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IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > Sizing lead screw and stepper motor
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  1. #61
    Join Date
    Nov 2013
    Posts
    4280

    Re: Sizing lead screw and stepper motor

    Hi,

    Twisting a very heavy large diameter ball screw takes much more force than turning a narrower one. Given the numbers it appears the weight of the X axis gantry may not have as much bearing on the acceleration force needed as it is divided up into small increments by the threads on the ball screw. You may have a very valid point.
    Yes, exactly. Many people do not realise just how significant the rotating mass is to the overall momentum. As of a couple of years ago I assumed the rotational inertia was just a small detail
    compared to the massive gantry....but no that is definitely not the case. That discovery has changed my entire appreciation of machine mechanics.

    Craig

  2. #62
    Join Date
    Jul 2018
    Posts
    6248

    Re: Sizing lead screw and stepper motor

    Hi Layton - I do moulds and deep bowls so need a good Z. It currently is ~300mm travel. If your into the math here's my motor calculator spreadsheet. The main reason for me to have rails on top of the gantry is to separate the 8 bearings that are tightly packed into a vertical saddle when the rails are mounted to the front. I used round rails on this machine to see how they go, I usually use square rails. I have designed raising gantry machines with no saddle but have not made one yet. Saddles are a PIT butt. The saddle on this machine is awesomely stiff but it's also the most expensive one I have made. Look up my Frankenrouter thread...

    Next machine will have one rail at the front bottom and one rail at the top rear (maybe) to help get the saddle front lower and to get the action line of the bearing patch closer to the shear centre of the gantry. Torsion and shear deflection play a big part in deflections in gantries as they are short beams not "long" beams. If the load trajectory goes thru the shear centre then the beam has no torsion only bending.

    What is Shear Center - Location of Shear Center with Examples - The Constructor

    Peter

  3. #63
    Join Date
    Jul 2018
    Posts
    6248

    Re: Sizing lead screw and stepper motor

    Hi Layton - Reading your text about inertial forces makes me want to explain something as I think you are not interpreting inertial force correctly. eg My thinking is that that huge mass of metal high up in the air creates a lot of resistance to inertial forces.

    An inertial force is created by the relationship of accelerations and masses. Its similar to gravity and weight. If a mass is not accelerating, then it does not experience an inertial force. If it is accelerating, then the inertial force opposes the motion. So if you're in an accelerating car going north you get pushed into the seat by the resulting inertial force pushing south. If the car is in steady motion then there is no inertial force and you are comfortable in your seat. If you go around a radius at constant path velocity there is a radial acceleration that pushes you to the outside of the radius. If you go around a radius and accelerate around the radius then there is a radial and tangential inertial acceleration to deal with.

    The inertial force acts over the entire mass but for calculation purposes it can be considered to act thru the centre of gravity of the mass object.

    Now accelerating rotating masses produces inertial torques so if the shaft is accelerating clockwise the resisting torque is anticlockwise. If the shaft is at constant velocity then no inertial torque exists. So in terms of machine design we only consider ine
    rtial forces when the masses accelerate. This is especially true for rapid moves as when cutting the tool is usually at constant velocity or near to. Hobby machines rarely get to 0.1g, high speed commercial machines can get over 1.0g some pick and place machine are 2g or more.

    Next its about toolpaths - toolpaths are a contiguous stream of small straight lines. There are no "curves" in a toolpath. But they get blurred together mechanically to form the smooth shape we expect. The motion controller is responsible for calculating the path acceleration and capping the path accelerations according to the axis set values. Some motion controllers forward calculate and back calculate motion and smooth it out but at hobby level it's a simple numerical capping. You can read up on motion control in the linuxcnc paperwork. Motion control is a very interesting subject. Trapezoidal, S curve, cycloids, jerk, snap crackle and pop to name some subjects Peter

  4. #64
    Join Date
    Jul 2018
    Posts
    6248

    Re: Sizing lead screw and stepper motor

    Hi Layton - to go further with this inertia thing. The machine does not supply "inertia" inertia is a resultant force resisting movement caused by accelerations. Say the gantry and Z axis weigh 100kg. A good set of linear bearings has a friction co-efficient of 0.004 so the force needed to move the assembly slowly is 100x0.004=0.4kgf. The seals on the cars have a resistance of around 2kgf so we need 2.4kgf to move the 100kg assembly. Little finger stuff. Same as you can push your 1 tonne car in a carpark....Now we want to accelerate the assembly say at 0.1g or say 1m/s/s if gravity is ~10m/s/s. At 0.1g the inertial force resisting the acceleration is F=mxa or 100x1 = 100N so we need 10kgf to achieve this feat. A gantry machine has two motors so we need 5kgf from each motor plus the torque to turn the screws plus some friction forces. Use the spreadsheet prior attached and you will see that this is the main torque requirement... Peter

    Inertia like gravity is a phenomenon created probably at atomic level by some subparticles that react to acceleration. Same as mass particles have gravity.

    Gravity - Wikipedia

    Inertia - Wikipedia

  5. #65
    Join Date
    Jul 2018
    Posts
    6248

    Re: Sizing lead screw and stepper motor

    Hi All - Were on a motion roll so Layton have a look at the Tinyg controller. Its the highest quality motion at hobby and maybe even commercial level you can get. The Tinyg motion code is open source and someone has implemented it into Linuxcnc since its open source. If you can get the Linuxcnc/tinyg algorithm combo your machine will be very very happy or consider using TG hardware... Since your doing carving motion control could be very important to you....Peter

    https://synthetos.myshopify.com/products/tinyg

    https://www.youtube.com/watch?v=pCC1GXnYfFI

    tinyg controls jerk (rate of change of acceleration in time) which is the main culprit to making machines dance. Its not the jerk per se its the uncontrolled peak (infinite) accelerations created by trapezoidal velocity curves...

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