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  1. #1
    I'm looking into the possibility of making a machine to aid me in circuit board assembly. I don't want to make it too complicated or I end up designing a machine that I can get off the shelf anyway or I end up reinventing the wheel and spending all of my time on the new wheel design instead of the projects I want to make with the thing.

    I need to move a table in X and Y with an accuracy of 100m (50m would be nice but like I say I don't want to reinvent the wheel). The table will move around under a gantry that will have the part picker/placer on it that will travel in the Z. The reason I am not planning the table to move in one axis and the picker on the gantry in the other is that I want to avoid the risk of having parts fall off my picker that is pneumatic but it's an option if I can sort the picker out to be as robust as possible and if it solves a load of other problems.

    200x300mm movement should be enough but given space that could be a bit bigger. I've been thinking something along the lines of a couple of those sliders with bearings in them on each axis with a stepper motor controlling them.

    I don't know if what I want exists off the shelf or if I should just settle on cobbling my own together.

  2. #2

    This sort of thing might do. I can take the motor off and replace with my own mechanism, but I am not sure what the T8 screw means in terms of movement. They say they have 42 stepper motors, is that 42 steps per revolution?

  3. #3
    T8.= Trapezoidal screw - that's likely to bugger your positional accuracy requirement unless you introduce some anti-backlash mechanism. Stepper "42" - can't see that in the link, but could be related to the frame size that translates to a NEMA17 motor - it's just an indication of size and expected power range. Most common steppers are 200 steps/1.8 degree/step resolution. On a T8 that's 25 steps per mm or 40 micron - but you could pretend that micro-stepping offers higher resolution. Of course that assumes everything about the screw thread linearity / accuracy, as well as the frame geometry. Do you really need 100 micron accuracy? I don't think so (I'm pretty sure that working with 0402 components I'm not that accurate by hand).

    How are you planning to feed your P&P - what's the component delivery mechanism? How are you going to achieve component alignment (and necessary re-alignment...particularly with vibration moving through the frame). Have you not thought of solder stencilling or solder-paste deposition as a "glue" for the components before placement?

    I spent a few months in a clean room pick&place PCB assembly room when I was "a lad" - okay, quite a few years ago, but working incredibly compact boards (and high layer-count PCs) and the early generations of SMD devices. P&P with a less than heavy investment can go spectacularly wrong quickly.

    I'd be interested to hear how you get on; unfortunately none of my projects get anywhere near complicated enough to make me desire such a machine myself.

    Edit: reference to 0402 - I tried to prove I could... I much prefer 0603 for sanity, or 0805 to rent out as student digs :).
    Surface tension of molten solder will draw components into alignment over distances substantially greater than your 100 micro accuracy.
    Last edited by Doddy; 19-12-2020 at 09:06 PM.

  4. #4
    What sort of backlash can I expect? 40m is plenty. These days IC's have pin pitches down to 0.5mm. I'm not looking for an automated solution. I tend to use a lot of the same parts so it would save a lot of time just to load one BOM line at a time so that the machine can automatically pick each part up once told where the first one on the tape is and be driven to the location where it is set down. My plan would be to feed it the coordinates of each part on the board but that I give it the OK before placing a part so that I can make a manual adjustment if required. So for the average passive that there are a lot of and that do not require too much precision it will probably work automatically and quickly but the IC's that I won't be able to pick up reliably anyway I can move precisely into position by hand after it goes to about the right location.

  5. #5
    Boards are stencil pasted before part placement. the paste indeed will hold parts in place which is why I'd rather move the board around than the PnP head.

    Large items that there are not many of I would still put on by hand. I'm trying to speed up the fiddly small parts that there are lots of and not try to do the easy big ones that would be near impossible to pick up and move like large electrolytic's as I can put them on faster by hand anyway

  6. #6
    Actually, thinking about it - you can design out backlash - you'll be moving from the component pick-up in one direction across both X/Y axis - so backlash doesn't really play as much a part as e.g. a router. You'll still suffer from screw accuracy, but I think you overestimate your accuracy requirement in any case. You can't eliminate backlash, but you can probably get it down to a reasonably tolerable level. You might find few people with experience in the DIY P&P domain - you might have to have a play here and come back with specific questions. Clearly you're going to be developing a lot of the system and software yourself - but I can't see anything that would particularly worry me if you're playing man-in-the-loop with the placement.

    We want vids :)

  7. #7
    If the backlash is known I suppose I can factor it in when I reverse direction just like I have to when using a manual miller and the Vernier's on the handles.

    Not sure what to do with the software yet as windows programming is not my thing but have been told VB is dead easy with lots of tutorials. It will likely be a work in progress starting with the mechanical build and a simple stand alone controller I can punch coordinates into followed by talking to it from a PC with a USB/serial converter.

    Yes it will be very much man in the loop, it's really pointless trying to over automate, more of a helping hand that saves me being bent over a table for 3 hours. I just built a board with 219 parts on it at an average rate of one per minute. A lot of time was wasted trying to pick up the 0603's that then just stuck to the tweezers. I just bought one of those hand vacuum pickup tools to make it easier but then started getting ideas.

  8. #8
    To stop backlash can't I put a threaded item that is rotation locked to the moving body with a spring between the two to keep the table or head on the gantry always pushed one way?

  9. #9

  10. #10
    cheap but without making it useless! basically you can buy pathetic hand tools for 2k that you just put your arm on and slide around from part pickup to placement but I see that as problematic as you need fine motor control and in the demo of how fast you can go parts are not dropped right and of course they neglect the time taken to hunt out the parts locations so they are just matching footprints for effect. If I can buy a good base with motors and drivers for under 300 then that's a really good start but a bit bigger than 300x160mm would be good as if I can put multiple boards on (they can easily be 200x100mm each) this further greatly speeds up the processing even as a man in the loop.

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