Gantry design question

[D.C.]

I wasn't planing on using cloth except maybe as a cosmetic wrap.
Carbon fibre also comes in ready made stiff boards and box section, you can laminate the boards/box to the aluminium honeycomb without needing to use a vacuum so it makes life a lot simpler.

Thanks for the offer mekanik it's very kind of you but I'm afraid I chickened out and decided to go for cheapy MDF job as my first build and then get more adventuous in a year or two.

After I've got a machine up and running I might buy enough to make up 50 cm beams of multiple different materials so I can test how each behaves side by side. Seems like a cheaper way of getting a definitive answer to the 'will it really work' question.

[martin54]

If you are just gluing a flat sheet of carbon fibre to the honeycomb rather than laying it up I would have thought you would lose a lot of strength, the box section they do is all unidirectional cloth which means all the strength is in one direction which could give problems. Like you say the only way to know for sure is to try it lol.

If you are going down the MDF route then try to make sure the components you buy can be migrated to the next build, especially expensive stuff like the electronics. That is what I have been told by some of the forum members.

[mekanik]

@DC
Have to agree with Martin,to get a ridgid structure i would make the mold and include webbing details then vacuum infusion,i suppose you could use the honeycomb to provide stiffening and to save the intricate mold work of the webbing detail but it still needs to cast in one operation.
Just out of interest what width were you thinking of making the Gantry.
regards
mekanik

[D.C.]

I was considering it for a 4x8 build.

I'm not sure you would lose that much strength, plenty of people make torsion boxes using only glue for joining for example. The vacuum process is really to squeeze the different layers of cloth together around a complex shape, when you are dealing with a ready made flat sheet you can achieve the squeezing the old fashioned way with weight or clamps. If you were to try to make the gantry in one piece then yeah you would need a vacuum to ensure good contact in all areas.

[martin54]

I was considering it for a 4x8 build.

I'm not sure you would lose that much strength, plenty of people make torsion boxes using only glue for joining for example. The vacuum process is really to squeeze the different layers of cloth together around a complex shape, when you are dealing with a ready made flat sheet you can achieve the squeezing the old fashioned way with weight or clamps. If you were to try to make the gantry in one piece then yeah you would need a vacuum to ensure good contact in all areas.

Sorry but that's not true at all, I could quite easily manufacture a complex shape without the use of a vacuum just by using the right weight cloth and simply laying the cloth around a mould or covering the part if I was just skinning it. The carbon fibre part that I made would look just the same as a part made using a resin infusion system but that's where the similarities end. The part produced by resin infusion would be far lighter & stronger than the part I made, what the vacuum actually does is remove all the excess resin which cant be removed from a traditional type wet lay up. The strength comes from the layers of cloth being closely bonded together, the less resin that is present the stronger the bond.

[mekanik]

Hi Martin
You beat me too it.
OCLV as per my Trek frame, when i put the vacuum on the concrete mold that i attempted i was gobsmacked @ just how compact the the laid up glass went then when it's infused its far superior to any hand lay up ( i ended up having to lay it up by hand eventually as i couldn't get the mold vacuum tight)
@DC
Sorry mate i must have misunderstood your intention, i thought you wanted a maximum ridgidity gantry using composite construction, if this was the case it would need to be one piece.
If i ever get the time i might design a 4' gantry and make a mold,as it's a good size it might be possible that other members of the forum might find this usefull.
Have a nice day guys
mekanik

[Tenson]

I think DC is talking about using pre-made ridged sheets. In that case they are already good and strong and he just needs to join them at right angles or whatever. Okay it might be a little weaker at those joins but he can probably do well with traditional methods like bolting each panel to some square section + resin.

[martin54]

I think DC is talking about using pre-made ridged sheets. In that case they are already good and strong and he just needs to join them at right angles or whatever. Okay it might be a little weaker at those joins but he can probably do well with traditional methods like bolting each panel to some square section + resin.

Yes from a previous post he said using pre made sheet or box section to sandwich the aluminium honeycomb. The premade sheet will be good quality & strong so may well do the job but not as strong as if made as one complete unit. You could actually make the gantry in one quite easily using traditional wet lay methods, might even work out better with the increased weight but wouldn't be as strong.

[D.C.]

Yes I was talking about using a premade sheet and strength wasn't really a great concern although I do agree that doing the whole thing as a complete unit would be stronger.

All a gantry has to do strength wise is hold it's own weight, the weight of the spindle + it's fittings and resist relatively small amounts of lateral forces and the occasional shock load, a single piece of 6mm plywood can easily cope with the strength requirements without failing.

The problem is rigidity and coping with those forces while still being in the correct position all along it's span to the nearest thousandth of an inch. Which is why I was thinking of CF sheets with the honeycomb acting like a torsion box, the strength of CF isn't needed but it's insanely good stiffness & vibration dampening properties are.

[Tenson]

I like the ingenuity but I think it's kind of a waste of expensive material. CF is very stiff for its weight, but steel is equally stiff. Since the weight isn't much issue, use steel. Want damping, then use steel box section with foam or sand inside. For sheets of aluminium or steel use constrained layers (I mentioned green glu before, bitumen and butyl rubber sheet is also good).

I design loudspeakers and in this area we pay a lot of attention to damping sheet materials for the speaker boxes. The BBC did a great amount research on this and concluded that constrained layer damping was most effective, although making it stiff was not the goal. So IMO use stiff materials and constrained layers, but don't worry much about weight.