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Thread: formula to size CNC steeper and servo motors

  1. #1
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    Default formula to size CNC steeper and servo motors

    Right now at this point of my conversion this is my biggest concern. I know the size and weight of tables being moved and speeds play into the factoring but is there a formula to determine the require size of a stepper or servo motor??

  2. #2
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    What kind of conversion are you doing?

    I don't think there's any simple formula, as an accurate estimate will require all the weights/inertias PLUS the coefficients of friction of the linear guides and drive mechanisms (ballscrews/leadscrews).

    Your best bet would be to look around to see if someone else has done a similar conversion, and if they were successful, use similarly specced hardware. Chances are that someone over at cnczone.com has done it.

    ...after all, there's a big difference between converting a Sieg X2 benchtop mill and converting a Bridgeport :-)

  3. #3
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    I have asked for help with several questions/process over on CNCzone but get a reply I guess I just need to make them in one small question at a time, so I posted a formula request question over there and Bob gave me a link to Gecko FAQ witch helps but still little concussed about what is meant by "TPI screw" in the formula do you happen to know? or how I get that value?


    I did talk with some people I know locally that have CNC plasma tables but they don't have any ideas about how to do a mill is what I am told

    My conversion is a old Kempsmith mill that has been changed to vertical mill at some point before I got it a few years ago , yes I know its old and yes I have heard just go buy a new machine but this is what I have and what I need to make work hopefully with a little advice here and there form the people on the forums I will be able to do this with out to much of a problem.

  4. #4
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    I was thinking it maybe stands for turns per inch but would like to be sure to make sure I figure for the correct motor

  5. #5
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    Quote Originally Posted by goldhunter_2
    I was thinking it maybe stands for turns per inch but would like to be sure to make sure I figure for the correct motor
    TPI is Threads Per Inch (just like what you find specified for screws and bolts). For a 20 TPI screw it would take 20 revolutions to move 1 inch.
    http://en.wikipedia.org/wiki/Unified...#Basic_profile

    To measure the TPI, take something 1 inch long (i.e., caliper set to 1.0" or a ruler) start the zero at 1 thread, and count the number of threads between 0 and 1".

    The lower the TPI, the 'faster' the screw. This means that it'll take fewer turns to move the table the same distance, but will require more motor torque.


    Though now that I think about it... you could probably just skip all the calculation mess and get some empirical data. For each of the axis handwheels, measure the distance from the center to the crank handle. Move the handwheel so that the crank handle is at 0, and start hanging weight from the handle. Once the wheel starts to turn and drive the table, multiply the weight (ounces) by the distance from center to crank handle (inches) to get the torque required to move the table (oz*in). That'd be the absolute *mininum* torque requirement for that axis.
    Depending on what your handwheels/cranks look like, there'd be easier ways (like a torque wrench).


    Of course, I may just be going off on a tangent here, as more detail into what specifically you're doing, and what your objective is when you're asking questions would help keep things focused. If you put more effort into typing out the question and providing background, the forum members will be better equipped to provide the answer you're seeking

    but is there a formula to determine the require size of a stepper or servo motor??
    Are you trying to figure out how big a motor just to move the table, or are you trying to figure out how big a motor to drive a specific load at a certain speed? I've yet to machine a crystal ball, so I'm relying on you to give the necessary details

  6. #6
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    sorry I like the people with crystal balls..lol my apologizes sometimes (most the time) my typed subject doesn't come out the same as what is in my head

    in answer to your question I am trying to be sure that the motors I buy to move my mill table and projects (mostly aluminum parts) will be at least big enough to do the job with out wasting money on something that is way to big. I'm not really trying to build the fastest thing out there just want to be sure it will work for repetitive parts on this big heavy table as the CNC would be more accurate over 10 or 20 of the same parts or at least that is my thinking

    I did my math a little different to figure out TPI but will go back and try with your posted method to see how accurate I was on the first try. my handle is about 8" form center I turned (complete 360 or 0 to 0 turns) it until each axes moved the table 1" this is what I got doing it that way

    my side to side motion = 4 TPI
    my in and out motion = 5 TPI
    my up and down motion = 10 TPI
    Last edited by goldhunter_2; 11-24-2009 at 01:38 PM.

  7. #7
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    Quote Originally Posted by goldhunter_2
    Right now at this point of my conversion this is my biggest concern. I know the size and weight of tables being moved and speeds play into the factoring but is there a formula to determine the require size of a stepper or servo motor??

    There was an extensive article on the sizing of steppers in the UK Magazine "Model Engineers Workshop" over three issues, Nov 2004, Dec/Jan2005 and Feb/Mar 2005. They were elaborated on, in the March and Aug/Sept 2006 issues. You may be able to get back issues on the net, but if you cant access then directly in the US, send me a PM and I can e-mail the scans of the pages. I have scanned them and keep them on my computer for ready reference, as I (slowly - very!!) convert my own mill

  8. #8
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    Ok I used the formula in the Gecko FAQ section and I think I must have done my math wrong or the assumed IPM of 120 is just to much to ask for a big machine like I have I am not really sure now. I'll put my math below if someone sees a error please point it out to me. I lowered the one IPM to 90 and still seems high RPM/watts . What are everyone else's large mills normally running IPM?

