Sizing of rails and motors


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Thread: Sizing of rails and motors

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    Member TTalma's Avatar
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    Default Sizing of rails and motors

    I am designing a small desktop machine. I am planing on a fixed gantry, and a working space of ~12"x12". One of the goals is to keep the cost under $200, and I am willing to give up some accuracy/speed to achieve that goal. For cost reasons I am planing to use unsupported rails, T8x8mm leadscrews. I plan to use a trim router for the spindle.

    I did some calculations and for stepper size and I think a NEMA 17, 1.5 amp, 59oz/in motors should be large enough. And 400mm long, 8mm rails should be large enough.

    Are the motors large enough? will I have a lot of deflection in the rails?

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    Member Biggs427's Avatar
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    Default Re: Sizing of rails and motors

    What would be the diameter of unsupported rails? What will you cut?

    Nema17 are on the small side.

    I had nema17 on my first cnc with 5/16 threaded rod, (1.4333mm pitch) but as soon as I upgraded to T8, the steppers were stalling more often.

    Try to find a small nema23 stepper like this one: https://www.omc-stepperonline.com/hy...rice&order=ASC

    A cheap TB6600 drive is more than enough to power it.



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    Default Re: Sizing of rails and motors

    Quote Originally Posted by TTalma View Post
    I am designing a small desktop machine. I am planing on a fixed gantry, and a working space of ~12"x12". One of the goals is to keep the cost under $200, and I am willing to give up some accuracy/speed to achieve that goal. For cost reasons I am planing to use unsupported rails, T8x8mm leadscrews. I plan to use a trim router for the spindle.
    $200 will be extremely hard to hit unless you have a lot of components laying about.
    I did some calculations and for stepper size and I think a NEMA 17, 1.5 amp, 59oz/in motors should be large enough. And 400mm long, 8mm rails should be large enough.
    While it somewhat depends upon what your will be doing with the machine you will likely want triple the oz/in values you have there. Actually what you are doing is a big factor, you might get away with steppers that size for a machining doing engraving if you can keep the drag in the system to the absolute minimal. The problem is any increase in stiction, rolling resistance tool load or whatever will lead to early stalls of the steppers.
    Are the motors large enough? will I have a lot of deflection in the rails?
    Nope, I would avoid such small steppers. As for deflection that always exists. How much depends on the size of the components and the spans you are dealign with.



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    Default Re: Sizing of rails and motors

    I have a larger machine with unsupported rails. Maybe it can be used as a comparison. My X axis is 40" long with 36" of travel using 1" rails on opposite sides of the machine so they are spaced 33" apart. They perform acceptably, although there is more deflection than I would like. I wouldn't want them any smaller or longer. Also note that my machine has rigid end supports. Deflection is significantly higher if the ends are loosely supported.

    I would feel a lot more comfortable with 12mm or 1/2" rails in a machine of your size. This is based on the assumption that deflection goes up 4X as the length is doubled and rail strength increases 8X as the diameter is doubled. Your machine has 1/3 the rail length, so it would have around 1/9 as much deflection. Rails that are 1/2 the diameter would have 1/8 as much strength. I think this is correct based on my physics classes from many years ago, but feel free to double check my math. Alternatively find a rail strength calculator.

    One area that unsupported rails falls weak on my machine is the Y axis. There are two 33" rails spaced 5" apart. The head can very easily twist as the rails flex a very small amount. It is a simple problem of leverage. I added a third supported rail about 12" up to add some strength.

    Steve



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    Default Re: Sizing of rails and motors

    May I recomend you just buy a Seinci mill instead? From memory, its the size of what you need and is probably the only sub 600$ machine I'd ever recomend. I think its 250$ + router!

    Luthier/Woodworker/Machinist in NS, Canada.


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Sizing of rails and motors

Sizing of rails and motors