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  1. #1

    Student organization looking to build CNC milling machine for school donation

    Hi guys, I'm the president of a student organization and we are in the process of designing a fixed gantry styled CNC milling machine to donate to our University. I was hoping yall might be able to help me to understand what I don't know (which is a lot).

    First questions are about the entire milling head area. Does a spindle motor capable of ATC need to be a servo motor with an absolute encoder? If I want it to be capable of everything from wood to mild steel (or maybe tool steel with conservative speeds and feeds and proper rigidity [which i will get to in a moment]) would 500-6k rpm be okay or should I try to incorporate a gearing system to increase spindle speed for those softer materials? How would I go about designing a gearing system? What taper would you recommend for light duty steel cutting with lots of aluminum and wood/plastic cutting? would 8 ft-lbs of torque be enough for machining mild steel with conservative speeds and feeds using that taper?

    rigidity questions: of course I recognize there isn't much replacement for pure mass in the rigidity question, however we intend to create a dual fixed gantry build (X and Z axis sandwiched between two fixed gantries). do you think that this, coupled with otherwise proper design might make for a setup that is notably more rigid than its single gantry counterparts? what other design considerations should I be thinking of for proper rigid, low vibration design? we intend to design the frame with various sized structural tubing and filling any section not filled with cables with sand or non shrinking grout to increase mass and reduce vibrations.

    Thank you in advance for your responses, I am sure I'll have more questions as this build progresses.

  2. #2
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    Re: Student organization looking to build CNC milling machine for school donation

    Hi 308 - This is a very ambitious project for students. I suggest you read Principles of Rapid machine Design by Eberhard Bamberg. On the issue of twin gantry or single its complex. Commercial machines use single gantry for access, a twin design will present problems for ATC and other things, like alignment. I think you will end up looking like commercial machines so pick a machine that exists that does what you want and use it as a benchmark. You need to create a very clear objective list for this machine & review often. It will be expensive but fun. You need to evaluate statements like "were going to fill in steel tubes with stuff to prevent vibration" you are solving issues that do not exist in a good design. You should have the tools to evaluate those things as you go along. I assume you will be doing FEA to evaluate the structure so you can do a modal analysis to predict vibration issues. If this is an engineering exercise it should be a clean sheet and only solve issues as they arise. For instance the "mass is great" idea is a non engineering approach. Mass and stiffness are not related. You need to think these things through very carefully otherwise you may as well buy a machine or you will spend much time and effort on things that do not matter in the end. Where your heading it will be cheaper to buy a machine vs make one. You will need to get lots of free machine time or donated time and resources if you think you can build one cheaper. Book about a year plus into the project to get it done Happy to help Peter

    Edit - Hi 308 I've been involved off and on with student projects for some decades. I recently supervised a group of students in a 3D printing project for instance. It's a great idea to make a machine and donate it to a uni. But is this going to be an ongoing developmental machine? I say this because if this is intended to be a useful machine it is unlikely to work properly out of the box and someone will have to work through its foibles to get it to a useful condition. The risk is that it will have many issues and then be forgotten and rust in a corner. If its a development machine that successive groups can work up then that's a good thing. If you intend it to be a useful machine at unboxing day you may end up with egg on faces so to speak. It happens to the best of us....

    I design machines for a living and even when a group of seasoned professionals get together and design a bespoke machine unless it uses entirely understood and prior developed principles and methods it will have issues. A bespoke machine is full of unintended consequences that the project manager has to have the experience and smarts to side step along the way. For instance if it is a training machine I suggest you don't use an ATC, it adds a complication and reliability issue that is not necessary. Tools have to be set and checked in an ATC and if not, broken tools, crashes and heart break occur too easily. One tool manually set by the person is a much better training scenario...

    You mention as the build progresses, you need to design something in CAD to maturity before you even think about building! To the nuts and bolts level please. It is a strange thing but you will design something you cannot assemble or will need rework to allow assembly. I am designing a router at the moment with the stiffness you speak of and I'm in the final phase of sorting bolt holes and access. It's not a trivial matter checking 100+ holes and fits!!

    Don't mean to rain on the parade just trying to give you a fair heads up. Peter

  3. #3
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    Re: Student organization looking to build CNC milling machine for school donation

    Depending on where you are, don't overlook safety interlocks as well. ANSI B11 is quite clear what you need to have, and it's not minor by any means. Most schools will not even accept used proven machines without the B11 standards being met, or for that matter some sort of NRTL electrical approval on the machine, that alone will cost 2500-5K for certification.

