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  1. #21
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    Default Re: Design input for newbie

    Thank you NIC 77,

    I will use the FE abilities in Fusion 360 to evaluate the ribbing. I have this far made a point of having internal or external sheets that meets the center of the rails to get good load paths.

    Good idea with the aluminium, it will be easier to work with as well.

    The size of the bed is 4'4" by 3' as drawn. This was partly driven by rack length and partly by plywood sheet size. The travel is roughly 3'4" x, 2'6" y and 5" z. I'm debating the z-travel with myself. This machine will not get me what I want, I'm not sure whether there is any point in making it not quite as limited. For extreme cases there is a built in adjustment to bit length in the spindel mount, there is four mounts above each other.



  2. #22
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    Default Re: Design input for newbie

    Try surplus or off-cut Mic-6, which is Blanchard ground, and you can get probably cheaper than the steel.



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    Default Re: Design input for newbie

    Hi,

    I've read here and it seems that the technically really good choices are considered to be Hiwin or THK rails. Does anyone have any expeiene of these or the alternatives on Ebay? I'm primarily looking at the ones with four ball races.

    I've also continued with the CAD and done some work on the x-axis rack and pinion. My ideas with the design have been:


    • Plenty of space for steppers and a good air supply.
    • A plywood spring that forces the pinion into the rack, it will be preloaded with a few millimeters.
    • Small radial loads on the stepper due to the long shaft, the lower bearing takes almost all of the loads.
    • Direct drive, good speed, 900 ipm @ 500 rpm , enough force, 0.5g with 270 oz in steppers and utilizing half of the holding torque, simple installation with few parts. Pulleys, belts, bearings, and shafts are more expensive than larger steppers. 360 ipm cutting will require 200 rpm, this means that derating due to rpm is limited even with moderate supply voltage.


    I will probably continue to work with the details of the bearing position. I want the load to be as much as possible in line with the spring to avoid bending. Possibly overkill but it will help to make the drive stiffer.

    I will probably turn it around to provide more space for the y-drive, still to be done.

    as always, please comment
    Sven

    Attached Thumbnails Attached Thumbnails Design input for newbie-x-drive-mounted-png   Design input for newbie-x-pinion-png   Design input for newbie-x-drive-mounted-detail-jpg  


  4. #24
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    Default Re: Design input for newbie

    I think the low resolution with direct drive will show up as poor cut quality, and rough motion.

    Gerry

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    Default Re: Design input for newbie

    Quote Originally Posted by SvenNorberg View Post
    Hi,

    I've read here and it seems that the technically really good choices are considered to be Hiwin or THK rails. Does anyone have any expeiene of these or the alternatives on Ebay? I'm primarily looking at the ones with four ball races.

    Sven
    Generally, BST Automation is recommended at this Forum. Fred responds fast as well. They customize everything for what you need.
    https://bstmotion.aliexpress.com/store/314742






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    Default Re: Design input for newbie

    nikdan, thank you



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    Default Re: Design input for newbie

    Quote Originally Posted by ger21 View Post
    I think the low resolution with direct drive will show up as poor cut quality, and rough motion.
    Really good point, I've thought about that earlier but it completely slipped my mind now. It would be interesting to learn what travel/full step and degree of micro stepping that is used by different machines and how good the results are?

    The pinion I looked at would give 0.01" / full step.

    /Sven



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    Default Re: Design input for newbie

    Ger21 is completely right on direct drive, routers for wood, steppers, and 0.01" full step resolutions.

    All machines need/want or lets say "work better" with about 10:1 or say 4:1 better resolution than actually desired final accuracy.

    A router "wants" about 0.01 mm resolution.
    A milling machine for steel about 0.005 mm step size => delivering about 0.01 - 0.02 mm actual local resolution.

    Absolutely everything bends.
    The frame bends, the screws wind (ie bend), the screw mounts, the screw bearings, the spindle mount, the spindle itself etc.

    Most of the time the bend is actually quite a lot .. when cutting fast, ie heavy cuts for roughing.
    Typically, this bend would be about 10x more than people think.
    The errors are mostly corrected on the finish passes where the loads are 10x-30x less, and the bend/error is 100x less.

    For design purposes, as a mental planning exercise, a lathe that supports 200 kg cutting loads can typically stand 100.000 kg loads without breaking.
    Table sized, 11-12" good lathe, circa 400-1000 kg mass.
    A modern large VMC, 40 hp / 25 kW, 4000 kg, might have 2000 kg max rated load on the table.
    And approx 1200 kgf rated push force on the ballscrews.
    But the theoretical capacity of the (35-) 45 mm linear bearings is 30-45.000 kg.

    The "right" way to design cutting machines is to plan for approx 50:1 loads vs "strength".
    Machine tools are typically max-loaded to 2% of their "strength".

