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    Default Design input for newbie

    Hello,

    I'm planing to get into the DIY CNC, partly because I finds it fascinating and partly because I want to make things with it. I'll try and outline the line of action that I intend to pursue. I'll be happy for any comment and will ask some specific question as well.

    I intend to make two machines.

    First a smaller to learn and hopefully help in making the next one. I'm looking at 3 axis eBay kits with SBR16 and SFU1605. Intended strokes are 36x20x4". I'm most comfortable with working with wood so I plan to use 5/8" plywood and work with closed boxes as much as possible. The machine will be located in a constant temperature basement with the possibility to control the humidity if necessary. I want this machine to be able to deliver accuracy of about 0.1 to 0.5 mm, i.e. 0.004- 0.02". Hopefully the lower one if used carefully. Speed is not a goal.

    The second one should be able to handle full sheets. I also intend to have a removeable bed. I'm thinking of having the bearing blocks for the z-axis on the y-wagon and the rails on the z-axis. I hope this would give a high stiffness with the bed and still a useable machine for foam work with a larger z-stroke, up to 16". I'm toying with the idea to laminate, glass or carbon fibre, especially the gantry to increase the stiffness.

    There is a few specific things that I really would appreciate some input on, apart from a general sanity check:

    Are torques of about 300 oz-inch reasonable to drive machine 1? Bear in mind that I'm not looking for speed. I expect the gantry to be about 50 lb and driven by two screws. I will use either a small router or a spindle, I would prefer a spindle but already have a small DW router that would do for starters. Some quick calculations shows that forces of at least 20 lb should be attainable, this would give a reasonable acceleration but would it be ok for cutting in wood with moderate feed rates?

    I don't have a computer with a parallell port and don't se any major point with getting one. Is a motion controller a good choice or simply overkill. It would be moved, possibly together with the stepper drivers, to machine 2.

    I'm starting to do some CAD-work and I will follow up here.

    any comments are appreciated
    Sven

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    Default Re: Design input for newbie

    What are you planning to use this machine for? (Let's talk about the first one; you'll know a lot more by the time you get to building the second.) If it's intended for cutting wood, then speed is something you want. Wood needs to be cut relatively fast, or you start getting burn marks - it can even catch fire if you go extremely slowly. Speed in stepper motors is not really a function of the holding torque (300 oz-in in this case), although that's the number people tend to focus on. It's more closely related to inductance - lower inductance at a given voltage makes for faster motion, and high-inductance motors can perform very poorly if paired with a low-voltage power supply and drivers. A random 300 oz-in motor might be fine for your machine - or it might not work at all.

    It's usually a good idea to build your frame first, before purchasing motors and drivers; things can change in the middle of a build, and you don't want to be stuck with stuff that won't work. Also, it will give you more time to research this - many people buy motor/driver/power supply "kits" on Ebay consisting of wildly mismatched parts and then complain when their machines don't work, or work poorly.

    You can probably find an older computer with a parallel port for free, or close to that. These are perfectly adequate for running CNC machines, and save your good computer from being tied up on long part runs, and you won't have to strip it of all the extraneous functions that interfere with running G-code. If you want, you can buy adapters to run a router off your USB port, but that will likely cost more than the old PC, and you'll still need a computer to dedicate to running your router.

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    Default Re: Design input for newbie

    Quote Originally Posted by awerby View Post
    What are you planning to use this machine for? (Let's talk about the first one; you'll know a lot more by the time you get to building the second.) If it's intended for cutting wood, then speed is something you want. Wood needs to be cut relatively fast, or you start getting burn marks - it can even catch fire if you go extremely slowly. Speed in stepper motors is not really a function of the holding torque (300 oz-in in this case), although that's the number people tend to focus on. It's more closely related to inductance - lower inductance at a given voltage makes for faster motion, and high-inductance motors can perform very poorly if paired with a low-voltage power supply and drivers. A random 300 oz-in motor might be fine for your machine - or it might not work at all.

    It's usually a good idea to build your frame first, before purchasing motors and drivers; things can change in the middle of a build, and you don't want to be stuck with stuff that won't work. Also, it will give you more time to research this - many people buy motor/driver/power supply "kits" on Ebay consisting of wildly mismatched parts and then complain when their machines don't work, or work poorly.

