Designing first CNC, advice needed


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    Default Designing first CNC, advice needed

    Hi, everyone.

    I am designing T-slot based 3-axis CNC router. It is planned to be rather big - 1000x700x260 working area with ~1400-1000-800 overall dimensions. First I planned to lasercut planar parts from 10mm aluminum, but was able to find local guys who only cut high-carbon steel, so those parts will be cut from 5mm steel, which should make them ~1.4 times heavier. For precision no profile is to be drilled, only steel parts, which are to be lasercut.

    The rails used are sbr20 1400-1000-500, all of which will be attached to 60120 profile. The table itself is planned to be made from 30120 with 6060 at both sides. As linear motion system I plan to use nema23 with sfu1605 on y and z axes and nema34 with sfu2005 on x.

    I need an expert opinion on that build. Are thereany mistakes? Will it be able to cut aluminum? Is the idea to replace 10mm aluminum with 5mm steel ok?

    All opinioins will be apreciated.

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    Designing first CNC, advice needed-cncrouter3-base-5mm-2-jpg   Designing first CNC, advice needed-cncrouter3-z-5mm-4-jpg   Designing first CNC, advice needed-cncrouter3-x-5mm-1-jpg   Designing first CNC, advice needed-cncrouter3-y-5mm-1-jpg  

    Designing first CNC, advice needed-cncrouter3-y-5mm-2-jpg   Designing first CNC, advice needed-cncrouter3-y-5mm-3-jpg   Designing first CNC, advice needed-cncrouter3-y-5mm-4-jpg   Designing first CNC, advice needed-cncrouter3-z-5mm-1-jpg  

    Designing first CNC, advice needed-cncrouter3-z-5mm-2-jpg   Designing first CNC, advice needed-cncrouter3-3-jpg  
    Last edited by AlexnderB; 07-24-2014 at 02:56 AM. Reason: adding pictures


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    Default Re: Designing first CNC, advice needed

    Hi, the basic design seems pretty standard fare, the things that stand out to me are the gantry, it's very tall, but isn't braced in any way, so you'll find you'll get quite a bit flex there. If you can add some right angle bracing that attaches along the length as well as to supporting the horizontal panels at the top and the bottom, you'll greatly stiffen it.

    Almost any machine can cut aluminium, with varying degrees that is. Part of the biggest problem faced with cutting aluminium is rigidity, and mass. Machines made out of thin section material, will resonate and vibrate a lot. This directly translates to noise from reasonably loud to screaming banshee ear piercing sounds, as well as very adversely affecting the cutting quality, which in turn translates to poor finishes, and lots of worn/broken cutters, and stress on the machine.

    The more mass you can put into the base and frame to add not only weight, but really stiffen and support it, and the stiffer and tighter you can make the gantry, the better it will be for not only cutting, but also the noise produced. Some people have used various materials such as epoxy, concrete, etc, to help with these problems

    Using 5mm steel I think you'll find that every single point on the machine that has any form of tolerance being even the slightest out, will result in the problems I highlighted earlier. Another god trick is for every part of the machine that is metal to metal contact, use a strip of the really thin double sided tape, it compresses down to nothing so won't affect anything, but it can greatly help with resonating and vibration noise.

    cheers, Ian

    It's a state of mind!


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    Default Re: Designing first CNC, advice needed

    Thanks for reply, will something like shown on the picture help? Maybe I should increase width ofsteel parts from 5 to 6?

    The advise for putting tape between connecting metal parts is something I wouldn't do myself for sure, so it was very helpful.

    Perhaps welding will help with gantry stiffness?

    Attached Thumbnails Attached Thumbnails Designing first CNC, advice needed-cncrouter3-x-w-gantry-brackets-jpg  


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    Default Re: Designing first CNC, advice needed

    I also think that the gantry sides are much too tall. They're also much too thin. I'd recommend at least 10-15mm steel sides, and I'd try tyo reduce the height by at least 25%, if not more.
    Or, make the sides out of box sections.

    If you can afford them, profile rails will give you a much stiffer machine, as round rails always seem to have a little bit of play in them.

    Gerry

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    Default Re: Designing first CNC, advice needed

    Thanks for the comment.
    As for profile rails - I don't want to spend more money on my first machine, as there can be other design flaws, which will diminish gains from using better rails.
    I've made some modifications.
    1 - gantry sides are now supported with 60120 profile parts, which are bolted at all length to them.
    2 - gantry lowered by ~120mm without changing working z-area, there was unneccesary height in previous designs.
    3 - width of all steel plates changed from 5 to 6mm, it should give them roughly 30% more stiffness and 20% more weight.

    Should I consider using 2 ballscrews with motors for x-axis instead of 1?
    Also I am worried about gantry weight, which is roughly 100kg for now, will 1 Nema34 with 4.9N/m holding torque be able to move it at decent speed?

