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  1. #1
    Hello everybody!

    I have been reading posts here and designing my own CNC for a while now. I have been sitting on this and not continued for quite some time, but I now have the opportunity to access a lathe and a well-equipped workshop for a couple of months, so I want to finally move this forward.

    Before I go into the design, I want to quickly state what I want to achieve with this machine: I want a relatively large (read: Around 1m x 0.7m x 0.1m) area machine to cut wood, plastics and aluminium. At the same time I really want to constrain the cost as much as possible, not wanting to go over a cost of 4000€ (at this point I should probably mention that I am located in Germany).

    Now with the information I have gathered, this is the design i came up with to satisfy these requirements:

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    The Profile is Motedis 45x90S B-Type Slot Aluminium Profile, The rails are HIWIN rails and I am planning on using ballscrews for the motion. For the Rails and Screws I plan on buying a bundle on Aliexpress which will likely dictate the exact dimensions of the machine.

    Now to the questions I am still having:
    • How would you go about connecting together the Profiles? Would angles suffice, or do you use something else there?
    • What would be the best way to mount the Rail Bearings? The green elements in my Model are just placeholders, since I struggle to find a good way to mount them.
    • How would I place Drag Chains on this design, to protect the cables? Would I need to make sure to have extra flat space to route these?
    • I was planning on using NEMA 34s with Timing belts to drive the screws. Would I need two of them for the Y Axis?
    • The Spindle Plate hanging on the Gantry looks huge to me, compared to the rest of the machine. Did I misdesign this?
    • Is a machine like this even capable of milling Aluminium? Would it be able to cut thin (not more than 10mm) steel sheets?
    • And finally, is this design even any good? I mostly worked off the information I found in various posts on this forum, especially for the gantry design, but I have no idea if the result has any glancing flaw.


    Thank you for any help!

  2. #2
    Quote Originally Posted by Chrono View Post
    Is a machine like this even capable of milling Aluminium? Would it be able to cut thin (not more than 10mm) steel sheets?
    Others will be able to say, but should be able to take light cuts in Ali. Steel is probably out of the question - for that you're better off converting an existing mill.

    Quote Originally Posted by Chrono View Post
    The Spindle Plate hanging on the Gantry looks huge to me, compared to the rest of the machine. Did I misdesign this?
    Can't see it in the renders? The carriage plates look reasonable - but obviously there isn't a Z axis yet which will change things.

    Quote Originally Posted by Chrono View Post
    I was planning on using NEMA 34s with Timing belts to drive the screws. Would I need two of them for the Y Axis?
    This size of machine should be fine with Nema 23, but that's just a mounting size. On your long axis, you can either go with a pair, or you can use a single motor with a timing belt connecting the screws. If a belt across the CNC isn't a massive inconvenience to you, my vote is for the belt - otherwise dual drive. There is a motor calculations spreadsheet on the forum which will help direct you to which motors to use.

    Quote Originally Posted by Chrono View Post
    How would you go about connecting together the Profiles? Would angles suffice, or do you use something else there?
    You can tap the ends of the profiles and stick large bolts in, you can use angles, or you can use T-nuts. There are probably other ways - look at a Profile supplier's website for inspiration (e.g. KJN).


    Quote Originally Posted by Chrono View Post
    And finally, is this design even any good? I mostly worked off the information I found in various posts on this forum, especially for the gantry design, but I have no idea if the result has any glancing flaw.
    Looks like you're on the right lines to me - I'd carry on by improving the detail of the CAD.
    Last edited by AndyUK; 29-04-2019 at 04:23 PM.

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  4. #3
    Quote Originally Posted by Chrono View Post
    • What would be the best way to mount the Rail Bearings? The green elements in my Model are just placeholders, since I struggle to find a good way to mount them.

    Hi Chrono

    Andy beat me to many bits of good advice, but I'll add:

    Whilst some of the details are missing the green plate method, provided the plates are thick enough should be a good approach, as you will be able to adjust the plates to get the ballscrews aligned properly. In fact being able to make adjustments for alignment of ballscrews and stepper/servo motor mounting should be your aim when designing the mounting points/plates (with rigidity of course :-) ).

    Your Z Axis carriage/box will likely need some side bracing of some kind to stiffen it - could be as easy as some triangular corner plates to tie the right angled joint together.

    Hope that helps.


