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  1. #1
    Subject of conversion in this thread is a mill from 1980's.

    Click image for larger version. 

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    There are some underlying issues that I would hope to address during the conversion:
    - huge slack on Z-axis (covered here in detail: http://www.mycncuk.com/forums/showth...g-a-conversion )
    - large slack on both X and Y -axis

    Have read as much as possible, trying to learn from other machinists build-logs. As a result, here is a draft shopping list
    - items in blue relate to axis/ballscrews.
    ITEM
    NO.
    PART NAME Length QTY
    1 Nema 23 stepper motor, 3Nm (425 Oz-in) - 3
    2 PM752 Microstepping Driver 3
    3 Power supply 400W - 1
    4 Coupling - 3
    5 MBA motor mount - 3
    6 X, Y, Z-Axis Ballscrew Fixed Support Block, FK-type
    3
    7 X-Axis 1605 / C7 ballscrew 581mm + ballnut 581mm (520mm usable thread) 1
    8 Y-Axis 1605 / C7 ballscrew 346mm + ballnut 346 mm (285mm usable thread) 1
    9 Z-Axis 1605 / C7 ballscrew 386mm + ballnut 386mm (325mm usable thread) 1
    10 Ball nut housing (X,Y,Z-axis)
    3
    11 X and Y-Axis Ballscrew Floating Support Block, FF-type
    2
    12 Z-Axis Ballscrew Floating Support Block, BF-type
    1


    In view of the mill, please can anyone spot here
    1) Something clearly wrong ? (something one would regret later?)
    - eg. size of motors

    2) USB vs. parallel port
    - that is something I keep on thinking. Have read about arguments for / against.
    - Kind of would like to go USB, but I have read parallel port is the right thing to do.

    3) Cables and sundries
    - worried about little bits missing from the list. (trying to buy all in one go, to save on shipping fees)


    For now I have left out from shopping list "pulleys" and "belts". Hope could have this machine running some day, then later add belts/pulleys. Not sure how this conversion is going to turn out, will it run or not.

  2. #2
    Motors and drivers are a good choice. What voltage is that 400W PSU?

    I would substitute the couplings (4) for timing belts and pulleys. It makes it much easier to mount the motors as no faffing about getting them on concentric, reduces resonance and allows you to choose the ratio to get either better resolution or speed.

    For (5),(6),(11),(12) make your own as you can use the milling machine to make them better than the commercial ones and much much cheaper.

    USB / Parallel. If you use USB I doubt you'll regret it, but why not try the parallel port first as it costs virtually nothing to try?

    For the stepper motor cables most people use 4-Core CY, 1.5mm^2 cable. Also add E-stop switch, limit switches and connectors for stepper motors.

  3. #3
    Quote Originally Posted by Jonathan View Post
    Motors and drivers are a good choice. What voltage is that 400W PSU?
    - The one I was looking at, is this. Is it any good?

    Wattage 400 Watts
    Input Voltage 240V AC
    Output Voltage 36V DC adjustable +- 2.5V approximately
    Output Current 11 Amps

    Quote Originally Posted by Jonathan View Post
    I would substitute the couplings (4) for timing belts and pulleys. It makes it much easier to mount the motors as no faffing about getting them on concentric, reduces resonance and allows you to choose the ratio to get either better resolution or speed.
    Think I just started to like belts and pulleys.

    Not sure if one wants more speed, at 5mm pitch. Thought of going lower pitch, even 2mm, but most of screws available seem to be 5mm.
    - Perhaps my preference is machine / cutter longevity and accuracy.

    Quote Originally Posted by Jonathan View Post
    For (5),(6),(11),(12) make your own as you can use the milling machine to make them better than the commercial ones and much much cheaper.
    In my calculations,perhaps half budget is for mounting hardware. On Ebay there seems to be great bargains, for example I like these mounts:
    http://www.ebay.co.uk/itm/Aluminium-...item3f148563db

    Good an idea to make home-made mounts etc., though worry a bit about increased complexity.


