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  1. #21
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    Re: WillMill - HSM Benchtop Composite Mill

    Morning William - FEA on a part basis will be spot on with stiffness if it is constrained and loaded realistically. In simsolid I can model the friction in connections and the bolt loads so it will model assembly deflections accurately. In your case it will need to be a bonded connection? How does F360 deal with assemblies? If the connection is bonded I assume it to be 50% efficient a a first guess.

    Using close pack fibres that are well stacked (hexagonal close pack or square close pack) theoretically you can get 90% plus but in reality 60% Vf is the top figure. Random bits I expect to be 40% Vf. Hard to get to 50%. Using CF will be uneconomical for you unless you can get scrap. I'd use a random scrap stack. Need specialist software to design laminates and FE laminates I use Strand7.

    You need to get some aluminium swarf, place in a known volume container on a scale weigh the swarf then fill the swarf volume with water and weigh. You can then convert weight fractions to volume fractions and you will know the Vf of the system. Same with sand if you go the sand route.

    With glass and CF laminates under 1atm pressure usually works out Vf=50%. Autoclaves at 6bar can get to 60% Vf in laminates. Internal hydraulic pressure and friction within the laminate prevent squeezing it much further. It is possible to go further, I did read Boeing went a lot further but you do need resin around fibres and the autoclave technology becomes complex. Peter

    https://www.compositesworld.com/news...s-to-tennessee

    there are recyclers in europe as well

  2. #22
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - heres a link to common bulk densities of stuff. take sand its dry bulk density is 1650kg/m3 its crystalline density is 2200kg.m3 (silicon dioxide) weight = density x volum so volume = weight/density so 1650kg/2200= 0.75m3. so the volume fraction of sand is 0.75 which is really good. Using the water fill test you can determine this yourself. So if you used sand E=70GPa then the E for the epoxy mix will be 70 x 0.75 x 0.5 = 26GPa . which is about the published stiffness of many EG mixes. Some get to 35GPa but I think they measure this in compression which is the stiffer direction. Peter


    https://www.simetric.co.uk/si_materials.htm

  3. #23

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - Glad to hear that FEA will estimate the stiffness in real life, assuming that one would constrain it correctly. F360 is great with assemblies, but sometimes there can be trouble when simulating something like a hollow body filled with a different material. That's what I've been having trouble with anyways.

    Great idea with using water to find the potential volume fraction of the swarf used. I'll definitely experiment a little with that. Oh that's genius, I can estimate the stiffness by knowing Vf (volume fraction) and E (Young's modulus) of the materials! Thank you Peter

  4. #24
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - Then just model the hollow and the core is a bonus. Have you found a suitable scrap CF yet? Its time to nail down the material you will use otherwise your time in FE is wasted. Peter

  5. #25
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Will,
    Here's a box of offcuts from one of my engineering customers. I get a box every 2 weeks or so from them. It adds up over time. I'm sure there must be a composite fabricator nearby that can do same. Any "dry" carbon offcut ask them to put in a box for you. Peter

  6. #26
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William and others - I was watching a video of the Taiwan Machine show from 2015 and saw this, a CF gantry on a Hiwin machine. This particular gantry is a cantilever. I'm sure we shall see more of these around. Peter



    I do like air bearings and intend to make a machine with these soon.

  7. #27
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - another article for you, but settle the material!! . One papers machine is about 30N/um the other is about 150N/um measured on VMC sorry that papers too big to upload.... Peter

  8. #28

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - Things have been moving a bit slow here, lot's of things going on right now. Yes regarding materials, the plan is to use woven carbon fiber for the skin of the components, and then use carbon fiber swarf next to the skin to maximize the potential stiffness. Aluminium swarf will be used for the core as a filler. The amount of carbon fiber used will depend on how much I can get my hands on. I'll focus on getting my supply of materials sorted today.

    Update: The revised z-axis is finished. It's a composite clamp for the spindle that mount's to a cast iron plate with a bolted connection. In this way the head can be trammed with shim stock or other means of "adjustments". I have though of a second configuration for the z-axis. I will post some pictures and do a little comparison of the two configurations.

    I received my 4.8 CFM vacuum pump with the mail today. I miss being in the workshop so I have decided to make my own vacuum chamber as a little excuse to getting my hands dirty again. I'll just mount some vacuum hose fittings on a thick acrylic lid and put on some silicone sealing paste. Then vacuum lid can the go on just about every pot. I'll do the same for a resin trap. This also saves me a bit of money ;-)

    The Mill Stiffness PDF was a great read!

    Regards William

  9. #29
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    Re: WillMill - HSM Benchtop Composite Mill

    Fwiw..
    I have actually built a mill, really a VMC, for milling steel, somewhat efficiently.
    2.4 x 2 x 1.5 meters in size, 1600 mm table, 1200 mm movement, 32 mm ballscrews.

    When I stood on top of the spindle, 85 kgf, originally it deflected 0.28 mm.
    After 5 revisions and 1000 kg more in tool steel and 400 bolts, the deflection was around 0.12 mm (v3,v4) before the latest revisions.

