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Thread: Ball screw size

  1. #25
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    Default Re: Ball screw size

    You know what, I'll make you a couple graphs if you like. My last response was a bit rude. That's what happens when you post after 4 beers. lol. Might take me a few days to get to it.

    Your gantry should weigh alot more than 40 kg. This includes everything that moves back and forth, including motors, including motors, rails, uprights, y and z axis, spindle, etc. If it is really light weight, also, it may not handle the best acceleration the motors can theoretically give you, it might flex too much. Just look at some Youtube videos to see what I mean. Really, these calculations have nothing to do with machine flex and assume a sturdy machine. Useful for picking ballscrews though. No problems, I can give you some graphs that have a few different colored lines for different weights.

    Like I said, it may be a few days until I get to it.

    Also, have you decided on a spindle? Any idea what the part of the Z axis that moves up and down might weigh? I can give you a Z axis graph too. If you're looking to do 3D carvings, you may wish for a fast Z, so perhaps 10mm lead on the Z and 10 or 20 on the rest. Also, you'd want to bolt some stiffeners to your gantry risers and Z axis plate, you could use steel angle iron. You could also bolt some square steel tube to the T slot on the two unused faces to make it stiffer. You'll see how much of a difference more weight makes after I do the graphs. You may rethink some things in the design after that.

    Of course, the graphs are just my opinion. You can't sue me afterwards .



  2. #26
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    Default Re: Ball screw size

    Oh that's really nice ! Thanks you so much for your help!


    Today I use a 800w spindle with my first small CNC, I will certainly upgrade it to 2.2KW so I can add ~6kg to the gantry. The weight I calculated is only for the X and Z gantry. I might have made a mistake but I double check and the X,Z weight should be around 40kg (46kg wih the new spindle). The total Weight should be around 120 kg. The weight of the Z axis will be around 15kg.


    I don't understand why it can't handle the acceleration if it is light. I was thinking the opposite: the moving gantry should be light. It will certainly be clearer for me with graph. If it accelerate too much it will bend?


    I just realise that my reseller sell also heavier aluminium extrusion (item Suisse). I already ordered this one: item Suisse
    you can find properties of each on the links. I'm afraid to have made a mistake. Will it make a big difference? Or will it be sufficent? I calculate bending for each and there is not a big difference (with 800 newton).


    Good idead to add steel.


    Thanks again for the graphs, I am not in a hurry



  3. #27
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    Default Re: Ball screw size

    There are a couple of things I'm not clear about. You mentioned 1250mm. But your long axis is 1800mm? Better to have your dual driven axis as the long one IMO. On my build I have this reversed, so I can load and unload some very specific parts easily, but I also have shorter travels than you and my gantry will be 800 lbs. The deflection of the gantry beam depends on the length cubed, so it makes a difference, especially if you are using T slot.

    Also, there's no way your gantry will be 46 kg. Something is wrong there. That's too light. Try a weight estimate in excel, adding all the parts up. It's for everything that moves, includes bearings, motors, cables, gantry uprights, X, Y, and Z, for all the axis, etc.

    Yes, if it's not built stiff enough, it will flex under acceleration. Lighter will get you better numbers in these calculations, but it practice, it won't. This is just an inertia estimate, helps to size your ballscrews, but doesn't estimate the machine stiffness.

    These graphs assume a ballscrew efficiency of 90%. They don't include bearing friction, or ballscrew preload torque, and probably a couple other things I'm not thinking of. Also, the graph you provided has half step microstepping. You may loose a small amount from the graph with a little more microstepping. Perhaps. Not much I assume. So a little wiggle room is needed. Also, these are separate from the critical speed calculations that we went through earlier.

    I picked an arbitrary motor inertia of 0.00008 kg*m^2 based on some other Nema 23s I looked at. I did not find a published value for this motor. I used a value of 0.000000123 kg*m^2 / mm for the 20mm ballscrews based on the THK rolled ballscrew table I have. Note that The 25mm ballscrews are over double this. I used a ballscrew length of 1876mm for the gantry (2 of them, with 2 motors), and a ballscrew length of 300mm for the Z (I just picked something).

    Also, I'm only human, it's possible I made a mistake somewhere (but in general, I think the advice is sound).

    For the gantry:

    Ball screw size-brice-o-1-jpg

    Ball screw size-brice-o-2-jpg

    For the Z:

    Ball screw size-brice-o-3-z-jpg

    IMO, for the X and Y, 20mm lead, an all up gantry weight of 300 lbs with a max speed of approx 500 IPM at an accel of 0.15G is what I would aim for.

    For the Z, 10mm lead, a max speed of approx 300 IPM at an accel of 0.15G is what I would aim for.

    So you see from the graphs that you can add some more weight to the gantry. Perhaps 3" x 1.5" x 1/8" square tube bolted to the bottom and 6" x 1.5" x 1/8" square tube bolted to the back. Big steel angle iron bolted to the gantry upright plates and Z axis plate. Also consider epoxy granite or high density polyurethane filling for the T slot. You'll have to figure out the weights of those and come up with a better estimate for everything combined.

    I hope this was helpful. Of course, it's just my opinion. I'm pretty tired right now. Rainy day, I should be changing my tires. I'll have another look at this later to see if I made any obvious mistakes. Just wanted to get this out of the way.



  4. #28
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    Default Re: Ball screw size

    Thanks for all! I inverted X and Y axis, you're right.
    I will take into account your comments and will probably go for 20mm lead.
    I can definitely move forward with the project. I really appreciate your help for the graphs and details, thanks!



  5. #29
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    Default

    Quote Originally Posted by joel0407 View Post
    Hi,are you using mach3,if so would you like to share the settings.I have built one and max ipm I can go is 30ipm before stalling. Thanks.



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