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Thread: CNC bench mill design for feedback

  1. #21
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    Default Re: CNC bench mill design for feedback

    You have a most interesting design here. I don't get to this side of the forum much so it is good to see a fresh design.


    Quote Originally Posted by oxomoxo View Post
    Hi @skrubol, lots of good questions :



    Yep, that's the central question !!

    Essentially it's an overbuilt (see below) yet not too big bench mill for a home shop intended at small mechanical parts, plastics, and brass / aluminum involved in animatronics projects.
    This is the unfortunate part as I think your design will be crippled by the choice of materials to build the machine. Imagine the same machine with Aluminum as the base material. You mentioned that there are plenty of CNC shops to do this in Corian, but most of those shops could do the work in Aluminum if they really wanted to. However you might be surprised how many machine shop out there would be willing to do your design in aluminum. The only difference being that they would a bit harder to find for somebody not tuned into the locals machining industry. A machine shop would also likely start with correctly sized pieces instead of gluing up flat stock for the thicker sections of your design.


    You assume quite smaller than it is :P .. The thickness of all ''flat'' frame components is about 40mm (to be exact : 38=2x19mm press laminated).

    I know the pictures make scale hard to judge for lack of reference, and the rendered aspect of the material gives it a toyish look but with a 70x65x60 cm overall size it's not a toy, Fusion tells me the weight of all Corian parts will be around 90Kg ..
    I wouldn't call it a toy no matter what is made of. I actually like the mechanical layout of the parts. Personally I'm really bothered by the Corian as the structural material. Just imagine what you would get if the machine was made out of aluminum. Or take it a step further and make it out of steel. If you go aluminum I really doubt that the machining time and cost would be excessively different.
    I sure won't be hogging steel at crazy feeds but I believe with some care I could do some small steel parts too (if needed) ..
    I've learned to be careful about what people mean by machining steel but I really doubt that you would get anywhere near desirable results in a timely manner. It may even have trouble with some of the more difficult aluminum and brass alloys. It really comes down to how badly the machine vibrates under load in these materials. I have no experience for a definitive answer but I'd strongly suggest that it would have a difficult time.


    That's a question I have too, I'm in relation with the material manufacturer to clear out the feasibility and a lot of technical questions, I will keep you updated.

    But, please consider that the chain of contacts are all on machined surfaces so, unless I miss something, that brings us to your next question :



    The way the mill is designed does not require a massive dimensional precision : that may influence in size or give some slight offsets to the build envelope but as long as X/Y squareness and Z parallelism (which are way more common than dimensional precision) are tight on the machine building the machine, the geometry should stay unharmed.
    Good mechanical design won't make up for weak material. If your goal was wood or plastics I wouln't be too concerned. The fact is many routers/mills are in fact made out of wood and do just fine for what their owners intended. In this case you want to build a machine out of a material that is structurally similar to wood and the expect it to machine materials beyond aluminum. It might meet your expectations or it might not, I can't say because I don't know what your expectations are.
    Anyhow, that's a good (crucial) point, to be considered with due diligence (including design time).



    The Y axis being about half the size of X axis, it substantially reduces the weight of moving parts and the global intent is to make the forces go down to the bench thus the overall "triangle/pyramid/cathedral" shape (in that one having both my parent be architects must have played a role :P ).
    Well we have your parents to thank for fresh design then!!!!!!
    Also one reason is benches are usually longer than deep, so the footprint is in relation with that to keep the machine facing the user.

    Again, this project is aimed at solving the "I want a serious yet affordable CNC mill but I don't have a Bridgeport to build it" type of situation, which is my case but I believe is quite common. So I try to leverage what's around and countertop shops outnumber the toomakers shops by far ...
    Maybe locally the counter top shops are large in number but I suspect that around here there is a 3 to 1 or better advantage for machine shops. By the way it helps to widen your search beyond toomakers shops. There are all sorts of metal working enterprises that can offer up services that might be employed by a tool builder. Not to advertise but one example locally is Nifty Bar, a business that specializes in processing steel plate. The point is you can literally order many parts needed for a machine build, example: http://www.niftybar.com/files/2314/8...000_1_2017.pdf. That material may be of no interest to this specific build but the point is a lot of stuff can be purchased these days. I know some of this stuff looks "expensive" but a sheet of Corian is not cheap either and that is before machining.

    As for not having "a Bridgeport to build it" that is actually a common problem so people are always looking for new ways to do things. As I've said the only real issue I see is the material choice. Have you considered fiberglass sheet goods, wood composite structures, aluminum extrusions or even bolted together steel? Aluminum sheet goods such as MIC6 can be had at prices that aren't that much different than Corian.
    Also, I try to think upfront the best I can but that's still a WIP, haven't built it yet and all questions / concerns / feedback are welcome for that reason
    Well lets look at it this way you can always build it as part of a learning process. Since there are few if any metal working machines built this way in the end you really need a prototype to see how it will work.

    I do have one other concern though, the actual table needs to have parts bolted or clamped to it. Corian would be terrible in such an application. Why not spend some money here on a tee slotted table which can be had online. IF those are too rich for you tooling plate can be had and is fairly accurate raw. I just see a corian table getting very ugly very quickly due the stress of threaded holes, clamps, swarf and what have you.