    Attached Thumbnails Attached Thumbnails Designing first CNC, advice needed-cncrouter3-low-6mm-jpg   Designing first CNC, advice needed-cncrouter3-low-6mm-2-jpg   Designing first CNC, advice needed-cncrouter3-low-6mm-3-jpg  


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    Default Re: Designing first CNC, advice needed

    Should I consider using 2 ballscrews with motors for x-axis instead of 1?
    Yes.

    The weight shouldn't be an issue at all.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Designing first CNC, advice needed

    I think most people recommend the 10mm lead ballscrews for faster travel. You won't really lose any real resolution because machine flex will be greater than the extra resolution from the 5mm lead screws.



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    Default Re: Designing first CNC, advice needed

    Look at all those holes! Why bother trying to stiffen up thin and flimsy uprights? Just build them right to begin with. Adding 1mm isn't going to help much at all, it's still way under what is reasonable for this type of design. Your gantry is very tall, reduce the size of the gantry, build it out of box section or maybe extrusion, greatly increase the thickness or your uprights and be done with it. There's a reason why you don't see others designs with those kind of proportions. Since cost is an issue, reducing the physical size of those pieces will allow you to increase their thickness for the same money. As far as the linear rails are concerned, the way your machine is gonna flex, you could get the same performance from MDF and gas pipe. In my opinion, adding a brace or increasing plate thicknesses by a mm or two is not going to solve your problem. Others have given good suggestions. Take them to heart, look at other people's designs and go back to the drawing board. Or you can just build it this way and struggle trying to make it work while you're saving up to build the next machine the right way

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    Default Re: Designing first CNC, advice needed

    Your gantry can be strengthen by simply adding another sheet of material per side and tie it to the existing part with a spacer between. By correctly placing and sizing your spacers you can get the strength of a box section. The scary part is how are you are attaching these sides to the horizontal member. This will need to have sufficient gussets giving you strength at the connection points. I'm in process of building mine as well. Good luck!


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    Default Re: Designing first CNC, advice needed

    Thanks to everyone for helpful comments.
    I guess one of my problems is trying to achieve foam and aluminum cutting machine in single build and for low cost.
    And another is the reason I came up with design of this kind - I don't want to drill or measure anything by myself. I wanted all holes to be lasercut and thus be placed precisely where designed. I mean there are no profile-to-profile connections except in the table and generally only rails are connected to profile, which should fit in slots. This is done ofthe lack of experience workingwith metal parts or any precise building.
    Now I will think what can be done to stiffen the gantry or how the machine can be redesigned.



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    Default Re: Designing first CNC, advice needed

    Designing first CNC, advice needed-cncrouter3-low-6mm-2xscews-crazygantry-jpg
    Maybe something like this will work?



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    Default Re: Designing first CNC, advice needed

    I would move the bearings and rails that fix the Z axis to the X axis from above/below the gantry, to the front face instead, it will not only make fixing the Z axis much easier to get accurate, but will also then allow you to run a flat bar that bolts along the top of the gantry, and affixes to the top of the gantry sides, which will greatly reduce flex.

    If you need to increase the gantry height to allow for the linear rails being moved, the extra support will cater for that, so long as you don't make the gantry too much higher as a result.

    I would also consider reducing the number of fixing points (holes) to the minimum required to keep everything rigid. As every hole/bolt, is a potential point for vibration to occur. Large bolts with plates or large washers will likely be better than a multitude of small screws, or rivets that will each "give" a fraction under load over time, but collectively can add up to non-inconsequential flex.

    Don't over design it, or over think it, just try to keep it simple.
    The basic rules are, racking will be directly proportional to gantry height AND inversely proportional to gantry width.

    X axis flex or twisting will be inversely proportional to the thickness of the gantry sides, AND how they are fixed to the linear rails and drivescrews along the Y axis, and the rear of the gantry.

    A flat bar running along the top of the gantry adds tremendous rigidity. Every screw or rivet is a potential vibration point, every vibration point will create noise, and reduce the quality of machining, and wear cutters faster. Make fixing points rigid, and simple, so they won't shear, strain, loosen, etc. You get the idea anyway I'm sure.

    And lastly, the more mass in the frame and base you can put, the better, the base doesn't move, so it's not possible to make it too heavy. The gantry should also be made as heavy as possible, that will be supported by the rails, and steppers chosen. If you end up with an overly heavy gantry, make it wider to spread the bearings further apart, and just increase the length of the machine to suit. Fixed properly the steppers won't care if the gantry is 50kgs or a 100kgs, it should just be a smooth rolling mass you could push up and down with your little finger.

    cheers, Ian

    It's a state of mind!


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    Default Re: Designing first CNC, advice needed

    Everybody have been very helpful here, thanks a lot.
    I have redesigned the gantry as well as z-axis part, made the gantry wider and lower not altering working area on z.
    Here is the current state of the project.

