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  1. #21
    Member jmdejoanelli's Avatar
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    Default Re: JD's CNC Router Build

    Hey guys, once again, lots of good notes here, thank you! Whether I go for a 3d puzzle of Aluminium or steel, and how much box section I end up using will likely be down to cost, availability, and how much time I want to invest in welding and such. The reality is I'll probably end up using a combination of all of the above.

    I used Blender to block out what I currently had in mind for the machine layout in the attached image. Obviously scale is rough, consider this to be a digital sketch on the back of a napkin.

    Orange: stand - separating this out as it doesn't need the precision of the actual machine base, so will probably be just a plain welded frame
    Green: machine base and walls
    Purple: gantry
    Red: z axis
    Dark blue: tool envelope
    Light blue: tool envelope for steel/aluminium fixture plate for Al machining

    Dimensions here ended up being 1700mm on x, 850mm on y, and 400mm on z (half of that is gantry).

    I realise that having the gantry spanning between the two walls like this will present challenges with regards to protecting the rails and ballscrews, but I figure I'm going to have to get crafty to figure that out no matter what.

    I had a thought whilst blocking this out. It seems like it wouldn't be that much additional effort to add a "bridge" that essentially completes the square of the machine base (the green part). That doesn't necessarily stop lozenging forces (I think that's the right word?), but it would certainly add a lot of global rigidity surely?

    Cheers,
    JD

    Attached Thumbnails Attached Thumbnails JD's CNC Router Build-cnc_machine-png  


  2. #22
    Member jmdejoanelli's Avatar
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    Default Re: JD's CNC Router Build

    From what I've seen, cut prices here aren't too bad, generally you can order to the mm and they charge something like $10 a cut which compared to the many hundreds of $$ for a 200x200 section is nothing, and for the smaller cheaper section, it's easy enough to get full lengths and cut them myself, since I'll actually be able to physically move them



  3. #23
    Member peteeng's Avatar
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    Default Re: JD's CNC Router Build

    Hi JM - It looks like you have two rails on the inside of the wall? A) I have modelled twin rails many times and they do not function any better than a single rail (unless they are round rails which you won't be using) B) Vertical orientation of your rails limits how you can adjust the rails. You need to think that through carefully. Its usual practice to "land" the gantry on the rails so its stable during assembly (eg they are on top of the wall) and can be adjusted easily. As drawn when you loosen the rail bolts to adjust them the gantry will fall, unless it's on a register or land. Then that land has to be perfect which again is difficult to do on two separate parts (or is the green part a single part?). Possible with the correct machine. More thinking needed. Peter

    In regard to the bridge idea if the walls are stiff enough no need. Bridges present problems in commercial machines as they make it difficult to load with cranes. To create a bridge of equal stiffness all the way along the wall it would need to be all along the wall. It could be at the back and front to compensate the end stiffness as the local stiffness at the walls ends is half the stiffness of the wall in the middle. There are machines with external frames called exoskeleton machines usually very advanced highly complex and difficult to build. They are built for research or specific machining requirements... My experience is that walls due to the size they need to be for drives and rails etc are adequately stiff for the job. The Z axis however is the perennial problem...

    Last edited by peteeng; 01-08-2025 at 11:13 PM.


  4. #24
    Member jmdejoanelli's Avatar
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    Default Re: JD's CNC Router Build

    Hi Peter, yes I had two rails on each side of the y axis, and you are right that they are unnecessary. I also didn't consider that the rails on the inner walls would just make things more difficult so I'll move those too.

    The green piece won't be a single piece, likely the bed, and the two walls will be 3 separate assemblies, with machined locating features and bolted connections between the 3 subassemblies. I figure that gets me close enough to where any further adjustments can be made while dialling in the linear rails. One question on that topic, with the two y axis rails being so far apart, what kind of procedure is normally used to make sure the two rails are parallel and coplanar? I imagine a dial indicator running on a carriage on one rail and reaching over 1400mm isn't really practical...

    I've updated the block model and attached it. Although, it's now fairly typical in it's topology and probably isn't super useful anymore

    By the way, I forgot to model it in, but the dead space at the back under the gantry I was thinking to build some kind of retractable tool tray so that space wont go to waste.

