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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by ack1 View Post
    Awerby, it is flexing side to side. Well front to back if you are standing in front of the machine.
    Is the gantry beam hollow and if so, what is inside it now?

    It goes to show that you can't always tell how sturdy something is from a pic. A thick-looking steel part could be made of thin walls welded together instead of being solid.



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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by ack1 View Post
    This is my machine when i got it.


    Side view of the gantry with the cap off. IF i remember correcly steel was about 8mm thick.



    Sometimes i do whole alu sheets of engraved pieces and if I go too fast (gantry makes a sudden change from forward to reverse or vice versa, in the Y direction) the gantry flexes just by its own. Specially if im in the middle of the table.
    The gantry span is somewhere around 2.3meter so its not surprising.
    These numbers are very small but you can cleary see what i mean. Look at number 7.
    Have you checked the g-code for the job, especially when it does the number 7? Sometimes CAM puts out crappy code... and sometimes the font, if translated from a Windows or Adobe font, can look "pixelated" even if it's a single line font. Sometimes on what appears to be a diagonal, CAM will spit out something like

    G1X0
    Y0
    X1
    Y1
    X2
    Y2

    and when you run this at speed it looks like it's a mechanical or electronics issue. Not saying that this is the case, but something to look at. Especially with some older machines.



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    Default Re: My carbon fiber CNC gantry

    check all the bolts of Router and Spindle motor..seems looseness..

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    Default Re: My carbon fiber CNC gantry

    I think Louie might be onto something. Looking closely at the results you're getting, the oscillations are suspiciously regular. The number "1", for instance, wiggles exactly the same way each time. If this were looseness or deflection, as I originally suspected, the wiggles would be more random. Check your code, and see if it's in "exact stop" rather than "constant contouring" mode.

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    Default Re: My carbon fiber CNC gantry

    Quote Originally Posted by louieatienza View Post
    Have you checked the g-code for the job, especially when it does the number 7? Sometimes CAM puts out crappy code... and sometimes the font, if translated from a Windows or Adobe font, can look "pixelated" even if it's a single line font. Sometimes on what appears to be a diagonal, CAM will spit out something like

    G1X0
    Y0
    X1
    Y1
    X2
    Y2

    and when you run this at speed it looks like it's a mechanical or electronics issue. Not saying that this is the case, but something to look at. Especially with some older machines.

    That's interesting. They use regular square and rectangle steel tubes. I was originally planning on using similar steel rectangle tubes for the gantry and frame on my build as they seemed to be relatively cost effective. I was unsure how thick the walls would need to be to deliver acceptable rigidity but I guess that it probably depends and few different things. That gantry looks fairly long in the pic.

    The good news with a hollow tube is that it can be filled to create extra stiffness. There is probably a lot of options that would help including more steel tubes to fill that cavity.

    If it were me, and I felt like experimenting, I would want to try reinforcing the center with strong steel bars and filling the rest of the cavity with a mix of epoxy granite (which apparently has a proven track record for creating rigid CNC bases with good vibration dampening) and a high shore hardness urethane rubber. I am sure others will have better advice that is based on actual CNC experience though.



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    Default Re: My carbon fiber CNC gantry

    Goemon , you keep going my friend , you are on the right path and will have a superior product when done .
    I am a marine project manager with an engineering background and fabricate for fun . Carbon has surpassed metals in just about every aspect in the marine environment,this including masts and the rigging used to hold it up as well as hull structures and sails .
    A carbon mast is both lighter and stiffer then its aluminum equivalent and localized reinforcing can be introduced into a design to either stiffen and area or to increase strength . Like with all new technologies you will get a fair amount of naysayers finding all the excuses in the world why it won't work ,but the early adopters usually get to enjoy the advances and understanding of the new tech .
    You will definitely produce a far stiffer component then a metal equivalent with a substantial weight saving ,which will give you the possibility of faster acceleration in the Y axis from point to point . This can add up to much faster job times if taken advantage of the weight saviing on a complex component .
    In the marine industry we deal with compression , tension ,torque and flex and carbon has been the best material so far to address all these loads . As you have continually pointed out ,the ensign is critical to achieving your goal of stiffness , and carbon can get you there the easiest .
    Just remember you may want to protect the bottom part of the gantry from micro impact from flying Swarf chips as this could eventually lead to a delamination failure from the continuous impact in the same area . Perhaps a sheet of stainless foil used for heat treating would work well here , it's available from McMaster and can be bonded to the gantry with ease .
    Please continue to describe the design and build , as I think the tech explains itself perfectly .
    Rocrat



