1. #1
    hi all and merry xmas, im gonna build my next machine in jan, 2.5m x 1.3m, im looking to go rack and pinion, could anyone give me details of nicely priced supliers and maybe a bit of info on ratio if you think it will be needed :)

  2. #2
    Wmh Transmissions are good for R&P http://www.wmh-trans.co.uk/

    Ratio will 100% be needed but how much will depend on pinion sizes and motor speed etc. Typical minimum is 3:1 ratio but not uncommon for 5:1 or higher if large pinion is used. To decide on the ratio required you'll first need to determine what kind of resolution and performance your looking to achieve.
    This will guide you to what Size pinion and ratio will be required. Also playing big part in the equation is motor speed and torque.

    Along with this you have to decide on how you achive the ratio. Gearboxs or Pulleys.? Both have there advantages and disadvantages.!
    Gearboxs are good because they easy to impiment and very clean setup, thou they can be expensive. They give high torque and are very strong robust units with low maintenance requirements and long life.
    Some disadvantages are that they can have higher backlash than pulleys and are less efficient so require larger motor torque for same speed has pulleys would allow using smaller motor torque.
    They can have other negative affects on the system in that because they are direct mount the resonance which can affect motors is higher so your drives need to be good ones which can deal with higher resonances on the system.

    Pulleys are good because they are cheaper and relatively high efficiency lowering motor requirements compared to GB. They lessen resonance on the motors and tend to run smoother than GB.
    Disadvantages are that it's difficult to achive high ratio because pulley sizes quickly get large. They are also more difficult to impiment taking more work to achieve and setup. They also need protecting from debris etc and require more maintenance than GB.

    Regards the ratio, pinion and motor speed/torque and resolution required then it's critical they are matched carefully.
    Large pinion means low resolution which means you'll need higher ratio to bring resolution back up. Higher ratio means higher motor RPM required to reach required speed.
    Higher speed if using steppers means lower torque because torque drops as Rpm rise.

    It's a careful balancing act between speed vs Motor torque and resolution. Ideally you want to be aiming to have the ratio/pinon size setup so that motor RPM falls in a safe zone of the motors Speed/torque curve while cutting at the upper end of what you think you'll be needing.
    Rapid speed isn't so important and if you aim to size for cutting there will be some left.

    The next problem with R&P is the fact it's so inefficient compared to ballscrews it's much harder on steppers so you'll need larger steppers. This brings it's own set of problems.?
    Large steppers tend to be slower than smaller steppers so motor RPM affects ratios and pinion size etc. Worse than this is the fact to get any decent speeds from large steppers means using high power drives which cost lot more money.!

    So for this reason often you'll see Servos used on R&P machines. These bring there own set of conditons and expenses into play which I won't get into now.

    But hopefully you'll be seeing that R&P needs careful attention and it's much easier to get wrong than using ballscrews.
    Last edited by JAZZCNC; 23-12-2015 at 12:38 PM.

  3. #3
    thankyou very much :) some very usefull information there and its given me the info of what i need to be looking for, i have in the mean time found a relativly cheap supplier of ballscrews that was mentioned in one of the threads, and he can supply a 2.5m one for a little extra, so maybe ballscrew is still an option and less brain taxing for me :)

  4. #4
    Quote Originally Posted by universally View Post
    thankyou very much :) some very usefull information there and its given me the info of what i need to be looking for, i have in the mean time found a relativly cheap supplier of ballscrews that was mentioned in one of the threads, and he can supply a 2.5m one for a little extra, so maybe ballscrew is still an option and less brain taxing for me :)
    Yes less taxing but still you need to approach with care.! Some of the same considerations apply.?

    Because the screw length is long you'll get whip if spun fast. This can be lessened by using larger diameter screw but doing so brings it's own troubles.?
    Large diameter screws have much higher inertia which severly affects performance and motors. So it's very important you don't over size the Diameter.
    But Smaller Dia screws bring there own set of issues, namely Whip and resonance. These affect performance because they have to be spun slower to avoid problems.
    There are ways to get around these problems like Rotating the ballnut but this gets more involved requiring either making your own or buying very expensive ballnut.
    Another option is to buy screw pitch higher than is needed to reach the speeds your looking for. For instance 20mm pitch instead of 10mm. Then apply a ratio like 2:1.
    This has the affect of halfing the screw speed but still giving the same travel speed and resolution has 10mm pitch. It also increases torque.
    There's small down side in that mechanical torque the screw provides is less than that a 10mm pitch would give using same motors but it's nothing worth bothering about. The positives far out way the negative.

    If your going to go with BS which I suggest you do. Then make sure you get them machined for FIXED BK bearings at each end and if you really want to stiffen up then have them machined for double Fixed bearings at each end. The end support really makes a difference to how long length screws perform so don't scrimp in this department.

    If you use the above setup you'll get away with 20mm Dia screws. Don't go below 20mm or above 25mm.

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