    You can probably find an older computer with a parallel port for free, or close to that. These are perfectly adequate for running CNC machines, and save your good computer from being tied up on long part runs, and you won't have to strip it of all the extraneous functions that interfere with running G-code. If you want, you can buy adapters to run a router off your USB port, but that will likely cost more than the old PC, and you'll still need a computer to dedicate to running your router.
    Thank you for your good points. Wood is the focus, any capability to cut aluminium or plastics would be a bonus, especially for machine 1. I was looking for motor size mostly to be able to fit them within the frame. I'll have to look into feed rates for wood, great point. I've never used steppers but I'm familiar with the idea of inductance, voltage and chopping drivers. I see this as solvable later on since it doesn't have to physically fit in the frame. I guess that it is the charts of torque vs. rpm that I need to ponder a bit?

    Given that a used computer and a BOB is very cheap compared with a quality motion controller I will really consider that. (Nothing to stop me from upgrading later.)



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    Default Re: Design input for newbie

    Quote Originally Posted by SvenNorberg View Post
    The second one should be able to handle full sheets. I also intend to have a removeable bed. I'm thinking of having the bearing blocks for the z-axis on the y-wagon and the rails on the z-axis. I hope this would give a high stiffness with the bed and still a useable machine for foam work with a larger z-stroke, up to 16". I'm toying with the idea to laminate, glass or carbon fibre, especially the gantry to increase the stiffness.
    Steel is stiff. You are thinking that lighter is better, which it is if you can get the same stiffness, but also, it doesn't matter as much as you think.

    Quote Originally Posted by SvenNorberg View Post
    Are torques of about 300 oz-inch reasonable to drive machine 1? Bear in mind that I'm not looking for speed. I expect the gantry to be about 50 lb and driven by two screws. I will use either a small router or a spindle, I would prefer a spindle but already have a small DW router that would do for starters. Some quick calculations shows that forces of at least 20 lb should be attainable, this would give a reasonable acceleration but would it be ok for cutting in wood with moderate feed rates?
    What kind of calculations did you do? The available torque at the max RPM you will use is the one to look at, not the holding torque. That's because you can only input one value for acceleration in the control software that is used throughout the speed range.

    http://www.cnczone.com/forums/steppe...nductance.html

    Read through the links posted by me in post #3 of that thread.

    If you disagree, let me know.



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    Default Re: Design input for newbie

    Given that a used computer and a BOB is very cheap compared with a quality motion controller I will really consider that. (Nothing to stop me from upgrading later.)
    If you plan on buying a Mach3 license, you're looking at $175. For about the same price, you can get UCCNC with a UC100 motion controller.
    Quality of motion (pulse stream) with the parallel port can vary from PC to PC. The UC100 takes that out of the equation, and UCCNC is arguably better than Mach3.

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    Default Re: Design input for newbie

    NIC 77, thank you for your reply. I see that you have been very helpful here.

    After reading the topics you linked to and having performed some math I've got a much better picture of what I should do and also some questions singled out:

    I really should have a high voltage and good drivers.

    Both 1605 and 1610 screws will provide the acceleration I think I want, 2 m/s2, 0.2g. 1605 have an edge regarding force but they would be pushed to give 200 IPM due to screw whip (?). 1610 would provide 300 IPM comfortably but require more torque to push the bit when cutting. I've found recommendation of 0.005-0.01" in chip thickness. That means that feed rates of 50-200 IPM is a reasonable range, thinking of 1/8" to 1/2" bits.

    Regarding the lighter vs. stiffer point that you pointed out, it is more a question about my having much better aptitude and equipment for wood work. I do intend to use oversize box sections to utilize the wood. I made a quick check now and found that a 200x200x15 (8x8x5/8) mm wood closed box has roughly the same stiffness as a 100x100x5 (4x4x3/16) mm steel beam. I don't know it you would consider that as a small or large beam for a gantry of 700 mm (28") between supports.

