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  1. #21
    Quote Originally Posted by Tenson View Post
    Hmm, surely the parallel connection will draw twice the current of a single winding, not twice that of the series connections. Similarly the series connection will draw half the current of a single winding, not half that of a parallel set-up.
    In parallel the current is twice the current for one winding. In series it is the same as the current for one winding as you have the same current flowing through both.
    GCSE Physics...

    Quote Originally Posted by Tenson View Post
    I still wonder though; why isn't it better to use the same screw pitch for all axis, so that providing the motors don't run out of torque, each axis will have the same speed and resolution?
    Each axis has a different mass and length of ballscrew, both of which affect the feedrate. So the same screw pitch on both will result in different speeds since other variables are still different. The resolution will be the same.

    Your Y-axis is 600mm (?), so I'd still go for 10mm pitch on Y.

    Quote Originally Posted by Tenson View Post
    I really don't want to go the rotating ball-nut route
    Why?

    Quote Originally Posted by Tenson View Post
    My question was really about why it is better to power each motor and ball-screw from a separate drive circuit, rather than just using both motors from one circuit. The motors can surely still individually stall, even with two driving circuits.
    Same answer, but to put it bluntly if one steppers stalls when you have two on one driver then you can say goodbye to the driver. If one stalls with two drivers it's a bit annoying as the gantry is bent, but not the end of the world.

  2. #22
    Tenson Jonathan's right on all counts here. I gave you the options he's giving you specifics about the rotating nut. If your wise you'll go with the rotating nuts at this length for best speeds verse's cost's.
    The rotating nut will give you best of both worlds and save you some money.

    The other reason for not using 1 drive with 2 motors is that if the pulse timing doesn't get to each motor at exactly the same time then you will slowly drift either infront or behind on 1 axis so all in all it's a lose lose situation and at best you'll be inaccurate and worst be letting out the magic smoke. . . Your choice.!!

  3. #23
    Hi guys,

    Jonathan, if you are genuinely willing to machine two rotating ball-nut/motor mount things for me at a good price then I'll go for it :)

    I'm sorry to be persistently stupid but I still don't understand the desire for 10mm pitch. This is my thought process: Two screws with the same pitch, turned at the same RPM, will give produce the same speed and distance of movement. In practice, one axis will have a greater load than the other, so it will need more torque for the screw to maintain the same RPM. However, with adequate torque (two screws, two motors) there should be enough torque to do that. In fact I'd expect a larger screw pitch to need even more torque. If the 10mm screw moves twice the distance with one turn, it will require twice the force to turn it, not to mention the greater mass of the load.

    Where is my thinking going wrong?

  4. #24
    A ball bearing screw is just that: a screw which runs on ball bearings. The screw and nut have matching helical grooves or races, and the ball bearings recirculate in these races. There is no physical contact between the screw and the nut. As the screw or nut rotates, and the rolling balls reach the trailing end of the nut, they are deflected or guided from this "pitch" contact by means of a return tube and returned to the leading end of the circuit.

  5. #25
    Quote Originally Posted by Tenson View Post
    I'm sorry to be persistently stupid but I still don't understand the desire for 10mm pitch.
    It's all about the speed vs torque. Remember with steppers as the speed rise's the torque drops away.
    So with a 5mm pitch screw and usable max torque for cutting feeds translated to rpm is around 1000rpm with nema23 motor will give about 5mtr/min if your lucky . Taking into account the safety margin for twin direct drive screws the safe area will probably be around 4-4.5mtr/min(Obviously this will depend on mass and friction of each axis). Now at these feeds you'll have low torque so any deep roughing or hard material will require even lower feeds.

    With a 10mm pitch the same 1000rpm gives you 10mtr/min with a similiar amount of torque. So obviously at the same 5mtr/min the motors are spinning half the speed so far more torque available so deep roughing cuts are no problem if the spindle and machine can handle them.
    The other reason as Jonathan pointed out is 10mm pitch means for the same speed the 10mm pitch screws are spinnig half the speed of 5mm so less chance of whip.
    Thou to be honest I don't hold much stall by this and Jonathan and I have debated this before.!
    While in principle he's right in physics terms but in practice it doesn't quite work that way and my machine is living proof as it use's all the things Jonathan disagree's with like: twin 20mm x 5mm pitch geared 1:2 with nema 34 and non AC drives on 80V.
    It will rapid at 12mtr/min if need be thou I restrict it by tuning to around 7mtr/min to save on screw wear as I don't require high rapid feeds and this allows higher acceleration which is much more usefull.
    It will happly cut at 7mtr/min and if you check the vertical post I put up you'll see it cut 10mm deep 1 pass in dense resin inpregnated MDF with no problem, it even cut full depth 13mm for half the length untill the very very worn 6mm cutter snapped and even then the machine didn't stall.
    So the critical speed issue is a non starter to me and I'm sure by the very fact of human nature most won't have the will to resist using all the available speed the 16mm x 10mm will give them so whip will be an issue because there's no strength in the screw. With 20mm you have more meat so better able to handle the slightly higher spinning if geared like I do.

    All that said if you don't require high resolution then I would still advise you to go 16mm x 10mm pitch as it's far more versatile and cost affective. Rotating nut, direct drive or connecting belts is an individual's decision.!! . . . All I'll say is you still have the sync issue whether rotating or spinning screw.!!. . Thou Jonathans obviously then man to tell you exactly how it differs to direct drive spinning screws.

