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    Default CNC bench mill design for feedback

    Hello,

    I've been doing some design lately on a bench mill (I call it the C-Mill) I'd like to build ...


    Some data :

    * Designed for flat material (Corian)
    * Barrel bolts assembly
    * 30x15x18 cm work envelope
    * 70x65x60 cm overall size
    Hardware is pretty standard
    * 80mm spindle
    * HGR20 Rails
    * Steppers NEMA23 TBD ...


    Here are some very early and quick renders for feedback :
    structure ? assembly ? material ? components ? steppers ?

    I'm considering open-sourcing the project.

    What do you think ?

    CNC bench mill design for feedback-draft-v86-ltf-jpg
    CNC bench mill design for feedback-draft-v86-left-jpg
    CNC bench mill design for feedback-draft-v86-front-jpg
    CNC bench mill design for feedback-draft-v86-top-jpg



    Thanks for your reading.
    PS: being my first post, if it needs to be moved elsewhere please do so.

    Similar Threads:
    Attached Thumbnails Attached Thumbnails CNC bench mill design for feedback-draft-v86-ltf-jpg   CNC bench mill design for feedback-draft-v86-left-jpg   CNC bench mill design for feedback-draft-v86-front-jpg   CNC bench mill design for feedback-draft-v86-top-jpg  



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    Default Re: CNC bench mill design for feedback

    So it's made from Corian?

    Corian can be brittle, particularly problematic with threaded holes near material edge.



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    Default Re: CNC bench mill design for feedback

    I wonder if a limiting factor will be the head itself and its attachment to relatively short rails?



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    Default Re: CNC bench mill design for feedback

    Hi guys, thanks for you replies,

    Quote Originally Posted by pippin88 View Post
    So it's made from Corian?

    Corian can be brittle, particularly problematic with threaded holes near material edge.
    Yep Corian or another solid surface material, the point is I don't have a way of producing the Corian parts myself, and there are a lot of Corian shops with big CNCs to order parts from.

    WRT brittleness, I accounted for that issue in the design, there are no (zero) "traction" mode fasteners, all barrel bolts and inserts are in "compression" (sandwich / clamp) through the material.

    Quote Originally Posted by hackish View Post
    I wonder if a limiting factor will be the head itself and its attachment to relatively short rails?
    For now the head is the least "stable" part (in terms of design). Currently the Z work range is 180mm (~7") and the rails account for that movement. The model being full parametric, extending that length would be a piece of cake

    There has been other comments and discussion (DYI-CNC group @FB) on the whole head design, some were suggesting a higher C frame with a downward plate holding the spindle (to prevent gravitational cantilever / nose bob issues) instead of the current more "classic" bench mill L layout.

    For now, my position is that C frame layout w/ plunger plate adds a lot of flex upon lateral strain (X/Y cuts) or would impose a stupidly overbuilt assembly to stay straight, adding much weight and complexity.

    I'd like to maintain the spindle nose as close as possible to a rigid horizontal plane, but I have some ideas about making the spindle support higher and doubling the spindle mount. This would make the head a complete square instead of the current triangle.

    What do you think ?

    Last edited by oxomoxo; 05-01-2018 at 03:42 AM.


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    Default Re: CNC bench mill design for feedback

    Hi there,

    I have finally had some time to push this project ahead ...
    Here are some updated pictures, I've notably modified the head and extended Z rails for increased rigidity.

    CNC bench mill design for feedback-draft-doublemount-v8-head-jpg

    And fitted the stepper mounts, balls crews etc.

    CNC bench mill design for feedback-draft-doublemount-v8-detail-jpg

    A general view :

    CNC bench mill design for feedback-draft-doublemount-v8-ltf-jpg
    CNC bench mill design for feedback-draft-doublemount-v8-right-jpg
    CNC bench mill design for feedback-draft-doublemount-v8-front-jpg

    Thanks for your comments and feedback ...