    Chris

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  6. #4
    Quote Originally Posted by Washout View Post
    Andy beat me to many bits of good advice
    There is a reason I picked which questions to answer and which to avoid!

    Washout makes a good point about the adjustment of things. I like your ballscrew placement idea on the gantry, but it doesn't leave a lot of room for height errors - the location of the top plate will be defined by the carriages, which may or may not co-inside with the height of your ballnut support. You may want to design with say, a 5mm shim under the BK/BF12, so that you can adjust the shims to get the right height. This is why the ballscrew on mine is on the back of the L shape, it allows me to adjust for height relative to the carriages - but leads to a slightly more flimsy 3-sides of a box shape.
    Last edited by AndyUK; 30-04-2019 at 01:30 PM.

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  8. #5
    Thank you two for the replies! I have been implementing some of the things you were talking about in my spare time, but much more needs to be done yet.

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    What I changed:

    • I added angles and connector plates. My plan is to mount the spoilboard on the angles.
    • I added the Z-Axis. Currently, due to the different heights of the Rails and the Screw-sleds, the plate holding the spindle has a weird U-Shape. I could cut into the Z-Carriage and sink the screw into the gantry a bit to make this flat, but thus sacrifice gantry rigidity. Any thoughts on this?
    • I also added Angles to help support the Z-Carriage, like Washout recommended.


    What I am yet to do:

    • I understand the thought about keeping the mounting plates adjustable, thus I will design out those green plates to make actual mounts. The same for the back bearings.
    • I want to add motors to both the Gantry for the Y-Axis and the Z-Axis. For the Y Axis I thought about mounting them in the sidepanel holding up the gantry. For the Z-Axis I thought about lengthening the top plate and hanging the motor there. Would this be a problem in terms of placing extra torque on the Gantry?


    I also have a couple more questions:

    • Should I put in two more profiles to brace the bed in the Y direction? Crossbracing will be hard using only Aluminium profile
    • How thick should the different Alu plate parts, like the bearing plates, sidepanels and Z-Carriage be? Currently, I have all those set at 20mm thickness.
    • What about endstops? I have access to a 3D printer, can I use that to print mounts for the endstop sensors, or would those be too flimsy? I do not see them having to take much force, but maybe I am missing something here.


    Quote Originally Posted by AndyUK View Post
    On your long axis, you can either go with a pair, or you can use a single motor with a timing belt connecting the screws. If a belt across the CNC isn't a massive inconvenience to you, my vote is for the belt - otherwise dual drive. There is a motor calculations spreadsheet on the forum which will help direct you to which motors to use.
    What would be the advantage using a single motor? I am guessing the, more or less, guaranteed synchronization of the ballscrews?

    Quote Originally Posted by AndyUK View Post
    I like your ballscrew placement idea on the gantry, but it doesn't leave a lot of room for height errors - the location of the top plate will be defined by the carriages, which may or may not co-inside with the height of your ballnut support. You may want to design with say, a 5mm shim under the BK/BF12, so that you can adjust the shims to get the right height. This is why the ballscrew on mine is on the back of the L shape, it allows me to adjust for height relative to the carriages - but leads to a slightly more flimsy 3-sides of a box shape.
    Regarding this, would it also work if I were to flip-down the part of the Z-Carriage in the back that currently is mounted to the Screw Sled? That way I could freely adjust the height. I would only probably have to do it behind the current place where the screw is mounted, since I would hardly be able to actually screw anything in the other way around. Also, regarding the washers, would I just simply place washers between the sled and what I want to mount? The question arises, since that would massively decrease the area of contact between the two.

    Again, thank you for the Replies!
    -Chrono

  9. #6
    Quote Originally Posted by Chrono View Post
    I want to add motors to both the Gantry for the Y-Axis and the Z-Axis. For the Y Axis I thought about mounting them in the sidepanel holding up the gantry. For the Z-Axis I thought about lengthening the top plate and hanging the motor there. Would this be a problem in terms of placing extra torque on the Gantry?
    As in, hanging the motor off the back of the carriage, the opposite side to the spindle? Don't think it'll be a problem in terms of torque on the gantry (else you're not going to be cutting much!), infact it may even help to bring your centre of gravity closer to the middle of the X axis bearings, which is where it should ideally be. The only issue I see is space, you won't be able to place the CNC up against something without loosing travel on X.