    Attached Thumbnails Attached Thumbnails Designing first CNC, advice needed-cncrouter3-low-6mm-2_x_screws-square_gantry-altered_z-jpg   Designing first CNC, advice needed-cncrouter3-low-6mm-2_x_screws-square_gantry-altered_z_1-jpg   Designing first CNC, advice needed-cncrouter3-low-6mm-2_x_screws-square_gantry-altered_z_2-jpg   Designing first CNC, advice needed-cncrouter3-low-6mm-2_x_screws-square_gantry-altered_z_y-jpg  

    Designing first CNC, advice needed-cncrouter3-low-6mm-2_x_screws-square_gantry-altered_z_z-jpg  


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    Default Re: Designing first CNC, advice needed

    I've been back to designing, and here is new design (draft without motors and screws). I want to use double srews with steppers on y-axis. Are there any obvoiuos problems with this kind of design?

    Attached Thumbnails Attached Thumbnails Designing first CNC, advice needed-cncrouter52-jpg   Designing first CNC, advice needed-cncrouter5-jpg   Designing first CNC, advice needed-cncrouter5-1-jpg   Designing first CNC, advice needed-cncrouter5-y1-jpg  

    Designing first CNC, advice needed-cncrouter5-y-jpg  
    Last edited by AlexnderB; 07-30-2014 at 02:10 AM. Reason: errors


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    Default Re: Designing first CNC, advice needed

    Why the radically different design?

    I suppose the best thing would be to ask what do you expect to be able to machine with whatever you build, and then go from there.

    The design you've just done looks like a massively scaled momus style cnc machine, a largely MDF/Plastic hobbyist machine, but being that large you'll find it very difficult to not only maintain accuracy, but also hard to reduce vibration and flexing. The Z axis in particular I'm fairly sure you'll regret unless you will only be doing very high materials. Bear in mind with each axis, you lose a lot of travel from tolerances for limit switches, the size of the bearings used, etc. So to get any reasonable depth of cut on the Z, you'll need an extremely high Z axis assembly, which apart from the weight, also then introduces design problems with the X and Y axis. It would be a nightmare trying to stop or reduce the "racking" that would ensue in such a situation.

    Unless your planning on doing limited depth of cuts, (80mm to 150mm is the usual design range) on really high materials, I'd go with the earlier design, but just refine it to reduce slop and vibration.

    Above all else, the best thing I can recommend is keep it simple, so you give yourself the best chance to actually build it. You may not want to hear it, but if you are successful in building a basic cnc machine, I would be very confident in saying within 2-3 years you'll most likely start building a better designed one to replace this one, or even augment it, after gaining real experience with using them, so don't over think it, have fun!

    cheers, Ian

    It's a state of mind!


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    Default Re: Designing first CNC, advice needed

    I've been trying to find an old link that had some great formulas for moving gantry design, and explained it really well, but for the life of me I can't seem to find it at the moment, this link however may hold some interest for you in calculating stress:

    cnc machine stiffness calculator

    cheers, Ian

    It's a state of mind!


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    Default Re: Designing first CNC, advice needed

    One of the requirements for the machine is being able to cut some landscape from foam for architectural miniatures, thats from where the heighth of the z-axis comes (at least 300m needed). This kind of design, I thought, will reduce problems which come from high gantry. The design is inspired by this one * and this **. I understand both are from steel tube, but looks like something similar can be made from extrusion.
    Also I really wanted a router able to cut aluminum, but thats more of a wish than a requirement. Same, or even more desired are PCBs.
    Thanks for comment, I will think over it.

    *
    **BUILD LOG: A sufficiently strong machine



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    Default Re: Designing first CNC, advice needed

    The thing with machines such as the one you linked to, (which looks pretty damn awesome I must say), is the balancing act for noise, vibration, inertia, repeatability, and accuracy.

    A heavy gantry will be great for heavy duty machining by virtue of the mass, but, because of the weight, every direction change, requires a lot to not only drive that mass, but also sufficiently designed strength in the frame to counter flexing, because of the inertia of moving such a mass.

    Because extrusion is pretty light, a box frame using that will be good for general work, but higher speeds or aggressive machining, or 3D profiling, will cause a degree of flex to be introduced. If your doing foam, and able to get very substantial depth of cuts in minimal passes, slower feeds aren't so bad, but consider if you want to do signs or plaques with 3D motifs on them, that requires fine resolution, you'll need to use smaller cutters, with significantly shallower DOC's.

    If you want the open frame type as you linked, and you don't have access to a welder, personally I'd make the frame out of as thick a steel square as is reasonable, and bolt it together with solid bracing so it can't give in any direction, and then shim with thin aluminium plate where you fit the rails, so you can get it all aligned easier. And then make the gantry out of reasonably strong alu extrusion with plates to brace along the top, and sides for maximum rigidity with making it too heavy.

    Something to consider anyway.

    Also, the reason I mentioned using thin alu plate or bar as a shim to mount the rails and gear on, is because steel square can vary dramatically over it's lengths, and without a large mill, can be a mongrel to surface accurately, alu being much softer is more amenable for this purpose.

    I have no idea what the actual formula would be, but based on my own experience as my own very rough rule of thumb, I'd think you'd want the frame to be at least 2-3 times the weight of the gantry, to help balance out noise and vibration.

    cheers, Ian

    It's a state of mind!


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