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This topic contains 98 replies, has 1 voice, and was last updated by  andrewM 4 years, 1 month ago.

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  • #1832

    Paul Kelly
    Participant

    You haven’t said, but I’m assuming you’re using NEMA23 motors?
    PK

    #1833

    Nick Witham
    Member
    #1834

    Nick Witham
    Member

    Oh, and BTW: I made a trip to a local machine shop. The manager told me they get most of their precision milling and machining done in Perth and shipped to Port Hedland, mainly due to labour costs. There is only one other machine shop in town, which is a huge mob, so im not likely to get any love from them.

    The first guy was pretty interested in my project and said if I produced a set of fab drawings, he’ll see what he can do. They have a plasma cutter there, and a mill. The Plasma cutter is CNC, but the mill is not. I think i’ll get him to mill the steel faces that are going to hold my rails. I’ve made a few design changes, which allows me to make things a bit more compact in my gantry, as well as having to weld a few locations. I can do this since I know I can get it milled afterwards. There are few minor geometery changes since im going to mill some of the faces, but I have accounted for them. So far, things are looking good on the structural side.

    #1835

    Paul Kelly
    Participant

    At 48V, 3A, my steppers output about 1.616Nm with a max 936rpm at the spindle.

    There isn’t a NEMA23 stepper motor on the planet that will give you 1.6Nm anywhere near 936RPM

    That motor is rated for 12.25W.
    1.6Nm at 936RPM =157W
    So you’re an order of magnitude out.

    The maths is similar for your power supply.
    At rest, that motor will draw about 300mA tops from your 48V supply (Steppers are constant power rotary solenoids, not motors. The drive will turn power off to the coils (or recirculate it) when the current hits 3.5A. At that point the voltage across the motor will be 3.5V, so it will be consuming 12.25W of electrical power. It can’t produce more mechanical power than that.)

    You also seem to be confusing speed and acceleration. It takes power to accelerate a gantry or head. But not much to keep it moving.

    Look, I’m going to bow out of this discussion at this point. Looks like you’re pretty set on the path just about everyone takes. ie 16×5 screws, high torque rather than high speed motors. Hell I did it that way the first time.

    Your machine will accelerate at about 900mm/s/s and rapid at about 6m/min. After a while you’ll limit it to 4.5m/min rapids and 650mm/s/s accel so that you have enough residual torque to get past that bit of swarf that’s lodged in the bearing or to run that heavy job without losing steps.)

    Just remember that all the pictures you’ve seen of cnc machines were posted pretty much the instant the machine was finished. All the “My machine goes this fast” bragging happens at that point too. Not too many people will post about their disappointment a year down the track….

    PK

    #1836

    Nick Witham
    Member

    Thanks Paul,

    There isn’t a NEMA23 stepper motor on the planet that will give you 1.6Nm anywhere near 936RPM

    The 936RPM is the corner speed.

    Look, I’m going to bow out of this discussion at this point. Looks like you’re pretty set on the path just about everyone takes. ie 16×5 screws, high torque rather than high speed motors. Hell I did it that way the first time.

    I guess sometimes people just have to agree to disagree. FYI, Im going with 1610 screws purely cos all the calcs I have done, then re-done then checked and re-checked and re-re-checked say that 1605 are too slow and 1620 isn’t torquey enough.

    I hope you don’t think im being ungrateful by not following your every suggestion. As you have said, it’s my build, and I need to be comfortable with what I spend my money on. You’ve already convinced me to go with a steel frame over aluminium, and now made me calc out my torque and feed rates to inevitability change from 1605 to 1610 screws, and i thank you for making those suggestions.

    Im likely to spend the $1000 this week on my screws and rails off a guy on AliExpress, so once I get them delivered in a month, I’ll get all my steel cut, then the fun really begins!

    #1888

    andrewM
    Participant
    #1937

    Creepyjoe
    Member

    Hey nick, how’s the build coming?

    #1938

    Nick Witham
    Member

    Creepyjoe, post: 2413, member: 674 wrote: Hey nick, how’s the build coming?

    Still waiting on parts from China. Once I get all the drive gear, I’ll put my order in to get my steel fabricated. Don’t really want to fabricate the steel until I get my drive gear just in case there are dimensional issues.

    #2217

    andrewM
    Participant

    Any more updates Nick?

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