    Quote Originally Posted by Jonathan View Post
    why not try the parallel port first as it costs virtually nothing to try?
    Good point, parallel it will be.

    Quote Originally Posted by Jonathan View Post
    For the stepper motor cables most people use 4-Core CY, 1.5mm^2 cable. Also add E-stop switch, limit switches and connectors for stepper motors
    Going now back to drawing board, I will soon come up with a more detailed shopping list. Thanks Jonathan for comments, they are very helpful.

  4. #4
    The proposed set up looks good for a backlash of around .015 -> .025mm if you get it right. Plenty good enough to remake the machine after the screws bed in and .025 mm start to hack you off, which it probably will

    A stepper does 400 half steps per rev. A 5:4 belt reduction on to a 5mm screw would give you 0.01 mm per half step, add one microstep for .005 mm and it becomes credible. You could reduce it 5:2 and avoid the springy microstep but that could seriously affect your rapid at 36 volts.

  5. #5
    Quote Originally Posted by diy-john View Post
    - The one I was looking at, is this. Is it any good?

    Wattage 400 Watts
    Input Voltage 240V AC
    Output Voltage 36V DC adjustable +- 2.5V approximately
    Output Current 11 Amps


    No good, why spend more on 70V drivers then use them on only 36V? The torque from a stepper motor is inversely proportional to speed and proportional to voltage, so if you get a 70V PSU you will get almost twice as much torque from the same motors. It's risking to operate right on the limit, so look for something just under 70V. One of the cheaper ways to do it is to use a toroidal transformer.

    Quote Originally Posted by diy-john View Post
    - Perhaps my preference is machine / cutter longevity and accuracy.
    Cutter longevity is a vague one... if you feed a cutter too slow it can break just as quickly as from cutting too fast. You need to estimate the feedrates required for the materials you wish to cut taking into account the range of cutter diameters. Then make sure it will go a bit faster to be sure. However clearly you don't want to optimise the system for a high feedrate, and thus low resolution, if the vast majority of the time you'll be using much lower feedrates. You could select different size pulleys for each axis and swap them round / experiment to find the best compromise.

    Quote Originally Posted by diy-john View Post
    In my calculations,perhaps half budget is for mounting hardware. On Ebay there seems to be great bargains, for example I like these mounts:
    http://www.ebay.co.uk/itm/Aluminium-...item3f148563db

    Good an idea to make home-made mounts etc., though worry a bit about increased complexity.
    It should be decreased complexity if you make your own mounts for pulleys. This is what I started out with on my milling machine as it was easy to make without CNC and does the job. You can see them, briefly, in some of my very old videos:

    http://www.youtube.com/watch?v=tasl9...eature=channel

  6. #6
    Quote Originally Posted by Robin Hewitt View Post
    The proposed set up looks good for a backlash of around .015 -> .025mm if you get it right. Plenty good enough to remake the machine after the screws bed in and .025 mm start to hack you off, which it probably will
    ...
    You could reduce it 5:2
    Robin, with this rate, the mill will be cutting a couple of atoms per slice - maybe.

    So far, added to shopping list some 48 teeth (around 75mm diameter) pulleys, and smaller 24 teeth (around 35mm diameter). Could make it into 5:2, guess it will be a bit more precise then.


    Quote Originally Posted by Jonathan View Post

    No good, why spend more on 70V drivers then use them on only 36V?
    Good Jonathan you say that, didn't even come to think about that. Updated into shopping list. So far only found some 50V PSU's, but will keep on looking, to add to the list.



    Shopping list colour codes:

    RED=NOT NEEDED, going DIY

    ORANGE=Maybe not needed (DIY?)

