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  1. #21
    Join Date
    Mar 2003
    Posts
    35538

    Re: Fixed Gantry Build

    Any ideas on how then to mount the carriages to the surface plate?
    Flanged blocks can be bolted from the bottom with the next smaller bolt size. Just drill and tap the plate from the top, and bolt them down.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

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

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

  2. #22
    Join Date
    Jul 2012
    Posts
    97

    Re: Fixed Gantry Build

    Quote Originally Posted by peteeng View Post
    Hello Jack - I used Izz so did compare torsional rigidity. Adding the internal brace stops the section from lozenging. This is a different approach then filling with epoxy. Adding the brace correctly in practice is quite a mission. I've done the same FEA work in the Maximus thread if you look that up. There is a critical tube thickness where this lozenging becomes acceptable. By acceptable I mean within the deflection target you set. So if you take the brace material and distribute it to the tube you may end up with an acceptable design. Plus adding a brace like you have makes the structure asymmetric so the deflection maybe different if you push in the other direction (so check this) . Since you are welding extra bits on for the rails I suggest you move the rails into the same plane as the top and bottom flanges. This will also minimise lozenging as the loads go directly into the flanges.
    ...
    As an afterthought the ends of your gantry are open, if you close them you will improve again maybe won't need the brace.

    UCB this is same for your design, close the ends of the gantry. Open ended sections are not very stiff. Cheers Peter
    Peter, thanks, "lozenging" was the word I was missing.
    The amount of engineering effort you put in your Maximus design is impressive, I'll read the whole thread later.

    In my simulations EG filling is even more effective than the partial bracing, and it also provides the much needed vibration damping - a hollow steel tube rings like a bell.
    Closing the ends is better than nothing, but much less effective than filling to prevent the lozenging.

    So I believe my initial point still stands: filling the gantry tube with EG is an easy way to stabilise the cross-section with the added advantage of vibration damping, especially in a fixed gantry setting where the extra weight does not matter.

  3. #23
    Join Date
    Jul 2019
    Posts
    8

    Re: Fixed Gantry Build

    Thanks for the heads up on the flange blocks! As for Z height I'll have a look at increasing it. It should be as easy as making the uprights taller. For the box section, enclosing each end should be easy enough.

  4. #24
    Join Date
    Jul 2018
    Posts
    6248

    Re: Fixed Gantry Build

    Hi Jack - If weight does not matter then picking a much thicker tube is more economic than using EG. The size of the steel tube and its weight indicates it will vibrate at a freq outside the usual resonance range of the machine. You have FEA capability so doing modal analysis should resolve these issues without using EG. It seems to me EG is a patch up approach vs doing it within the steel or alum paradigm. Perhaps a better thought is to start somewhere else then using a std box section? I'm playing with inventor at the moment and things like Fusion 360 have form finding algorithms. Start with an arbitrary block and it will find the "ideal" form. Steel SHS and RHS like Al extrusions are used because they are convenient in some way. Perhaps there is a better topology for a gantry?? Does solidworks FE have a form finding feature? UCB keep building....Peter

  5. #25
    Join Date
    Jul 2018
    Posts
    6248

    Re: Fixed Gantry Build

    Hi UCB - To address the weld distortion issue you could soft solder the parts together. There are many solders in the 200-250degC range that would provide the required strength. At this temp there's hardly a colour change, a propane torch will do the trick easily. The loads in the structure are nowhere near the material strength due to the large stiffness requirement. We are looking for joint stiffness and efficiency with no distortion, metal to metal contact provides this. Silver solder definitely provides the answer, I do this alot. But softer solders will work and are cheaper and easier to do due to the lower temp. This also allows very thin things to be connected together easily... Cheers Peter

    https://en.wikipedia.org/wiki/Solder_alloys

  6. #26
    Join Date
    Jul 2019
    Posts
    8

    Re: Fixed Gantry Build

    Apologies for such a delay in updates! I spent a few months away from home at some job training, and then got stuck working on some car projects as well as moved a couple states over. Anyway I've made a few big changes to the design/construction.



    1. Material choices - I had a difficult time sourcing the steel gantry HSS locally (smaller town) and no machine shop wanted to play ball. In lieu of steel I looked further into Aluminum tooling plate. for under $300 I got a 30" x 6" x 2.5" piece for the gantry, and an 20" x 18" x 1" piece for the table. Checking them on the surface plate showed less than a thou of deviation over ~20". These also proved very easy to handle and drill/tap.

