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  1. #681
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    Default Re: Milli a new composite mill kit

    Hi - I changed the base geometry a bit and now the static stiffness is X 31 Y 45 and Z 67 N/um I'll leave it at that. Its clear that making the base thicker makes it stiffer but it also makes it heavy. The base is now 127kg and I don't want it to be any heavier. A few more holes to take some weight off and its done. Geometry will change for the drives, have to add gutters for the ballscrews etc. Get the Mini to the next level... may get a golden duck or a shiny gem? or maybe a wizard thingy...Peter



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    Default Re: Milli a new composite mill kit

    Morning all- Here's a drawing of Milli-6 Peter

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-milli-mini-jpg   Milli a new composite mill kit-milli-mini-side-jpg   Milli a new composite mill kit-milloi-mini-6-jpg  


  3. #683
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    Default Re: Milli a new composite mill kit

    Hi All - Bonus entry...
    Waiting for computer to solve a problem... I added some N23 motor drives to see what they look like. Milli will use 60 or 80mm servos. Had two hack away some of the gantry for the drive nut but when I brought it into FE it still is stiff at 28N/um so OK. I did some quick calcs to check motor sizes and the small servos will drive this thing quite fast so that's good. Probably use a 3mm pitch ballscrew so lots of grunt for chomping steel....Even though I have been aiming at 70GPa material if I use 40GPa CSA it will be OK at 16N/um I think... so onward and upward. Have to concentrate on YaG for a little while now... Peter

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-x-axis-mm7-jpg   Milli a new composite mill kit-mm7-motors-jpg  


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    Default Re: Milli a new composite mill kit

    Not to distract you Peter, but this thing is starting to look like a good candidate for a horizontal mill.

    If the spindle were a unitized motor - say one of those 4-pole BT30 12krpm high-torque things - being able to mount it on the bottom of the gantry horizontally would be amazing.

    You can ignore this... just late-night wild dreams.

    -Ralph

    P.S. - just mounted the column tonight, epoxy shim is curing right now. Getting closer to some post-modification data for you to chew on.



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    Default Re: Milli a new composite mill kit

    Afternoon Ralph - Yes I can see what your saying. If a horizontal spindle was used the long axis would have to be extended (as it would chew into the envelope severely) I think but that's no biggy or I make provision for bolting the columns on the bed in different positions. I think the config has many opportunities... Once YaG is built I want to make a large router. Perhaps use this scaled up to 4x8' size. News - I have been permanently loaned an 80W laser to play with. Its been stuffed in the back of a factory for some time and no-one was interested in buying it so the owner has "given" it away... I picked it up yesterday. Another thing to play with... Peter

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-laser-jpg  


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    Default Re: Milli a new composite mill kit

    Hi Ralph - I'm thinking of downsizing Milli-Mini to a micro so the base weight is not so big.... being my first mill I need to keep it simple! So will be ER16 power head and 20mm rails to see what size that brings... Peter



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    Default Re: Milli a new composite mill kit

    Quote Originally Posted by peteeng View Post
    Morning All - Have completed a whole new series of designs called the Mini. The Crux was that every time I changed the geometry I seemed to go backward with the X stiffness. I decided to scale down the machine till the base got to 100kg, a 2 person lift or I can block and tackle from the roof. This means a table size of 250x500mm. Then the X dropped to 14N/um which drives me mad. Plus the Y&Z get better at 50N/um... Then it struck me the columns are not wide enough! The penny dropped so the Mini then got "wings" I just copied and flipped the columns around and booleaned them together adjusted the base and now I get X 28 Y29 and Z69 N/um... very good. So make the columns square and keep adjusting until they work.... The current wing design is a bit complex to make. Plus smooth out the base and maybe make a bit thinner. Holy Grail Batman!! Peter
    Peter, I had arrived at essentially they same column shape.

    Seems very stiff. Not super easy to cast but not the hardest design either.