    My table has a original 6" wheel(3" form center) my in & out , up & down use a home made arm that is 10"diameter (5" form center)

    (up/down) 10lbs is 160oz x 5" movement arm = 800 in-oz of torque
    10 TPI x 90 IPM = 900 RPM 800 in-oz x 900 RPM/1351 = 532.938 watts

    (side to side)
    4 TPI x 120 IPM = 480 RPM 480 in-oz x 480 RPM/1351 = 170.54 watts

    (up & down)
    5 TPI x 120 IPM = 600 RPM 800 in-oz x 600 RPM/1351 = 355.2923 watts

    Is my math wrong? Is my IPM just to high? or does it look right and I just need to throw some gears into the equations ?



    NzOldun,
    I understand about very slow projects I have one motor that I am still looking for parts for but I have just about all of them now and its only been three years ..lol I actual just found a deal on a atlas 12x36 lathe an tooling I couldn't pass up so spending that money will slow my mills cnc conversion down but the way my math is going trying to figure these motors I have plenty of time to figure the correct motors
    I did a web search and found the magazine home page I see where to subscribe to it but not order any back issues. Does this magazine normally have pretty good articles for metal machining projects?

    Robert

  9. #9
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    Quote Originally Posted by goldhunter_2
    Ok I used the formula in the Gecko FAQ section and I think I must have done my math wrong or the assumed IPM of 120 is just to much to ask for a big machine like I have I am not really sure now. I'll put my math below if someone sees a error please point it out to me. I lowered the one IPM to 90 and still seems high RPM/watts . What are everyone else's large mills normally running IPM?

    My table has a original 6" wheel(3" form center) my in & out , up & down use a home made arm that is 10"diameter (5" form center)

    (up/down) 10lbs is 160oz x 5" movement arm = 800 in-oz of torque
    10 TPI x 90 IPM = 900 RPM 800 in-oz x 900 RPM/1351 = 532.938 watts

    (side to side)
    4 TPI x 120 IPM = 480 RPM 480 in-oz x 480 RPM/1351 = 170.54 watts

    (up & down)
    5 TPI x 120 IPM = 600 RPM 800 in-oz x 600 RPM/1351 = 355.2923 watts

    Is my math wrong? Is my IPM just to high? or does it look right and I just need to throw some gears into the equations ?



    NzOldun,
    I understand about very slow projects I have one motor that I am still looking for parts for but I have just about all of them now and its only been three years ..lol I actual just found a deal on a atlas 12x36 lathe an tooling I couldn't pass up so spending that money will slow my mills cnc conversion down but the way my math is going trying to figure these motors I have plenty of time to figure the correct motors
    I did a web search and found the magazine home page I see where to subscribe to it but not order any back issues. Does this magazine normally have pretty good articles for metal machining projects?

    Robert
    Robert,

    Their web site is www.myhobbystore.com and you can navigate to their back issues from there. If you cant get the issues, email me on grant.notley@xtra.co.nz

  10. #10
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    Default Still more questions than answers....

    [QUOTE=goldhunter_2]Ok I used the formula in the Gecko FAQ section and I think I must have done my math wrong or the assumed IPM of 120 is just to much to ask for a big machine like I have I am not really sure now. I'll put my math below if someone sees a error please point it out to me. I lowered the one IPM to 90 and still seems high RPM/watts . What are everyone else's large mills normally running IPM?

    My table has a original 6" wheel(3" form center) my in & out , up & down use a home made arm that is 10"diameter (5" form center)

    (up/down) 10lbs is 160oz x 5" movement arm = 800 in-oz of torque
    10 TPI x 90 IPM = 900 RPM 800 in-oz x 900 RPM/1351 = 532.938 watts

    (side to side)
    4 TPI x 120 IPM = 480 RPM 480 in-oz x 480 RPM/1351 = 170.54 watts

    (up & down)
    5 TPI x 120 IPM = 600 RPM 800 in-oz x 600 RPM/1351 = 355.2923 watts

    Is my math wrong? Is my IPM just to high? or does it look right and I just need to throw some gears into the equations ?



    One thing to consider - when you gear something down, you increase torque - so think about your gear ratios when thinking about torque. 500 Watts is not all that much for a servo - I have worked with motors from 50 to 1500 Watts. Bigger question is this - is that machine good choice for running at 120 IPM? How long is teh table? To get high acceleration takes more torque, takes more watts, takes more money. If your machine was not designed for CNC it probably won't want to operate that fast, and unless it already has ball screws it will be very expensive to get it running that fast...

    Is this an old manual machine? probably has acme screws and brass nuts. Going to have some backlash and they won't like high speeds and high acceleration. Better give us a little more info and a few pictures.

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