    2. Gantry - With change in material I redesigned the gantry as well as the uprights. I elected to put the ballscrew on the top to minimize the distance between the gantry and cutter. As for the gantry supports I'm switching to 2" x 2" x 0.250 wall tube. I'll either double it up in the Y axis or use some .250 sheet to provide additional support beneath the gantry, and improve rigidity in the Y axis. I'll also be able to shim either end of the gantry as needed relative to the table. Weld distortion will still be a factor, but that's a future problem for now. The pieces that mount to the surface plate will be 1/2" thick ground flat bar stock. You may notice the spindle overhangs the table on the left. The idea behind this is to accommodate a tool height probe I'll mount to the surface plate, and won't take up any space on the table. In order to reduce the footprint of the machine, and because it's frankly easier, I elected to use a pulley and belt on the X axis



    2. Y-axis - I had some trouble with mounting the bearing blocks to the table. I was getting quite a bit of run out in the Z axis as I moved the table in the Y. I believe this was due primarily to not having a sufficiently rigid table causing the middle to bow up in between the blocks. In lieu of that I decided to mount the rails to the surface plate and increase the thickness of the table. Re configuring this axis resulted in a deviation of only about ~0.0008 over the 18 inches of travel in the Y. The rails do overhang the surface plate though. If this causes any issues in the future I'll troubleshoot. I will have to figure out what to do about the stepper motor. As it stands, the motor interferes with the table by about 1/4" which I can fix either by clearancing the table or possibly mounting the motor lower and using a pulley and belt. I'm leaning toward clearancing the table.

    Below is the progress I've made so far in construction.

    First up was the Z axis, as I felt is was the most forgiving. This turned out to be correct, and I learned a lot in its construction.




    The table was up next. Unfortunately I don't have any pictures of the first iteration, mounting the blocks to the plate. The biggest hurdle in the table was drilling and tapping the surface plate. Without the ability to use a drill press keeping the holes centered and straight proved a bit challenging. Everything eventually went together though without too much fuss. I did royally screw up two of the bolt holes in the table though. It was a late night, and I didn't measure twice lol.



    The spacers are just 5/8" tooling plate. They're bolted to the bearings and the table is bolted to the spacers with 8 1/2" socket cap bolts. Using 5/8" spacers instead of much more pricey 15mm spacers necessitated the use of ~.035 of shims between the table and ball nut.



    The gantry went together very easily. I indicated the top rail to be level each area the ball screw bearing blocks would be mounted, then drilled and tapped the tooling plate. Repeated the indicating for the lower rail (relative to the upper one) and got it mounted.



    Mounting the ball screw was straight forward. It attaches to the Z axis plate via a leftover 5/8" piece of tooling plate. I left the plate a bit long to accommodate the drag chain for the Y.




    As of now I'm waiting for parts for the gantry supports, and spindle. In the mean time I need to tackle the electronics enclosure, and assemble the Z axis.

  7. #27
    Join Date
    Jul 2018
    Posts
    6248

    Re: Fixed Gantry Build

    Hi UCB - Looking really good. The sawn edges of the plates have high spots. Its worth running some wet and dry or a stone over the surfaces to lap them a bit. On first fit and pull down the surfaces will sit on the high spots and pull up but over time the high spots will crush and the parts could loosen (this is called running in by some) once lapped use loctite to bond the surfaces together and they will never loosen. Peter

  8. #28
    Join Date
    Jul 2019
    Posts
    8

    Re: Fixed Gantry Build

    Thanks for the tip Peteeng! I started with 400 grit and worked up to 1500 where the blocks mount, and re-indicated it to the rail. Anyway, some updates:

    I got the X-axis supports made. They're 2 x 2 x 0.250 HSS welded to 0.375 ground plate. I took everything very slow with the welder and constantly checked for warpage and that the uprights were square to the surface plate. After mounting the gantry I did have to shim one upright by 0.010 and the other by 0.017 to get the gantry perfectly square. Definitely not the prettiest welds, but it'll do. Toward the back you can see the combo bearing block/nema 23 mount for the Y-axis. On each end of the gantry you can see mounts for inductive limit switches for the Z-axis.



    To level the gantry I welded a nut to one of the uprights and can adjust it fairly easily with a 1/4-20 socket head cap screw. None of the weight actually rides on the bolt, since the gantry is bolted to the upright. Initially I was going to use shims, but that proved to be a PITA to adjust. Using this method I was able to get each end leveled using a tenths indicator, and it seems like the surface plate has around a 0.0015 bow upwards toward the middle. For now that's good enough though. Squaring the X/Y will likely be done using a 345 triangle once I have everything wired up, or if I get a hold of a larger machinist square.



    The Y-axis limit switches will be mounted with angle iron to the surface plate and will read a 3/8 bolt threaded into the table.