    Cheers,
    Josef

    Attached Thumbnails Attached Thumbnails JD's CNC Router Build-cnc_machine_v2-jpg  


  5. #25
    Member peteeng's Avatar
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    Default Re: JD's CNC Router Build

    Hi Josef,
    Don't be too worried about "convention" it exists because it works. The model is very useful. You need to include the cars and their holes so you start to think about how to connect all of this together . Its not a trivial exercise & it will consume lots of time. Geometry blocks such as you have drawn cannot be connected together easily. I see you now have a rail on top and on the front of the gantry. You need to check that the machinist can set this up and machine the two lands in the one operation. This will create accurate lands. Having rails in different planes makes it harder to align the rails unless you use machined registers. Then there are the wall rails - You have them in the middle and they need to be at a side, probably the inside of the wall. This connects the rail to the wall better in shear then if it's in the middle. Ideally rails live on deep webs, not on a membrane which will deflect and vibrate. Here's some shots of the machine I am building now. It's a little bigger than yours but will do what you describe easily. No welding...This build is the 7th Scoot and the design is fully developed. I now want to move onto composite builds. Steel has its limits...Peter

    Scoot-S3 one of my customers builds
    scoot7 walls - levelling the base
    scoot txt - preparing to screw the inside wall onto the ribs

    edit - I looked at the V2 again and see you have the gantry rails on the front. The purple was a bit deceptive for me... We usually talk about a machine configuration or classification and part topography. I've attached asme B5 which discusses most of the classifications...

    Re: aligning rails. You use the gantry and a dial gauge. If you have access to geometric lasers they are the best way. Use an engineers level to level things if you have one. They are extremely accurate. String lines across corners will tell you if walls have "wind", not as wind in trees but wind as in winding something up... All doable. If the romans and egyptians and mayas can make things to 0.01mm you can.

    Attached Thumbnails Attached Thumbnails JD's CNC Router Build-scoot-s3-jpg   JD's CNC Router Build-scoot7-walls-jpg   JD's CNC Router Build-scoot-txt-2-jpg   JD's CNC Router Build-scoot-series-3-jpg  

    Attached Files Attached Files
    Last edited by peteeng; 01-10-2025 at 06:12 AM.


  6. #26

    Default Re: JD's CNC Router Build

    Nice progress. Check out the PrintNC design for some geometry/layout inspo. The overall design has some shortcomings like using only one block per rail, among other details, but in general, the tubes-stacked-on-tubes layout is really hard to beat for economical rigidity. Obviously your y-axis rail tubes would be much taller to get the high-rail design. The 3 or 4 horizontal base tubes could be 100x100 or so.

    The other element of printNC i would consider deviating from would be the gantry rails on top and bottom. Just a little more work machining the rail mounts on both sides to get everything squared up rather than along the front face. Although it is a more efficient way to get good bearing spacing and has some overall packing advantages for the ballscrew, etc, too. (PrintNC doesn't call for having the rail surfaces machined, it's simply a lower accuracy design)

    Last edited by catahoula; 01-10-2025 at 05:39 PM.


  7. #27

    Default Re: JD's CNC Router Build

    Also you could use a cast aluminum tooling plate for the work table, something like this: https://www.midweststeelsupply.com/s...minumplateatp5
    Maybe 20-25mm thick. For workholding, you can drill and tap a grid pattern of holes, like a fixture plate
    The style linked above come pre-machined so they are quite flat, and you could use the machine, once built, to do the final surfacing to the machine's alignment.



  8. #28
    Member jmdejoanelli's Avatar
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    Default Re: JD's CNC Router Build

    Thanks for the asme doc, Peter, there's a lot of very useful information in there! Also the Scoot design looks like what I'm going for more or less, but perhaps with a bit more bulk. Sorry about the v2 image, I'd forgotten to include the wireframe in the export which makes things a lot clearer. Positioning of the rails is only rough to get the topology right, the aim of the model is just to have a picture to point at to help in conversation before I start with CAD.

    I have somehow not come across engineering levels before, my jaw dropped when I saw you can get 20um/m division resolution for under $100! I'll certainly be getting one of those (maybe more accurate than 20um/m obviously).

