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  1. #1

    Join Date
    Nov 2022
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    Building frame from GRP

    Hi,

    I've been thinking to build new machine, 800x600 workarea, fully supported SBR 20mm round linear rails, ballscrews, hy200 2220 0210 ax08 steppers, chinese controllers and grbl with arduino/mega. Mainly use parts from my previous machine.

    Now i'm obsessed with building machine frame and possibly gantry from GRP. I have plenty. I think im going with 800g vowen rowing and chopstrand mats with polyester resin.

    I havent seen any of these type machines build? Is there anything that makes that kind of table/frame poor? I will be using mdf as sacrifial / wacuum table top.

    Heres pic of my current setup and quick sketch of machine I've been thinking about to build just to clarify dimensions. Not sure yet if im going to single y axis stepper or using dual.

    I mainly work with <12 mm birchplywood & mdf. I'm going for AMB FME 1050 digitally controller router as tool holder..

  2. #2
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    Nov 2013
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    2195

    Re: Building frame from GRP

    Hi,
    rigidity is the key design/build target of any machine. I see no reason why GRP cannot be used to advantage.

    Typically most GRP projects are designed to maximise strength.......but strength is not the same as stiffness. Before committing a lot of time and materials to build
    a design use an FEA program to determine the rigidity or lack thereof.

    The only weak point I see is those round rails. They are about as rigid as soggy spaghetti.

    Craig

  3. #3
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    Jul 2018
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    4470

    Re: Building frame from GRP

    Hi Marrti - Issue 1) CSM and WR produce laminates that are resin dominant and are not very stiff. Good plywood is much stiffer then this sort of laminate... Issue 2) Polyester resin shrinks 8% by volume. Read the data sheet to prove this to yourself. In laminates this means they will change shape dramatically from the moulded shape in say the first 3 or 4 months of it curing. Especially large flat surfaces they will cup big time eg a 100mm length of resin will be 92mm after its fully cured. This shrinkage occurs due to the excess styrene that is supplied within the resin to stop it curing in transport and in "open" moulds the styrene evaporates from the surface.... over time the excess resin desorbs from the laminate. This can be seen in boats that print through and greyhound.

    Fibreglass laminates can be used successfully if you do the following 1) Use a 100% solids epoxy resin as these have minimal shrinkage so will produce volumetrically stable laminates if correctly post cured 2) use infusion to produce high fibre volume fraction laminates using stitched cloth not woven cloth. (Woven cloth has "crimp" which means its volumetrically & structurally inefficient) This will maximise strength and stiffness. If you do this you will get laminates in the order of 20-25GPa stiffness at 1800kg/m3 using E-glass

    I have been testing glass laminates in the boating industry for over 30 years so I can tell you its difficult to get over 25GPa from a glass laminate. What your talking about may get to 10GPa but its the shrinkage that's the killer issue.

    Now since you are going to make moulds you may as well look at high performance concrete (called engineering grout) as you can get 30-40GPa out of the bag at much less cost and hassle. The grout I use has virtually no volume change, is 34Gpa out of the bag just add water and is 2200kg/m3 density. So much easier then trying to make a high tech glass laminate. Cheers Peter

    I would recommend the following:
    1) Build a machine in good quality plywood like formply used for casting concrete. Easy to work with, very good result. Built a few machines like this. all good E~18Gpa but being 18mm thick or thicker its really stiff in flexure.
    2) build a machine using UHPC or CSA concrete as attached. There are many examples of cast bases here
    3) If your really keen on fibreglass you will have to use carbon fibre to get the stiffness and learn how to infuse to compete with concrete. CF will get you to 70GPa stiffness same as aluminium... at 1500kg/m3
    4) if you have access to milling machines go straight to aluminium, stiffer then the prior suggestions absolutely stable and easy to work with 70GPa at 2700kg.m3

    Keep at it. Peter

  4. #4

    Join Date
    Nov 2022
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    2

    Re: Building frame from GRP

    Thank you for your insights.

    Infusion lamite will be too fiddly and epoxy too expensive. I'm only thinking this because i have lots of e-glass to spare due previous projects.