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    Default Re: Milli a new composite mill kit

    Hi All - I have attempted to shrink Milli. But the size of things is driven by the bearing package size. So even though I have gone to 20mm cars the size did not change much. So the ER16 looks silly on the arm. I made the table smaller but then it too does not scale. So I made it wider and shorter. Now its 300x350... But the base did not decrease at all!! Still 127kg. There is a space under the bridge that gets wasted if you don't make the table long... Interesting. Will get this into FE soon.. Peter

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-milli-micro-1-jpg   Milli a new composite mill kit-micro-er16-jpg  


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    Default Re: Milli a new composite mill kit

    Hi All - In the process of shrinking it seemed a good idea to make Milli shorter and longer. But it works out the dead space "behind" the columns drives a lot of the geometry and base weight. I radically changed the base and it fell over, then the bearing constraints fell over and now its a mess! Is that a hint? I'll try to heal the wounds to see if I can get the base weight down... Peter

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-milli-long-jpg  


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    Default Re: Milli a new composite mill kit

    Do the constrains break in CAD assembly or in FEA?

    Last edited by ardenum; 01-29-2021 at 08:06 AM.


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    Default Re: Milli a new composite mill kit

    Hi Ard - The constraints failed in the CAD assembly. As I drastically modified the geometry various relationships failed in subassemblies / assemblies. To go to FE I export the geometry from Alibre to simsolid via step or parasolid file. The contacts have to be reestablished and specified in FE. Peter



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    Default Re: Milli a new composite mill kit

    This shuffling of the x and y axis work areas is ultimately why I went with a fairly square work area of 460x560mm. Of course, mine isn't a benchtop machine. If you boost the x length then the gantry cross section needs to increase substantially to maintain the Y & Z stiffness, which increases the y-offset from column position and requires longer Y distance too in order not to waste space between the columns, as you have found. Boosting Y travel means it's facing sideways on the bench and takes up a lot of space. But I'm guessing that in the most likely end-use scenarios a machine >100kg is probably best on it's own floor stand, anyways

    nice find with the laser, by the way



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    Default Re: Milli a new composite mill kit

    G'Day Cat and others - Yes I have looked at a couple of aspect ratios and have come to a couple of conclusions. Milli's intent is a benchtop machine but she has always struggled with her weight and size. I think its a stand machine.... its over 300kg at the mo. This is due to the 20N/um stiffness target and how I restrain the model. Currently using the "post" if I hold the table or the bench the stiffness triples.

    So playing with the aspect ratio has lead me to 400x400x400mm travel. Original aim was 500x500x500mm, was just a line in the sand but 400^3 seems to work well. I realised with this config that one axis has to be the distance from the tool to the back of the base. Otherwise you are wasting space behind & under the bridge area. So I tried to minimise this distance by shrinking the carriage, gantry and columns in the Y dirn (X is gantry travel dirn, Y isorthogonal to gantry). Plus I tried to shrink the footprint, currently 700mm deep by 900mm wide so would fit on a stout bench...

    I built 3 "long" versions then went square. Surprisingly the static stiffness was acceptable even with a 600mm "long" table. I think the triangularish gantry/carriage design is quite stiff. I also learnt that the table was not stiff enough and its now 50mm aluminium so much betterer. It could be 40mm steel as well. (up to now its been 32mm).. so the last big hurdle is the base weight. Its 137kg at the moment and I want it to be <100kg. maybe two pieces, maybe sandwich? maybe beryllium... Once thats concluded I think its polishing time... Peter

    My surprise is how much stouter this is compared to other benchtop machines. Its more in line with small VMC's. So my target stiffness maybe too high or benchtop machines are too compliant... I don't know of a machine with an equivalent 120mm thick solid aluminium base 700x700mm !! The gantry and carriage size is driven by the bearing size. Would have to go to 15mm bearings to make it smaller and that does not sound good....

    Ralphs Makini base is 700x500x125mm deep cast iron but generally 20mm thick and its 102kg...

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-milli-1-jpg   Milli a new composite mill kit-milli-2-jpg   Milli a new composite mill kit-makini-1-jpg  


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    Default

    Yeah, how I started to see the design question is in terms of which axis has minimal travel (the cantilever). Router layouts work best with square x&y cause Z is the cantilever. Vertical mills work best with long x&z cause Y is the cantilever. The rising gantry design that you're working with doesn't really have much of a cantilever and thus may be the best layout for a true cube with even travels in all axes. So choosing gantry, column and spindle size will determine the minimum Y travel, then the other axes can be made to match and the whole thing scales in proportion from that ratio.



