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  1. #1

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    Lightbulb WillMill - HSM Benchtop Composite Mill

    Goals


    • Mill aluminium moulds
    • Good surface finish in aluminium
    • Have a very damp machine
    • Get experience with casting composites
    • Have fun!



    Requirements


    • Max deflection: 70N/?m
    • Positional accuracy over 100mm: 0.05 mm
    • Part flatness over 200mm: 0.03 mm
    • Vibration dampening properties
    • Must contain dust and chips
    • Benchtop machine



    Specifications


    • Gantry clearance: 120mm
    • Part size: 300x600mm
    • 1.5 kW HF spindle



    Features


    • Square linear rails and ball screws
    • Epoxy granite base
    • T-slot table



    I have not specified a part accuracy, but I'm fairly certain that I can get the positional accuracy to be better than 0.05 mm. The goal with the tolerances is to be able to machine somewhat of a press fit in aluminium, and I'm fairly confident I can achieve that by using precision milled stock (0.01 mm flatness) to mount the linear rails onto. I think the flatness of the t-slot table and aluminium stock to mount the linear rails on, will be a little better than specified by the manufacturer, to give them some room for error. Therefore the machine will probably be a little more precise and increase my chance of milling "somewhat" of a press fit. The base of the machine will either be a thin t-slot table cast into epoxy granite, or just a solid piece of cast iron t-slot plate. The t-slot table will also have a flatness of 0.01 mm. The linear rails for the gantry will simply be mounted directly to the t-slot table.

    I've specified a maximum stiffness of 70 N/?m for the whole machine, this means that the machine should not be able to deflect more than 0,001 mm from where it previously was, every time we apply 70 N.

    Now there's still a lot for me to figure out, but for now I'll start designing the gantry for the machine. I'm still not sure weather or not I will use epoxy granite for the gantry or if it will be made of a composite consisting of fibers. - It depends on what I can get my hands on for cheap/free.

    Any
    questions and input are always welcome

  2. #2
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - Your gantry clearance will have to be much bigger for a mill.. unless you are only doing plate work. but you state moulds so how tall will they be... Plus 70N/um is very stiff, look at Bamfords work that was his design target and look at the size of his machine to do the job...Peter

  3. #3

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    Re: WillMill - HSM Benchtop Composite Mill

    You're right, it's very stiff. I'll aim for 70N/um at the moment and then perhaps adjust that spec a bit if it proves to difficult. For what I'm intending to use the machine for, 120 mm clearance is almost the minimum required (tool length considered) for what I intend to do with the machine. I'm a little concerned with how stiff I'll be able to make the z-axis when it's fully extended, so that's mainly the limiting factor of the gantry height. When I get further into the design I'll be able to see just how much deflection it will have and decide weather or not it would make sense to increase the gantry height. I intend to have two vices to hold metal pieces on and if stiffness becomes a problem I could simply put the vices onto some kind of riser block. This would increase rigidity as the gantry would be retracted further, but we will discuss options like that if it becomes relevant.

    Yesterday I was doing some simulations on the gantry. I've continued with the design today and have currently ended up with a gantry that has a deflection of 2 to 5 microns (depending on where the forces come from) with a force of 1400N.
    1400N should the force required to cut steel on a small scale.
    I have simulated the gantry with a composite that has an estimated modulus of elasticity of 50 GPA and a density of 2 g/cm^3. I've force is applied to the front faces of "abstract" 20x20mm square rails 90 below the lowest point on the gantry, and the whole gantry body weighs in at just 13 kg. At the moment I'm not sure how stiff my composite will be, as I haven't made it yet. My plan is to mix carbon fiber cutoffs together with washed aluminium swarf from a machine shop. Aluminium has a young's modulus of 70 GPa and steel has a young's modulus of 200 GPa. Look up "modulus of elasticity" or "Young's modulus" on google, if you'd like to learn about "bending stiffness" as a material property.