    The round-rail chinese kit machines get away with being woefully under-strength and non-rigid by only being able to work at 1/10 of their capacity in terms of load or push per axis and total material removal rate or MRR.

    Wood as such is potentially very strong - as strong as or sometimes more so than steel - by mass.
    So a 10 mm steel spar needs the same mass in wood- for equivalent strength.

    BUT .. for cnc machines, you need rigidity not strength. 50:1, remember.
    Making bigger sections makes the member vastly more rigid, for minimal cost in mass and materials.

    So double the table thickness. It is 8x now more rigid.
    Etc..
    If an alu or steel gantry is say 15x15 cm, make it 30x30 or 35x35 in wood.
    Double the "size" for same wall thickness == 6x or more rigidity.

    Tubes and combined walls - Foundation Solutions - ArcelorMittal Distribution

    If you play with the calc and increase section size by 2x, you will see 6x more bending moment or rigidity.
    And the free-length rigidity of a piece is length pwr 3.
    So double the length == 8x less rigidity. So you need 8x more mass or 2.x more section size.

    This is easy to measure.
    Make a member, stand on 2 round rollers on rigid surface, place a load in middle. Measure with dti.

    Final advice:
    Your plan is good.
    Make the first machine as proposed.
    Test in use, test with dti for rigidity.

    Now add additional stiffeners for depth via industrial epoxy.
    Test in use, test with dti for rigidity.

    If your plan is for commercial-use with full sheets ..
    make it any way you want version 1.
    Usually...
    then use common industrial stuff and steel and a proper spindle for version 2, with ac servos.



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    Default Re: Design input for newbie

    Quote Originally Posted by ger21 View Post
    I think the low resolution with direct drive will show up as poor cut quality, and rough motion.
    Gerry I'd agree with this. My setup with my 8-start ACME screws gave nice motion (1" lead) but I do notice on larger sweeps and arcs, faceting, even with the 10X microstepping on the GeckoDrive. I would think at least 3:1, and maybe up to 7:1 reduction would work better with the R&P setup.



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    Default Re: Design input for newbie

    For rack and pinion and stepper motors, 2:1 or 3:1 gear reduction typically, look at the CNCRP kits for some inspiration.

    If using servos, greater gear reduction using low backlash planetary gears.

    Look at 1Jumper10's build log for that:

    http://www.cnczone.com/forums/cnc-wo...are-forum.html

    You could use a low backlash planetary for a stepper but the gear ratio would need to be much lower (2 to 3 or 5 absolute max), and the cost is more expensive unless you can find good surplus ones. If you end up looking at the stepper motors that have gearboxes built in, make sure to look also at the backlash they have.

    You also need to spring load the pinion into the rack. I think some very high end machines use precise alignment instead, but for a DIY machine, this helps to prevent binding and eliminate backlash when you can't get everything aligned perfectly.

    But, I will say that I haven't seen a design like that, it's a unique concept, and original. If you can come up with something like that then you may be able to come up with something original that incorporates gear reduction and spring load. Also, for a machine like this, I would just use straight rack, not helical.



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    Default Re: Design input for newbie

    Quote Originally Posted by NIC 77 View Post
    For rack and pinion and stepper motors, 2:1 or 3:1 gear reduction typically, look at the CNCRP kits for some inspiration.

    If using servos, greater gear reduction using low backlash planetary gears.

    Look at 1Jumper10's build log for that:

    http://www.cnczone.com/forums/cnc-wo...are-forum.html

    You could use a low backlash planetary for a stepper but the gear ratio would need to be much lower (2 to 3 or 5 absolute max), and the cost is more expensive unless you can find good surplus ones. If you end up looking at the stepper motors that have gearboxes built in, make sure to look also at the backlash they have.

    You also need to spring load the pinion into the rack. I think some very high end machines use precise alignment instead, but for a DIY machine, this helps to prevent binding and eliminate backlash when you can't get everything aligned perfectly.

    But, I will say that I haven't seen a design like that, it's a unique concept, and original. If you can come up with something like that then you may be able to come up with something original that incorporates gear reduction and spring load. Also, for a machine like this, I would just use straight rack, not helical.
    They have Chinese NEMA23 gearboxes up to 7:1 ratio, with reasonable backlash ratings.

    Modern industrial CNCs actually use two motors, master and slave, to control backlash. They'll work in tandem during a move for max acceleration and force, and opposed when at rest. I'm not a big fan of the spring loaded mechanisms because they push the pinion into the rack, the rack an pinion are designed with a fixed center distance; the lubricant that is supposed to be used helps take up some of that clearance. That said it may be a wise idea to source a spring-loaded split pinion.



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Design input for newbie

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