  6. #26
    I see, so the motors drop in torque is not proportional to the increase in RPM? If you get more than double the torque at half the RPM then I can understand the argument for 10mm pitch vs. 5mm.

    Jazz, have you tried not using the 1:2 gearing and just spinning the motors faster? If you found the slower motor speed is better then I should just follow your advice and go for 10mm pitch ;)

    I'd like to go for rotating ballnuts if Jonathan will be kind enough to make the parts for me, as it would be nice to keep the drive motors within the machine frame, rather than stuck on the end. Also I could use ballscrew without end-machining since it would just be held in a clamp.

    I can get 2510 ballscrew from Zapp, so can you make a rotating ballnut assembly for that Jonathan?

  7. #27
    Quote Originally Posted by Tenson View Post
    I see, so the motors drop in torque is not proportional to the increase in RPM? If you get more than double the torque at half the RPM then I can understand the argument for 10mm pitch vs. 5mm.
    A good approximation is that the torque is fairly constant upto a particular rpm (the 'corner speed') after which the torque does drop proportionately. We've come to the point where hand waving explanations are insufficient as there's too many variables. It will depend on the chosen motor and thresholds.

    The graph in the 3nm motor datasheet is helpful:

    http://www.zappautomation.co.uk/redi...H88-3008BF.pdf

    (They've just gone up in price by 20%!!)

    Also Irving's spreadsheet is very helpful to understand which factors affect the feedrate:

    http://www.mycncuk.com/forums/showth...otor-do-I-need

    Even when critical speed isn't a problem (as is the case with the rotating ballnut) you'll find the 10mm pitch screw wins over 5mm as the torque required to accelerate it is less, due to the kinetic energy of the rotating screw being 4 times less if the screw is only rotating at half the speed.

    Quote Originally Posted by Tenson View Post
    I'd like to go for rotating ballnuts if Jonathan will be kind enough to make the parts for me, as it would be nice to keep the drive motors within the machine frame, rather than stuck on the end. Also I could use ballscrew without end-machining since it would just be held in a clamp.
    I'll work out a price and let you know asap. Though it's theoretically possible to get away without end-machining with rotating nuts I wouldn't advice is as the surface area of the screw in contact at the clamp will be very small. It adds very little to the price with linearmotionbearings2008, so I got my ballscrews cut with the standard end machining, except the same on both ends with the threaded portion. That way you can put a nut on either end to clamp it.

    Quote Originally Posted by Tenson View Post
    I can get 2510 ballscrew from Zapp, so can you make a rotating ballnut assembly for that Jonathan?
    Yep I can do any size, that's what I'm using:

    Click image for larger version. 

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    If you want 2510, although I think you'll be fine with 1610, you'd be much better off getting it from linearmotionbearings2008 on eBay. He sells 1605,1610,2005,2505 and 2510.

    This is the version I did which is suitable for 16mm or 20mm screws:

    Click image for larger version. 

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    Last edited by Jonathan; 26-03-2012 at 04:56 PM.

  8. #28
    Quote Originally Posted by Jonathan View Post
    Though it's theoretically possible to get away without end-machining with rotating nuts I wouldn't advice is as the surface area of the screw in contact at the clamp will be very small.
    I thought about that too, but it seems easy to overcome. Double-nut the end of it or pot the thread then clamp. I hate waiting for long deliveries so I'd probably get it from Zapp!

    Yep I can do any size, that's what I'm using:

    Click image for larger version. 

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    If you want 2510, although I think you'll be fine with 1610, you'd be much better off getting it from linearmotionbearings2008 on eBay. He sells 1605,1610,2005,2505 and 2510.

    This is the version I did which is suitable for 16mm or 20mm screws:

    Click image for larger version. 

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    The bracket on the right of the picture (and in the final picture) looks good for me as I'd want to mount it under the gantry. If you think 1610 will be enough then lets go for that, though the saving seems small (about 15 each screw).

    I've started ordering some parts. My plan is to make the Z axis first, then the X and then finally make the machine frame and Y axis. I ordered a 2.2Kw spindle, 250mm ballscrew and some rail and bearings. I'll then make some 10mm plates to hold it all together.
    Last edited by Tenson; 26-03-2012 at 05:19 PM.

  9. #29
    Quote Originally Posted by Jonathan View Post
    Even when critical speed isn't a problem (as is the case with the rotating ballnut) you'll find the 10mm pitch screw wins over 5mm as the torque required to accelerate it is less, due to the kinetic energy of the rotating screw being 4 times less if the screw is only rotating at half the speed.
    Okay you have less rotational inertia from the screw, but twice the pitch means half the driving force on the nut so your max accelerations go down.
    Sounds a bit swings and roundabouts to me :naughty:

  10. #30
    Quote Originally Posted by Tenson View Post
    I thought about that too, but it seems easy to overcome. Double-nut the end of it or pot the thread then clamp. I hate waiting for long deliveries so I'd probably get it from Zapp!
    Your joking my last order came special machined from china in seven days. I'll bet you 10 between Zapp and Jonathan you can't get these screws delivered and machined in less time.!!

    Quote Originally Posted by Robin Hewitt View Post
    Sounds a bit swings and roundabouts to me :naughty:
    Agreed.!!

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