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    Default Re: My carbon fiber CNC Gantry

    I'm glad you posted that video, when I first this I thought, ok you could do it, but why?

    Really interesting video, given that the one they made at the factory was very specifically engineered and manufactured, how comparable would the box sections you are using be for gantry use?

    cheers, Ian

    It's a state of mind!


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    Default Re: My carbon fiber CNC gantry

    Very interesting thread for me, as I am just about to build a 4000mm long composite gantry. I have been procrastinating back and forth about it, and maybe just to use steel, but am pretty committed by now to doing it in composite. I therefore list my plan below, for the purpose of getting everyones comments. All comments and ideas gratefully accepted - to ensure I am not overlooking something....

    - Composite gantry beam 4000mm long x 600mm high x 400mm deep [RHS - rectangular hollow section].
    - Construction: Sandwich construction - 5mm foam core, internal laminate = 2mm quadraxlial glass fibre, external laminate = 4mm quadraxial carbon fibre & glass fibre (50%/50%), with transverse internal gussets/bulkheads every 300mm (bonded in).
    - Resin system = epoxy, infusion, room temp.
    - Method: vacuum infusion + hotbox post cure cycle. [make up core material as C-Section, lay-up inside C-Section, bond in transverse gussets, close C-Section by bonding on last side panel, vacuum infuse external laminate in one hit].
    - Thermal expansion goal is to achieve approximate equivalence with concrete [13x10-6 m/(mK)] and steel [12x10-06 m/(mK)], as side frames are steel and machine base is concrete floor. Thermal expansion co-efficient of epoxy GRP = 36x10-06 m/(mK), epoxy/carbon = 2x10-06 m/(mK).

    Metal threads will be achieved by either/or
    1) 50x8mm steel flatbar laminated into the core of the sandwich construction, or,
    2) Drilling beam and bonding in threaded inserts (with plexus/crestabond/etc)

    Datum faces (machine beds) for 35mm linear rails & 24x24mm gear rack will be produced by one of the following methods, either/or:
    1) Shimming components in place and then back-filling gaps with "Chock-it" [CHOCKFAST ORANGE (PR-610TCF)], or,
    2) Epoxy-concrete machine beds, molded on a 4000mm long flat table produced with self-leveling epoxy (make flat table mold using self-leveling epoxy, lay in the epoxy/sand/fibre mix on the table mold, place the gantry beam on top to bond the epoxy concrete machine beds to the beam.

    ??? forgotten something?...

    Like I said, all comments and ideals thanks very much.

    Regards,
    Jono



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    Default Re: My carbon fiber CNC gantry

    Thanks for the post. I for one am very interested in the use of composites and such, as I have zero experience with the materials. As such I strongly suggest that when you get started on this, you start a build thread so that we can get another example of a build from beginning to end. Once a few proven builds get some time on this forum I think we will see more people considering alternative builds.


    Quote Originally Posted by jono5axe View Post
    Very interesting thread for me, as I am just about to build a 4000mm long composite gantry. I have been procrastinating back and forth about it, and maybe just to use steel, but am pretty committed by now to doing it in composite. I therefore list my plan below, for the purpose of getting everyones comments. All comments and ideas gratefully accepted - to ensure I am not overlooking something....