    Attached Thumbnails Attached Thumbnails CNC bench mill design for feedback-draft-doublemount-v8-head-jpg   CNC bench mill design for feedback-draft-doublemount-v8-detail-jpg   CNC bench mill design for feedback-draft-doublemount-v8-ltf-jpg   CNC bench mill design for feedback-draft-doublemount-v8-right-jpg  

    CNC bench mill design for feedback-draft-doublemount-v8-front-jpg  


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    Default Re: CNC bench mill design for feedback

    What are you hoping to be able to cut with this?
    Corian is not very stiff. Flextural modulus of 1.2 MPSI, Aluminum is 10. Better than most plastics anyway (and I have owned a router made of PVC thinner than the sections it looks like you're planning, 12mm I assume?)
    Do you have any spec on flatness? The material data sheet doesn't mention that. Also, what's the precision a countertop place holds? It may be good enough to line up a sink, but not be good enough for a mill.
    Any particular reason for the Y axis on the X rather than vice versa?



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by skrubol View Post
    Any particular reason for the Y axis on the X rather than vice versa?
    Actually, this is what I like the most in this design. This way the foot print of the machine is a real foot print, not like in some other bench mills. I also think this design is more rigid and less prone to vibrations this way. Is there any disadvantage you can see in his design of X/Y?

    I don't like the choice of material, would not build a machine out of Corian. It is suitable as waste board, table top or similar, but not a machine. Perhaps a 3D printer would look nice if made out of Corian, but for a CNC I would chose steel or aluminium.

    https://www.youtube.com/c/AdaptingCamera/videos
    https://adapting-camera.blogspot.com


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    Default Re: CNC bench mill design for feedback

    Hi @skrubol, lots of good questions :

    Quote Originally Posted by skrubol View Post
    What are you hoping to be able to cut with this?
    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.

    Quote Originally Posted by skrubol View Post
    it looks like you're planning, 12mm I assume ?
    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 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) ..

    Quote Originally Posted by skrubol View Post
    Do you have any spec on flatness? The material data sheet doesn't mention that.
    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 :

    Quote Originally Posted by skrubol View Post
    Also, what's the precision a countertop place holds? It may be good enough to line up a sink, but not be good enough for a mill.
    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.

    Anyhow, that's a good (crucial) point, to be considered with due diligence (including design time).

    Quote Originally Posted by skrubol View Post
    Any particular reason for the Y axis on the X rather than vice versa?
    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 ).

    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 ...

    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

    Last edited by oxomoxo; 05-17-2018 at 10:23 AM.


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    Default Re: CNC bench mill design for feedback

    Oxomoxo,

    As I said, I like your design, and especially the fact that you try to keep the moving weight down, this is definitely very good. That's why I also designed my CNC with moving table and not moving gantry, which is more popular and sexy looking, but less rigid.

    One thing I haven't seen touched here is the actual table. Which sort of fixtures are you planning to use? Or will you add a T-slot table on top? Also, depending on the materials you plan to work with, be aware that especially aluminium requires some serious hold down, so a heavy wise is needed. Those are normally also pretty high, so before you start you need to consider Z clearance. In any case, even if you like Corian, I'd reconsider the material, especially for the table. There is no point in using Corian for the table which you have to hide under a sheet of T-slot aluminium. Better to use that from the start and save you some extra height waste.

    https://www.youtube.com/c/AdaptingCamera/videos
    https://adapting-camera.blogspot.com


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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by A_Camera View Post
    Oxomoxo,

    As I said, I like your design, and especially the fact that you try to keep the moving weight down, this is definitely very good. That's why I also designed my CNC with moving table and not moving gantry, which is more popular and sexy looking, but less rigid.
    @A_Camera thanks for your comments, to reply on the material choice I'd say it's a mitigation of convenience (mainly access to tools and materials) and fit for the job, sure this design and material won't fit "any" job you could clamp to the table but it surely could prove quite capable in the end considering the thickness of the material.

    I'm not completely sure I have the all knowledge to run proper simulations but I'll try to, since they are available in fusion.

    Quote Originally Posted by A_Camera View Post
    One thing I haven't seen touched here is the actual table. [..] I'd reconsider the material, especially for the table. There is no point in using Corian for the table which you have to hide under a sheet of T-slot aluminium. Better to use that from the start and save you some extra height waste.
    And, you point this right ... workholding is pretty much unaddressed right now, the only thing I now for sure is Z travel is 180mm so it could accommodate a vise, and if I have a corian plate made it will be covered in threaded inserts.