    I have really tried to find data on cutting forces, the best I have found was a rough estimate of about 1 lbf per 1/16" of thickness for wood. Given the useage I intend, mostly shets up to 1/2" is seems that something like 20 lpb ~= 100 N is in the ballpark. There doesn't seem to be a problem to push the bits through wood force wise?


    I really appreciate the discussion here. I apologise for anything that is unclear due to my being used to the SI-system and not having English as my first language, please ask if needed! I will try and upload my calculation when they are written up properly. Now I will continue to do the cad.

    Last edited by SvenNorberg; 11-15-2017 at 02:23 PM. Reason: spelling


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    Default Re: Design input for newbie

    Quote Originally Posted by ger21 View Post
    If you plan on buying a Mach3 license, you're looking at $175. For about the same price, you can get UCCNC with a UC100 motion controller.
    Quality of motion (pulse stream) with the parallel port can vary from PC to PC. The UC100 takes that out of the equation, and UCCNC is arguably better than Mach3.
    Thank you for your comment. I will have to look into the software and controller bit properly. Fortunately they don't have to be packed into the hardware. As an engineer I find the idea of the motion controllers very tempting.



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    Default Re: Design input for newbie

    Are you talking about enclosing your motors inside the wooden torsion boxes you're using as the structure of your machine? If so, have you thought about heat build-up? Steppers often run quite hot, and if they're in a closed box, that's going to make them even hotter, although you might not realize it until something melts down.

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    Default Re: Design input for newbie

    Thank you for your concern. I will not enclose the motors, but I understand why you read it so. I simply meant that the drivers and so on doesn't have to be mechanically attached to the structure.



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    Default Re: Design input for newbie

    Sven - Welcome to the forum!

    Quote Originally Posted by SvenNorberg View Post
    Thank you for your good points. Wood is the focus, any capability to cut aluminium or plastics would be a bonus, especially for machine 1.
    While there is nothing wrong with a wood framed machine to cut wood, foam and some plastics I get concerns when Aluminum is mentioned. This is mostly due to the need (in my mind anyways) to use a coolant / lube with aluminum which of course is less that ideal around wood framed structures. You can certainly do some dry but chip weld with aluminum can be significant. This especially if you have a router on a machine that can't easily hit the right combinations of spindle speeds, feed rates and horse power. So the first reason I tend to reject metal machining on wood framed machines has to do with fluids. The send reason is cost effectiveness relative to a build out of steel.

    The reasons I suggest steel very often in these forums is that it can be had extremely cheap at scrap metal prices. Steel often is bought as scrap for as little as 7 cents a pound by recyclers in this country. That means there is opportunity to buy scrape steel rather cheap and it is often given away. More so it means the value of drops at a steel supply are suppressed. Drops can be had for less than 60 cents a pound / hundred weight. This works to ones advantage when building a small machine because you can get suitable materials at a significant discount over buying prime stock. In other words finding a suitable beam for a 20" Y axis should be pretty easy and not break the bank.
    I was looking for motor size mostly to be able to fit them within the frame. I'll have to look into feed rates for wood, great point. I've never used steppers but I'm familiar with the idea of inductance, voltage and chopping drivers.
    Innovation here is pretty constant so putting off a hardware buy to the last moment can make sense. As for motor size they are usually well documented but the manufactures and beyond that they fit into standard frame sizes.

    As for steppers the same advances that have lowered the cost of computing hardware and even things like stepper drivers has also impacted servo motor drives. Servos can be considered a real possibility that simply wasn't practical before. Like others have suggested it can pay to wait until you have a finalized design before getting too concerned about the drivers.

    Another interesting problem with small machines is the opportunities to reach max speeds are often limited by the travels available. You end up doing lots of accelerations up and down and not often reaching full speeds. This is more often seen in rapid speeds rather than feed speeds.
    I see this as solvable later on since it doesn't have to physically fit in the frame. I guess that it is the charts of torque vs. rpm that I need to ponder a bit?