  4. #4
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    Re: Student organization looking to build CNC milling machine for school donation

    Quote Originally Posted by Daedalus308 View Post
    Hi guys.... we intend to create a dual fixed gantry build (X and Z axis sandwiched between two fixed gantries). do you think that this, coupled with otherwise proper design might make for a setup that is notably more rigid than its single gantry counterparts?....
    Hi 308 - In response to this question:
    1) shopbot cnc started making routers a long time ago in this configuration. They soon changed to a conventional config. So there are evolutionary reasons why this config is not any better
    2) If you consider that the Z axis is a vertical structure and then you have to attach another rail structure to the back of it, it makes sense to have those rails vertical too. It is simpler and simple is best in any machine
    3) A machine design is about stiffness especially for cutting steel as you mention. So every part of the machine has to be justified on a stiffness basis. If you have two parallel members not connected with a shear structure they behave independently. There is no global gain in stiffness, which I think is the perceived gain in this config.

    The carriage in the middle can only transfer load across the gap via the lead side deflecting. But you have to make the lead side very stiff for your application so it does not deflect much. This makes the lagging beam redundant in the limit case. This also goes for the transfer of moments if you use rails. Rails are rigid in the sense that they resist transverse moments. So the lead beam will resist most of the applied moment and only transfer a small moment to the lagging beam. A way around this is to use round shafting bearings so that they cannot resist moments and allow the two beams to moment share. But shafting has less capacity and probably these days you would not use them. If you do an FE of the gantry this will show up the load share issue. It will show up better if you do a non linear geometric analysis as a couple of these things can be non linear if the beams are not very stiff and you don't model the bearings correctly.
    4) In a fixed gantry design the beauty of it is to have a hugely stiff gantry that does not move ie no inertial loads to cope with. So you may as well have a very big single beam or even a cantilever like a bridgeport machine. Which depending on the size of this machine you will need a bigger beam than you expect.

    So: how big is this machine? benchtop or 4x4ft? and are you aiming at 0.1mm, 0.01mm or 0.001mm accuracy?

    In terms of machine stiffness this is measured as the "static and dynamic stiffness" The dynamic stiffness is hard to measure but the static stiffness is easy. You are going to need a stiffness at the tool in the order of 10-100N/0.001mm Small hobby mills are around 1-5N/um and the owners of these try to make them stiffer. So I expect 10N/um is minimum.... Peter

  5. #5
    Quote Originally Posted by peteeng View Post
    Hi 308 - This is a very ambitious project for students. I suggest you read Principles of Rapid machine Design by Eberhard Bamberg. On the issue of twin gantry or single its complex. Commercial machines use single gantry for access, a twin design will present problems for ATC and other things, like alignment. I think you will end up looking like commercial machines so pick a machine that exists that does what you want and use it as a benchmark. You need to create a very clear objective list for this machine & review often. It will be expensive but fun. You need to evaluate statements like "were going to fill in steel tubes with stuff to prevent vibration" you are solving issues that do not exist in a good design. You should have the tools to evaluate those things as you go along. I assume you will be doing FEA to evaluate the structure so you can do a modal analysis to predict vibration issues. If this is an engineering exercise it should be a clean sheet and only solve issues as they arise. For instance the "mass is great" idea is a non engineering approach. Mass and stiffness are not related. You need to think these things through very carefully otherwise you may as well buy a machine or you will spend much time and effort on things that do not matter in the end. Where your heading it will be cheaper to buy a machine vs make one. You will need to get lots of free machine time or donated time and resources if you think you can build one cheaper. Book about a year plus into the project to get it done Happy to help Peter

    Edit - Hi 308 I've been involved off and on with student projects for some decades. I recently supervised a group of students in a 3D printing project for instance. It's a great idea to make a machine and donate it to a uni. But is this going to be an ongoing developmental machine? I say this because if this is intended to be a useful machine it is unlikely to work properly out of the box and someone will have to work through its foibles to get it to a useful condition. The risk is that it will have many issues and then be forgotten and rust in a corner. If its a development machine that successive groups can work up then that's a good thing. If you intend it to be a useful machine at unboxing day you may end up with egg on faces so to speak. It happens to the best of us....