    - Composite gantry beam 4000mm long x 600mm high x 400mm deep [RHS - rectangular hollow section].
    That is a big beam.
    - Construction: Sandwich construction - 5mm foam core, internal laminate = 2mm quadraxlial glass fibre, external laminate = 4mm quadraxial carbon fibre & glass fibre (50%/50%), with transverse internal gussets/bulkheads every 300mm (bonded in).
    Interesting but it strikes me as a bit thin. I base this on the little bit (very little) I understand from boat building. There structural parts are often built up from many lamination that can end up inches thick. A 4mm shell laminate just strikes me as very thin to get any significant stiffness and freedom from vibration over that distance.
    - Resin system = epoxy, infusion, room temp.
    - Method: vacuum infusion + hotbox post cure cycle. [make up core material as C-Section, lay-up inside C-Section, bond in transverse gussets, close C-Section by bonding on last side panel, vacuum infuse external laminate in one hit].
    Again interesting and my experience here is so think as to be useless but wouldn't it be fear easier and lead to a much stronger beam if the beam was built up around a foam core? Of course 4000mm is a long stretch for foam so that core might need an internal support beam. In any event it just seems like doing two C beams and then trying to join them afterwards is going to be more difficult and time consuming.
    - Thermal expansion goal is to achieve approximate equivalence with concrete [13x10-6 m/(mK)] and steel [12x10-06 m/(mK)], as side frames are steel and machine base is concrete floor. Thermal expansion co-efficient of epoxy GRP = 36x10-06 m/(mK), epoxy/carbon = 2x10-06 m/(mK).

    Metal threads will be achieved by either/or
    1) 50x8mm steel flatbar laminated into the core of the sandwich construction, or,
    2) Drilling beam and bonding in threaded inserts (with plexus/crestabond/etc)
    How to handle this is a very interesting subject. I can't claim to have the right answer here but do have some experience with Epoxy Granite machine tool bases and parts. The biggest problem with thread inserts is that they do not have a strong resistance to pull out. The other problem is that threaded inserts, bars or whatever need to be thick enough to properly hold threads. So you end up taking about plates that 8 to 10 mm thinks that have to be molded in or inserts similarly thick. This brings up real questions about how you will place such inserts and keep them bonded in place.

    I know in Epoxy Granite solutions thread inserts are weak points in a structure. In your design I would see threaded inserts as an even weaker solution due to the thin walls you are talking about on the carbon fiber laminate.
    Datum faces (machine beds) for 35mm linear rails & 24x24mm gear rack will be produced by one of the following methods, either/or:
    1) Shimming components in place and then back-filling gaps with "Chock-it" [CHOCKFAST ORANGE (PR-610TCF)], or,
    2) Epoxy-concrete machine beds, molded on a 4000mm long flat table produced with self-leveling epoxy (make flat table mold using self-leveling epoxy, lay in the epoxy/sand/fibre mix on the table mold, place the gantry beam on top to bond the epoxy concrete machine beds to the beam.

    ??? forgotten something?...

    Like I said, all comments and ideals thanks very much.

    Regards,
    Jono
    Like I said above I really want to see another machine build of this sort.

    If you have access to the CAD software and engineering experience I would suggest modeling your beam for stiffness and vibration. Vibration is key here as that can have a very negative impact on machining quality. I have. a gut feeling (very gut feeling) that you will need a beam that is thicker than 4 mm on the shell laminate to get a suitably stiff beam over 4000 mm. What might save you here is the overall size of the beam but you will need plenty of internal gusseting to keep the beam walls from vibrating.



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    Default Re: My carbon fiber CNC gantry

    We've been making carbon fibre gantries for some time… and just stopped.

    In our application, we had to make a 10 metre wide machine which would move at 1 metre/sec or faster and support a weight of 100 kgs with moderate accuracy. The flex and the torsion of the gantry had to be low. Yes, we're not talking about mill accuracy but adequate for the purpose, say ± 0.3mm.

    Carbon or aluminium were the choices with a monocoque carbon gantry seeming like a good choice compared with a fabricated aluminium one. The layup engineering was done by outside contractors with experience of masts and other high load carbon products.

    The gantry was made from pre-preg and nomex core. My experience is that pre-preg is fast and clean and offers much better integrity than resin infusion where you can quite easily get failures in the infusion process giving bubbles and delimitation.

    The mould was fairly immense and used a bladder to inflate to 3-4 atmospheres instead of vacuum bagging. The entire mould was fitted into a fabricated autoclave with heaters to get it up over 120ºC.