    For everyone reading along, Peter has helped me a lot already, and I'd like to mention that a lot of design workflow and inspiration has come from reading Peteeng's thread on one of his http://machine builds, there is lot's of great advice and information to read there. You can see some of his router kits and read about some exotic and machinable composite materials that he has made here: https://cncrouterkits.com.au/tetrium/.

    Now regarding the gantry... I think it's lacking a bit of torsional stiffness, so I'll get working on that.

    - William

  4. #4
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    Re: WillMill - HSM Benchtop Composite Mill

    Good morning William - The scope is coming along. BUT its time to go back to the beginning. At first I imagined you wanted to build a moving gantry machine. They are also called bridge and double column machines. The machine exists to move the tool around so there are two fundamental things to discuss. A) The tool holder and spindle B) the machine foundation (or structure). Now you are clear its a mill we understand it needs to be very stiff. Your 70N/um is a clear indication of that.

    A) The tool holder and spindle - what do you intend here? A R8, Morse taper, ER collet, BT-20 or 30 etc etc..... This is the pointy end of the machine so its really important to get it settled.

    Now ideally a machine is a pyramid with broad foundations and the tool at the top. Pyramids are statically and dynamically stable. Skyscrapers however are unstable and a gantry machine is equivalent to a skyscaper. This has to do with its bearing arrangements. Its difficult to place the tool load within the kern (or the centre) of the bearings. If you designed a chair where the seat was outside the 4 legs when you sit in it it would fall over. This is exactly what a gantry machine could do if we did not use bearings that where captive to the machine. So you need to look at other machine configurations before you commit to the gantry style of machine.

    B) A moving gantry machine is inherently unstable. Ideally the tool point should be within the footprint of the bearings at all times. This makes it stable. With a 3 axis machine it is possible to have 2 of these within the foot print but not all three (I think). So have a look at mills in the table size you are thinking of and broaden your thoughts on the machine config. There are a couple I've seen and I can't find their image at the moment but you should have a think about this. A gantry machine is used to maximise the working envelope vs the footprint. If you do not have a foot print issue then you would not use a gantry config. Plus its for very wide tables. 400mm is not very wide.

    as an aside I just found a machine builder in Denmark - maybe useful for you https://cnc-nordic.dk/

    Enough for now. Peter

  5. #5
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    Re: WillMill - HSM Benchtop Composite Mill

    Some alternative configs and some structural muses - Peter
    Attached Thumbnails Attached Thumbnails combo mill.JPG   mill 1.JPG   big mill.JPG  

  6. #6
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - I have been mucking about with alternative geometry for gantries (or any element really) Especially for casting as making hollows can be tricky. So I made two models to compare. One is from 2mm thick steel with an internal isogrid. The other is a hollow 5mm thick with the same profile (100x150mm) I placed a 100N load at the same place and there deflections are nearly the same. But the isogrid weighs 15.7kg and the hollow weighs 20.8kg. Triangles work!! Peter

  7. #7

    Re: WillMill - HSM Benchtop Composite Mill

    Quote Originally Posted by Williamlii View Post

    Yesterday I was doing some simulations on the gantry. I've continued with the design today and have currently ended up with a gantry that has a deflection of 2 to 5 microns (depending on where the forces come from) with a force of 1400N.

    1400N should the force required to cut steel on a small scale.

    - William
    Wow. Where are you getting these numbers from (polite answers only please)? I'm trying to think of cutters of this size that could take cutting forces equivalent to the weight of one seriously fat bloke without breaking - or even deflecting noticeably, let alone anything like 2-5um. Then there's the machine itself....

    There is a danger you might end up blowing smoke up each other's ass if you aren't careful.