    I design machines for a living and even when a group of seasoned professionals get together and design a bespoke machine unless it uses entirely understood and prior developed principles and methods it will have issues. A bespoke machine is full of unintended consequences that the project manager has to have the experience and smarts to side step along the way. For instance if it is a training machine I suggest you don't use an ATC, it adds a complication and reliability issue that is not necessary. Tools have to be set and checked in an ATC and if not, broken tools, crashes and heart break occur too easily. One tool manually set by the person is a much better training scenario...

    You mention as the build progresses, you need to design something in CAD to maturity before you even think about building! To the nuts and bolts level please. It is a strange thing but you will design something you cannot assemble or will need rework to allow assembly. I am designing a router at the moment with the stiffness you speak of and I'm in the final phase of sorting bolt holes and access. It's not a trivial matter checking 100+ holes and fits!!

    Don't mean to rain on the parade just trying to give you a fair heads up. Peter
    Hey Peter! Sorry for the later reply, cnczone hadnt notified me of any responses. First i wanna thank you for the lengthy reply, i really appreciate the thought that went into your response. I'll try to address your statements one at a time.

    As far as the monetary efficiency goes, we recognize that it will be more expensive than an equally effective off the shelf machine, but this is an exercise to get students some hands on experience so we're totally okay with that! Plus, we have one major sponsor Currently and are continuing to look for more.

    Also, thanks for the reading material! I'll get on that this weekend to see what i can learn!

    As far as FEA goes, no one in our group of students have really done FEA before and we dont know how to at this point which is unfortunate for design analysis. Ill see if we can try to learn anything about FEA to help us.

    We intend for the machine to be operational on delivery, but likely not perfect. We hope that an ATC (rack style so as not to interfere with the gantry) gets made for future use, i just want to make sure that infrastructure is there to accommodate it. Same concept for things like coolant, automatic oiling, safety guards etc.

    We recognize that precision alignment of 2 gantries will be difficult, but one of my favorite sayings is "if you cant make it perfect, make it adjustable" so in addition to allowing for adjustment, were designing in some concepts to reduce potential issues.

    I see what youre saying about finding an existing machine so ill be sure to look into that!

    Were currently using fusion 360 to design each and every part and put em into one assembly folder as a finished machine. I learned the importance of full design a few years back with a belt grinder i designed.

    Hopefully i was able to address everything, thanks for all the advice! I'll try to look into everything you mentioned!

  6. #6
    Quote Originally Posted by underthetire View Post
    Depending on where you are, don't overlook safety interlocks as well. ANSI B11 is quite clear what you need to have, and it's not minor by any means. Most schools will not even accept used proven machines without the B11 standards being met, or for that matter some sort of NRTL electrical approval on the machine, that alone will cost 2500-5K for certification.
    Thanks for the consideration! I hadn't even thought of that and i will look into that as soon as i can! Great point!

  7. #7
    Quote Originally Posted by peteeng View Post
    Hi 308 - In response to this question:
    1) shopbot cnc started making routers a long time ago in this configuration. They soon changed to a conventional config. So there are evolutionary reasons why this config is not any better
    2) If you consider that the Z axis is a vertical structure and then you have to attach another rail structure to the back of it, it makes sense to have those rails vertical too. It is simpler and simple is best in any machine
    3) A machine design is about stiffness especially for cutting steel as you mention. So every part of the machine has to be justified on a stiffness basis. If you have two parallel members not connected with a shear structure they behave independently. There is no global gain in stiffness, which I think is the perceived gain in this config.

    The carriage in the middle can only transfer load across the gap via the lead side deflecting. But you have to make the lead side very stiff for your application so it does not deflect much. This makes the lagging beam redundant in the limit case. This also goes for the transfer of moments if you use rails. Rails are rigid in the sense that they resist transverse moments. So the lead beam will resist most of the applied moment and only transfer a small moment to the lagging beam. A way around this is to use round shafting bearings so that they cannot resist moments and allow the two beams to moment share. But shafting has less capacity and probably these days you would not use them. If you do an FE of the gantry this will show up the load share issue. It will show up better if you do a non linear geometric analysis as a couple of these things can be non linear if the beams are not very stiff and you don't model the bearings correctly.
    4) In a fixed gantry design the beauty of it is to have a hugely stiff gantry that does not move ie no inertial loads to cope with. So you may as well have a very big single beam or even a cantilever like a bridgeport machine. Which depending on the size of this machine you will need a bigger beam than you expect.