  2. #22
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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by oxomoxo View Post
    The very purpose of engineering being to determine fitness of material/process to the purpose, "very" and "a LOT" doesn't carry much signification : with the right forces in the right place one could tear down the building the mill will be into.
    How about too much?
    However, thanks for your concern, both questions (material strength and thermal stability) are to be considered, and I'm discussing them with Corian's technical staff atm.

    WRT strength, the purpose (and expectations) of the machine is crucial :

    But still, I want to run some simulation to confirm that everything is right as my intuition says, especially at dowel nuts assembly points.
    One thing to consider is unintentional damage that is a machine crash.
    There are multiple things to consider when it comes to thermal expansion : dimensional precision and geometry (squareness/parallelism)
    Here is some meaningful data I have :

    * the mill will be operated in a somewhat thermally stable environment (home shop)
    * the thermal expansion factor of Corian is 39*10^-6/L(mm)/°C which builds up to 0.025mm/°C on the longest dimension (Z Axis) or 0.00156mm/°C across the thickness of the material.
    * the mass of the mill (Corian parts weights about 90kg) "should" give ''a lot'' of (sorry no data available on conductivity right now) thermal inertia to flatten out transients if any.
    * the topology of the mill (open frame) means dimensional precision is essentially a mater of relative movement/positioning of spindle and table. No ATC so tool origin will have to be checked at change and tends to correct error/variation along the machining.
    * to hinder geometry the mill would have to be heated in such a way that there would be large temperature gradients across the frame (non uniform expansion) and for that purpose possible heat points (motors) are thermally insulated from the frame besides being mechanically insulated (plum couplers).

    That's the state of my research so far ..
    What do you think ?
    One gotcha with thermal issues is that it can be very localized especially if the material doesn't conduct heat well. The end result is expansion in one place resisted by cool parts else where. So you get twisting and binding as thing move out of alignment. This can by the way be a problem on any type of machine built in any manner. If you get enough expansion in one part of the frame you will either have your linear rails bind or the expansion will cause them to attempt to rock on the corian mounting surfaces. So yeah thermal isolation of hot parts would be a requirement. My general experience here has been on high precision lathes that never saw a piece of Corian, however that doens't mean that it wasn't a problem.

    In any event it will be interesting to see this machine in operation. Best of luck with it!



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    Default Re: CNC bench mill design for feedback

    Thanks for your appreciation of the design, so far people seem to dig the frame structure and thats a pretty encouraging point.

    Corian however seems to hit a general resistance as a first reaction, depite the "massively overbuilt for the job" factor ... My take at this is that "it's a metal's metal's metal's world" we're in here, so I guess I'll have to insist on the aims of the machine for each new commenter until I actually build it and publish some results, oh well ...

    Quote Originally Posted by wizard View Post
    If your goal was wood or plastics I wouln't be too concerned.
    I've said a couple of times that plastics would be the most part of the jobs, but still we've seen lots of aluminum machined on aluminum extrusions based routers.

    How would you compare this design to say, an OpenBuilds C-beam based fixed gantry router that's proven to be machining aluminum ? Remove the motors and you're left with a 4Kg extruded frame that can still take some aluminum with a wood router as a spindle ...

    My perception, and intent with the design is that it will be stiffer (rigidity) and heavier (dampening) and that for precision machining it will yield much better results.

    Quote Originally Posted by wizard View Post
    I've learned to be careful about what people mean by machining steel but I really doubt that you would get anywhere near desirable results in a timely manner
    Thats a matter of the usual trade-offs : capacity vs time ... by "care" I meant very conservative CFM (steel ? yeah, why not, but definitely small and slooow). I've machined small steel parts with tools so small you wouldn't believe. This machine is not a high speed machining center by any mean and I don't run a job shop, I would qualify it a "serious makers' tool", would that make sense ? And yes the range of operations will have to be established for the different materials I'll be working with to obtain desired quality.

    Quote Originally Posted by wizard View Post
    Good mechanical design won't make up for weak material
    Again, expectation / purpose & fitness .. Good mechanical design won't make a material stronger for sure, but can make a tool usable within a certain range of operations, and help defining said allowable range(s) for a construction / material, isn't that engineering ?

    Quote Originally Posted by wizard View Post
    Imagine the same machine with Aluminum as the base material
    Done that I may build a smaller/tabletop version at some point. If the project catches up and gets some traction ...

    Anyhow, I'll open-source the whole project as soon as I stabilize the desing. The CAD files are parametric and almost all the design is based on the thickness of the material so if you get inspired by another material solution you could change that quite easily and build yourself what you want without waiting for me

    Last edited by oxomoxo; 05-22-2018 at 09:26 AM.


