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  1. #1
    Member UCBones's Avatar
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    Default Fixed Gantry Build

    Hey all,

    Planning on building a fixed gantry build designed to cut plastic/wood/aluminum with reasonable precision in a relatively small form factor, and at a price point around $1500. At this stage I have a basic plan laid out, and am looking for feedback, but this thread will serve as the "build thread" for the project as well.

    THE DESIGN
    The base of the machine is a 24" x 24" Cast Iron surface plate, that I got on a good deal in the local classifieds. The plate has the usual light scratching, but the previous scraping marks are still easily visible, with no discernible warpage as measured with a precision straight edge and one thou feeler gauge. I know granite is very popular for higher end DIY builds, and isn't susceptible to corrosion and has better vibration characteristics, but the plate was super cheap, and I can readily drill/tap it without the need for special bits/inserts. I'd estimate the weight in the 150-160 lb range, and it should be plenty sturdy enough for the things I plan to machine. In addition I believe the CI plates are a bit more thermally stable.

    The X gantry is 3" x 6" 8020 extrusion that I plan on filling with epoxy granite. I debated going with HSS, but decided that trying to get it machined around here would be tough with limited options, and it would be relatively easy to swap out for something else in the future is need be. The gantry will sit on 2" x 4" rectangular HSS. I haven't decided on a wall thickness, but probably something between 1/4" and 3/8". On the top will be a 3/8" plate that the extrusion will sit on along with support brackets (not pictured) on either side as well as the back. I had to get a bit creative on the mounting to the surface plate, and I'm afraid this will be the weak link if I encounter any serious vibration/deflection issues. It again will be 3/8" plate, fastened on both the top and side surfaces. I'll likely extend the plate in the Y axis to improve longitudinal stability. Although only one bolt is pictured I plan on securing it with 4 7/16" grade 8 bolts on either side, and once I get everything exactly where I want it I may consider pinning it with some dowels.

    All of the rails are offshore 20mm HIWIN clones, and while I know there are better options out there it's just not in the budget. These should provide a reasonable amount of rigidity, and for the moment will get me in the game. The X axis will be 750mm (600mm rails in the model at the moment) which should allow for just over 600 mm of travel. The Y axis are 500mm, and though shorter than the plate would allow the machining table I plan on using is 12" x 24" x 1/2" aluminum plate, and 500mm rails net me over 300mm of travel. The Z axis is loosely laid out in the model, but I'm considering purchasing a premade axis off of ebay that uses 15mm rails, and looks to have decent machine work.

    As for motion control the X and Y axes are setup with 16mm dia. 10mm pitch ballscrews. the consensus seems to be that this is a solid middle ground between resolution and movement speed. The Z axis is setup for a 5mm pitch ballscrew, as movement speed isn't as critical in this axis. For motors I've gone with offshore 2.2 Nm NEMA23 steppers driven by generic stepper drivers and a 12.5 amp 48 V power supply with shielded cable throughout. I've gone with MACH3 as the control software and have a super cheap USB breakout board for now just to play with. It doesn't have much in the way of I/O and I plan on using inductive limit switches, and will require I/O for the cooling and spindle so a final board will be decided in the future. I've already purchased the basic electronic setup, and have successfully tested all 3 motors in MACH3.

    The spindle will be a generic Chinese 2.2Kw unit unless a used or new old stock unit falls in my lap. For the time being this is on the back burner until I get some axes in motion.

    For the moment I've made a few purchases of parts that so far includes:
    Stepper motors
    Drivers
    Power supplies (24V and 48V)
    USB breakout board
    Shielded cable
    X, Y, and Z axis rails
    X, Y, and Z axis ball screws
    Surface plate

    Any and all feedback is appreciated. I have a large amount of experience in CAD (Catia, SW, SE, Fusion360) but very little experience in the CNC world. 99% of the CNC I've done has been waterjet cutting on OMAX's and a little bit of 3D printing. This has thus far been a daunting, but rewarding learning experience, and I'm excited to make my first chips!





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  2. #2
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    Default Re: Fixed Gantry Build

    Looks like you put a lot of thought into this. I have a few comments to ponder!


    First Mach 3 is getting very old and has been unsupported for a very long time now. There are many options these days for a hobby machine. This isn’t a big deal as retro fit at a later date isn’t too difficult. This is just a standard warning for people that might not be familiar with the state of Mach3.

    As for the machines table, I’d turn it 90 degrees and flip the linear rails. This would allow for more rigid mounting of your gantry supports and frankly a lot less machining / fabricating. Your table can then be as long as required to get your working area.

