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jackd

Extended Axis Strength/Flexing concerns

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Hi all! I've been trawling through countless forums and post but i cannot find a solution.

I'm looking at making a diy (or at least pricing and making a CAD model) of a Ultimaker on a slightly larger scale. NOT HEAPS! Just a little bigger.

The X&Y extruder bars would be 500mm 6mm(d) Steel Rods and the outer X&Y would be X=600mm 10mm(d) and y 700mm 10mm(d).

Z axis is a mystery, likely to be 12x3 700mm trapazoidal threaded bar:

http://www.ebay.com.au/itm/111012363194?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649

Should I increase the Z support bars to 12mm or more? I though 10mm might be enough as well as extra support on the opposite end.

So would 10mm thick bars be strong enough? I'm planning on using 6mm(+) plywood for the frame and converting to direct drive.

Really just looking for advice on the strength of rods and any help would be greatly appreciated!

Thank you for your time

Jack.

 

 

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Its nothing to do with the strength, its the stiffness that will cause you a big problem.

I think the UMaker drive layout is only good for print bed size maybe 350x350mm

Any more and the vertical flexing will likely exceed the layer height (meaning the nozzle

may well decide to start scraping and banging into the top layer in the middle depending

on the speed its going and so on - which wont always be constant). You will also run into

accuracy issues printing quickly as the XY rods will also bend laterally during fast moves so

your geometry will likely end up being thrown out if you print really fast.

Issue is that if you increase the diameter of the two 6mm XY rods, the weight will go up with the

square of the radius. So you will loose all of the benefit of having a light printhead.

Having very thick rotating bars for the outer slides will help, but only a bit. Another problem is going to be

that the bowden will be much longer, and potentially getting quite nasty curvature at the limits of travel. All

of which will give bad printing results.

So just for example...if you calculate the beam deflection, to have the same stiffness as a Ø6x300mm

XY bar (the standard ones), in 500mm length, the required diameter is Ø9mm !!

The moving mass of this is triple the standard one (not that you can buy a 9mm rod, but you see the point).

Or you can just use Ø6x500mm rods, and accept that for any given printhead accel your printhead will

be deflecting 4x as much as the standard one. So by extending the Ø6mm rods by +200mm length you

are decreasing their stiffness 400%. This is because in the beam bending equation, the term for the beam length

is "beam length cubed".

So thats a compromise you will have to accept if you want to keep the Ultimaker drive arrangement

with a larger format. It will probably print not as well, because of the long bowden, and the printhead will

be less accurate for a given speed. But if you are happy with that, of course it will still "work".

If you REALLY want to do it, I recommend making it rectangular format, keeping

perhaps the Y axis short, that way you keep it at least very stiff in one direction.

C.

PS> I suppose this might be helpful:

http://www.engineersedge.com/beam_bending/beam_bending2.htm

 

 

 

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The solution here is to use hollow tubes instead of solid 6mm axles - just use a larger diameter. Using a hollow aluminum or 4310 tube that's 12mm or 18mm OD is going to drastically increase your stiffness while keeping weight constant - maybe even lowering it (depending on your wall thickness).

I ran some numbers for this a while ago when I was scoping out a large format 3D printer I'm working on and the stiffness/weight looks great for aluminum tubes. Apart from the easy availability, there isn't much going for solid drive axles. The only complication is finding tubes with a good enough surface tolerance, because purpose-built hollow drive axles are incredibly expensive. I suspect, however, that high quality aluminum extrusion or DOM 4130 aircraft tubing is going to be good enough. Carbon fiber tubing might also be an option because it can be had with a great surface finish, but unless you've got good connections, finding that cheap will probably be tough.

 

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I spent a LONG time searching for hollow tubes earlier this year, the problem is that

such tubes tend only to be available in quite large sizes. So its no problem to find

20mm diameter hollow prescision ground hardened rod, with an internal diameter of

say 12mm.

However the production difficulties of either drilling out solid bar (very very expensive, needs

to be "gun drilled") or actually grinding a thinwall tube sucessfully are such that its

economically not feasilble.

The only possible source I found was Sandvik, who could do them but I never got a response

from them regarding a quote. Probably because they are only interested in making a run

of 500 tubes...not 10.

http://www.smt.sandvik.com/en/products/tube-pipe-fittings-and-flanges/tubular-products/precision-tubes/thin-wall-precision-tubes/

I also looked into glass & other ceramics, some of which have very high stiffness and hardness

but are basically extortionately expensive and not really available in lengths over 200mm or so.

http://www.coorstek.com/markets/chemicals/ceramic-tubes-rods.php

 

Carbon fibre tubes will not give you anywhere near enough surface hardness, since that property

will be determined by the matrix, which is basically epoxy resin.

 

 

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