My MPCNC threaded rod mod build

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This topic contains 13 replies, has 6 voices, and was last updated by  Dan 3 months, 3 weeks ago.

Viewing 14 posts - 1 through 14 (of 14 total)
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  • #26824

    David Keranen
    Participant

    This is test run .The build has a threaded rod mod on x and y for milling . Have a beaglebone black with a cramps board running linuxcnc.The cramps board is from pico-systems.This board can also run a 3d printer Everything is printed out of abs

    #26827

    Ryan
    Keymaster

    What are you doing about whip and backlash?

    Remember above 60rpms and you lose torque with the steppers.

    #26828

    David Keranen
    Participant

    I’m using a nylon bushing Hillman 8804338 tapped for 5/16 thread , it doesn’t seem to have any backlash at all . This is the second CNC machine I used this arrangement on .Made wood gear clocks with first machine .As for the threaded rod whipping about it doesn’t seem to occur on my machine, of course when I purchased it I tried to determine if it was absolutely straight.

    #26847

    Very interesting topic. Please keep us posted, I think it could solve the only issue the MPCNC has (in my opinion): a limited torque.

    I would worry that the threaded rod wobbles quite a bit at high speed though. You should consider some acme rods or ball screws instead, also because the pitch would be longer. I guess your CNC moves pretty slowly with these rods.

    Did you use one or two motors per axis?

    #26875

    WarHawk8080
    Participant

    So need a video of this in action!

    #26923

    David Keranen
    Participant

    I found the information I needed on google groups for the motors. Charles Steinkuehler I believe designed the cramps board and did the software for it .So there are two drivers for X and two for Y . They called it a gantrykins . So I have 5 A4988 drivers. I think there was someone else on this site that was working on the same thing, but I haven’t found it again.

    Was going to post a video of my machine but I still have issues . It may be caused by vibration. The gear printed about right with the pen but when I tried it with a 4 flute .062 end mill wasn’t good at all. Have to finish my table for the machine and bolt it down and maybe that will solve it . The gears in the picture I did on my first machine but the machine is much heavier . Hope I can get them to turn out on the MPCNC.

    Attachments:
    #26925

    Ryan
    Keymaster

    Yes, you have to bolt it down, and a 4 flute bit is not recommended for such high rpm spindles.

    #26945

    David Keranen
    Participant

    I had to shorten my z axis had it too long

    #30337

    Derek
    Participant

    What would I need to change in the marlin firmware if I was going to do X, Y threaded rod conversion?

    #36993

    Dan
    Participant

    I know this thread is a few months old, but I’ve been wondering along similar lines. The MPCNC that I built works fantastic on foam, moderately well on thin wood with a 1/16″ end mill, and not so wonderfully with a 1/8″ bit on thicker wood. If I slow the machine way, way, way down I can cut through 3/4″ ply but it’s still not real pretty and it takes forever.

    Don’t get me wrong I think the design is outstanding and have had a lot of fun building it. I’ve made a few CNCs of various designs over the years but this one was very cool to print and assemble. Many thanks to Vicious1.

    But as I was looking at some acme rod I had here, and then to the MPCNC, and then back to the rod, and then back to the MPCNC I got to wondering about modifying it to have two threaded rods on the X and Y axis. Just back of the napkin thinking; I would re-design the top of the end posts to hold the four steppers, two sets over or under or next to each other and then run the rods across. I don’t think it would be too crazy a modification. I think it would have to have four rods, two for each axis; trying to push a long pipe with one threaded rod in the middle I think would cause all kinds of warping. Any thoughts?

    #36994

    Ryan
    Keymaster

    Dan, changing the drive system will have nothing to do with your cut speeds. Can you give more information on your build dimensions, spindle, information on your bits, and what you consider way way way down?

    I just uploaded a video milling some soft wood, is this slow to you? So I can get an idea of your frame of reference.

    #36997

    Dan
    Participant

    Hi Ryan,
    That is beautiful work. You really have that one dialed in. I’m sort of old school in speeds and think in inches per minute – so the IPM on some of my other machines when cutting plywood has been in the 50 to 80 ipm range. When I was using the MPCNC and cutting some 3/4″ plywood last weekend I had to drop it down to around 10mm/sec, or say, somewhere in the low 20s for ipm. Please check my math, lots of Sudafed on board – spring allergies. This was also with very shallow cuts.

    Some other issues that I’m sure are contributing to a slower cut on my MPCNC is the spindle (I’m using a 500w spindle from Amazon with a cut speed around 10,000rpm), and the CNC was built large with a footprint of about 3’x4′.

    If I pushed the MPCNC much above 20mm/sec I was getting ovals. My cut depths were pretty shallow at around .0625″ (sorry, again I think in inches).

    Dan

    #36999

    Ryan
    Keymaster

    A large build and a questionable spindle and you are still getting great times.

    I am still learning every single time I make a cut. I have never had any reason to push the machine, it isn’t my style. I don’t really have the time to just test settings so as I use it I slowly change parameters and sure enough every time my CAD gets better and my cuts get faster.

    25.4mm/s = 1″/s

    50″/min ~ 21mm/s

    So I’m guessing for less than half the price you are getting about half the speed of your other builds? And back to the screws, at this point the belt can easily handle 60mm/s before the rpms start to lower the torque and are firmware limited at 190mm/s. This Is why I asked all the questions, the belts are never usually the speed issue, and horizontal screws are not a good option for a bunch of reasons. If you need more speed for some reason just get larger pulleys.

    #37001

    Dan
    Participant

    I’m learning too each time I turn any of these CNCs on. It’s magic that any of it actually works. And, yes, the cost for building the MPCNC was incredibly low compared to the other CNCs. From the moment a decision was made to start printing the parts to the time when it was completely up and running was less than two weeks. And that was just tinkering in the evenings and on the weekends. It’s an awesome design.

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