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Gantry Style CNC from plywood

Discussion in 'CNC Mills/Routers' started by C-CNC, Nov 1, 2018.

  1. C-CNC

    C-CNC Well-Known
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    C-CNC published a new build:

    Read more about this build...
     
  2. GeoffH

    GeoffH Journeyman
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    Making a CNC Router from very thin plywood is a big challenge for you. You will need to produce stiff box sections, and use a good quality glue. There are a few Plywood CNC builds on this site but using much thicker, hardwood Plywood. I guess the CNC size will be small?
    Anyway good luck with your project.
     
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  3. C-CNC

    C-CNC Well-Known
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    oh got the size wrong. 2.4cm... I also added the size in the summary...
     
  4. GeoffH

    GeoffH Journeyman
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    That's good. The model that you have attached seems to be incomplete, I'm guessing that the gantry is not cantilevered?
     
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  5. C-CNC

    C-CNC Well-Known
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    No it isn't cantileverd... To save time I only created one side... The model is also just a rough model to get the idea how i want to do it.
     
  6. C-CNC

    C-CNC Well-Known
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    Could anyone suggest a spindle that quietly still powerful but costs less than 150 €/$ ?
    Im currently thinking about a 400w 48volt spindle with an ER11 colltet for 86€ / 97,74USD. I thought its enough for the start and it could cut aluminum at low speeds.
    By the way, yesterday I tested the x-axis and achieved an feedrate of about 400mm/min (thats around 16"/min) with acceptable torque.
     
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  7. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    I would suggest using a braced rail. While you'll have plenty of stiffness in the 2.4cm plywood, the rod rails you have shown won't offer much and will be the weakness in your system.

    rail.jpg
     
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  8. C-CNC

    C-CNC Well-Known
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    Yes, the model is a bit outdated... im using for the x-axis SBR25 Rails(one for each side) and for the y and z axis SBR20 Rails
    I will post the fusion360 CAD model of the x-axis soon. But i got a lot to do for school right now...
     
  9. stargeezer

    stargeezer Master
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    The 400 watt spindle will be a great starting point for your router as long as you are satisfied with it's abilities. Limit your cutter diameter to no bigger than 1/4 inch ( or 6mm) and you will be fine. There is a ton of import 1/8" Cutters on ebay that are very cheap and sharp enough for starting out. When you decide to start cutting aluminum you will need better quality bits, but my son cut up to 1/4" aluminum with a 400w spindle motor and very cheap carbide bits. Just limit your speeds an feeds to what the setup you build can handle with good results. 400 m/minute might be a good rate on wood with one cutter but it might be a disaster when cutting aluminum.
     
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  10. C-CNC

    C-CNC Well-Known
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    I just found a brushless 500 watt, 48 volt, ER16 motor and thought brushed motors make a lot of noise ...
    The price is 128USD / 112 €, so 20 USD / 30 € higher. (yes USD to EUR = 1.15 to 1 but ebay dosnt think like that...)
    It's a higher price, but less noise and bigger collets...
    I am not shure if it would be a huge difference so what do you think ?
    (I know it's from cheapo china but I don't have that much money and chinese stuff isn't always that bad : {48V CNC 500W Luftkühl Spindel Motor Brushless + 52mm Clamp + Speed Governor ER16 | eBay} i added the cost of the powersupply in my calculation.)
     
  11. Rob Taylor

    Rob Taylor Veteran
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    You'll almost certainly get better results by relying on design rather than material properties on this one. Use thinner plywood for the moving parts, more like 10-15mm rather than 25mm-class stuff, and make sure all of it is in torsion box sections; the base, the columns, the gantry bridge, everything. The thinner wood will save a lot of non-useful weight, whilst the design will add significantly more rigidity than you would have gotten with the thicker material. You could also screw on some cheap aluminum angle on the edges to add some extra stiffness and rail mounting surfaces.