    Actually my first "idea" was to make a skin from grp with steel "rebar" frame to have something to bolt rail on and cast concrete inside grp frame to have weight ja mass. and use to motors for y axis.

    but.. Lets say that we make
    1) a neat frame with polyester resin and heavy and loose woven e-glass cloth with nice gelcoat and vacuumed in mold (shrinkage 8%) ,
    2) place anf glue rebar and steel frame with putty inside frame to have something to make threads on and
    3) cast low shrinkage concrete grout (like Fescon JB600/3) which has shringake less than 5%
    and let it sit for a while

    grp skin should tighten around conrete?

    Ill look for uhcp and csa.

    I'm not aiming to super high accuracy, mainly doing arts and furniture with birchply and Im happy with my Stepcraft d840 as my main workhorse (with amb 1050 and hf500) so I think that if I can get to steady performance like stepcraft machine that i use to work I'm happy.

  5. #5
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    Jul 2018
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    Re: Building frame from GRP

    Hi Maarti - Sorry you are describing a path of pain and disappointment. Nothing is more expensive than a failed project. low shrinkage Portland cement is still too much shrinkage for machine parts, plus it expands then shrinks during cure, this can upset your "precision" mould. Do not use portland cement products for machine parts. Plus Portland is unstable over time and it will crack all over as time passes.

    Mixed media parts are risky as steel has a thermal expansion totally different to resin and fibreglass. Threads can be placed into laminates (and plywood) easily with no steel inserts. Accuracy is a relative statement but if your base warps 1mm or 5mm then you'll be a very unhappy camper... Do not make decisions on low cost. There are no "low cost" solutions in machine building otherwise others will be doing it. Research more, try to understand the underlaying reasons for using materials and try to establish what you are trying to actually do. All answers are available in the forum. Peter

    You say you have a D840. Stepcraft claim this can "mill" aluminium so I suggest you go down the aluminium plate path with milled aluminium parts. This will be a reliable and stable machine.... if done right it will be an excellent machine.

    your lowest cost approach will be using premade materials eg plywood or plate aluminium. The manufacturer of these has made all the decisions, quality assurance and sorted the time dependent issues.... if your obsessed with GRP then you must use epoxy correctly cured to get stable parts. Keep at it. Peter

  6. #6
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    Jul 2018
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    Re: Building frame from GRP

    Hi Marrti - The other thing with mixing steel with low modulus materials is that the steel will become a dominant loadpath. Steel is E=200GPa Al=70GPa, woven FG=say 10GPa so steel is 20x stiffer, so the load will go down the steel not the GRP.... To illustrate the shrinkage deformation think about steel welding and how much the steel moves around when you weld. Steel shrinks 2% by volume going from liquidous to solid so imagine what a 5% shrinking concrete mass could do!! So pick your construction medium and stick to it. Another approach (maybe) if your really keen on Polyester is to make parts at near nett shape, fully cure at 80degs C for 12 hours this will drive off all the styrene and shrink it fully then final machine to size in your router.... I wouldn't do this but maybe it will work. This is like welding up a steel frame then stress relieving it, then final machining it... speak to your resin supplier you could use 50C for 24 hrs or even lower. I cure epoxy at 30C in our honey warmer for 48hrs and it gets to full hard. But every resin is different and your supplier will be able to spec the lowest temp to achieve full cure plus desorb all styrene. Before I made a machine part like this I would make a large flat panel, post cure as intended and see if it stayed flat...You mention vacuum so keeping it under vacuum on the mould thru the heat treat is desirable. Peter

    I just looked up some actual hand laid PE CSM laminate modulii results and it was 8GPa. Infused CSM was 15GPa. I think 10GPa will be a top value for hand laid WR/CSM.... Resin is about 3Gpa so you need to minimise resin in the laminate, glass fibre is 69GPa by itself... F17 plywood is 17Gpa and F27 is 18Gpa so much better than your proposed GF laminate.... keep researching. Peter

  7. #7
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    Nov 2013
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    Re: Building frame from GRP

    Hi,
    the cheapest material with the greatest stiffness is steel....hands down. Youngs modulus of 205GPa....it just cant be beat for rigidity. Can be drilled, tapped, welded, forged......
    The next best material is cast iron. Youngs modulus of 110GPa.

    Once you start welding (steel) then you must allow for stress relieving and finish machining. Neither is cheap but the end result is superb and lo and behold the majority of the worlds
    machines are made of cast iron and/or steel. The reason is simple: they are the cheapest materials to manufacture highly rigid machines.

    If you want to re-invent the wheel in the form of mixed materials or use of a (largely) unproven material then you might expect some poor results before you latch onto the right
    way to use those materials.

    Craig

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