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  1. #21
    Looking good and glad you haven't been put off by the other miserable B*****ds:heehee:

    I'd go with the first option but if it was mine I would put the y-axis lead screw between the blocks, like you have done on the x-axis. The gantry could also do with a bit of stiffening to prevent racking, either another plate on the other side or a length of 8020 bolted to the ends and the y-axis plate would really improve it.

    the lower gantry fixing may also be a weak spot and again I would use some 8020 (or angle plate) to strengthen the base plate and prove a larger fixing surface to the gantry.

    How did you get on with thk rails?

  2. #22
    Quote Originally Posted by Ross77 View Post
    Looking good and glad you haven't been put off by the other miserable B*****ds :heehee:
    Mea culpa, you go that way after a few years in this game :whistling:

    It looks perfectly serviceable now that Mike has cut his tolerances to 'whatever is achievable'.

    Mike, I have bags of Belleville washers and springs left over from preloading my 16mm ball screw nuts and bearings if you want some (assuming no one has pinched them). True zero backlash is a delight after you have experienced the alternatives, I often find myself idly rocking the X axis handle a thou either way and feeling it move the table


    Miserable git

  3. #23
    Other things have got in the way of my replying earlier, but in the interim I have been doing a lot of reading - mostly on the web, but a few books. Nothing I have read has made up my mind what kind of machine to build - or even what is needed to get the sort of accuracy I want. As an example of this, I was just looking at a Renishaw (dental milling) site where they said that too rigid a frame worked against accuracy. (something to do with compliance missmatch)

    The upshot of this is that the only certainty is the need to machine parts to try different things out - I need a (CNC) milling machine to make one. As I enjoyed converting my Proxxon, hopefully I can find something that will both be useful, and become the basis of a conversion.

    By the way, Ross77, the THK rails are fine. They have been swaddled up in protective paper against use in the, hopefully, not too distant future.


  4. #24
    Have you got a link to that article? I know that just adding more mass can lead to vibration problems but not heard that a machine can be to stiff !!!!!

    Is that the type of machine you are after? I know what you mean about needing one to make one. I'm in the same position (although nearly there now)

  5. #25
    Link is

    Probably a little lighter than I want, but not much. The sort of thing I want to make may be (examples to show range)

    Frame side in aluminium machined from 2mm alloy plate, ribs "T section" 2mm by 2mm with up to 60mm unsupported.
    Compressed air turbine in titanium (a bugger to machine with anything) 26mm dia 6 grams weight - must be inherently balanced (hence need accuracy)
    machinable castable wax model to try casting iconel model of same if titanium doesn't work.

    The most common needs are accuracy and a really high spindle speed.


  6. #26
    What a fascinating machine. An upside down mill using a die sinker's rib cutter to machine ceramic. Presume "non-Cartesian" means they have done away with the need for slides by using two, offset rotation centres. I've never been a particular fan of "skimp on the design then try to fix it with software", but that looks like it might just work

  7. #27
    I can't find the reference at the moment, but I think that it is a sort of stabilised tripod with five legs - two of them are cross braces. Each leg can be individually jacked up or down giving X, Y, Z and two limited axes of rotation. There was something vaguely similar in the form of the "hexapod robot milling machine" on Youtube.

    Of course, I could be wrong and it may be something else entirely.


    Edit:- Robin, I have just been thinking about your two offset rotation centers - and fix it with software - what a great idea, have you patented it yet?
    Last edited by leadinglights; 05-06-2010 at 08:29 AM.

  8. #28
    Simplest would be a single, horizontal, swinging arm with a bearing at one end for the workpiece and a bearing rigid to the machine at the other.

    The available work area would be an odd shape, the maths would be tricky, but replacing linear bearings with good old taper rollers could solve a lot of rigidity issues especially if you dumped their C shaped frame in favour of something tripedal.

  9. #29
    Quote Originally Posted by leadinglights View Post
    Edit:- Robin, I have just been thinking about your two offset rotation centers - and fix it with software - what a great idea, have you patented it yet?
    With a bit of clever design you might be able to use bearings that were physically larger than the offset. A bit of luck on ebay could make it quite solid at a reasonable price, large bearings with no particular purpose don't attract a lot of bids

  10. #30
    How big do you need? Ive got quite a few under the bench . if theres one the right size then it yours for the cost of the postage.

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