    So: how big is this machine? benchtop or 4x4ft? and are you aiming at 0.1mm, 0.01mm or 0.001mm accuracy?

    In terms of machine stiffness this is measured as the "static and dynamic stiffness" The dynamic stiffness is hard to measure but the static stiffness is easy. You are going to need a stiffness at the tool in the order of 10-100N/0.001mm Small hobby mills are around 1-5N/um and the owners of these try to make them stiffer. So I expect 10N/um is minimum.... Peter
    Just like my last reply, I'll try to respond to your points one at a time

    1) Do you have any idea what models or years that was produced? I would love to be able to see some of those and research how the end users liked them.
    2)I'm not really sure what you mean here, I do intend to have the Z axis traveling on rails, one pair per side attached to a gantry. Perhaps we could further discuss this so I can better understand what you mean?
    3) could you expand on this? i'm not familiar with the concept of a shear structure
    4) This is intended to be a standalone machine with a 2ftx2ft working area (subject to change), I would like to achieve a positional repeatability of at least as accurate as 0.001 inches, so about 0.025mm repeatability for a machined part (given otherwise correct machine setup, proper tooling, speeds, feeds, etc.). In the end I would like to achieve stiffness equal to or greater than that of a Haas minimill (I don't know the numbers, but i do have personal experience to use this as a reference).

    Thanks again for the thoughtful responses! The information you are giving me is enlightening and it is greatly appreciated!

  8. #8
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    Re: Student organization looking to build CNC milling machine for school donation

    Hi 308 - As far as a machine goes you cannot make many things adjustable. Adjustability is a two edge sword. What you are aiming at and talking about requires machined fits, adjustability will work against you in the long term.

    Fusion 360 has FEA especially for students. Being engineering students I imagine they and you have studied structures and am familiar with manual beam calculations. I spent the early part of my career pre FEA so did all of this by hand, I've even calculated FE meshes by hand pre auto meshing. Building something is one thing but also designing something the way you will do it as a professional is important. Every part in a mill should have a calculation behind it, even the nuts and bolts and oil. Its ambitious, keep us posted and aim high gravity sucks. Peter

    Hi 308 - As an aside. I suggest you work on your language as well. If you used words and expressions as you have done at management levels you will never get the funds approved for your project. Words like "hope" and "adjustability (as a solution)" send shivers up management spines!! You have to have clear objectives, non conflicting ideas and reasonable solutions in the pitch. Publish some images here we'd like to see them... Good luck

    and tell me briefly why the twin gantry is a good idea?

  9. #9
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    Re: Student organization looking to build CNC milling machine for school donation

    As far as the monetary efficiency goes, we recognize that it will be more expensive than an equally effective off the shelf machine,
    Do you have a complete machine shop at your disposal, and the knowledge and abilities to use it?
    If not, you'll most likely not only spend more, but also have a far LESS effective machine than had you purchased one. A far better option for students would be to buy a HAAS, and teach the students how to use it, and how to make parts and products.
    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)

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    Re: Student organization looking to build CNC milling machine for school donation

    Please point to a twin gantry machine picture so we know what you mean ... ??

    The most rigid machine possible is a twin-column milling machine using a moving table, usually y moves.

    The two fixed columns have half the unsupported distance vs a c-frame, thus are 8x more rigid, each.
    And there are 2, they can be anchored to the floor, the bridge can be heavy as it is clear of stuff.

    Drawback of moving table is needing double the table size for the support below.
    Larger footprint in y, but much more rigid (anchored to floor), easy, simple, cheap.

    I suggest defining a plan and wants/desires/abilities diagram, with time/costs somewhat marked in.
    Making the machine mechanically is trivial, given you have people with skills and can get secondary bits like motor mounts/brackets machined locally (school) for free and have measuring stuff.

    Imo, the better you make the machine mechanically, exceeding common VMCs specs, the more successful it will be.

    There are endless complex challenges, most of which cannot be solved well in less than 6 months, even with many clever people working on each one.
    Toolchanger will be an issue.
    IO will be an issue.
    Sensors will be an issue.
    Controller will be an issue. The issue.
    Docs will be a big issue.

