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Thread: Ball screw size

  1. #21
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    Default Re: Ball screw size

    For critical speed calculations, it really depends on the straightness of the ballscrew. The Nook calculator:

    Metric Critical Speed Calculator | Nook Industries

    is over conservative compared with other calculators based on their assumptions. Of course in practice, you may be able to achieve a little bit more.

    I use a THK .pdf spreadsheet to find the root diameters to use and also the moments of inertia for calculations for rolled ballscrews. I couldn't find the exact one I use just now, but it is floating around online. Other manufacturers could have a small difference from they have published but it is about the same.

    2010, 16.4mm root diameter, end fixity "B", 1800mm length, gives 612 RPM.

    Converting the units from metric and using the NCalculators calculator

    Lead Screw Critical Speed (N) Formula & Calculator

    gives 900 RPM, as was previously mentioned. Like I said, the nook one is over conservative. Probably you will achieve between 600 and 900 before whipping with 1800mm between the bearings.

    To answer your question about what to do, I don't have enough info. That is why I was vague. Also, I don't know what your goals are for speed and acceleration.

    I'd need to know the weight of the gantry, including all moving parts, that includes Y-Z assembly, motors, linear rails, ballscrews, everything that moves. The rotor inertia for the motor. The lengths of the screws. And a torque vs speed graph for the motors using the same power supply voltage that you will be using. Also what your goals are for top speed, I can guess at a good acceleration from there. So really, what kind of machine are you building?

    I used to make graphs for people, but really, my motivation to help people with these kinds of things has declined since I was mocked for doing so in another thread.

    If you want to do the math there is a sample equation in post #21 of this thread with some links to equation sources as well.

    http://www.cnczone.com/forums/linear...118-cnc-2.html

    If those speeds are slower than your goals, then you are looking at 2020 ballscrews or rack and pinion. IMO, the screw length is getting to the point where you might want to consider it.

    Perhaps make some posts giving more specifics about your design, steel tube, weight estimates, speeds you want to achieve, what materials you want to cut, and then the advice people give can be more relevant.



  2. #22
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    Default Re: Ball screw size

    Thanks for these explanation, I appreciate! I’ll try to give you more information and to be clear.
    In attachment you can find a quick sketch of what it will looks like. I use aluminium profile 40 series and 20mm aluminium plate (6082).
    It will mainly be used to cut wood. I would like to test cutting aluminium and acrylic too.
    I use 4X KL23H2100-35-4B with a 48V PSU and a gecko G540. Here is the specs of the motor:
    381 oz In. Holding Torque
    1.8° /200 Steps Per Rev.
    2.55V Rated voltage
    3.5 Amps Current Per Phase
    0.73 ohms Resistance
    Inductance 2.8 mH
    I try to calculate with your formula (from: http://www.engr.sjsu.edu/bjfurman/co...iaformulas.pdf) to calculate the motor inertia but I don't understand what is "motor armature". Is it the length of the motor?

    I was expecting a speed around 600/700RPM ( I was thinking that it was a decent speed for the size.(It is?)), a resolution of ~0.01, an accuracy of a least 0.1mm.

    The gantry including every parts will weight nearly (a bit less) than 40 kg. The screw length of the X axis is 1250mm.

    If you need other informations don't hesitate plz.

    Thanks.

    Attached Thumbnails Attached Thumbnails Ball screw size-grph-png   Ball screw size-protov5-jpg   Ball screw size-ballscrewrm2010-png  


  3. #23
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    Default Re: Ball screw size

    For that equation armature is really the wrong term, should use rotor. It's referring to the inertia of the motor, easiest way to get that is probably to find it listed on a similar motor. Should be around 700 Kg-cm2.



  4. #24
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    Default Re: Ball screw size

    Quote Originally Posted by skrubol View Post
    For that equation armature is really the wrong term, should use rotor. It's referring to the inertia of the motor, easiest way to get that is probably to find it listed on a similar motor. Should be around 700 Kg-cm2.
    Yep, it is a number supplied by the motor manufacturer, and it depends on the type of motor whether armature is the right or wrong term. It is the inertia of the spinny bits of the motor, no arguments there.

    Sounds like you know what you're talking about. Tell you what, I'm very busy. Please feel free, with my blessing, to make this gentleman some graphs.

    Quote Originally Posted by NIC 77 View Post

    I used to make graphs for people, but really, my motivation to help people with these kinds of things has declined since I was mocked for doing so in another thread.
    Apparently any high school kid can do this math, and I contribute nothing of value to the zone. That's almost an exact quote to what was said about me on this forum. And I'm still pissed!

    It might take me an hour or two to go through all the specifics for this build and make and post some graphs. After I was mocked in another thread, I tell you this, I no longer give a sht. My time is valuable to me.

    If the OP wants to give me a $20 USD Paypal donation, I will do him some math and graphs. Otherwise, I have provided sources and examples, and sample calculations free of charge.

    This is a direct result of me being made fun of in another thread! Normally I do this for free but have since reconsidered

    I was pretty clear about this in my last post.

    Really, just looking to see if any bright high school kids are out there who are good with math and happen to visit this forum.



  5. #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 .



  6. #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



  7. #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.



  8. #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!



  9. #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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