    Use a high performance glue that will also add significant structural support to the whole thing whilst keeping weight down- and can be spread in very thin sections to keep joints from floating too much and won't flow out under screw tension. Not 100% sure what that would be, off-hand, (polyurethane could be a good option?) but there are likely more options in Germany than a lot of other places anyway for those high performance materials.

    At some point, put blasting sand or pool filter sand inside the torsion box sections to help with damping (plywood is fairly resonant for a composite wood product- look at DIY audio applications and musical instrument bodies!). If your axis motors can handle the weight of it, put some in the gantry and carriage too. Adding the MDF for the spoilboard will help there, it's quite damping too.

    Consider your options for not using wood for the gantry carriage/z-axis body; it may be too small a volume to practically use wood. To create a stiff enough structure, you may end up creating too large an overhang, creating a significant moment on your gantry axis mechanicals. It's such a small area that you could probably just use laminated aluminum square tube or something along those lines and get a similar rigidity to plywood in 1/3 the volume, or something. On the other hand, maybe a simple slab of 24mm plywood would be sufficient to do everything because it's so short. Maybe you could use 5mm plywood with the right structure and keep it all shrunk down flat. There are a few possible options, and you can't really know unless you a) try them, or b) run some calculations. Just something that jumps out to me to be aware of though.

    That spindle actually looks pretty decent for what you're doing. 500W, if that's close to actual usable power out and not just marketing, is plenty for basically any material that a plywood machine is gonna cut. Brushless is almost certainly going to be higher power density, likely smoother running and quieter, more usable torque, all that good stuff. I'd keep poking around a little more to see if you can find a 240V EU model, but it may just be the 110V US models that can be found in the lower ~$100-150 range. For cutting wood, you'll definitely appreciate the larger collet options.

    Gearing down your steppers between 2:1 and 4:1 will improve both absolute torque and acceleration (at the cost of top speed rapids) for smoother interpolation and contouring, so maybe consider how feasible it would be to do pulley plates and motor standoffs instead of direct mounting if you're intending on doing a lot of arcs, circles and organic forms vs simple plate-making.
     
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  12. C-CNC

    C-CNC Well-Known
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    Im currently running my motors at 800steps per rev (1/4 microstep) that has no reason just thought it could be good.
    The threaded rod got a pitch of 2mm/rev (0.078 inch per rev = 12.7 revs per inch)
    The motor is running at 3.33 revs/sec = 6.66mm/sec = 400mm/min or 15.7inch/min
    Wouldn't it be better to turn down the revs / sec so increasing the torque of the motor instead of gearing it down?
    Thats what I think, but I'm not a professional, so correct me if I'm wrong.
     
  13. C-CNC

    C-CNC Well-Known
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    I only used this wood cause these plates were meant to be used to be shipping boxes at my dads work, but they never used them so my dad said i could use them..
     
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  14. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Build looks great so far. Excellent work. :thumbsup:
    .
     
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  15. C-CNC

    C-CNC Well-Known
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    For a 14year old or an adult o_O:D Thanks anyway !
    I newer thought i would get that much support and feedback :thumbsup:
     
  16. GrayUK

    GrayUK Openbuilds Team Elder
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    You just came to the right place for that sort of thing!! :thumbsup:
     
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  17. Andreas Bockert

    Andreas Bockert Veteran
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    With 2mm / rev you certainly don't want to step down. The typical openbuild screws are 8mm/rev.

    When picking a spindle you'll want something that goes low in the RPMs. Especially if you can't get to run faster than 400mm/min. Ideally, when running 10k RPM you'll want to go about 3000mm/min to get a decent chip. At 10k RPM and 400mm/min you'll be rubbing the bit and generating a lot of toasty dust and burning your carbide...

    What drivers are you using? If you add a fan to them you can get pretty far with DRV8825 drivers. I ran my machine at 4000mm/min with DRV8825 drivers. With 1/8 micro stepping you should be able to hit roughly 2000mm/min before the GRBL/Arduino is maxed out.

    So, add a fan, max your currents and see how high you can push things...

    Maybe I missed it, but do you have a plan for tackling backlash?

    (Btw, in the description you still state that it's 2.4mm plywood, not cm).
     