    Quote Originally Posted by Chrono View Post
    I added the Z-Axis. Currently, due to the different heights of the Rails and the Screw-sleds, the plate holding the spindle has a weird U-Shape. I could cut into the Z-Carriage and sink the screw into the gantry a bit to make this flat, but thus sacrifice gantry rigidity. Any thoughts on this?
    I'd certainly be putting a second pair of bearings on that Z axis and lengthening the rail to retain your travel. Remember that the moment on that axis is Force x Distance to pivot - currently the pivot is the centre of your single bearing, the forces are at either end of the same bearing. With two bearings, the pivot becomes the point between them, and the forces are acting on the bottom of the bottom one, and the top of the top one. Your deflection in that direction will be significantly reduced.

    Quote Originally Posted by Chrono View Post
    How thick should the different Alu plate parts, like the bearing plates, sidepanels and Z-Carriage be? Currently, I have all those set at 20mm thickness.

    What about endstops? I have access to a 3D printer, can I use that to print mounts for the endstop sensors, or would those be too flimsy? I do not see them having to take much force, but maybe I am missing something here.
    20mm Ali plates for the main structures sound reasonable. Theres a spreadsheet on the forum for calculating gantry deflections that you should be looking at, and it'll give you an idea of how to approximate the cutting forces involved, and what deflection you'll expect from different materials. Regards endstops, I wouldn't be worrying too much about them just yet, but a 3D printed mount for the limit switches sounds like a good idea. With any luck they'll cut the motion before they take too much force, but having them break rather than damaging the linear motion components or your gantry sounds good ;)

    Quote Originally Posted by Chrono View Post
    What would be the advantage using a single motor? I am guessing the, more or less, guaranteed synchronization of the ballscrews?
    Spot on - once you've set the relationship between the two screws with the timing belt, you don't need to worry about the gantry racking, or homing the motors independently every time you use the CNC to ensure a perpendicular X and Y. If you miss steps, they're missed on both sides - which is probably safer. Then theres the reduced complexity in limit and homing switches, and you don't have to buy yourself an extra servo/stepper driver which is £50-100 right there. You can also use a 4 axis controller and still have a spare axis for development later on.

    ... basically, I'm mildly regretting going for dual motors. But I didn't want the timing belt across the entire machine, so I'm paying the price!


    Quote Originally Posted by Chrono View Post
    Regarding this, would it also work if I were to flip-down the part of the Z-Carriage in the back that currently is mounted to the Screw Sled? That way I could freely adjust the height. I would only probably have to do it behind the current place where the screw is mounted, since I would hardly be able to actually screw anything in the other way around. Also, regarding the washers, would I just simply place washers between the sled and what I want to mount? The question arises, since that would massively decrease the area of contact between the two.
    That sounds like how I've designed mine (see the build log).

    Washers? I'd use some shim material which maintained the surface area.
    Last edited by AndyUK; 02-05-2019 at 10:33 PM.

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  11. #7
    Sorry for the long radio silence, things have been quite busy here and I could barely find the time to work on this.

    I took the advice with the Y-Axis Ballscrew and the Z-Axis sleds to heart and changed the corresponding geometry:
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    On the to-do-list are the following things:
    • Add a mounting point for the Z-Axis NEMA 34 behind where the Y-Axis ballscrew currently connects to the Y-Axis sled.
    • Add an "Omega-Drive" belt structure to the front to run the two X-Axis Ballscrews from one NEMA 34 stepper. The "Omega-Drive" structure is there to maximize surface contact between Belt and Gears
    • Finally add that back ballscrew-bearing plate to connect it to the base structure
    • Add a Spot for the Y-Axis Motor to sit. Due to clearence with the Y-Axis sled I am still unsure where to put that, but maybe even below the X-Axis rails on the bottom?

    Do any of these plans throw up a red flag from something I have not considered yet?

    I have also been talking to the mechanical engineers in my workplace, who came up with a plethora of ideas to alter the design, so I am seeking a second opinion on some of these:
    • Substitute the Aluminium Profile for Steel Box section and weld it. The warping should be controllable in a machine this size, is what I am hearing.
    • Change the Y-Axis gantry to a design that has a Profile up top and one below with the ballscrew sandwiched in between. The rails would go to the front and both profiles would be held together at the sides, like they are now, and with an extra plate at the back. Alternatively manufacture this out of steel section.
    • And now to the weirdest one to me: Mount the gantry to the X-Axis like it is now, but only on one side. Then have the other side be out of thin steel with only a loose connection to the gantry beam, thus only inhibiting rotation, but not translation. This would "solve" the problem of the gantry being over-constrained, since the second beam would only brace against torque, but allow the gantry to move over it, in case the rails are not exactly parallel. I know that in theory this makes sense, but it still seems counter-intuitive to me in terms of rigidity.


