Carbon Fiber pultruded tubes for X axis on Cartesian

RCarlyle

8mm carbon fiber tubes are much more flexible than 8mm steel rods. (Lighter but less stiff.) If you do carbon fiber, you need to increase the diameter to compensate for higher elasticity.

Also, only sliding bushings are appropriate, rolling LM bearings will destroy CF rods over time.

briskspirit

@rcarlyle What about CF rods?

RCarlyle

@briskspirit tubes will usually perform better than rods in this application since all meaningful loading is transverse/bending. The center of the rod lies on the neutral axis for bending, so it contributes minimal bending strength. In other words, tubes are lighter with nearly the same bending strength. This goes for steel and CF, but hollow linear rods are too expensive to be worth using from what I've seen. CF tubes are reasonably cost effective if you use CF.

The problem is the modulus of elasticity or stiffness of the material per unit cross sectional area. If you have a fixed rod diameter or area (eg 8mm OD requirement) and a heavy carriage, then you'll get the highest performance from the material with the highest modulus of elasticity. Stiffness/weight ratio won't dominate, since we have a fixed amount of diameter to work with. Steel beats CF for that. It's when you can move to a larger tube diameter that CF beats steel.

If the carriage is very light and the weight of the rods themselves dominates the moving mass, it's more complex and you really need to do some math to find which is better.

briskspirit

@rcarlyle I don't know how to calculate that... ~700 grams of mass for carriage, 2 rods 385mm each of unsupported lenth. So maybe the only way for me to try and "feel" Thanks for your explanation!

RCarlyle

@briskspirit it's a pretty straightforward excel calculation if you want to try... I don't have my spreadsheet at the office nor time to work it through right now, but the basic idea is to look up generic beam deflection equations and add up the loads.

Calculate 4th moment of inertia (I) for the rod/tube
Calculate mass per unit length for the rod/tube
Look up modulus of elasticity (E) for the rod/tube material
Use E, rod mass per length, L, and I to calculate the rod deflection at center span due to rod self-mass for a given acceleration (doesn't matter what the accel is, can just use 1 m/s for simplicity)
Use E, HALF of carriage mass (since you have two rods), and I to calculate the rod deflection at center span due to carriage mass for the same acceleration as above
Add the two deflections together to get the total rod flex at that acceleration
Compare the result for CF tubes vs steel rods and see which has less deflection

RCarlyle

If that sounds like something you don't want to try, then yeah, just do it by feel

briskspirit

@rcarlyle "it's a pretty straightforward excel calculation"... Thanks, now you've made me feel dumb...

antlestxp

i ran 10mm carbon rods on my 300x300 corexy for a little while. the hope was to improve accuracy with the reduction in weight. it ended up not working well. in my opinion it was not worth the trouble. it is impossible to get accurate diameter rods without spending a ton. then you need to use igus style bushings that may go from being to tight to too loose as they move along the axis. then there is the flex they have. it may not seem like much in hand but when I ran the bed leveling it the results showed my bed was much more warped then it really was. I ended up going back to steel rods. I am thinking about trying some machined alloy rods in the future.

briskspirit

@antlestxp Wow, thanks! Valuable opinion!

Dougal1957

Unless of course you can get the carbon tubes custom made and the ground to size after curing (fortunately I know someone that can do this for me just not sure what he will charge me) I will ask him again and see what the cost will be but I suspect that a single MGN12 rail will be the better option price wise

briskspirit

@dougal1957 I am ok with "precision" of steel rods, just thought to save some weight

Dougal1957

without doubt they will be lighter you do need to consider if you need to increase the diameter tho to achieve the desired stiffness and only you can determine that

RCarlyle

Ceramic-coated aluminum works fine with bronze bushings and anodized aluminum works fine with igus bushings. Those are both options if you want to try aluminum. Just use a larger diameter than your current steel rods... it’s possible (although may be hard to source) to get sliding bushings with thinner walls so you can put a 10mm rod bushing in the same pocket as an LM8UU bearing.

I think I would suggest plastic wheels on carbon fiber rods rather than trying to put a cylindrical bearing on them. Less sensitive to diameter variation.

gtj0

This was my attempt to use 10x8x500mm CF ~~rods~~ roll wrapped tubes.
https://www.youtube.com/watch?v=y-h5bUulqL4

Here are the tubes.
https://smile.amazon.com/gp/product/B00RWYANKY

Tolerances DO vary. One batch I got from a particular manufacturer was so bad I returned them. The ones I got from ARRIS though required only minimal work on 2 of the 4 tubes with 2000 grit sandpaper and a quick polish and they were smooth end-to-end in the Igus bearings.

When I decided to upscale to a 500x500mm print area I tried 12x8x1000mm tubes but the tolerances were so bad I gave up and went with the Igus 17mm rails.

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