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  1. #21
    Quote Originally Posted by Jonathan View Post
    I would mount the stepper motor to a block of aluminium since the posts are not particularly strong, wont conduct much heat from the motor and also look like a grandfather clock. Also you might have difficulty fitting the belt/pulley between two posts? It would certainly limit your choice of ratios.
    Probably negligent, but instead of making the ballnut mount match the circular profile of the nut, I'd make the mount closer to a rectangle so it fixes to a greater area of the bed since that should be stronger.

    fair points, i will certainly be changing the ballnut mount but the screw mount is, although debatable in need of more thought (access to y axis gib screws has been somewhat compromised)

    cost also needs to be taken into account so i may well use standoffs just to get the machine running and then cnc a new mount

  2. #22
    got some more feutering done on the x axis design this evening

    firstly i modified the stepper/ballscrew mount by lowering the motor giving space for gib screw access holes and then added a rib on the back for some extra rigidity

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    then i drew a 1204 ballscrew and crude representations of thrust bearings for the ballscrew mount
    a 450mm long screw leaves just over 38mm overhanging the table travel extremities

    still have to decide on a pulley ratio so i can incorporate it into my design, any thoughts?

    could the moderator please change the title of this thread to 'centec 2 ballscrews' as its getting far of the original topic and it would be a shame to have to start again

  3. #23
    things have been slow to progress lately due to lifes consistent interuptions but on another note...
    I now have ballscrews!
    2x 1204 & 1x 1605. 250mm, 450mm and 450mm respectively
    also got my hands on a nice 20mm 2mt drill bit to drill the hole in the mills base casting for the z axis screw
    Click image for larger version. 

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    now that i have me some screws i need to get them machined. i have arranged for a friend to do this for me as the boxford doesn't have much in the way of change-gears and im afraid id screw it up (pun intended).

    but before i can do that i still need to order bearings and pulleys so i can factor them into the design. this leads me to a few questions...

    1) pulleys, is there really any difference betweel xl and htd or other types of pulley?

    2) bearings, i see alot of people using angular contact bearings, wouldn't ball thrust bearings be a cheaper alternative or is their something im missing?

    3) i see that using belleville washers is also quite common for preloading the bearings, would an easily available split washer not do the job?

  4. #24
    Quote Originally Posted by jonnydeen View Post
    now that i have me some screws i need to get them machined. i have arranged for a friend to do this
    Why couldn't you get them machined by the seller? Assuming you bought them from China it should be very cheap. It's not easy to do it yourself due to the hardness of the surface.

    1) Different tooth shapes which are more/less accurate (backlash etc). HTD is generally considered the best for this application.
    2) Could be, but I think they'd be harder to mount and pre-load accurately? Also possibly lower efficiency, but I'm not sure.
    3) No since it wouldn't apply an even force on the bearing ring. Belleville washers may not be nessecary - it depends how you mount the bearings. The BK/BF blocks don't use them, instead they rely on the bearings being ground such that when they are tightened together the correct pre-load is applied, or they use shims. If you do use Belleville washers then you can put the ballscrew in tension, which is good since it increases the stiffness of the system a little (or a lot depending).
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  5. The Following User Says Thank You to Jonathan For This Useful Post:

  6. #25
    thanks for the reply jonathan

    firstly, im getting the screws machined free of charge, why pay if i dont need to?

    as for pulleys, i will take your word for htd and give them preference so long as i can find a set to fit the cramped x axis

    now, the bearing selection is an interesting predicament...
    the largest id magneto bearing i could comfortably implement would be E5 (16mm od)
    i can easily get 8x16x5 ball thrust bearings

    so, do i turn 12mm ballscrews down to 5mm to use magnetos or 8mm and just use ball thrust bearings. or in other words is 5mm sufficient if i ever hope for this machine to cut steel?

    and as for bellville washers, point taken

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