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  18. C-CNC

    C-CNC Well-Known
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    I put the drivers into the description. The drivers can handle up to 4A and 4.5 Amps peak.
    Im using universal GCode sender to controll the Arduino.
    When I tested the motors with a helical drilling operation the motors got really warm. After 25-30 minutes they warmed up to about 50-60°C(125-140°F) so I had to stop the operation and let the motors cool down. I do not know if attaching heatsinks can solve the problem.
    I ran them with 2 Amps so at the rated current -> I won't be able to add more current to increase the torque without frying them.
     
  19. Andreas Bockert

    Andreas Bockert Veteran
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    60C should be within operating range. I think that steppers are ok even at 90-100C.
     
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  20. stargeezer

    stargeezer Master
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    First of all I am with you on every cost saving step you have taken and applaud your efforts to build what you need while finding alternative materials that might have ended with filling up more landfills. You are proving that spending loads of money is NOT required to build a CNC. Great Job!!

    One place I'd urge you to re-examine is your stepper mount. The photo on the Build page shows a gap between your mount and the face of the stepper motor flange. I think that you need to either put a spacer between the motor flange and the mount, cut open the mounts inner bearing space open to clear clear it to give you a flat mount or use washers to fill the gap on each bolt you use. My thoughs are that mounting like you show puts lots of stress one the stepper motor flange that may cause it to part off. The biggest issue is that I doubt those bolts can ever be tightened enough as you show them.

    Just some thoughts you did'nt ask for.

    Larry
     
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  21. C-CNC

    C-CNC Well-Known
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    I already disassembled all the parts on the steppermount that were meant to reduce vibrations. I will use some washers to release the stress on the stepper motor

    Thanks for the great thoughts !
     
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  22. Rob Atha

    Rob Atha Well-Known
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    I made my first cnc this year, used 18mm ply throughout and it’s a pretty solid build. I’ve looked into using x-frame rails but the cost for some pieces are extortionate and to be honest don’t look rigid enough for what I need.

    I’ve used both my Mikita trimming router but have now opted for a cheap Chinese spindle to cut down on nose and vibration, and it worked well. Although people have mentioned sbr20/25 rails, I opted for sbr12, I believe unless you are mating a monster of a machine, anything larger that this would be overkill.

    91052F11-CFEF-4504-B582-8D28FB1C6BAB.jpeg

    These are just some cuts I’ve managed to achieve with my machine for some Christmas lanterns I’m making and with the setup I’ve used I think they turned out really well considering I’ve only spent £150 to make the machine
     
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  23. Rob Taylor

    Rob Taylor Veteran
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    Some roughness as you'd expect, but that's super impressive, actually, for 150 quid.

    I think people wildly underestimate how much force a large number of ball bearings spread over multiple planes of a fully supported rail can take. People are out here using 20 and 25mm THK/Hiwin rails when 12mm and sometimes even 9mm rail would be just fine for their machine. It's hard to judge visually when things are engineered like that, common sense has to take a back seat to actual numbers.

    (Also, YORKSHER! :thumbsup:)

    Yes and no. I'd use full or half-stepping on 2mm pitch screws- that's 0.01mm or 0.005mm resolution, respectively- not that you'll actually see that out of the machine, it's just the theoretical output on the nuts themselves. This will also reduce the frequency that your controller has to send at, which is usually a good thing for reliability. As you say, it'll also give you more torque.

    However, gearing achieves more than purely adding torque, it also balances the power transfer between the moving carriage and the motor rotor- you don't want them to get too far away from each other in terms of momentum, or you end up forcing the motor to do a lot of work that you don't actually see out of the machine. Remember, every time you decelerate the carriage, you're basically trying to force its kinetic energy back into the motor... Which the motor doesn't like very much. Gearing helps even the two parts out so it can speed up and slow down much more quickly and efficiently (which it's doing a lot any time you're cutting curves!). On your machine, it might be overkill, but just thought I'd mention it anyway, since inch-thick plywood could be a lot for the motors you're using.
     
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