    Cat, the base of the PrintNC machine does look nice. I was wondering about how to approach the base and workholding, considering I want to cut both wood and Aluminium. It does seem like a complete Aluminium plate would be a good idea, since it also means I could potentially run coolant. I can make a wood work plate that mounts to the aluminium base when I do wood work. I honestly thought a solid Aluminium base would be a lot more expensive than it is.



  9. #29
    Member peteeng's Avatar
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    Default Re: JD's CNC Router Build

    Hi Joseph - A tool plate base is a great start. The walls can be tooling plate as well - keep at it. Peter



  10. #30

    Default Re: JD's CNC Router Build

    Yeah those cast and machined Al tooling plates are weirdly reasonable in price. Nice way to boost mass and rigidity



  11. #31
    Member jmdejoanelli's Avatar
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    Default Re: JD's CNC Router Build

    I was back in the office today, and got the chance to ask about the laser capabilities in our prototyping area. Turns out it's not quite the beast I thought it was (it's the size of a van, so I made some assumptions). Apparently it will still cut up to 10mm steel plate though, which might be more than enough. I'm quite fond of the idea of paying cost price for steel plate and then doing the laser cutting myself plus I can do some cool stuff with laser cut puzzle pieces.

    Cheers,
    JD



  12. #32
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    Default Re: JD's CNC Router Build

    Hi,

    I'm looking at C3 grade ballscrews, they seem like the best precision I can get for a cost I can stomach.
    That I'd really like to see. Be careful, there are a number of suppliers, Chinese mostly, who are claiming C3 and C5 ballscrews for little more than C7's. I call BS.
    I can smell it from here, and I'm 9000km from China!. Price C3's from THK, NSK etc and then ask if what you are being offered is credible.

    I use 750W Delta servos direct coupled to 32mm 5mm pitch THK double nut (BNFN) C5 ballscrews. Rated rated torque I get 0.27g acceleration which is fast enough to make your arse pucker
    and at 3000 rpm do 15m/min. I can and have tuned them up to 5000rpm for 25m/min, but its just too scary fast. I detune them to 15m/min, and often run my machine at half that even.
    Good acceleration is good for toolpath following, I use 0.15g, but speed anything much over 10m/min is wasteful in my experience.

    Craig



  13. #33

    Default Re: JD's CNC Router Build

    Hey Joeaverage, any idea what acceleration you would expect from this combination?
    X axis:
    - THK BIF2005 C5 ball screws, NEMA24-5amp-3.5Nm(495oz.in) stepper motor running 48V PS. Total moving mass of about 33kg (running on shs25 THK rails).

    I understand this might be difficult to estimate but I am currently running 600mm/s2 which is only 0.06g. I have always wondered if I am still too conservative with my machine. Cheers

    Last edited by crjohnson; 01-15-2025 at 02:15 AM.


  14. #34
    Member peteeng's Avatar
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    Default Re: JD's CNC Router Build

    Hi CR - Do you mean the Y axis? convention has the gantry as the X axis so only one motor there and the Y axis has two motors if its a gantry machine? I expect theoretically you could move it much more then 0.06g. But if you describe the machine a bit better the maths can be done. Since you have the machine you just bump up the accel until it complains. Peter

    So I think your saying that the Z axis assembly weighs 33kg and it is being driven by a 3.5Nm stepper with a 2005 screw?

    - - - Updated - - -

    Hi CR - Do you mean the Y axis? convention has the gantry as the X axis so only one motor there and the Y axis has two motors if its a gantry machine? I expect theoretically you could move it much more then 0.06g. But if you describe the machine a bit better the maths can be done. Since you have the machine you just bump up the accel until it complains. Peter

    So I think your saying that the Z axis assembly weighs 33kg and it is being driven by a 3.5Nm stepper with a 2005 screw?

    - - - Updated - - -

    Hi CR - Do you mean the Y axis? convention has the gantry as the X axis so only one motor there and the Y axis has two motors if its a gantry machine? I expect theoretically you could move it much more then 0.06g. But if you describe the machine a bit better the maths can be done. Since you have the machine you just bump up the accel until it complains. Peter

    So I think your saying that the Z axis assembly weighs 33kg and it is being driven by a 3.5Nm stepper with a 2005 screw?