    GRAY=check quantity

    PALE BLUE=Ballscrews+ballnuts as units

    YELLOW=New since last list


    ITEM
    NO.
    PART NAME QTY
    1 Nema 23 stepper motor, 3Nm (425 Oz-in) 3
    2 PM752 Microstepping Driver 3
    3 Power supply 50V 68V, at least 400W 1
    4 Coupling 3
    5 MBA motor mount 3
    5 Aluminium motor mount 3
    6 X, Y, Z-Axis Ballscrew Fixed Support Block, FK-type 3
    7 X-Axis 1605 / C7 ballscrew 581mm + ballnut 1
    8 Y-Axis 1605 / C7 ballscrew 346mm + ballnut 1
    9 Z-Axis 1605 / C7 ballscrew 386mm + ballnut 1
    10 Ball nut housing (X,Y,Z-axis) 3
    11 X and Y-Axis Ballscrew Floating Support Block, FF-type 2
    12 Z-Axis Ballscrew Floating Support Block, BF-type 1
    13 Pulley for 12mm width belts. AT5 -type. 24 teeth. Total width circa 22mm. Total diameter circa 37mm. For X, Y, Z -axis 3
    14 Pulley for 12mm width belts. AT5 -type. 48 teeth. Total diameter circa 75mm, for X ,Y Z -axis 3
    15 Belt 12 mm 12AT5 length circa 450mm, for X, Y-axis 2
    16 Belt 12 mm 12AT5 length circa 330mm, for Z-axis 1
    17 E-stop switch 1
    18 Limit switches 6
    19 Connectors for stepper motors 3
    20 4-Core CY, 1.5mm^2 cable



    Nice video Jonathan. Makes it easier to understand how the pulleys are setup.
    - 6, 11, 12 ... not quite sure how involved a procedure it is to make them DIY? (and how big a saving can be made, and can one create those without CNC? )

    Think the shopping list starts to look quite good (ready to go?). Really appreciate, your help has been great. Thank you!

  7. #7
    Here is a new draft of the ballscrews. Lengths change a bit, from going to pulleys/belts instead of directly driving the screws
    .Click image for larger version. 

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  8. #8
    A ball screw is best held in tension by Belleville washers to compensate for thermal expansion, one thrust race at either end.

    If you lengthen the XY shafts at one end you can keep the handles.

    Run the table to it's ful travel and ask yourself where the CNC parts are going to fit in the available space.

    Experience tells me that being new to this you are very unlikely to believe this is necessary. You feel a compelling desire to see it cutting, don't want to go back to the drawing board now and spend extra money on machining screws. I only mention it so I can say, "Told you so" later on :naughty:

  9. #9
    Yes Robin, back to drawing board it is. Really appreciate your advice, glad I didn't yet buy wrong length ballscrews.

    Went to garage, moved X/Y -axis back and forth full travel. Need to lengthen the ballscrews, pulleys would now crash to the table.

    Not only that, but realized that it might be possible to extend X/Y -axis travel a good couple of inches, with minor changes.
    - Had not really thought, where one would fit the ballnut, and where screw would be mounted. (attach ballnut to upper part (the one sliding), or lower part vs. the other way around, that kind of things)
    - Below is a sketch of the Y-axis movement (current vs. possibly future). Changes would be: 1) a longer ballscrew; 2) attach ballscrew to the table that does not move; 3) attach ballnut to the table that moves, at the centre.

    Click image for larger version. 

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    Quote Originally Posted by Robin Hewitt View Post
    Belleville washers to compensate for thermal expansion, one thrust race at either end.
    This is all new to me. Thanks will look for pics from google. Another thing I don't quite understand yet, is the construction of the "fixed mount" for the ballscrew. (still thinking whether to buy ready made, or DIY).

  10. #10
    Quote Originally Posted by diy-john View Post
    Yes Robin, back to drawing board it is.
    Wow! If you are okay to take advice you can make one hell of a machine. I got it right on my third set of screws when the penny finally dropped.

    Problem is, you need a CNC mill to make a pretty CNC mill which is why I was suggesting you cobble something together first. Hope you have a lathe, really do need a lathe.

    I used pairs of Belleville washers to apply a preload of 500 lbf. One pair for each screw to hold the axial bearings in tension, one pair for each double ball nut.

    That means there is no play in the system, apart from the column bending, so long as the load does not exceed 500 lbf. It was a lot of work but is quite delightful to use and all self adjusting.

    The XY screws appeared much too long until it was all together, have a pic...
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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