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  1. #1
    Ross77's Avatar
    Lives in Devon, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 9-10 years. Has a total post count of 750. Received thanks 27 times, giving thanks to others 52 times.
    I'm looking at options for a 4th axis for a small mill (Novamill). I have a cheap belt driven one but the holding and resolution are not great.

    So that leaves a modified worm driven rotary table, servo with brake or a harmonic drive.


    I also want it to have through hole capacity and use both collet and chuck options. 100mm max. diameter. and be capable of gear cutting

    My initial thoughts are:

    1. Worm dive is simple but have to live with the backlash and have to adjust periodically.

    2. Servo motor and brake seems quite simple and would be good resolution and accuracy. brake can hold for indexing or fixed milling but dynamic milling
    would be reliant on the servo holding torque

    3. Harmonic drive, Good accuracy and torque but heard that they are prone to damage and can flex out of position under load.

    What would be the best choice for good resolution and holding force?

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 18 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,486. Received thanks 284 times, giving thanks to others 7 times.
    For something compact enough for a Novamill, you'll probably have to build something yourself.

    It's worth looking at the SkyfireCNC options (If the info isn't on their website, I do have the PDF documents somewhere).
    I also got as far as some sketches for a compact big bore 4th axis for my Triac, and my conclusion was a suitably adjustable worm and wheel was going to be the best compromise. Custom made also gave the option to mount the motor at a suitable angle, so as to minimise overall height of the unit (probably about 30 degs off horizontal, so the motor sticks out the front, but doesn't mean the axis has to sit higher to give clearance for the motor, or having the motor sticking straight up)
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    I drew up and made a 4th axis using a hollow shaft ER32 collet chuck had two backplates threaded to fit to use either a 3 jaw or 4 jaw chuck and it works great.

    Click image for larger version. 

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    Phill

  4. #4
    Ross77's Avatar
    Lives in Devon, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 9-10 years. Has a total post count of 750. Received thanks 27 times, giving thanks to others 52 times.
    I had come to the conclusion that I would have to make something bespoke. The Skyfire one looks good and specs are what im looking for. Guess it will be a pricey.

    Why did you discount the harmonic drive?

    I do have an optical indexer that I could use to cut a nice accurate gear in steel and then turn the worm in phosphor bronze and make up a spring loaded tooth to take out the backlash

    Phill
    What resolution do you get out of that set up?

  5. #5
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 18 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,486. Received thanks 284 times, giving thanks to others 7 times.
    The problem was getting a suitable harmonic drive.
    I was aiming for about a 6" chuck, but with as big a bore as possible, and such harmonic couplings were pricey, unless you want to take the gamble with the plethora of used offerings on eBay.

    I came to the conclusion that many CNC 4th axis use worms, and they generally don't have many problems with backlash, as they are designed to be adjusted, which is key. The problem is making it adjustable, while having a big motor attached.

    For inspiration, you may want to check out the original InTurn design thread over on the Mach forums. It used belts, and a brake - https://www.machsupport.com/forum/in...72469#msg72469
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  6. #6
    This thread made me think "is there such a thing as a worm drive equivalent of a ballscrew?" .... it turns out there are - see some links here: https://www.practicalmachinist.com/v...e-worm-281127/ and here: https://www.detlevhofmann.de/product...units/?lang=en

  7. #7
    Quote Originally Posted by Ross77 View Post
    I'm looking at options for a 4th axis for a small mill (Novamill). I have a cheap belt driven one but the holding and resolution are not great.
    A geared stepper and a belt tension system will sort that out for little work or cost.
    You think that's too expensive? You're not a Model Engineer are you? :D

  8. #8
    Quote Originally Posted by phill05 View Post
    I drew up and made a 4th axis using a hollow shaft ER32 collet chuck had two backplates threaded to fit to use either a 3 jaw or 4 jaw chuck and it works great.

    Click image for larger version. 

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    Phill
    Realy like ur build , wondering if u can help me with some code , i need to know how u read position and write position on lcd .
    Thanks .

  9. #9
    Description taken from S Ward's web site: http://www.worldofward.com/rotarycontroller/shop/.

    The controller is intended to drive rotary table, lathe head indexer etc.
    Allowing easy, error free division without the use of division plates.
    As well as division the controller allows movement by angles (0.01 to 360 degrees),
    continuous motion (for machining), positioning (jog mode) and a sophisticated
    program mode which allows complex sequences of movements and events to be
    executed.
    Multiple controllers can be connected together to allow sophisticated machining
    operations – e.g. automatic gear cutting, one controller dividing whilst the other
    moves the cutter.
    Input is via a 16 key keypad whilst a 4 line by 20 column LCD display provides
    information in a user friendly format.

    Features.
    • Jog mode. Keys move CW/CCW in 0.01, 0.1, 1.0 and 10.0 degree steps.
    • Division mode. Divide a circle into any number of divisions from 1 to 9999
    inclusive.
    • Degree mode. Any number of degrees from 0.01 to 359.99 in 0.01 degree
    steps (note: actual accuracy depends on mechanics).
    • Continuous drive. Table is driven continuously CW/CCW at one of the
    (selectable) 5 speeds.
    • Program mode. Allows complex sequences to be carried out. (Up to 10
    programs can be stored)
    • Configurable for any worm drive from 1:1 to 9999:1.
    • Five speed settings.
    • Configurable for all common stepper / driver combinations.
    • Five machine profiles.
    • Backlash compensation.

    Phill

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