    After some months of use, it was obvious that there was some sag in the gantry so the next versions used a heavier and heavier layup. I can't be sure, but my guess is that sag is a fact of life unless you have a truly massively thick layup.

    In spite of the fairly substantial mould, there was some distortion in the finished product because of the pressure used in the bladder. Attaching parts is somewhat problematic since accurate location is difficult with the mould.

    The gantry is also noisy. Because it is hollow and fairly stiff, any vibration is amplified like beating a drum skin. Unless you add something like Kevlar, this is a fact of life with carbon.

    What made us stop this method of construction was mainly the space required for the mould and autoclave but also the noise and sagging. We decided that for our requirements, fast and light, that aluminium would be better. Note, that cost was not a decisive factor but there's no doubt that the alu version is going to be cheaper.

    For a CNC machine, honestly, I can't see the point. You normally don't need more speed than you can get from a lead screw because cutting tools won't really go very fast. You do need to damp vibration and carbon doesn't look great there. You will need some compression strength and carbon fails there too.

    There's no doubt that carbon is sexy as far as outward appearances go and in some applications where weight and strength are critical, carbon offers a lot but where weight is not a big issue, then metal looks more interesting and at a fraction of the cost and complexity.

    D



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    Default Re: My carbon fiber CNC gantry

    Hello Dermot, thanks for that info, very interesting. My beam is for a gantry router, and it is moving at 1.0m/s and with acceleration of 2 m/s2 (designed for, at least). What was the cross-section dimensions of your beam? I have a couple of comments re your points below:

    Quote Originally Posted by DermotMcD View Post
    We've been making carbon fibre gantries for some time… and just stopped.

    In our application, we had to make a 10 metre wide machine which would move at 1 metre/sec or faster and support a weight of 100 kgs with moderate accuracy. The flex and the torsion of the gantry had to be low. Yes, we're not talking about mill accuracy but adequate for the purpose, say ± 0.3mm.

    Carbon or aluminium were the choices with a monocoque carbon gantry seeming like a good choice compared with a fabricated aluminium one. The layup engineering was done by outside contractors with experience of masts and other high load carbon products.

    The gantry was made from pre-preg and nomex core. My experience is that pre-preg is fast and clean and offers much better integrity than resin infusion where you can quite easily get failures in the infusion process giving bubbles and delimitation.

    The mould was fairly immense and used a bladder to inflate to 3-4 atmospheres instead of vacuum bagging. The entire mould was fitted into a fabricated autoclave with heaters to get it up over 120ºC.

    After some months of use, it was obvious that there was some sag in the gantry so the next versions used a heavier and heavier layup. I can't be sure, but my guess is that sag is a fact of life unless you have a truly massively thick layup.D
    I would have expected that additional laminate thickness would have only limited effect on deflection, and may possibly increase deflection by increasing weight. The cross-section needs to increase in size to reduce deflection. For example, if you use a steel RHS and get excessive deflection, increasing the wall thickness will not have much effect, but choosing a bigger RHS will certainly reduce deflection. Of course, this is problematic if you have already made a mold...

    I suppose that we are not really going to achieve zero deflection on large structures, and are more interested in a consistent deflection which is not affected by tool forces.

    Quote Originally Posted by DermotMcD View Post
    In spite of the fairly substantial mould, there was some distortion in the finished product because of the pressure used in the bladder. Attaching parts is somewhat problematic since accurate location is difficult with the mould.

    The gantry is also noisy. Because it is hollow and fairly stiff, any vibration is amplified like beating a drum skin. Unless you add something like Kevlar, this is a fact of life with carbon.

    What made us stop this method of construction was mainly the space required for the mould and autoclave but also the noise and sagging. We decided that for our requirements, fast and light, that aluminium would be better. Note, that cost was not a decisive factor but there's no doubt that the alu version is going to be cheaper.