    At the moment I needed to take off the front beam, since the auto toolchanger was too large, and I need to put a spacer in to accomodate it.
    Once the beam is back in place, I can measure current deflection.
    Anything around 0.05 mm will be great.

  10. #30

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Hanermo - That sounds like an beefy machine. 0.12 mm deflection is more than what I would have expected for a machine that can mill still "somewhat efficiently" as you state. I'd love to see a clip of the machine milling some steel.
    On another note, that means that my machine is way more stiff than it has to be. This is of course not bad news, it means I can slack a little with the composite the machine will be made of. It doesn't has to be as stiff, which potentially means I don't have to use any carbon in my composite mix... I'll stick to the plan for now: Aluminium swarf, carbon strands and woven carbon fiber skin.

    Thank you for the numbers, good to know.

  11. #31
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William
    1) Resin trap - You need to select a suitable container to go inside your vacuum pot. eg a icecream container, a std 10L bucket or something that is readily available and a throw away. I use std 10L buckets in mine. I made my trap by laminating fibreglass around a large plastic bucket. If resin flows into your trap (and being a learner it will) its a pain to clean it so by having the resin flow into a container you just throw that away. Plus make it big enough to get your hand in there. You need to wax it very well to make resin removal easy. Do not seal the pot with silicon. You need to be able to get the lid off and on easily. Use some foam or plastic sheet and cut a seal. Test your containers, vacuum will collapse thin or non circular objects.

    2) You have not designed the entire machine yet. The entire machine will deflect quite a bit more then you think once it is fully modelled. A machine can never be too stiff it always works out less stiff in reality then on paper for various reasons.

    3) You may plan to use CF cloth for the outside but do you have a source yet? and why go to all this trouble yet use a cast iron plate for the Z Axis? What is CF swarf? If you have a suitable supply of CF then you can make a slab of CF for the Z axis. This is then in-line with your objectives....Regards Peter

    Re: woven cloth vs stitched cloth. Woven cloth has non-straight fibres (kinked) and therefore is less stiff (at least half the stiffness) then stitched cloth which has straight fibres. So try to get stitched cloth offcuts not woven. If you get woven then place this internally not near the surface. Plain weave is often used for its appearance on the surface as this is the "look" but structurally weaves are very inefficient.

  12. #32

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter
    1) Good idea, I intended selecting some kind of pot that would be bigger than a throw away container. Waxing the mixing cups or container for easy clean up, that will definitely make my life a little easier. Great tip Peter.
    The silicone will not make a permanent seal between the lid and the pot, it will act as a gasket. There are special types of silicone for this purpose.

    2) You're absolutely right, the machine will deflect more than what it does right now, and even more in reality. And yes, the stiffer the better, but if the end result is overkill, then I don't necessarily need to add carbon strands into the composite (unless I get a free source to these)...

    3) The cast iron plate is not what gives the z-axis most of it's stiffness. That would be the clamp. The cast iron plate will be precision milled with a 0.01 mm flatness and the linear rails will be mounted to it. I'm not sure just how precise my mill can mill parts flat, since I don't have a surface plate to verify it. I'll buy a straight edge and test this to see weather or not I could mill the plate flat myself, but until then I'll use a flat milled cast iron plate to mount the linear rails of the z-axis to. By carbon fiber swarf I mean carbon fiber strands. Some fiber strands mixed with cutoffs just looks a bit like swarf to me.

    Thank you for explaining the difference/properties between woven and stitched cloth. I will use stitched cloth.

    I've contacted every company in Denmark somewhat near me, that delivers or uses carbon fiber, for carbon cutoffs.

    (the z-axis will also have two screws for the clamp at the top)
    Attached Thumbnails Attached Thumbnails Z-axis Illustration.PNG  

  13. #33
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Will- The composite industry talks about short fibres say <50mm milled fibres <5mm long fibres all the rest. Woven cloth, stitched cloth, multiaxial cloth. I still don't see why you want to use cast iron but so be it. Peter

  14. #34

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter I have not found any source of flat milled composite slabs. But you right, and it annoys me a little too. I'll make the saddle and z-axis "backplate" of composites too then. I hope my mill will make them flat enough, but if they end up with a flatness of 0.03 mm, then so be it.

  15. #35
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Will - Given your resources you can only do what you can do with your mill. So probably time to give your mill an overhaul in prep for the new mill build. Sharpen your tools before you do a job is always a good plan. In your objectives it says "experience casting composites" so CI is in conflict with that objective. Plus this is a design exercise. There is no value in making a compromise in this project. You do the best you can on paper and at the end you may have to make compromises due to practical or economic reasons. But right now there is no point in compromising. Design the best composite mill you can then figure out how to realise it.

    The practical side of infusion of parts is still a huge learning curve and your months away from that unless you start parallel projects... such as making a slab of G10... who will be your composite supplier of wax, fabric, vac bags, tacky tape, breather cloth etc etc??

    Peter

    https://www.professionalplastics.com/contact they have a european distributor. G10 or FR4 should be readily available but you can make it yourself now you have a vac pump.