    Gcode implementation and features and proofs will be a big issue, will depend on the controller, and are about 10.000x more complex than you probably think.
    SW:
    You probably want css/inverse mode or feed/rev, rigid tapping.
    Probing. How ?
    Look into this. It won´t be pretty.
    Toolsetter.
    Test it, prove it, a wood tester machine of 500$ is fine for 99% of proofs.

    IO for mechanical tests of oil, air pressures, any interlocks, servo faults, axis enables, etc.
    Toolchanger IO, spindle IO.

    A basic 3 axis servo lathe needs about 130 wires and 40+ IO.
    (Like mine).

    At the low end, a chinese 4-axis offline controller wired to some mechanicals gets you a machine.
    Using better mechanicals and a stiffer frame gets you a better machine than most here. Very basic.
    Next come the 5 serious problems I outlined.

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    Re: Student organization looking to build CNC milling machine for school donation

    Quote Originally Posted by Daedalus308 View Post
    Just like my last reply, I'll try to respond to your points one at a time

    1) Do you have any idea what models or years that was produced? I would love to be able to see some of those and research how the end users liked them.
    2)I'm not really sure what you mean here, I do intend to have the Z axis traveling on rails, one pair per side attached to a gantry. Perhaps we could further discuss this so I can better understand what you mean?
    3) could you expand on this? i'm not familiar with the concept of a shear structure
    4) This is intended to be a standalone machine with a 2ftx2ft working area (subject to change), I would like to achieve a positional repeatability of at least as accurate as 0.001 inches, so about 0.025mm repeatability for a machined part (given otherwise correct machine setup, proper tooling, speeds, feeds, etc.). In the end I would like to achieve stiffness equal to or greater than that of a Haas minimill (I don't know the numbers, but i do have personal experience to use this as a reference).

    Thanks again for the thoughtful responses! The information you are giving me is enlightening and it is greatly appreciated!
    Answers:
    1) That was 20 years ago and everyone would have moved on by now. Call Ted Hall I'm sure he has time for students or someone there can help. He has a successful business so the users must like them
    2)Please publish some images to ensure we are on the same page
    3) An "I" beam consists of flanges and a web. The web transfers shear between the flanges, the web is the shear structure. We make I beams to improve the structural efficiency of the material. In a twin gantry design as I think you are designing you are using two tubes as the flanges but you do not have a web. I think you expect that two widely spaced tubes are stiffer then one large central tube. This is incorrect thinking. If you do the correct math this will become apparent

    Mind experiment : You have two parallel beams well supported at the ends (your twin gantry). There is a stick in the middle that slips in between. You push on the lead beam, how much load gets transferred to the lagging beam via the stick? If the lead beam is very stiff not much, so the lagging beam is redundant. So in the twin beam approach you eventually have to have two beams the size of the correct beam. The twin gantry is actually evaluated in Bambergs thesis and rejected for being less stiff. I rest this issue here.

    4) Speak to HAAS and ask them the stiffness of their minimill. HAAS is a philanthropic organisation with great student interest. The stiffness number will scare you I think... Plus look up accuracy and repeatability they are different.

    I think you need to define what this machines real reason to exist is. A HAAS is a venerable machine to emulate but so far I can't see you getting there. A machine has to have a strong reason to exist otherwise it will rust in a corner, if in fact it gets built. You need to expand the vision. Companies and people are happy to help and money is not the issue, there is more money in the world now then there ever has been. For instance why do you want to cut tool steel? Make the machine out of carbon fibre? do you realise that by the time you are a good engineer, mills maybe dinosaurs? taken over by 3D printers? Companies like HAAS are looking for the next stuff not the stuff that was done 20 years ago. If you where tasked to take a mill to the moon to cut some moonium would you make it out of 2T of steel? What does your sponsor want? Think beyond your thinking... that opens doors

    I'm not invalidating your current trajectory I think there's an opportunity here bigger than you think...
    There are a million reasons to halt this project, you only need one really good reason to have it happen Peter

  12. #12
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    Re: Student organization looking to build CNC milling machine for school donation

    Heres a nice mill. Peter

    https://www.youtube.com/watch?v=iMd_8rqUlns
    Attached Thumbnails Attached Thumbnails mill.jpg  

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