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    Default Re: Milli a new composite mill kit

    Afternoon All - Today I finished the second coat on my workshop floor!! Yeh now in a few days I can get serious with shelves and tools. Want to make a surface plate for these cast parts. I'm thinking of a urethane cast table. Can get PU in very hard and it does cure level from experience. Or I go to my stonemasons and get a slab of granite and lap it flat... Watch this space... So Milli-Long-4 has the motors roughed in and the gantry relieved to take the car bolts. This version scores X29 Y26 and Z38. I'm thinking of changing the target spec to 30N/um for all axis, seems reachable now. But the main issue at the mo is to reduce the base weight to a minimum... less then 100kg at least. Two pieces? Have to run some ideas using Milli-Long-4 as the reference model.... The long is now out of date as its square! 400x400x400 so maybe call it Milli-400. Peter

    floor won't be shiny for long

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-milli-long-4-cad-jpg   Milli a new composite mill kit-milli-long-4-x-jpg   Milli a new composite mill kit-milli-long-4-y-jpg   Milli a new composite mill kit-milli-long-4-z-jpg  

    Milli a new composite mill kit-rhs-shed-jpg  


  16. #696
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    Default Re: Milli a new composite mill kit

    I know this is premature, but you're gonna be selecting motors soon, I'm currently trying to figure out servo sizes using rexroth's indrasize software (https://www.boschrexroth.com/en/xc/p...ze/indrasize-2) It's pretty straightforward, if you know what you're putting in the brackets.

    Will you be using any software or just pen and paper to chose yours?



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    Default Re: Milli a new composite mill kit

    Hi Ard - I'm a pen and paper man. I've already done a prelim calc and a small servo will do it nicely. I have to check the Z axis as this is being lifted but a 3mm screw will be very powerful. Z will need a brake. A 3mm pitch with 2.39Nm will push 459kgf so that's good. As usual its the required acceleration thats the question mark. I'm working with 2m/s/s ie 0.2g probably too low... anyone have any ideas on a good accel for this type of machine? Peter I'll tidy my calcs and publish soon...

    Actually not premi as I have to figure out the motor real estate. Using 60 or 80mm motors makes difference. Looking for a good servo mount as well...Using the 25mm rails made them higher and this makes the 16mm ballscrews easier to "fit" Peter

    Attached Thumbnails Attached Thumbnails Milli a new composite mill kit-servo-jpg  


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    Default Re: Milli a new composite mill kit

    maxtenso on aliepress sells SYK mounts for bearings and motors at https://maxtenso.aliexpress.com/store/5080383

    This one looks particularly sturdy https://aliexpress.com/item/4000225807196.html

    for servos there's lichuan https://aliexpress.com/store/1907567 or wenling yuhai is popular in the east https://wlyuhai.en.alibaba.com/?spm=a2700



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    Default Re: Milli a new composite mill kit

    Hi peteeng,
    when I did the calculations for my new build mill I found that the rotational inertia of the ballscrew constitutes 80% of the inertia with the armature
    of the servo about 15% and the linear momentum only 5%.

    Each machine will be different but I'm guessing that your machine will behave similarly, that is dominated by the ballscrew.

    This is, in part, because I've used 32mm diameter ballscrews. The rotational inertia of a ballscrew is the fourth power of radius.
    The single biggest improvement in acceleration can be achieved by reducing the ballscrew diameter. Naturally there is a limit before
    the ballscrew becomes a source of flexibility.

    I have been experimenting with my servos, 750W, 2.4Nm rated. With rated torque only I can achieve 0.25g accelerations, if I were
    prepared to accept overload conditions I might get as high as 0.75g.

    As it turns out I think 0.25g will be adequate for me, it is still the fastest and highest accelerating machine I've ever had.

    Craig



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    Default Re: Milli a new composite mill kit

    Hi Craig - For Milli at 0.2g using 16mm the torque for the screws is 1/10 the torque for the linear motion. For the table only, need 0.3Nm. 42kf table with 50kg job. But I'll work with 0.5g for fun and see what happens when I tidy the calcs. Its the Z axis thats the hardest in this design, as the gantry has to move up and down (50kg plus) plus provide cutting forces... Peter



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