  16. #36
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Will and others - I have been designing a small mill and have come back to looking at gantries. As usual the issue is square gantries lozenge (change to a diamond shape ie shear deflection). In the Maximus thread I determined that the corners needed thickening or to use a diagonal brace. Others have filled their tubes with EG, placed internal webs etc. This has pointed to the fact that gantries are shear dominant and are not strictly beams in flexure. This is called short beam behavior. Short beam theory is very different from long beam theory. FE takes this into account mostly but large shear deflection is best looked at via non linear solvers not linear solvers. But that's getting technical.

    So I was looking at a 10kW laser video yesterday and noticed the gantry was triangular (flat bottomed isosceles, point upward) and this brought me back to the brace I had used in Maximus. But what if I just left off the back square bit so the loads went directly to the bearing corners vs around the back of the square section? (called shear flow or shear lag) So I modelled a simple gantry this morning SQ vs TRi and tri wins hands down. SQ 0.62mm TRI 0.45mm nearly 30% stiffer. The question is how do I build this shape in aluminium plate???? Always a hurdle to new things perhaps its time to make a mould...

    The gantry modelled is aluminium 6mm thick 1100mm wide. The tool plate is 200mm wide and 25mm thick with a 200mm cantilver. The SHS gantry is 150x150x6mm The tri is 150mm high and 150mm wide and 6mm thick. The tri is lighter and stiffer, win win...

    Peter

    Will maybe do something similar in F360 and confirm the result?

  17. #37

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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - Well my resources doesn't have to be a limiting factor... The idea of buying a cast iron plate was to find a supplier that could also grind it or make it flat with a precision mill. Your're right Peter, I need to remember that this is a design exercise, and that building the machine will a project for another time. Designing the machine already requires a few projects, like measuring the stiffness of the composites I'll be able to build in my garage (I'm really exited about testing my own composite one day!). I'll design the dream machine, and make compromises when the time comes to build the machine. I've spent most of the day learning about some of the imperial threads used for sealing connections to connect fittings and hoses to. I've also made a little list of things to buy and contacted some sellers on Alibaba to buy a whole bag of barb tee fittings... I've mostly been looking at consumer suppliers, but when i took a look at professionalplastics.com I was blown away. What a wide selection of materials. I just contacted them regarding potential suppliers or warehouses in EU.

    Triangles, of course! Why have I never considered a triangle? Great idea Peter. I can confirm the results. I made a quick search for carbon triangles, and found this interesting trapezoidal tube which left me wondering how strong it would be compared to a square. It turns out this shape has about 30% less deflection compared to a square. http://www.dodge.com.tw/rw_products_6e6086ab.htm

    Profile: 50x80mm - 400mm long.
    Load: 700N in the middle of the front face 40 mm below the lowest point of the gantry.
    Weight: Both profiles weight the same +- 100 g.

    Square deflection: 0.004931 mm
    trapezoidal deflection: 0.003433 mm

    Interesting... If I increase the beam length from 400mm to 700mm the trapezoidal profile is only around 20% stronger instead of being 30% stronger. When I think about the beding forces it makes sense. The trapezoidal shape that I stimulated is not perfected for it's job, so it could potentially be stronger... Trying to think of any scenarios where a trapezoidal tube would be practical or useful compared to other shapes, but I can only think of the "cool" aesthetic of the angled faces.
    A right triangle will be more practical, but I was just wondering how much of a difference the trapezoidal shape would have compared to a triangle.

    Regards William
    Attached Thumbnails Attached Thumbnails Rectangle.jpg   Trapezoidal .jpg   Trapezoidal Carbon.PNG  

  18. #38

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    Re: WillMill - HSM Benchtop Composite Mill

    Hello everyone - As mentioned the other day I contacted Jingjiang City Jianken High-Speed Electricmotor Co., Ltd. about a CAD model for their JGD-85 2.2 kW ER-20 spindle. After I few messages I had gotten my hands on a drawing of the spindle with some information that I thought someone could benefit from - so I'll post a picture of the drawing here.

    The spindle has a housing that protects and holds everything together. I was wondering what the wall thickness of the housing was for FEA purposes, and the drawing states that the wall of the housing has a thickness of 2.5 mm, but this doesn't seem right to me?? Does typical "consumer" HF spindles only have a wall thickness of 2.5 mm? - or has something gone wrong in their engineering department? With cutting loads and vibrations I would have though that the spindle housing would be at least be 4 mm thick.

    - William
    Attached Thumbnails Attached Thumbnails JGD 85 - 2.2 kW.jpg  

  19. #39
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi William - a triangle is a fixed geometry. It can't change shape. So the more the trap moves to being a rombus the more compliant it becomes. Even the trap is more compliant then a tri. Even in FE we use square mesh to model various things because it can lozenge and twist. If we use triangular mesh it can "lock", say if we are doing a plastic flow problem and we can't get to a solution. Using square mesh allows the material to flow and allows the solver to keep solving... Peter

  20. #40
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    Re: WillMill - HSM Benchtop Composite Mill

    Hi Will - You have reverted to saying "stronger" vs correctly saying stiffer or more rigid. Peter

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