    Given that a used computer and a BOB is very cheap compared with a quality motion controller I will really consider that. (Nothing to stop me from upgrading later.)
    Low end CNC controller are another area seeing rapid improvements in system quality and availability. It seems to be safe to say that many of these solutions have eclipsed the BOB and Mach3/4 combo. This is another thing that can be put off until most of the machine is assembled. In any event don't dismiss some of the other possibilities such as MASSO, Acorn with Centroid software, UCCNC, KMotionCNC/KFlop and the beast LinuxCNC. Then there is the whole world of GRBL and GRBL derived solutions such as SnapCNC. I've probably missed dozens of solutions here and didn't even mention the dedicated CNC controllers one can purchase from China for a song.



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    Default Re: Design input for newbie

    wizard - Thank you, it is good to be here, it seems really friendly and helpful!

    Thank you for your good point about fluids. I will stay with wood for now, that is mostly due to my being used to work with it. The second machine might be another issue, but then I will be more used to the CNC part of the build. One new thing at a time is enough.

    Regarding motors, drivers and so on, I will if possible do the layout to be able to handle both nema23 and nema34 and settle for nema23 if I have to. The rest of the electrical parts will have to be a separate study later on. I probably will continue to think about it though.



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    Default Re: Design input for newbie

    Yes, steel beams (square steel tube) can often be found cheap. Over 90% of the steel used in the machine I'm currently building comes from the scrap yard at 30 cents a pound, although I did get some of the 3/4" x 6" flat plate I'm using for 15 cents a pound. I had to buy the 10" x 0.5" flat plate and 1.5" x 0.5" flat bar I'm using new as it wasn't available at the scrap yard.

    The problem is making it flat, and having the tools to work with it.

    Ger21 is making a machine from wood, Biggs and others have made some machines from wood. Way less work overall to use wood I think.

    If you're going to make an 8" x 8" wood box for you gantry, I'd make it like a torsion box inside, and I wouldn't worry at all about the weight of it. I can't really give you any good tips about working with wood to build a machine, but others here can, if that is what you decide to do.

    I'd use 10mm or 20mm lead ballscrews for the X and Y, there's no way I'd use 5mm. You want to cut wood with this, so I think you'll like the extra speed. I'd also use 10mm for the Z axis, and you'll want your Z to move fast if you're doing 3D carving work, or it will slow everything else down.

    The rails you mentioned, SBR16?, are the round type supported rail. Many people have built machines on here with the round supported rails, but I have not heard one person say that they were happy with them. Many people said they regretted using them. I'd say any square type rails (even less expensive made in china ones) are better than the round (supported or unsupported) rails. This is based on things I have read in the forum, I have not used the round rails myself.

    It really depends on what your expectations are for the machine, and matching them with your budget and time invested, to be happy at the end of the day even if it's not as good as someone else's. That's why I think you should just build one smaller machine to start off with. Also, you might want to ask yourself if it's actually the building of the machine you want to do, or if you'd prefer to have one already made that you can just use. Check the local classifieds, you might find a good used one for less than the cost of building one.

    I wouldn't worry about the cutting forces. Also, I believe you will be making multiple passes to cut through 1/2" wood. Ask Ger21 about it, he does it for a living, and can give you better advise.

    You haven't mentioned what kind of spindle you plan on using? If you plan to cut aluminum, even occasionally, the little flakes of aluminum you get will be ingested in a router and destroy it. So I'd use a spindle, even a cheap Chinese one, and not a router.

    It is possible to cut aluminum with a sharp carbide bit and very shallow depths of cut without any lubrication, and on a machine that isn't as good as others may be. It will just take forever to do it. If you plan on cutting aluminum with any significant thickness or frequently, my advice would change, and I'd tell you to use a square steel tube with internal ribs and perhaps filled with epoxy granite for your gantry.

    I hope this was helpful.



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    Default Re: Design input for newbie

    Are torques of about 300 oz-inch reasonable to drive machine 1? Bear in mind that I'm not looking for speed. I expect the gantry to be about 50 lb and driven by two screws. I will use either a small router or a spindle, I would prefer a spindle but already have a small DW router that would do for starters. Some quick calculations shows that forces of at least 20 lb should be attainable, this would give a reasonable acceleration but would it be ok for cutting in wood with moderate feed rates?
    Yes, 300oz motors are fine. I'm using 250oz motors, with an old drive, and 24V, and get decent performance. With modern drives, and a 48V power supply, you'll get very good performance with 300oz motors.