    I've been looking at T-slot aluminum (machined, not extrusions) on alibay and small screwless vises, checking the Luiz Ally style vise ... lots of options.

    I'll probably also end up with a vacuum plate for flat material which could stay not too high ...

    But for proper clamping I'm open to any suggestion in the 100mm (width) range, low profile, allowing me to keep both my kidneys ... I'm also checking the local second hand market.

    Here is a render along with a familiar object (soda can) for scale :

    CNC bench mill design for feedback-draft-doublemount-v33-scale-jpg

    Attached Thumbnails Attached Thumbnails CNC bench mill design for feedback-draft-doublemount-v33-scale-jpg  


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    Default Re: CNC bench mill design for feedback

    The Y over X is interesting, probably makes sense here given how wide the column flares. Typical mills have narrow columns, so the most efficient use of material is probably to have a narrow base.
    Only reason I assumed 12mm is it's the largest thickness I've seen Corian in. (I've only seen 6 and 12mm.) Are you planning to have the machining done after the material is glued up?
    Just a warning, I'm not a Mech E, but if I were going to do FEA, the basic simulations I would run run are to ground a part to the table, apply your load to the end of the spindle (or vise versa,) and simulate with a load in the X, Y and Z direction (3 different simulations.) There are many more to run, but those will give you a good start on where your weaknesses are. If it's not going to be rigidly mounted down to something, the base should be left unconstrained I think.



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by skrubol View Post
    The Y over X is interesting, probably makes sense here given how wide the column flares. Typical mills have narrow columns, so the most efficient use of material is probably to have a narrow base.
    For now, material usage efficiency is ... not assessed, and that could be worth the effort because Corian is not cheap, and it's probably optimisable, I'll be able to see where after I run some simulations, there's a whole branch of the software that does exactly that.

    Quote Originally Posted by skrubol View Post
    Only reason I assumed 12mm is it's the largest thickness I've seen Corian in. (I've only seen 6 and 12mm.) Are you planning to have the machining done after the material is glued up?
    Corian makes a 19mm but it's only available in (off) white (thus the white renders, to get used to it).
    And yes, laminate first then cut. Having to press/glue precision parts multiple times after cut sounds like a lot of parameters to handle and possible fails.

    Quote Originally Posted by skrubol View Post
    Just a warning, I'm not a Mech E, but if I were going to do FEA, the basic simulations I would run run are to ground a part to the table, apply your load to the end of the spindle (or vise versa,) and simulate with a load in the X, Y and Z direction (3 different simulations.) There are many more to run, but those will give you a good start on where your weaknesses are. If it's not going to be rigidly mounted down to something, the base should be left unconstrained I think.
    I take note, thanks for the tip.

    I guess I'll need to know how much toque the steppers can actually deliver and how they do add up in typical usage.
    I also need to test the fastening method (dowel nuts) to rupture point and I'll try to virtually break the column if that's possible .. to see where that goes ... could be fun :P



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by oxomoxo View Post
    For now, material usage efficiency is ... not assessed, and that could be worth the effort because Corian is not cheap, and it's probably optimisable, I'll be able to see where after I run some simulations, there's a whole branch of the software that does exactly that.



    Corian makes a 19mm but it's only available in (off) white (thus the white renders, to get used to it).
    And yes, laminate first then cut. Having to press/glue precision parts multiple times after cut sounds like a lot of parameters to handle and possible fails.



    I take note, thanks for the tip.

    I guess I'll need to know how much toque the steppers can actually deliver and how they do add up in typical usage.
    I also need to test the fastening method (dowel nuts) to rupture point and I'll try to virtually break the column if that's possible .. to see where that goes ... could be fun :P
    Not sure about your table size but you can get nice steel t-slotted tables quite cheeply:

    https://rover.ebay.com/rover/0/0/0?m...2F370753429171

    C6 Milling Table 240x110mm - Arc Euro Trade

    X3-160 Milling Table - Arc Euro Trade

    https://rover.ebay.com/rover/0/0/0?m...2F273088730958



    Sent from my XT1562 using Tapatalk



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by oxomoxo View Post
    I guess I'll need to know how much toque the steppers can actually deliver and how they do add up in typical usage.
    I also need to test the fastening method (dowel nuts) to rupture point and I'll try to virtually break the column if that's possible .. to see where that goes ... could be fun :P
    Basic simulations will just show linear deflection, so it doesn't matter all that much how much force you're applying, if you double it, the deflection will double. It's more useful in the design phase for seeing where your highest stresses are and reinforcing those areas. For a feasibility study you would need to know pretty close how much force you will have.
    Simulating failures are more complicated simulations I believe, as they are in the nonlinear realm.