    I printed 3 pieces for a "shelf" for the drag chain to mount to behind the gantry. I'll likely use some cable channel to run the wiring down the left upright and to the control box. Unfortunately I bought the kind that the links do not unlatch. I'll likely buy a new one in the near future to make adding/removing cables easier.



    Tomorrow I take delivery of a hinged plastic electrical enclosure. it's about 15.5" x 13.5" x 9" internally, which should accommodate all the control electronics, and VFD. I'll make a more detailed post about that once I tackle it, but I've got plenty of shielded cable, panel connectors, cable glands, ferrules, and power bus bars. Power for the 24V, and 48V PSUs will be provided through a C14 plug and power for the VFD through a separate C19 plug. For now I printed an enclosure for the start/pause/e-stop switches so they're easily accessible.


  9. #29
    Join Date
    Apr 2008
    Posts
    82

    Re: Fixed Gantry Build

    Hi UCB.
    I like your design, very similar to mine.
    One thing i suggest you check when you start cutting is flex in your x gantry.
    It looks pretty thick but may still flex a bit.
    My x gantry is 18mm thick steel and i got flex in mine so i added 2 braces top and bottom to form a c channel shape and it made a huge improvement.
    Im cutting aluminum with mine.
    See how you go, even a brace in the middle of the gantry can offer a lot of extra support.
    Cheers.

  10. #30
    Join Date
    Jul 2019
    Posts
    8

    Re: Fixed Gantry Build

    Hey Tiller, the gantry is 2.5 inches thick, and a quick FEA in Fusion shows around 4 tenths of deflection in the gantry with a 300lb load. This is what my steppers could in theory produce with 100% efficiency. If it proves to be an issue I can always put a piece of 2x2 HSS behind it. I won't be pushing this machine too hard for the next year or so since I'll be limited to a 1.5kw spindle on 110v power.

    Updates:

    I assembled the rest of the X-axis and Z-axis, and began the arduous process of wiring everything.



    The Z-axis motor mounts like the Y-axis, and drives the ballscrew with a pulley. A single homing switch on the Z along with a soft limit will keep it in bounds.



    I tried to keep things relatively neat, and printed some cable holders, seen below on the upright in the bottom right of the image, and screwed down to the back of the plywood table.



    The electronics enclosure isn't exactly glorious cable management , but turned our fairly neat. All driver, and signal wires are shielded, with the shielding grounded to earth through the aluminum back plate everything mounts to, or the plug panel. I was careful to not to loop any of the shielding or grounds, and will hopefully avoid any EMI issues. A 24V pc fan pulls in cool air and blows it across the stepper drivers. Two separate plugs are used for 110VAC in, with one being dedicated for the VFD. This should prevent any breaker trips, as I'd be at the top end of one 20A circuit. One of my first projects will hopefully be to engrave a plug plate.




    I have the spindle and VFD all wired up, but am waiting on a new spindle mount to arrive, since I wasn't overly satisfied with the one provided in the Chinese kit. If anyone has any recommendations I'd be open to suggestions!. I like the RoverCNC 80mm mount, but the bolts holding it to the plate sit behind the spindle. I have to imagine that would make tramming it in pretty difficult, as you'd have to remove the spindle for every adjustment. Anywho, this week I plan to get the spindle mounted, and start making chips!

  11. #31
    Join Date
    Dec 2003
    Posts
    1206

    Re: Fixed Gantry Build

    Thats the first time I have seen proximity switches used in that manner.I have always encountered them at 90 degrees to the direction of travel of the axis and triggered by a significant piece of steel moving past the sensor.Let us know how they work as it might open up some possibilities.

  12. #32
    Join Date
    Jan 2008
    Posts
    1523

    Re: Fixed Gantry Build

    90 degrees is a better choice.

    I have mine inline. They have taken a hit or two from a fast moving axis...
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  13. #33
    Join Date
    Apr 2008
    Posts
    82
    Quote Originally Posted by pippin88 View Post
    90 degrees is a better choice.

    I have mine inline. They have taken a hit or two from a fast moving axis...
    Had same thing happen.
    Wondered why my x axis wouldn't home and it was because my x crashed into the sensor and kept on going. Bent the bracket right back and broke the sensor. Didnt realize because of the bellows covering everything.

  14. #34

    Re: Fixed Gantry Build

    Thanks for sharing this build I'm enjoying the thread and looking forward to any updates. I've just started heading down a very similar route with the build I'm working on. For me, I like the Idea of using an old 600 x 600 surface plate as a base as it gives just the size of machine I'm looking for. I will start another thread to document it as soon as i make a bit of progress.

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