    Speaking of the gantry if you are going to make those complex supports then you most likely have access to build the gantry beam out of steel sections. Your surface plate gives you a good starting point. As such you might as well make the machine as stiff as the budget allows and the plate can support.

    If you do go 8020 I’m not sure filling the cavities will do much for you. The big problem with extrusions is the thin webs where you mount the linear rails. In that regard there is a huge difference in the various extrusions available from manufactures. If you go the extrusion route prefer a heavy series.



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    Member peteeng's Avatar
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    Default Re: Fixed Gantry Build

    Hi UC - Filling the extrusions with "epoxy granite" is an expensive exercise and does not improve performance relative to the cost and effort. Your much better off using steel or aluminium heavy square tubing (unfilled) . Looking at your images I'd make the connection to the base plate a lot stiffer. Peter



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    Member Chris D's Avatar
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    Default Re: Fixed Gantry Build

    Here is my thought...

    Large industrial machines that are configured like this are called either a Bridge or Double Column machine. Gantry machines are "moving bridges".

    Just a pet peeve of mine.

    Otherwise the design looks good. I see one thing I would change right away and that is the single screw you are planning to hold the column to the base with. One screw always creates a pivot point, even if the object has registration surfaces. I would highly recommend using two screws.

    You may also want to show the final design once you have the Z axis drive system in place.

    Chris D.



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    Member UCBones's Avatar
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    Default Re: Fixed Gantry Build

    Thanks for the feedback everyone! I'll try and respond to everyone's points!

    Wizard:
    I haven't sunk but $20 into the mach3 board and have just played around with their demo software, so I'll be sure to check out the CAM forums here to see what everyone recommends.

    As for flipping the table 90 and switching the rails, if I'm reading this right you're saying to put the 750mm rails on the surface plate and 500mm rails on the bridge? that'll leave about 75mm of rail hanging off of either end. Would that be an issue? I'll try and draw something up in Fusion tonight based on what I think you're saying and post back.

    I certainly could have the bridge made from steel. There's a local yard that I can buy by the pound, and find just about any kind of structural steel you can imagine. My concern is finding a place to get it machined. I know of a couple machine shops around here but they're more focused on large volume parts. I'll have to stop by the scrap yard anyway and maybe I'll take a look at what they have and give some local machine shops a call.

    Thanks to both Wizard and peteeng on the epoxy granite info! In term of expense is there anything wrong with getting a marine epoxy and play sand? I've seen a few builds use that with success. More than strength I'm looking to cut down on vibration, but if it won't be much of an issue then I won't bother.

    Thanks to Chris for the nomenclature correction! Precision of language is important! Only one bolt hole was depicted as more of a reference. What I plan on doing is using 4 7/16" fasteners on each side, and I've updated the model to reflect that below. I still haven't decided if I'll go with the pre machined Z axis. I'm thinking I will initially, and maybe machine a custom one once I get the machine running on the pre-built. After that I can use the pre-built on a future CNC plasma table






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    Default Re: Fixed Gantry Build

    This looks like a pretty good design, but you might want to rethink some details. Mounting the Y-axis rails like that will be problematic; you're depending on T-nuts to hold the rails absolutely straight, flat and parallel to each other, which is a lot to expect. I'd suggest making up a plate that would mount to the extrusion, with channels milled into it, which would provide a flat base for the rails and ledgers they can rest against to ensure straightness.

    Also, those red-colored mounts between the base and the extrusion will have to be machined to exactly the same height and absolutely square, or the whole thing won't cut straight. Do you have access to a milling machine? I'm not seeing this working too well without one.

    Andrew Werby
    https://computersculpture.com/


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    Default Re: Fixed Gantry Build

    Hi UC - epoxy filling success is gauged by what? Sure you can do a successful fill but did they check the harmonic performance before and after the fill? Vibration is caused by the motors so you can isolate the motors more effectively then by modifying the structure. Learn about machine vibration vs doing stuff because people do it.

    "GME's New 80/20 CNC Build - My Design" a lot of effort was done in this thread (later part page 7 onward) and discussion about vibration, will help you in this design. Peter



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    Default Re: Fixed Gantry Build

    Peter, a lot of vibration can be due to steppers, but certainly not all.

    Material cutting can induce problem vibrations resulting in chatter and poor surface finish. This is usually much more obvious with metal machining than wood.

    There are a plenty of papers on use of epoxy granite in machine tools.

    I agree that quantifying the effect is non trivial for the hobbyist and usually not done.

    The same can be said of much of DIY machine design. Many design choices are made based on lack of understanding, what looks good etc rather than relatively simple good engineering principles.