    Edit: Also it's really hard to figure out machining linear force values. You'd think there would be a simple formula to estimate (with spindle torque, tool size, radial engagement as inputs,) but I've never been able to find one.



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by jkkmobile View Post
    Not sure about your table size but you can get nice steel t-slotted tables quite cheeply
    Thanks for the links @jkkmobile the last two are quite in the range (somewhat too long though)

    The aluminum t-slot table I mentioned earlier is this one
    https://www.ebay.com/itm/T-Slot-plat...m/183221484635
    Which would be the exact size of the XY travels

    Quote Originally Posted by skrubol View Post
    Simulating failures are more complicated simulations I believe, as they are in the nonlinear realm.
    I guess I spent too much time around game engines where physics and simulation basically means "ability to bump around and shatter stuff" haha ... :P

    Quote Originally Posted by skrubol View Post
    Edit: Also it's really hard to figure out machining linear force values. You'd think there would be a simple formula to estimate (with spindle torque, tool size, radial engagement as inputs,) but I've never been able to find one.
    This one I stumbled upon ... but I still have to take a dive and skim out what I need (not much on cutting tools, more structural stuff).
    http://160592857366.free.fr/joe/eboo...am%20Tolly.pdf

    Still no cutting tools on this one but more diversity of problems :
    http://www.dbc.wroc.pl/Content/7154/...e%20Design.pdf

    I'm not sure but maybe considering the cutting tool as a simple rod in a compound action (like a spiral plunge cut) could give a close enough idea ? I'm not saying I'd be able to derive an actual formula but maybe setting up a simulation scenario ... I apparently need to dig this further ..



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by oxomoxo View Post
    Which would be the exact size of the XY travels
    You usually want the table a little bit longer in at least one axis (usually the long axis) to give more fixturing flexibility when working on a piece at the limits of your machine (a place to fit clamps or whatever.)



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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by skrubol View Post
    You usually want the table a little bit longer in at least one axis (usually the long axis) to give more fixturing flexibility when working on a piece at the limits of your machine (a place to fit clamps or whatever.)
    Yes, especially since I'm considering designing a fourth axis (A), I'd add at least a couple of inches on the sides ... but the spindle part of the fourth axis design is kind of touchy to me, it's a pity there is no standard part aimed at housing bearings for quick and simple spindle assemblies (something like the LM series linear bearings housing but with a tight fit), that would ease up so many projects (I can think of a few reasons why though) ...

    Last edited by oxomoxo; 05-18-2018 at 03:15 AM.


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    Default Re: CNC bench mill design for feedback

    Littlemachineshop sells parts for mini (7x) lathes including headstocks.
    https://littlemachineshop.com/produc...ory=1023914534



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    Default Re: CNC bench mill design for feedback

    Corian is very brittle, and can easily crack if the forces are in the right place.

    It also expands and contracts a LOT with temperature changes.

    Gerry

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    Default Re: CNC bench mill design for feedback

    Quote Originally Posted by ger21 View Post
    Corian is very brittle, and can easily crack if the forces are in the right place.
    It also expands and contracts a LOT with temperature changes.
    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.

    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 :
    Quote Originally Posted by oxomoxo View Post
    Essentially it's an overbuilt yet not too big bench mill for a home shop intended at small mechanical parts, plastics, and brass / aluminum involved in animatronics projects.
    The thickness of all ''flat'' frame components is about 40mm (to be exact : 38=2x19mm press laminated).
    With a 70x65x60 cm overall size it's not a toy, Fusion tells me the weight of all Corian parts will be around 90Kg ..
    But still, I want to run some simulation to confirm that everything is right as my intuition says, especially at dowel nuts assembly points.

    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 ?

    Last edited by oxomoxo; 05-19-2018 at 08:43 AM.


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CNC bench mill design for feedback

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