    For a CNC machine, honestly, I can't see the point. You normally don't need more speed than you can get from a lead screw because cutting tools won't really go very fast. You do need to damp vibration and carbon doesn't look great there. You will need some compression strength and carbon fails there too.
    From my research it seems that composites (both carbon laminate, GRP, and especially resin/aggregate castings) are better at vibration damping than steel/Al, maybe 5 times better, and this is a primary reason for trying to use it (maybe incorrect, more comments pls?). We are routering soft materials and certainly need speed, and long travels, so more speed than a long ballscrew can do (i.e. we need 1 m/s) before buckling, so use gear rack. We do need speed, and this would be typical for a lot of gantry routers/mills.

    An important reason to consider composites is to keep weight down. In steel my 4000mm gantry design comes in at a ton, and therefore the performance comes down and the machine/servo spec goes up (costs go up). My composite design can be less than half that weight, and will perform to specs (? hopefully). It seems logical that as a gantry machine gets larger, and if it needs speed, then keeping gantry weight down is a primary concern.

    Another reason to consider composites is the issue of how do you actually build a large steel or aluminium gantry beam? RHS is not available for that (not big enough), so you are welding, and therefore not straight anymore, and also full of residual weld stresses (RHS also has residual stresses?). I have just been quoted $4000 for having my welded steel side frames stress relieved in an oven, and there are not many large ovens around. So, maybe use thinner sections? folded and riveted to avoid weld stresses? but still you need to provide machine beds for rails etc, which may need welding on, and which have to be machined, which is more difficult for a large (long) machine bed, and has cost. So, it is interesting to consider if the composite materials can be used to eliminate weld-stresses (and the weld stress relieving process step and cost) and also eliminate machining cost by using epoxy/aggregate/fibre cast machine ways. [of course, machine ways could be bonded onto a steel/Al beam using modern composites structural adhesives....]

    I don't agree that a carbon tube or rectangular section has no compression strength, this is not correct.

    These are some of my thoughts, maybe sensible, maybe not...

    regards
    Jono

    Quote Originally Posted by DermotMcD View Post
    There's no doubt that carbon is sexy as far as outward appearances go and in some applications where weight and strength are critical, carbon offers a lot but where weight is not a big issue, then metal looks more interesting and at a fraction of the cost and complexity.

    D




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    Default Re: My carbon fiber CNC gantry

    Hello Jono,

    What was the cross-section dimensions of your beam? I have a couple of comments re your points below:
    Approx 320 x 240.

    In fact the mould was made from stock U channel sections and a custom corner quarter round so we could scale it up or down by using different channel sections or multiples of same.

    I suppose that we are not really going to achieve zero deflection on large structures, and are more interested in a consistent deflection which is not affected by tool forces.
    Agreed. The biggest problem we wanted to avoid was torque twisting of the gantry so that a constant Y movement resulted in a curve. We wanted a deflection of less than 5mm with a centre load of 85 kgs. i.e. someone sitting in the middle!

    I think if we had taken the original carbon layup specs and doubled or trebled them, we would have got a better structure long term. Most people who spec carbon are after minimum weight and might say a 0.9mm web separated by 6mm of nomex from another is adequate for a skin but they're probably designing an America's cup hull which can sink as soon as it crosses the line while we're doing machines which might have a 25 year service life… or more.

    As it was, it's fit for purpose as far as the customers are concerned but there are so many variables outside our control that after 15 years, I would prefer to look down another road.

    We do need speed, and this would be typical for a lot of gantry routers/mills. It seems logical that as a gantry machine gets larger, and if it needs speed, then keeping gantry weight down is a primary concern.
    Agreed. However I think that using fabricated or welded aluminium truss sections may be better in many ways than composites. A competitor built a similar machine and the all up weight was about 500 kgs (probably 5 times ours) and the cost of driving this at any reasonable speed was insane.

    I don't agree that a carbon tube or rectangular section has no compression strength, this is not correct.
    I'm only quoting my carbon guru here. I was interested in using a carbon "conrod" in an oscillating mechanism and was told off big time. You'd imagine that the light weight would keep the forces down but apparently that type of end on load does not work.

    These are some of my thoughts, maybe sensible, maybe not...
    Well we're all groping in the dark to some extent or other!

    D



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