    In the mean time I will work on getting the steps on my to-do list done and report back once I am done with that!

  12. #8
    Cool, changes look to be in the right direction. Z axis will be much more stable - even more so if you can space those bearings apart a bit.

    Quote Originally Posted by Chrono View Post
    Add a mounting point for the Z-Axis NEMA 34 behind where the Y-Axis ballscrew currently connects to the Y-Axis sled.
    Woowwww there cowboy! Hold up a second! A Nema 34 for the Z axis? That's a little overkill. Get a much lower inductance Nema 23, gain yourself some performance by not having a big inertia to overcome (and take the extra mass from the moving carriage), and give yourself more space up on the Z. Something like 2-3Nm - even that will be more than sufficient.

    Quote Originally Posted by Chrono View Post
    Substitute the Aluminium Profile for Steel Box section and weld it. The warping should be controllable in a machine this size, is what I am hearing.
    Certainly do-able. I planned to do the same myself initially. But the welding will warp it, and mild steel box section is nowhere near flat to begin with, so either way you'll need to mill the faces you want to mount rails to. Or, you can epoxy them, but that won't be easy to get the two faces parallel. Then you have to consider how to attach the box section to the gantry end plates, which probably means capping them off and tapping them etc.

    Rigidity? Look at my build thread. I simulated a steel L shape out of a box and a rectangle, then I simulated a slightly smaller Ali HD profile. Although the young's modulus of Ali vs Steel says you'll get more deflection from Ali, when you take the profile shape into account its really minimal. I think I went from 3um deflection in steel to 4um in Ali (but the real numbers are in the build log). Either way, its nothing to be concerned with.

    Profile Ali is flat enough to mount straight to and just shim where and if needed. It also has nice ends which can be tapped in about 5mins and bolt directly onto your end plates.

    Weight? Every extra kilo is lost acceleration!

    Next question is cost - In my case, the aluminium was only marginally more expensive. Really not much in it - Approx £40 for steel, and £50 for Ali. The ease of Ali just won the day for me by a long margin.

    Quote Originally Posted by Chrono View Post
    Change the Y-Axis gantry to a design that has a Profile up top and one below with the ballscrew sandwiched in between. The rails would go to the front and both profiles would be held together at the sides, like they are now, and with an extra plate at the back. Alternatively manufacture this out of steel section.
    So, two tubes with a large gap between? From memory of my deflection calculations that's a far less rigid structure. The L shape is good because it resists deflection both up and down, as well as twisting around it's own axis. I think that design would perform poorly in both situations.

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  14. #9
    Wow, took me 4 weeks to finally find time and gradually work on those 4 items, huh... I really need to find more time for this :D

    Anyways. I did get everything in order, and took your comments into account, so let's go over the changes.
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    Y-Axis Motor Mounting:
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    For a long time, the only place i saw to mount the motor without it interfering with anything was below the base. This would have been achieved by lengthening the side part below the linear bearings/ballscrew. Luckily, after taking a step back and looking at the model again, I found a much better place to do so, right next to the Ballscrew for the Y-Axis itself. The spot there is pretty much perfect, since it is below any part of the Z-Axis' top part, even if the Z-Axis motor is mounted on the back, but at the same time, it is above the lowest part of the Z-Axis, meaning it does not add any constraints for the workspace inside the machine. I would plan to get those mounting plates(the green ones) cut from steel sheets, since I can get that rather cheap.

    Z-Axis Motor Mounting:
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    The second picture carries a lot of annotations, but I will go into the important parts again: I found two possible mounting spots for the Z-Axis motor, both are shown. Following your advice, I went for a NEMA 23 Motor on the Z-Axis, instead of a NEMA 34, like I am using for both the X- and Y-Axis. I now have the option of mounting the motor on the back, possibly compensating some of the torque the spindle places on the Y-Axis carriage. This, however, means that I need one more part to assemble the carriage, since I cannot just take two bent sheet metal parts. I would also need quite a significant amount more material, as you can see. The mounting on the back of the plate carrying the Z-Axis Bearings, would simply be using unused space that would already be there, and it would be relatively center on the axis, but possibly add to the torque of the spindle, since it is a bit forward. Using slots in the mounting bracket, I could easily change the height of the motor in that position to fit the mounting point needed, however.