  8. #8
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - Now you have settled on a machine static stiffness you can apply 70N or 700N to the tool point and expect the machine to deflect 70N = 0.001mm or 700N = 0.01mm. Machines are designed to be stiff and very linear. The machine components will be very low stressed at these load levels so stress is not important at the moment. 140kgf is a very, very large tool load so would be unexpected. So maybe 700N is a good "design" load (vs actual tool load). I think you need more Z which will mean more deflection and is this a moving gantry design or a fixed gantry design? The bearings will have compliance and the base structure will have compliance so you need to start adding these to your model....

    1400N / 0.005mm is 280N/um which is a very stiff component. A 20mm square car has a stiffness of ~200N/um Regards Peter

  9. #9
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - If you are considering a mouldless composite construction then consider using G10 or FR4 laminate. They are made to high tolerances and have good mechanical properties. A good Eglass quadaxial laminate will have same properties as these if infused. I have made laminates from 1mm to 55mm thick with about the same properties. So you could have these machined although shops don't like machining it, mill it in your shop or have it water jet cut to rough or finish. Flat construction has drawbacks so making moulds will allow shapes and features that you can't get otherwise. Peter

    Perhaps a combination of G10 and laminated metal wil be a good compromise....

    https://www.professionalplastics.com/G10FR4SHEET

  10. #10

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    Re: WillMill - HSM Benchtop Composite Mill

    HI Peter - My space for the mill is very limited, so it has to be an moving gantry mill. I'd like it to be similar to the machines that Datron builds. Interesting pictures and constructions Peter, I'd also like to design a small scale 5 axis some day, but that would be far into the future... I have not settled on a spindle yet, but I will likely end up with a high speed spindle that has a common spindle diameter of 80 mm. I was initially thinking about buying a common 1.5 kW HF spindle with ER collets and then upgrade to a nicer spindle in the future. At the moment I'm not sure weather or not I should buy 1.5 kW, 2.2 kW or 3.0 kW, but I'll read up on how much power is required when it becomes relevant. If I end up needing 3 kW i'll just update the clamp from Ø80mm to Ø100mm. Interesting simulations Peter, I had never really though about that kind of triangle construction for a gantry, that's a great and simple idea!

    Peter the leverage of a moving gantry and the forces applied to the gantry's bearings on the base is something that I have though about a lot. I didn't know that 20 mm rail cars "only" had a stiffness of 200 N/um, from the beginning I've intended to use roller bearings for increased stiffness, but I might just have to choose bigger rails, that should solve the problem. I'll look into this when I know what the z-axis will look like. Great input. I've worked a bit on the gantry in the mean while...

    Yesterday I did something I thought was a little clever... I decided to redesign the gantry from the ground up using shape optimization in Fusion 360 (their generative design features are for premium users only). When you make a shape optimization simulation in fusion 360, it's optimizing the shape for 1 load, so I just combined results from multiple simulations and created 1 solid shape from that. Now the optimization workspace in F360 currently doesn't consider shapes for different kinds manufacturing, it just find's the critical load "inside" the body that's simulated, so it doesn't always create results that makes sense in the real world. See the linked picture. Now what is interesting is that I was able to make a gantry about 1 kg lighter than the first design and have about 20% less deflection. The body of the gantry currently weighs 14 kgs with a density of 2,00 g cm^3. Another thing that's really great about using shape optimization in the way I described, is that you get an end result that deflects equally when the load comes from the directions you simulated. This method is probably being used by machine designers everyday, but I though it was worth sharing anyways


    Hi Muzzer, welcome. I've done a lot of reading the last two weeks on machine design and all sorts of materials that be used to build machines... I'm by no means an engineer and I'm in deep water with this project, but I read in an article somewhere that chatter can be generated when the end mill deflects more than 0.02 - 0.04 mm (somewhere around there, can't remember exactly but it was almost the width of a human hair). Now I don't know much about how much force cutters can take at their given size, but there's lot of variables to consider. I saw a guy on YouTube compare old end mils with newer end mills that were made with better materials. All of the compared cutters was of course the same size. Some of them were roughing end mills and others had variable flutes. The strongest cutter could take something like 4 times the load compared to the weakest cutter and potentially more, so there's lot's of stuff to consider before before going bezerk with a long finishing end mill for a roughing operation. If the machine deflects while taking a cut, then I could imagine that the machine's body would move back and fourth as well, as the edges off the tool cut into the material, which could produce vibrations and chatter. Therefore I think a stiffer machine would allow to tool to be used more effectively. Any thoughts? Take a look at this video for example, in my head the cutter should break under these cutting loads:


    Regards William
    Attached Thumbnails Attached Thumbnails Gantry front.png   Shape optimization.jpg  

  11. #11
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - Choosing a HS spindle such as you are talking about is a poor decision for a mill. Those are router spindles not mill spindles and is inconsistent with your dialogue. If you are using Datron as a benchmark you will see that they are not using such a spindle. You need to research this area more. If space is limited and parts are 300x600 (so you need a table 400x700 minimum then consider a moving column design or a fixed 700mm high axis like a photo prior vs a gantry or bridge. This will be a stiffer approach. Plus synchronizing two columns seems easy but if you are to cut steel (is steel in there?) this is not a trivial consideration (small red flag being flown here) Driving two screws means that they will always be at least the screw tolerance delta from side to side, this introduces chatter and inaccuracies. Driving each axis with one screw is better for your concept letting the bearings do their work properly. Daltron will have very precise screws with parallel scales for displacement feedback. I have not seen them cut steel (has anyone seen this?), they are light cut flyers....Peter

  12. #12
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Willkiam - Datron publish some interesting white papers. I found info that they do cut steel. Light fast cuts, so maybe 80mm spindle OK. Anyone have experience in this application? Peter

    https://www.datron.com/project/cnc-c...n-white-paper/

  13. #13

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - That was a great little read. I've seen plenty of videos of people cutting steel with HF spindles, I found a guy that had build a very stiff fixed gantry frame and had excellent results:
    .

    Now regarding power it looks like a 2-2.5 kW spindle will be great for my machine, but I'll have to do a deep dive on HF spindles, when I have to decide which one to get.

    When I have designed more of the gantry I'll simulate the rails according to their actual stiffness and decide which size of linear rails will be fit for the job.
    Didn't know that Datron had displacement feedback, that sounds very clever and advanced. Your comment made me think about the "synchronization" problem with having two ball screws for the X-axis. I initially thought of wiring up two motors in a way that would allow me to use a single driver for the motors, but then I realized that this would't allow me to adjust how much the specific motor rotates according to it's given signals. If a tell two motors of the same model to rotate 360 degrees, does anyone know how much they can differ from the desired amount of rotation?
    It could be that motor 1 rotates, 360,01 degrees, and motor 2 rotates 362 degrees. Is this a real world problem, or just something that I should ignore? - I guess it depends on what motors you get your hands on, but does anyone know just how "precise" generic stepper motors are compared to an identical model. If both turns 370 degrees then they would be off by the same amount, and I could just use a single driver to account for that. What if I used servo motors instead? Any input is very welcome.

    I might just end up with a single ball screw under the machine base to move the gantry back and fourth... I'll think about this and make then decision and comparison when it's time to design the base of the machine.

  14. #14
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - The 20mm rail will be fine for your mill. Its the foundation that's the issue. It needs to be very stiff. Using single driver is possible and I have done it to test. Cheap small machines use a single driver BUT it may have mid range resonance which has been my experience. A driver receives feedback from the motor (back EMF or back voltage) this is used to change the waveform to improve smoothness. If two motors are used on one driver this signal can be complex and the driver does not work properly. Especially at high speed (more back voltage). So I recommend one motor one driver.

