string?" question, but my current router is so far out of the norm I don't feel that I can extrapolate from what I'm doing at the moment, and I don't know what kinds of numbers are reasonable. For starters, I would assume profile cutting, say, 9mm ply with a 6mm cutter in a couple of passes. Currently, with speed limited by machine rigidity, I would cut that at maybe 600mm/min.
    Many thanks for any help available!

  7. #17
    I'll save you a lot of headache and just say use 3Nm motors run at 65-70Vdc on 75-80vdc drives and you won't have any problems cutting anything.

    Cutting speeds will depend on many factors like spindle power and tool material but 4500-5000mm/min wouldn't be out the way with 2.2Kw spindle using carbide tooling.

  8. The Following 3 Users Say Thank You to JAZZCNC For This Useful Post:

  9. #18
    Can you suggest a supplier of a kit of parts to do as you have described Jazz? for a 4 axis machine?

  10. #19
    I'm planning to re-use a set of Zapp SY60 3nm motors, 68V linear power supply and analogue drivers which should do for starters, then, and I'll see how it goes. Can always add another motor if needed.
    This whole design process is interesting: we are urged to study, analyse, carefully consider. I used the spreadsheet to get better insight into motor sizing but the answer, in practice, comes down to "SY23 because SY17 are too small, SY34 are too high inductance, and you might as well go for 3nm as they cost barely any more than the lower power motors in that frame size." Power supply sizing is similar.
    Ian - you could look at the Zapp web site for a kit. The price (at least when I bought) is the same as buying the individual parts but at least someone has identified a bunch of bits that play well together.

  11. #20
    I have the much reviled ebay 4 axis kit all on one board which i bought to get me going

    having said all that its been absolutely fine and reliable but now i'm using it more I thought i would get a proper set up with individual drivers so in the event of problems i can switch a faulty one out

    I was looking at cnc4you's site but his drivers are only up to 50v with a 36 v power supply
    I'll have a look at zapps site


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