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  1. #11
    Quote Originally Posted by mbele View Post
    Hah, back to the drawing board...

    1. Z axis travel is too short, I use following calculation:
    (max tool stickout - min tool stickout) + max stock height + clamping height + safety
    minimum: (35-10) + 25 + 10 + 5 = 65mm

    2. To implement apron type chip guard, I need to extend Z axis overhang, and since arrangement was optimised to make tooltip as close as possible to the linear components, arrangement does not make sense any more - Y overhang needs to be extended for 60mm and axis travel is only 100mm

    3. rigidity of X axis ballscrew shaft is 7x less than Z axis ballscrew, so there's no point in beefing up Y and Z if the percentage of their impact on the whole system rigidity is small. X axis rigidity can be improved by increasing shaft dia, but that raises inertia, so 2.7Nm/840gcm^2 stepper would work better than 400W/360gcm^2 AC servo, but that puts me in the no-feedback scenario, so there's no point in using servos on other axes...
    1 thing I'd say is use Angular Contact bearings if you are going to use Chinese DFU nuts & screws.
    I used thrust bearings on my first small benchtop mill and on my second I've gone with Angular contact.
    I would NEVER go back to thrust. They don't sit on the end of the ballscrew correctly and this causes a wobble you can't get rid of. Rigidity was crap no matter how tight all the axis were set up.

    The new on with the AC's is rock solid and all the axis are set as 'light touch'. It really is chalk and cheese just because of a difference in bearing types used. I did however have to make my own bearing housings from scratch because of the sizes of AC bearings available.

  2. #12
    Thanks for the hint dazp1976. I was thinking to go with BST Automation BK supports with AC bearings - they are a bit more expensive than "standard" ones, but nowhere near the brand name prices I saw on ebay.

    Re ballscrew shaft rigidity, 20mm dia 1100mm travel shaft has about the same rigidity as 8mm dia 180mm travel shaft :)

  3. #13
    Quote Originally Posted by mbele View Post
    Thanks for the hint dazp1976. I was thinking to go with BST Automation BK supports with AC bearings - they are a bit more expensive than "standard" ones, but nowhere near the brand name prices I saw on ebay.

    Re ballscrew shaft rigidity, 20mm dia 1100mm travel shaft has about the same rigidity as 8mm dia 180mm travel shaft :)
    Sounds good.
    I was glad to have the Seig X2 to make my own parts for the new bigger PM25 size machine.
    I'm using budget and Dunlop AC bearings sizes 12x32x10mm & 10x30x9mm with DFU1605 all round. Both brands work well.

  4. #14
    I decided to make base out of 150mm thick granite slab, with glued-in steel inserts that are going to be ground to provide accurate rail mounting surfaces.

    I toyed arround with twin tube + expanding concrete fill, and while it's much better solution in weight/deflection context, I cannot predict what will happen with it after welding and I couldn't find a place where I could get it stress relieved. Here's the link to a shop that has expanding mortar https://www.moertelshop.eu/buy-durfill-cheaply if anyone is interested, they also have ingredients for a special concrete with extremely high youngs modulus https://www.moertelshop.eu/buy-refra...gate-cheaply_6 (check the description).

    Full size granite slab is to heavy to handle for me, so I need to reduce weight but keep it from bending while/after the rail supports are machined and before it's mounted securely.

    Here's what I came up with:
    Click image for larger version. 

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    Click image for larger version. 

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    It also opens up possibility to make it as a construction from thinner slabs...
    I realy like the overhang variants, but am I asking for trouble?

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