    Don't use 1605 screws, though. 1610 will give far better performance.
    I'm looking at 3 axis eBay kits with SBR16 and SFU1605.
    These round rails and bearings will be the weakest part of your machine. My machine uses skate bearings on pipe for the X and Y axis, and 19mm round rails for the Z axis, and the Z axis has the most play in it.

    While a 16" Z axis would be OK for foam, it'll compromise the performance when cutting anything harder.


    I made a quick check now and found that a 200x200x15 (8x8x5/8) mm wood closed box has roughly the same stiffness as a 100x100x5 (4x4x3/16) mm steel beam.
    The gantry on my new machine will be an 8"x8" wood box. It's made from 4 layers of 1/4" MDF, so 25mm thick. It's 1700mm long, but the unsupported span is about 1200mm. I calculated the deflection to only be about .001-.002" with a 100lb load, if I recall. I haven't had a chance to assemble it yet, but here are some pics of the components.
    http://www.cnczone.com/forums/cnc-wo...ml#post1168468

    Gerry

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    Default Re: Design input for newbie

    What a nice machine you are doing! Very interesting with several spindles, I'll try and forget that ASAP.

    Quote Originally Posted by ger21 View Post
    These round rails and bearings will be the weakest part of your machine. My machine uses skate bearings on pipe for the X and Y axis, and 19mm round rails for the Z axis, and the Z axis has the most play in it.

    Don't use 1605 screws, though. 1610 will give far better performance.
    I appreciate these issues. My first machine is foremost a learning experience. I don't want to blow the budget and I want a build that isn't to difficult or time consuming. The rather cheap and ready to use kits are thus very tempting. There seems to be a premium to be paid for both 1610 or linear rails instead of SBR, or a more complicated build. I lean towards 1610, it is not such a large difference, but to stick with the SBR:s. One issue is that there are much more 1605 kits than 1610 available. Any input about the priority between pitch and rail would be appreciated.


    Quote Originally Posted by ger21 View Post
    While a 16" Z axis would be OK for foam, it'll compromise the performance when cutting anything harder.
    The idea was to have the z bearing blocks on the y-axis and remove the bed when using large z, thus the stiffness should be ok for normal use with about 4" of z-travel and most of the z-rails above the gantry. This is for machine 2 anyway. The first one will be 3-4" z-travel probably.



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    Default Re: Design input for newbie

    On my first build, I actually had it so the table top sections could be lowered. Kind of what you're thinking. In practice, I never used it.

    Yes, by putting the rails on the part that goes up and down you can get it so the first 3" of travel gives you close to the same result as using a shorter Z, however it won't be as good because all of the weight up high can cause some twists during accelerations, depending on how stiff everything is.

    16" is excessive. 3 to 4" is not enough. Let's say you want to cut through a 3 inch piece with a 3 inch long bit, and you also want to cut through a 1 inch piece with a one inch long bit, well you can't do them both with only 3" of travel.

    If you are looking at cutting foam objects, probably better to cut from 5" sheets and glue the pieces together after cutting.

    So a 5" foam sheet with a 5" bit, an extra inch for clearance, say 11 inches of Z travel total. Then you could have a 5" thick spacer table that fits on the lower table, and you would have 6" travel when not doing foam work.

    The problem there is that if you are building a machine with plates at the sides of the gantry, as many do, they will still need to be much taller, and they will flex more as a result, so you'd want your gantry to ride directly on beams.

    So what I mean is you wouldn't build it like this:

    Design input for newbie-like-3-jpg

    You'd build it like this:

    Design input for newbie-like-jpg

    The only point I'm making is that you wouldn't use the tall gantry risers on a machine like that.

    For only 30" x 20", a removable 5" table on top of your normal table, I don't think that's a big deal. You could make it from 4" x 4" solid wood then put a 1 inch spoil board on top, not sure how much you can lift, but I wouldn't have a problem taking that on and off.

    Wood is inexpensive and easy to work with, yes if that's what you want to do as a project to have fun with, I don't see a problem. I'd just make everything big and as a torsion box wherever you can.