  15. #35

    Default Re: JD's CNC Router Build

    Hi Pete, my Y axis is driven by 2 x nema23 motors each with the exact same ballscrew. But in the comment above I still meant to describe the assembly which moves on the x axis. Ie. spindle motor+mount, 2 large alloy plates, 2 bearing rails, 8 carriages, z-axis motor and ballscrew etc etc.

    I did actually try upping the acceleration (using UCCNC) and didn't run into any trouble. I tried 750, 1000, 1500, 2000 and 3000! I left it at 2000 for now. I tried the same tests on the Y axis and the only effect it had was making the whole machine+enclosure+frame shake a fair bit when doing multiple 1mm movements.

    It will be interesting to see if the machining quality has changed at all.



  16. #36
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    Default Re: JD's CNC Router Build

    Hi,

    Hey Joeaverage, any idea what acceleration you would expect from this combination?
    X axis:
    - THK BIF2005 C5 ball screws, NEMA24-5amp-3.5Nm(495oz.in) stepper motor running 48V PS. Total moving mass of about 33kg (running on shs25 THK rails).

    I understand this might be difficult to estimate but I am currently running 600mm/s2 which is only 0.06g. I have always wondered if I am still too conservative with my machine. Cheers
    In fact no, it is not difficult to estimate, the laws of physics are quite straight forward and quite plain. The only real 'guess' is the stepper torque at speed. All steppers lose torque the faster they go.
    If you choose low inductance steppers (a must in my opinion) then you might expect 1Nm at speed, say 1000rpm. Also going to guess that the rotational inertia of the stepper is Jstepper=0.5 x 10-4.kg.m2

    The rotational inertia of a 20mm diameter ballscrew of 1300mm length:
    Jballscrew=0.5 m.r2 where m=mass of ballscrew, and r =radius of ballscrew
    = 0.5 (0.01)2.PI.8000.(0.01)2
    =1.25 x 10-4.kg.m2

    The effective rotational inertia of a linearly accelerating mass (m) acted on by a ballscrew of 5mm pitch (p):
    Jlinear= m.p2/(2.pi)2
    =30. (0.005)2/40
    =0.1875 x10-4.kg.m2

    Total effective rotational inertia:
    Jtotal= (0.5 + 1.25 +0.19) x 10-4.kg.m2
    =1.94 x 10-4.kg.m2

    The first thing to note is that its the ballscrew which dominates (64%) the inertia followed by the stepper armature (25%) followed by the gantry mass (10%). This result surpises most people, they imagine
    with some conviction that reducing the mass of the gantry is paramount to reducing inertia....but no. The bottom line is make the gantry as heavy as necessary to make it stiff. Even a very heavy gantry
    has minimal impact on the overall inertia.

    The inertia of a ballscrew is proportional to the fourth power of its diameter. Thus even a small change in diameter (say up one size to 25mm, or alternately down one size to 16mm) will
    have a very VERY significant impact on inertia and therefore acceleration.

    Lets complete the acceleration calculation assuming the stepper has 1Nm available:

    dw/dt =T/Jtotal
    =1 /(1.94 x10-4)
    =5154rad/s2

    Converting to linear motion by recognizing that v=w.p/2.pi:
    accel= 4.1m/s2 or 0.4g

    This is a very good result. I rather expect that while your machine might do 0.4g that you will detune it to the region of 0.1g to 0.2g. This will still be any amount
    fast enough and also ensure some headroom that will prevent lost steps.

    By the way, you should aim for a much higher voltage to drive your steppers, I would suggest 80VDC. All steppers have inductance, and it is that inductance that severely
    limits the speed they may attain. The conventional means to overcome inductance is higher drive voltage. There are plenty of good and cheap Chinese 80VDC (and higher) drivers
    on the market. Take adavntage of them. You'll want an 80VDC supply. Many people choose switch mode on the basis of cost, but if you want rugged with excellent overload proerties
    then get a linear supply:

    https://www.antekinc.com/ps-10n80-10...-power-supply/


    Craig



  17. #37
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    Default Re: JD's CNC Router Build

    Hi,
    just as a sanity check we should consider the kinetic energy of the complete moving gantry.

