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  1. #1
    I have been considering a rebuild of my MD machine and I have been musing on how to counter various problems. One of them is the crap falling on the leadscrew on the 'X' axis. It is under the bed of the machine. Would it not be better to mount the leadscrew above, so gravity helps keep the crap off it ?
    If there were a sturdy gantry, you could hang everything off it like a travelling crane.

    Is this a reasonable idea or am I too stoopid to see the flaws ?

    I am also considering a design which pulls the Z axis as near as possible to the Y axis, with the stepper motor for the Z axis acting as a counterbalance to the spindle, to minimise twisting forces. This would mean a toothed belt in the drive chain of the Z leadscrew. What problems might there be here ?

    I have already got a quote from Chai for the bits I need for rails and screws, and will probably start building in Late February, or when the bits get delivered. At the moment I am completely undecided how much of the original machine to use. It will probably be little more than the steppers and the spindle mount, though there might be some profile I can use.

    I welcome your comments.

    Cheers,

    Rob

  2. #2
    Quote Originally Posted by cropwell View Post
    I have been considering a rebuild of my MD machine and I have been musing on how to counter various problems.
    Rob don't bother mate just sell it complete and start again.!! . . . . I've been down this road with Micheal marino who was doing the same and he'll agree with me you'll just be wasting your money.
    For very little more money you'll build a far far better machine than an upgraded MD could ever hope to be.!!. . . . . You can't turn a Sow's ear into Silk purse no matter how hard you try.!!

    Bit of advise is don't order anything until you have a finalised design and direction to head in.!

  3. #3
    Listen to Jazz's advice or you may be taking wrong steps in the wrong direction!!! Good luck with whatever you decide. G.

  4. #4
    Hi Chaps,

    There are (at least) two problems with selling my current machine. a. Overcoming the shame in admitting I bought it and b. I wouldn't wish it on anyone else ! The wee beastie is really only a collection of bits of ali profile and some trapezoidal screws. When I mean rebuild, I only intend to use the steppers, spindle mount and T slot plate. Some of the remainder of the machine might furnish profile and motor mounts, but the rest would go into my non-ferrous collection.


    Chai has quoted me on supported rails and bearings, leadscrews, ballnuts and end bearings. I have requested they are only machined on the driven end, so I can turn them to the final fit. For the small cost of the rails and leadscrews, I have been generous in length allowance. The machine can only be 600 x 500 x 200 MAX. I have a MIG and 4 saws capable of slicing steel, as well as 4 angle grinders (there is a short but very boring story as to why 4 of each !).

    On a small machine, the usual practice seems to be to drive the long axis with one leadscrew centrally under the machine. How problematic would it be to put it to one side ? Could any skewing be sorted with a parallel motion mechanism of wires and pulleys like draughtsmens' tables ?

    My main thought at this early stage is 'why put a leadscrew where it gets shit thrown at it ?' Are there any major mechanical disadvantages to starting the design as a gantry with the long axis screw at the top ? I think that I probably ought to get some drawings done to clarify my thoughts.

    Cheers,

    Rob

  5. #5
    Rob,

    There's no shame in buying an MD machine, or any starter machine. At that stage you need to be sure this hobby is for you and that is one of the ways to do it. You will also learn a lot about what is important and what factors are required for a really good machine.

    Many, myself included, have considered pulleys and wires to slave the other side but in the end I've not seen many builds like that. In the end with wire tensioning, stretch and setting up concerns etc. most opt for just buying another screw and belt slaving or double motors etc. I would suggest you do the same. At 500-600mm wide it be well worth it to go to double screw.

    Going back to your thoughts - if you put a single leadscrew at the top in the centre, it will have to be very high to clear the Z axis and stepper. The drop to the gantry will be so long that you will loose considerable stiffness with the mount.
    If you put a pair to each side, but still high up then you will have long supports up from the bed to mount the steppers to (assuming raised gantry design), again loosing stiffness.
    If you put a pair to each side, just above the bed then you have a fairly optimal position in that the driving force is similar in height to the cutter. The stepper mounts are short and simple. I think if you use side shields down each side of the bed (e.g. perspex), or dust extractor for wood you will be OK.
    You can also run the screws even lower down, outside the gantry sides and run the screw level / lower than the bed, and this also works. I think it is not quite as good as the previous for cutter force offset, but this might easily be outweighed if the mounting is more convenient and can be made stiffer.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  6. #6
    For the parts you intend to slavage, weigh up what they would cost new. Now you know what it's worth to you in bits, would you get more for it complete on ebay, so put it on there for a week with a break even starting price and see if you make a profit, if it fails to sell then strip it down.

  7. #7
    I hear what you say Eddy, and I might go that way, just take off the motors and spindle mount and sell the frame. I don't actually use the T slot bed, there's always a sacrificial MDF board.
    I want to keep the controller box I made.

    I am probably going to have a twin screw long axis and would need to buy a stepper and driver. I have got 4 Astrosyn MY103H702 steppers sitting in a box (bought for a project which never happened - cos I bought the MD machine). These are smaller than the 57BGYH7630's on the MD machine, but two of them should be OK on the one axis, but it might be as easy to get another 57BGYH7630.

    The driver is a Tonman PD2064M and I will need one to drive the slave motor - anybody know a supplier that is not in Exeter ?

  8. #8
    Quote Originally Posted by cropwell View Post
    I hear what you say Eddy, and I might go that way, just take off the motors and spindle mount and sell the frame.
    If you're going to eBay it, sell it as a complete working system and you'll get a lot more for it. CNC Routers seem to go for over the odds on eBay at the moment, regardless of the build quality...
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  9. Don't bother with the Astrosyns, they are too low torque and too high inductance to be useful in any reasonable machine. I too have a box of 4 that never got close to a machine...

  10. #10
    Ebay the lot and start again trust me on this you will only regret not doing it.!!

    Irving's correct those motors are no use for anything much more than cutting air.!

    Forget offset screws wires and all that rubbish again it's just doomed to failure and those that have done it have only done it to try get around the mess they made of first attempt.!! . . . . And that mostly that doesn't work so ends up getting scraped anyway. . . . .They just tend not to mention this bit.!!

    Also depending what you want to cut and how machines built will determine if you need twin screws or not. If just cutting wood and softer materials then 500-600mm is no problem for a single centre driven screw machine. Harder materials or wider than 700mm then go with with twin screws.

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