    I am unsure what spot to choose here. The top mounting position has quite a couple things going for it, but the back position does offer torque compensation at the price of significantly more complicated carriage assembly...

    X-Axis Motor Mounting:
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    The X-Axis mounting was easy to do, since I already had a good talk with one of the mechanical engineers in my workplace about that one. It is a simple Omega Belt Drive structure, driving both ballscrews from the same motor, just as you advised me to do. The two pulleys on the movable arms are tensioned using springs, not only to tension the belt, but also to increase the contact area of belt on both the ballscrew- and motor-pulleys.

    Finally: The Back bearing mounting plates:
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    These are basically the same as on the front and just simple metal pieces to hold the bearings of the ballscrews. The problem I currently have with them, however, is this: They are three-dimensional 20mm plates with pockets milled out of them. A better idea I had, to make them easier to manufacture, was to instead buy some longer Ali extrusion in the X-Direction, to bring the front and back edge of the machine more in line with the front face of the ballscrew-mounting-blocks, both in the front and back. Then I could use simple flat sheets of metal with some shimming underneath to equalize any tolerances. This would make those mounting brackets easier to manufacture, but add some "dead-length", when it comes to the ali extrusion, where it becomes longer, but the rails or axis are not. Still, I think that would be a better approach.
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    To-Do:
    Normally I would now lay-out what was on my mind, when it comes to next steps, but I cannot come up with anything concerning the CAD model, it looks pretty much done to me. What I need to now look into, however, is the electronics side of things: Find out what I still need in terms of controllers, motors, Power delivery, etc. and get some rough pricing, as well as model it into the CAD as needed. Where would be a good way to start here? For the gantry and machine design there were some helpful collection threads on here, comparing different gantry designs, for example. Is there also something like this when it comes to the electronics?

    As always, thanks for any help!
    -Chrono

  15. #10
    Quote Originally Posted by Chrono View Post
    Normally I would now lay-out what was on my mind, when it comes to next steps, but I cannot come up with anything concerning the CAD model, it looks pretty much done to me. What I need to now look into, however, is the electronics side of things: Find out what I still need in terms of controllers, motors, Power delivery, etc. and get some rough pricing, as well as model it into the CAD as needed. Where would be a good way to start here? For the gantry and machine design there were some helpful collection threads on here, comparing different gantry designs, for example. Is there also something like this when it comes to the electronics?
    Suggest you start here:

    http://www.mycncuk.com/threads/1524-...otor-do-I-need

    Once you're up to speed with the description, open the spreadsheet and customise to your needs (its setup for a smaller machine). Find reasonable values for your cutting forces and repeat for each axis in turn (increasing the moving weight as you go - ie. Z has a small moving load, Y has slightly more, X even more). I rebuilt the sheet from scratch to show all axis at once with individual inputs which helped.

    Next is the leadscrew calcs - lookup typical efficiencies of the system you're using, and what each of the measurements mean. This will give you screw inertia and critical speeds. From your screw pitch, you then have your maximum permissible speed on that axis. Tailor the ballscrew configuration until you're happy with the maximum speed.

    Now you have to decide what max cutting speeds and rapid speeds you want. I aimed for 5m/min rapids - some people here can get 7.5m/min. Cutting speed depends on the material (as does the cutting force).

    Then you can start plugging motors into the spreadsheet. Find a few example motors online, look at their specs, and put them in the sheet taking best guesses where needed. Then play with your power supply settings, and the sheet will tell you if the motor is up to the job or not. This is how I came to the conclusion that NEMA23s on each axis were more than suitable for my application.

    The motor drivers are then somewhat dictated by the current required, and the PSU is calculated by summing the maximum currents on all axes, then multiplying by a stepper usage factor (I think its 2/3rds? but you'll find this in the Power Supply threads). The voltage for the PSU is dictated by the inductance of your motors - the sheet will tell you an ideal voltage, and the drivers will have a maximum voltage - aim somewhere between those two allowing for variations in mains supplies.

    Rest of the electronics are essentially personal preference. Many ways to skin a cat...

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