    Standard stepper motors are 5% accurate mechanically with 200 steps per rev. You can get 400step/rev motors as well (I use these on one of my machines) and you can get precision grades. A stepper moves pole to pole and this is an electrical pole so its a bit wobbly on position. So 360/200= 1.8degs. So if you have a 10mm lead and direct drive 10/200= 0.05mm per mechanical step and 0.05*0.05(%)= 0.0025mm so your motion accuracy +/-0.0025mm So the possible delta side to side is 0.005mm from the motors. This is within your stated tolerance so not a big deal at the moment. Its more of a dynamic movement issue (walking/racking one side will always lead the other and as it moves, this changes ( Look up stick slip behavior ) and a bearing arrangement issue.

    You do need to move on and flesh out the entire machine. A machine is complex and inter related. The first milestone in a design is to have a general arrangement that at face value meets the scope and has no obvious fatal errors. You are not getting close to that yet. Peter

    Please note the the steel video is a fixed gantry and will be significantly stiffer then your current moving gantry design. Its a very good space frame design and its steel. It sounds very solid. Peter

  15. #15
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William -The space frame mill in the video is very interesting. It's a great solution to the machine stiffness problem vs whats called open loop structures ie large blocks of material and cantilevers. Here's a paper on closed loop machines. Also included is a nice small extrusion closed loop machine. Peter

  16. #16

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - Oh you meant that the base had to be stiff, and not the rails... Yes, the base will end up very stiff indeed, no worries. Thank you for your explanation of drivers and motors peter, that settles it. I'll have to use two drivers for two motors, even though they are synchronized.

    The I'll design the machine will not affect the shape of the gantry much depending on weather or not I use a single or two ball screws. But point taken, I agree that this should already be sorted out. It initially was, but you made a few good points that made me lean more towards using a single ball screw instead of two.

    Ouhh, interesting paper. I really like these types of machines, but there's also a lot I don't like about them too. The paper introduced to quite a lot of new constructions in parallel kinematics that I have not seen before. I do hope these machines will get accepted in the industry in the future, all though it doesn't seem like it.

    Regards William

  17. #17

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    Re: WillMill - HSM Benchtop Composite Mill

    Hello, I'm back with some design updates. I've been tinkering a bit around with some FEA and shape optimization in an attempt do design a rigid Z-axis/clamp. Again this has been designed for a material with a Yong's modulus of 50 GPa, and the results a rather impressive. For the simulations I have used a hollow tube to represent a 2.2 kW JGD-80 spindle (about 8 kg) , although I'm sure it will be a little stiffer than what I've made it in CAD. At the moment the Z-axis also functions as a Ø80mm clamp for the spindle, and it weighs in at just 7 kg. The idea with the current z-axis is that it's very damp as a composite, and I'm hoping it will do wonders compared to aluminium clamps. I'm not sure how well a composite material will grip the spindle when it functions as a clamp, therefore I could potentially cast a sleeve into the casting. I'll have to figure this out if this becomes the winning design. The screws for the clamp will of course have washers if this end up as the final design. Another very noteworthy thing to mention is that I've increased the initial z-axis travel so that it can travel 150 mm. The gantry will therefore also have to be higher than it currently is. The Z-axis clamp/plate is currently a bit tricky to cast, I'll have to sleep on it and figure out whether or not it would be worth it to go though the trouble. I do have a simple solution for casting it, but it does make the mould a little more complex.

    Other alternatives and materials. I have multiple designs that I think could work well. The simplest of them all would be to buy a stress relieved cast iron plate and mount two generic Ø80mm clamps for the spindle. A more exotic method would be to laminate metals together using epoxy and glass veil as Peter suggested as an option. This would also be a more damp compared to solid aluminium. Some articles mention that epoxy granite has up to 10 times better dampening properties than cast iron. My composite material will made be made of aluminium and carbon swarf and will hopefully have a similar dampness to that of epoxy granite.

    At the moment I really like the current design. It has superior damping properties compared to conventional materials, and I think it looks really cool and professional. I know pretty doesn't cut, but it makes me happy..

