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  1. #31
    Its been a while but I’ve been updating my design and have made a number of changes to include many of the improvements suggested in this thread. But I have also been persuaded to widen the table to 1250mm (4’) so I can machine a half sheet (seemed a good idea at the time).

    So I have now put together a preliminary BoM for the machine along with weights and I am very concerned that it has evolved into a monster. Lol.

    Excluding ball screws and misc nuts and bolts, but including spindle these are the approx weights:

    X Axis 21Kg
    Gantry 76Kg
    Base & Table 149Kg

    So my next issue is to work out how to drive these masses, What would be the best combination of driver, motor and ball screws. Would the 60BYGH401-03 4Nm steppers work and are 16mm screws to flimsy?

    Jim

  2. #32
    Hi Jim,

    I wouldn't be concerned with the weight 76KG for the gantry is not excessive. I would always take a slightly slower heavy gantry over a skinny fast one anyday. Mass can be your friend if matched with right motors and screws.
    Regards the 16mm screws.! . . . Depends is the only answer I would give without knowing the full dimensions. I'm pritty sure they will handle the mass/force's etc but length will determine if they are suitable without whipping. . . Give more info if you can.

    Don't know those motors.! Are they Nema23 or 34's. . . . Personally I prefer 34's for larger machines with weighty gantry's. They don't quite spin as fast as 23's but the torque remains higher up the power curve, giving strong torque without much drop-off as the rpm's rise then drops off the clifff really fast around 8-900rpm.
    23's tend to drop gradual as the rpm's rise but will carry lower torque to higher rpm's 1000-1200rpm + is not unusual.

    Again without knowing the full details it's hard to give a definate "Yes" or "no" but @4nm I think you'll be ok for most single ballscrew setup's. Ball screws are very effiecent so allow less powerfull motors than other linear drive system's IE:R&P etc.

  3. #33
    We need to know the travel of each axis, and the pitch of the 16mm screws to work out if they are suitable. The main limiting factor is in theory the speed at which the screw starts whipping - critical speed.

    The only 4Nm Nema23 motors I know of are these which are the same part number:

    http://www.cnc4you.co.uk/index.php?r...&product_id=72

    Their inductance is nice and low. The rotor inertia isn't stated...
    I'm intending to get a few of them for my router so that I can put the ones from the router back on my mill. Not sure how much difference it'll make over the 3nm. I'll be interested to see what they're like.

  4. #34
    The X axis ball screw would need to be 1600mm long and the Y axis (gantry) 1500mm long. I was hoping to use 10mm pitch screws, so it seems your limited to either 16mm or 25mm. Whip is a concern.

    Click image for larger version. 

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    The motors are the Nema 23s on ebay from CNC4YOU and appear to have a low inductance and 4.0N.m hold torque, but I dont know if thats enough and what Ill actually need.

    Click image for larger version. 

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    Are there any simple rules of thumb to work out your requirements regarding motors and screws?

    And one word of advice for anyone designing a machine, make sure you have lists and lists of standard size materials. I have gone through the pain of having to redesign this because I didnt check on the sizes of steel or MDF. Doh

  5. #35
    Personally I wouldn't go with 16mm at these length's unless like Jonathan you used a rotating nut setup.
    They are just too close to the edge for my liking and would need to be setup and aligned absolutly perfectly and kept this way, even the slightest miss alignment at this length would show up as vibration easily creating whip as the speed rise's.

    I wouldn't go with twin 25mm screws either bit OTT and hard on the motors, esp little 23's.

    I see you intend using belts and pulleys so you could always go with 20mm x 5mm pitch geared 1:2. If your wanting reassurence this works good then think I've mentioned it before.? . . I have a machine that use's this setup but with a single 6nm nema 34 motor driving 2*1500mm screws joined with belts. It works very very good and cutts every thing from paper to Ali and steel with super repeatabilty and accurecy.

    I will guarantee you from experience with my original 2 x 3nm setup that 2 x 4nm 23's geared 1:2 on 2005 screws will handle every thing you can thro at it,. . then some.!

    Seen as your using belts and for simplicity sake my first choice from experience would be 2005 geared 1:2 but if must use 16mm or need higher rapids than 10-11mtr/min then go for the more involved rotating nut setup. . . . Both will work good.

    OR find 20mm 10mm pitch screws.?

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  7. #36
    Rick, A silly question perhaps... most of the utube vids I've looked at seem to position the work at 0,0 giving the forces you describe in the pic 2. Would I be right in thinking that if the work was positioned mid Y (above the ball screws) with x=0 or where ever, it wouldn't be so so much of a problem? And, would that imply that best accuracy / precision would be in the centre of the table rather than at the edges??