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    Default Re: Design input for newbie

    Quote Originally Posted by SvenNorberg View Post
    I apologise for anything that is unclear due to my being used to the SI-system and not having English as my first language
    Embrace the SI-Units, it's the one true system. Imperial system is antiquated.

    As I'm currently building my own CNC I thought I'd give a few suggestions I've found out so far...
    - If you have the time and possibility to use steel, do so. It cheap and stiff. The weight of the gantry is normaly a small issue.
    - The stiffness of the side-plates (assuming you are bulding a gantry style cnc) will be the weakes parts, where most deflection will arise. You can raise the sides/bearings to minimize the height of these plates.
    - Spread the bearings as far apart as possible.
    - Buying parts from china takes about 60 days, which is a long wait when you need something. 'Biltema' is just as cheap and has most of the parts available.

    Building the machine 1 I'd suggest you either buy expensive and good quality parts you can transfer to machine 2, or buy all parts as cheap as possible and take the cost as a learning experience.
    It all depends on how secure you feel in your decision. Going for the middle ground is probably not ideal.

    Best of luck!



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    Default Re: Design input for newbie

    It's so nice to discuss things. I was thinking of having the rails at the level of the removable bed but why have risers at all? It will be much easier to make a sturdy sidewall than risers. Thank you.



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    Default Re: Design input for newbie

    Quote Originally Posted by SvenNorberg View Post
    It's so nice to discuss things. I was thinking of having the rails at the level of the removable bed but why have risers at all? It will be much easier to make a sturdy sidewall than risers. Thank you.
    True, but with the risers, you can actually make the gantry bridge wider than the riser span and get up to the whole extent of the table edges. When you have sidewalls, you have to add the width of Z carriage to get the same travel, which would increase the unsupported span between the sidewalls as opposed to the risers, requiring a larger footprint for the same cut area.



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    Default pictures

    Hi everybody,

    I've got some CAD done today. (My first work in Fusion 360, interesting software.)

    I have modelled this around HGR20 rails. 1300/1000/400. I'm thinking of increasing the y-rails to 1100. It would give better stiffness for y-forces but I would lose the direct connection between the y & z bearings blocks. The rails and bearings will be the less expensive variant.

    The z-axis is modelled around a 65 mm spindle.

    I've found that I can get rack & pinion quite affordable. There will be two racks on the x-axis and one on the y-axis. The z-axis will have a balls screw. These parts are not modelled yet.

    All structure are 15mm plywood. I plan to use ground steel plates glued to the plywood in positions of high local loads, i.e. under bearing blocks. I'm thinking of making the y-axis out of an metal plate.

    I would appreciate any comments.

    /Sven

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  20. #20
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    Default Re: Design input for newbie

    I like it. I like the fact that you have made everything like a torsion box.

    I like the ribs in the gantry but I think you could add more internal ribs in perpendicular directions to the ones you have. Plywood isn't too expensive and weight won't be an issue.

    When you add the rack and pinion, your design will obviously change a bit. Make sure you have a belt reduction system for the rack and pinion, 2:1 or 3:1, preferably spring loaded.

    You'll want a spoilboard, perhaps MDF, on top of the table. You can use the machine to cut it flat as a last step when you are done building.

    I don't think you have nearly enough Z travel. Personally, I think you should try to be able to cut a 3 inch piece with a 3 inch bit all the way to the spoilboard, and also a 1 inch piece with a 1 inch bit all the way to the spoilboard, and still have a bit extra to allow for positioning the machine and setting zeros, etc.

    You mentioned using ground steel plates....Aluminum flat bar typically comes pretty flat. If you use this instead you might make due with off the shelf aluminum instead of more expensive ground steel.

    I like the boxy structure you've come up with, you're thinking about this the right way. What's the size of this machine?

    You know, you could always make the gantry smaller, and cast it from epoxy granite? That's another option, but it's a different type of machine design and much much heavier.

    For the wood structure you have, you could consider filling it with high density polyurethane when you're done, that only weighs 4lbs per cubic foot and may provide some vibration dampening, although I'm not certain it would make a difference.

    Like I said, I like it, you're getting the point of building it like a torsion box.



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Design input for newbie

Design input for newbie