    I'll return with some alternative solutions and compare the results.
    - William
    Attached Thumbnails Attached Thumbnails Dimensions.jpg   Simulations.jpg   Combined shape optimization meshes.png  

  18. #18
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William,
    Found this today good read. Your 70N/um is way up there but as usual aim high....Peter

    https://books.google.com.au/books?id...N%2Fum&f=false

    you do need to include bearings : 1) you have to be able to assemble the assemblies 2) bearings take up real estate and their packaging size may drive other dims c) you need to think about tramming the spindle. How are you going to do that? These things get sorted at assemble level. Parts are always very stiff, assemblies are 30% to 50% efficient in stiffness at least.

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    Re: WillMill - HSM Benchtop Composite Mill

    Hello William - On the point of modelling. You are using a "design" material with E=50GPa this is possible from CF offcuts using infusion to consolidate the laminate. The process would be to model the part hollow to establish what thickness the skin needs to be to get to what deflection you need. Now F36o uses isotropic theory to do the modelling. Isotropic theory says that the shear modulus (G) will be about E/(2.6)= 19GPa. Unfortunately composites have a G~9GPa so the isotropic FE will under estimate the shear deflection by about half. So if there is a dialogue in F360 in which you can enter in the G separately use 9GPa.

    Aluminium swarf, from tests it is unlikely you will get a volume fraction better then 45% so E=70*0.45*efficiency. The efficiency of longish bits is about 0.6, Spherical bits is 0.5 and long fibres is 1.0. So your filler will be 70*0.45*0.6= 18.9GPa say 19GPa isotropic. This will bring down your filled volume stiffness a fair bit over the 50GPa you are using. So make a hollow part and see how its deflection changes vs the solid part. Determine if there is a thickness that does not influence the deflection much say 10% delta the solid deflection. That's the skin thickness you want to aim at.

    So casting would be to make the mould, lay in the CF at the design skin thickness, infuse, let cure. Next day fill with Al swarf infuse, next day release part done. Peter

    CF by the way E laminate = 200GPa*0.45* 1.0 = 90GPa if all fibres in one direction. If in 2 directions then E=90/2=45GPa if in random directions its a bit better then 45 so 50GPa is good.

  20. #20

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - Great numbers regarding machine stiffness. You're right 70N/um is perhaps a too high then. I'll see how strong I can make the Z-axis without adding absurd amounts of weight. Then when I get to simulate the whole gantry including the z-axis we can determine if we need to work on adding more stiffness (probably not). I've heard that FEA overestimates the stiffness sometimes, and if we then also include the "lost" stiffness by simply having joined parts, then the 70N/um would drop to something quite a bit lower in reality, right? I'll take a closer look at that book soon, seems like it's packed with great information that I can use right now.

    Oh yes, the Z-axis cannot be trammed as it is right now, I totally made a brain fart. In my head I could just tram the whole z-axis, but by combining the clamp into the same part I forgot that this removed the possibility tram the spindle separately. I'll have to revise the current design. I know that the bearings and actual thickness take up an important role, but since you point it out, I think I'll try and get a cad model from the manufacture. Then everyone can benefit of having that information available.

    Thank you for mentioning the thing about fusions calculations on the shear modulus. It's currently at 26 Gpa, I'll edit it to 9 GPa under "manage materials" - Thank you Peter

    Volume fraction. That's a great word. I've been a little worried about how much a vacuum back could compress aluminium swarf in a mould, but you introduced me some numbers that sounds realistic. It would indeed be a lot better to use milled aluminium strands. I've thought a bit about crumbling random aluminium swarf with some kind of simple and crude process to make a more compact and dense swarf.... If we assume that I were to make a "pure" aluminium composite, I was hoping that I could get swarf dense enough to have a volume fraction of 70%. Do you think this will be possible by using strands?

    Great idea, using woven carbon fibers for the essential skin thickness. Would you use the unidirectional type? It just makes sense, because it will give us the optimal stiffness if pointed in the right direction. Perhaps a mix of unidirectional and twill will work out better.

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