  8. #37
    Quote Originally Posted by mocha View Post
    Rick, A silly question perhaps... most of the utube vids I've looked at seem to position the work at 0,0 giving the forces you describe in the pic 2. Would I be right in thinking that if the work was positioned mid Y (above the ball screws) with x=0 or where ever, it wouldn't be so so much of a problem? And, would that imply that best accuracy / precision would be in the centre of the table rather than at the edges??
    Have you posted in the right thread.? Not sure what your saying or meaning.?

  9. #38
    Critical speeds with BK/BF bearings (approximate):

    16mm dia, 10mm pitch, 1600mm long - 800rpm = 8m/min
    16mm dia, 10mm pitch, 1500mm long - 910rpm = 9.1m/min
    20mm dia, 5mm pitch, 1600mm long - 1000rpm = 5m/min
    20mm dia, 5mm pitch, 1500mm long - 1150rpm = 5.75m/min
    25mm dia, 10mm pitch, 1600mm long - 1220rpm = 12.2m/min
    25mm dia, 10mm pitch, 1500mm long - 1400rpm = 14m/min

    So given that in my opinion 20mm diameter 5mm pitch screws are the worst option regardless of the gearing since the critical speed is quite low. Also the moment of inertia of a 20mm screw is 2.44 times greater (since it's proportional to the radius to 4th power) than the 16mm screw. That means the motor has to impart that much more kinetic energy into the screw to accelerate it to a given feedrate. The pitch of the 20mm screw is half that of the 16mm screw it clearly has to spin twice as fast. As the motor torque is proportional to the angular velocity of the screw and the inertia you loose big time since the torque required is now a factor of (10/5)*2.44=4.88 greater between the two options!! This doesn't take into account cutting forces, which would make the figure look not quite as bad ... but still it's a massive difference.

    Clearly I can use a similar argument as the above to say why the 25mm screws are too big for those motors...unless you rotate the ballnut. Rotating the ballnut is a win-win situation since screw whipping is no longer an issue and the moment of inertia of my rotating ballnut assembly on the RM2510 screw is about a third of the inertia of my 2m long screw. With that I get 15m/min (well over the critical speed) confortably and I'm limited by the 3nm motors, not the screw. For your size machine I reckon 16mm diameter with rotating ballnuts might be a good option...

    If you can find 20mm diameter, 10mm pitch screws then that's potentially good option (though still the inertia is greater as above), but I bet it's going to cost you more than a rotating ballnut setup on the 16mm screw.

    This does of course all depend on how fast you want to go. If you're not trying to get the land speed record then the RM1610 screws would be fine...
    Last edited by Jonathan; 13-09-2011 at 07:04 PM.

  10. #39
    Don't want to get into an argument Jonathan but I know from real world experience all these calculations means nothing in DIY cnc.

    All well and good if building cnc machine using high precision machinery built to exact specifications with top notch components all perfectly aligned.!. . . .but in the real world of DIY using a drill press and rudimentry tools trying to align screws etc using primative methods then it don't work like the nice calculations predict. . . . The extra beef of thicker screws takes up the slack of DIY building no matter what the calculations say should happen.!!!

    My setup being a prime example that your figures dont work in the DIY world, . . . It happly spins 20mm screws at twice the critical speed you quote.!! . . Those figure say it shouldn't . . . but it does and has done for the last few years without any problems.!

  11. #40
    Quote Originally Posted by JAZZCNC View Post
    The extra beef of thicker screws takes up the slack of DIY building no matter what the calculations say should happen.!!!
    Perhaps, but you can't deny the simple calculations I did in the previous post demonstrate a significant torque disadvantage from using a bigger screw with lower pitch.

    Quote Originally Posted by JAZZCNC View Post
    My setup being a prime example that your figures dont work in the DIY world, . . . It happly spins 20mm screws at twice the critical speed you quote.!! . . Those figure say it shouldn't . . . but it does and has done for the last few years without any problems.!
    I know, my RM1610 ballscrew happily exceeds the critical speed by quite a lot. The thing is the critical speed formula doesn't take into account the position of the ballnut, which acts as another support hence leading to the calculation being an underestimate in a lot of cases. This is partly the reason I concluded that 16mm screws are optimal - from a torque point of view they are far superior to the others and in reality it seems the critical speed is higher than what I worked out, so ultimately you'll end up much better off than with the other options.

    The calculations of critical speed may not mean as much as they imply, but I feel they mean more than nothing and give a good basis.

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