    Lets assume the stepper is doing 1000rpm.

    w= (1000/60).2.pi
    =104.7 rad/s

    The moment of inertia for the two rotating components, the ballscrew and the stepper armature is:
    J= (1.25 + 0.5) x 10-4
    =1.75 x 10-4.kg.m2

    Thus the kinetic energy of the rotating componets is:
    KErotational= 0.5 . w2. J
    =0.959Joule

    At 1000 rpm, with a 5mm pitch screw the velocity is:
    V= 1000/60 . 0.005
    =0.083 m/s (83mm/s)

    Ergo the kinetic energy of the linearly moving mass (30kg gantry);
    KElinear= 0.5 . 30 . (0.083)2
    =0.104Joule

    Thus the ratio of rotational kinetic energy to linear kinetic energy is 0.959/0.104 = 9.2:1

    Consider what that means......a 30 kg gantry has approx one tenth of the energy of the ballscrew and armature combined.

    If you made the gantry 60kg, ie doubled it the calculation would change, but not a whole lot, say two tenths of the energy.
    So within reason you can almost ignore the gantry mass, or rather think more about how stiff you can make rather than worry about its weight.

    This is contrary to our commonly held understanding, but the numbers don't lie.

    Craig



  18. #38

    Default Re: JD's CNC Router Build

    Thank you for the detailed reply! I knew the maths was there but also knew there was a difference in that verses real-world expectations. I just wasnt sure how big that gap was. I will see how the machine performs with the 0.2g settings soon.

    I've also limited my machine to 1500mm/min, so its not the fastest machine but is doesnt have to travel far either.

    I bought all my steppers and drivers from omc stepperonline and tried to pick the lowest inductance units at the time. The drivers are only rated to 50VDC so I cannot go any higher at this stage.

    https://www.omc-stepperonline.com/di...r-motor-dm556t
    https://www.omc-stepperonline.com/p-...s-24hp39-5004s
    https://www.omc-stepperonline.com/ne...s-23hs32-4004s



  19. #39
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    Default Re: JD's CNC Router Build

    Hi,
    as one last illustration of your design choices, let imagine that you want or can find a 25mm ballscrew cheap, and think 'yes, I'll use that instead'.

    If I redo the calculation but with a 25mm ballscrew rather than 20mm the result is the acceleration (with the same stepper and gantry) is 0.21g.
    This is also a very credible result, if your machine achieved 0.2g I'm sure you'd be more than happy. So either a 20mm OR a 25mm ballscrew would be OK.

    The real point is that if you increase the size of the ballscrew to 25mm from 20mm you halve the potential acceleration. Thus it is important to understand
    just how sensitive acceleration is to the size of the ballscrew and chose the screw accordingly. You can get it wrong with a too large ballscrew. In most case if you overbuild something in a CNC machine
    it does no harm, but you can go too far with a ballscrew.

    Craig



  20. #40
    Member peteeng's Avatar
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    Default Re: JD's CNC Router Build

    Hi CR - UCCNC is a trapezoidal velocity planner. Which motion controller are you using? So the values set in the axis configs are the target values. When the controller commands a move the motor applies full torque at that point in time (which for a stepper depends on its rpm) and the motion controller looks fwd and calculates the acceleration and velocities predicted by the motion and the code. If the motion gets to the max velocity first the velocity is clipped at that speed (this is ideal for constant velocity machining). If it gets to the max acceleration first it continues at that set accel until it gets to max velocity (say on a long straight path). But on short moves such as 1mm it probably will not get to max vel or max accel before it has to move again hence it will run slow. In UCCNC general settings you have control linear errors, linear addition lengths and unify lengths. If you are doing coarse work its good to understand these settings as you can run your machine much faster if you slacken these off as the controller will make a wider path, combine short moves into longer moves and decimate moves that are not needed. For instance I use Fusion for my CAD/CAM and have found its post prefers to create 100's of small arcs on curves such as txt engraving. These small arcs bog the controller down, so in the post settings I set the minimum rad to say 2000mm. This means the post never makes radius gcode only small polyline code. This allows the controller to move very quickly along a polyline vs trying to move around a rad which consists of even more finer polylines... Good luck - Peter



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