  4. #24
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    Quote Originally Posted by oxomoxo View Post
    Thanks for your appreciation of the design, so far people seem to dig the frame structure and thats a pretty encouraging point.
    It is always good to see somebody take a different approach that on the surface looks to be acceptable. I see a lot of good in this design especially from the standpoint of a DIY trying to build a shop up.
    Corian however seems to hit a general resistance as a first reaction, depite the "massively overbuilt for the job" factor ... My take at this is that "it's a metal's metal's metal's world" we're in here, so I guess I'll have to insist on the aims of the machine for each new commenter until I actually build it and publish some results, oh well ...
    We use a significant amount of plastics of various types at work and this is likely where my reluctance comes from. In this case I'm working automation within the medical devices industry. The various plastics certainly have their good points and bad points. I just have a hard time imagining them as structural components.

    Im however open minded and would live to see the results of this build. I do wonder though if you have considered other commonly available plastic sheet goods such as lexan, PVC and so forth. Does Corian have a structural property that sets it apart from these materials that caught your eye?


    I've said a couple of times that plastics would be the most part of the jobs, but still we've seen lots of aluminum machined on aluminum extrusions based routers.

    How would you compare this design to say, an OpenBuilds C-beam based fixed gantry router that's proven to be machining aluminum ?
    .
    Actually there is an interesting thread over in the DIY router forum that hit upon issues people have had with C beams. Here is the problem lots of things get "proven" every day. Those proofs don't always translate into answers the mainstream can accept. In the same regard we often see proofs of the most pathetic routers machining steel. Unfortunately the results are not acceptable machining to most. B
    Remove the motors and you're left with a 4Kg extruded frame that can still take some aluminum with a wood router as a spindle ...

    My perception, and intent with the design is that it will be stiffer (rigidity) and heavier (dampening) and that for precision machining it will yield much better results.
    It may very well do that. Im just really nervous about the Corian.
    Thats a matter of the usual trade-offs : capacity vs time ... by "care" I meant very conservative CFM (steel ? yeah, why not, but definitely small and slooow). I've machined small steel parts with tools so small you wouldn't believe. This machine is not a high speed machining center by any mean and I don't run a job shop, I would qualify it a "serious makers' tool", would that make sense ? And yes the range of operations will have to be established for the different materials I'll be working with to obtain desired quality.



    Again, expectation / purpose & fitness .. Good mechanical design won't make a material stronger for sure, but can make a tool usable within a certain range of operations, and help defining said allowable range(s) for a construction / material, isn't that engineering ?
    Sure it is engineering. Over on the router forums one guy is working on a carbon fiber gantry. I can understand from the engineering standpoint that it might be stiffer. Will it work well? Will the cost be manageable? Is the time to completion reasonable? I don't know the answers to those questions.


    Done that I may build a smaller/tabletop version at some point. If the project catches up and gets some traction ...

    Anyhow, I'll open-source the whole project as soon as I stabilize the desing. The CAD files are parametric and almost all the design is based on the thickness of the material so if you get inspired by another material solution you could change that quite easily and build yourself what you want without waiting for me
    That would be very generous on your part! Sadly i don't have the money for this idea but it would be nice to see this design built out of a number of materials to see how they perform and where the costs go. Even the thought of plywood has flown through my mind.



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by wizard View Post
    It is always good to see somebody take a different approach that on the surface looks to be acceptable. I see a lot of good in this design especially from the standpoint of a DIY trying to build a shop up.
    The transition from no machine tool to decent machine tool is often quite a step ... and you point this right as it is one of the aim of the project, if I can help some folks making that step it would be great.

    Quote Originally Posted by wizard View Post
    We use a significant amount of plastics of various types at work [...] The various plastics certainly have their good points and bad points. I just have a hard time imagining them as structural components.
    Yep, that's exactly what I was saying : the culture is set in such a way that plastics should "stay" on the receiving end ..

    Quote Originally Posted by wizard View Post
    I do wonder though if you have considered other commonly available plastic sheet goods such as lexan, PVC and so forth. Does Corian have a structural property that sets it apart from these materials that caught your eye?
    Corian is based on PMMA (acrylic, which is harder, stiffer and tougher than most common plastics) and rock powder (alumine tri-hidrate) for mass ..


    CNC bench mill design for feedback-plasticproperties-jpg

    As I see it, so is Corian to other plastics as cast iron is to other metals ...

    Quote Originally Posted by wizard View Post
    I'm just really nervous about the Corian.
    This is sounding silly :°)
    In watchmaking a lot of machine tools for working hardened steel pivots are (were) made of brass ...

    Quote Originally Posted by wizard View Post
    Sure it is engineering. Over on the router forums one guy is working on a carbon fiber gantry. I can understand from the engineering standpoint that it might be stiffer. Will it work well? Will the cost be manageable? Is the time to completion reasonable? I don't know the answers to those questions.
    The question of cost still has to be cleared and yes this is also engineering ..

    Quote Originally Posted by wizard View Post
    Even the thought of plywood has flown through my mind.
    Haha! Literally anything but ... let me guess : plastics ?

    Thanks again for your support and challenging questions.

    [EDIT:If anyone knows how to get rid of this squashed formatting and how to prevent it in the first place, let me know]

    Attached Thumbnails Attached Thumbnails CNC bench mill design for feedback-plasticproperties-jpg  
    Last edited by oxomoxo; 05-23-2018 at 07:11 AM. Reason: Post is squashed :/


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CNC bench mill design for feedback

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