    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)


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    Member peteeng's Avatar
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    Default Re: Fixed Gantry Build

    Hi Pippin - Yes agreed, but chatter is a machine design shortcoming just as machines that vibrate are a shortcoming. Epoxy granite construction is interesting but I can't see it as an economical solution to machine building. It certainly is damp and heavy which solves various problems. In the end it's up to the Maker to decide on the construction philosophy and to prove the pudding (or not). The Maker has various objectives, budgets and viewpoints with any machine and I encourage people to keep making things. I've been building various machines for 30 years and I like to feel I'm still getting better at it and I try to share the knowledge gleaned from 30 years of successes and failures, mine and others. Happy to help Peter



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    Default Re: Fixed Gantry Build

    A few things.

    The 20mm rails are fine but the carts might need over sized balls to be tight.

    I just re did a few 16mm rails and it really worked well. I think they run the same size balls so if you have standard preload carts I have a few thousand over sized balls.

    Same with the ball screws. I did not see if your going double nuts but if you have single you will want to re pack them for positive preload. It is easier to just get double nuts and shim them.

    On the gantry you might be just as well going with steel. I have a hunk of 2.5" X 4" 1/4" wall I might be able to surface for you and it will be much stronger. This hunk is a little heavy but most any steel part will be much stronger.

    On the gantry ball screw, it is best to put it closest to the point of load which is your cutting tip. It will work better on the bottom or between the rails.

    Nice looking model BTW.

    youtube videos of the G0704 under the name arizonavideo99


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    Default Re: Fixed Gantry Build

    Quote Originally Posted by peteeng View Post
    Hi UC - Filling the extrusions with "epoxy granite" is an expensive exercise and does not improve performance relative to the cost and effort. Your much better off using steel or aluminium heavy square tubing (unfilled) . Looking at your images I'd make the connection to the base plate a lot stiffer. Peter
    I would argue that any unfilled/unbraced square section is unsuitable for a gantry. Torsional stiffness of hollow tubing is poor.
    I think epoxy-granite is a good way and relatively simple way to stiffen one. At least for me it's easier than soldering a diagonal plate or rebars inside a tube.



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    Default Re: Fixed Gantry Build

    UCBones.

    I think we have mis communicated here in a couple of ways.

    With respect to expiry granite filling aluminum T-Slot extrusions, there are several problems I will try to clear up.

    1. The fill is concentrated at the center of the structure so it doesn’t offer a lot for the effort involved. It is a bit more difficult than stuffing sausage due to the small crops sections in those hollows.
    2. If you look closely at the extrusion cross section you will notice that there is nothing but what amounts to sheetmetal holding up one side of the T-Slot. How important to you that this is to you I can not decide but it is a mechanically weak design. More so how weak it is varies a lot with the extrusion supplier and the series of extrusion purchased.
    3. People seem to want to believe that Tslotted extrusions make mechanical assembly easier, I don’t buy into this at all. This especially on a gantry beam with the rails relatively close together. As mentioned above this is often addressed by a properly machined plate to mount the rails on that is then mounted on the extrusion.

    As for the table, hopefully this clears up what I meant:

    1. Turn the table 90 degrees
    2. Instead of you current linear rail mounting arrangement flip the rails and bearing blocks over.
    3. This puts the rails on the table and the bearing blocks on the surface plate. No other axis need change.
    4. This does allow you to move the gantry supports (columns) in to make for a more rigid mounting to the surface plate. I really don’t like the current arrangement at all, especially running screws into the edge which risks break out.
    5. Moving the gantry supports in board a bit makes it far easier to manufacture something that doesn’t require machining. Your drawings pretty much indicate parts that will require a mill to get right.
    6. No matter what your u buy, aluminum extrusions, steel tubing or something else, they will ship in a raw form with no guarantee of the various faces being square. You can design in provisions to help with alignment or hope that a high quality machining job gets things as square as needed. How square is needed is another issue which only you can answer.

    In any event you have a nice start scoring that surface plate. I just really don’t like how the up fights for the gantry beam are implemented. If you want to do this build without the use of a machine shop then you need easy ways to overcome the lack of precision in those uprights. There are a couple of ways to do this but you might want to put grouting the beam to the uprights.

    Remember the goal here is to build machines that meet your expectations as far as performance goes. We can’t really guess what your expectations are and, so we offer advice based on our best guesstimate of what you need.

    About Mach 3, if it becomes too much of a problem the switch to another step and direction implementation isn’t really that big of a deal. I only bring it up because of its unsupported nature and the fact that we have a lot of good to better options these days. Mach 3 has along history in the hobby world thus is widely known, but things change. Personally something that is supported and decoupled from the PC is the way to go these days.



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