Carriage material

I'm building a very large Delta printer from MakerSlides (hexagonal, flying extruders, enclosure that also acts as to stiffen the construction) and I wonder if the carriage material does have a huge impact on printing. Sure less mass is better that's why I've went with carbon arms but when it comes to carriages that go just up and down does it really make financial sense to make them from carbon rather than from aluminum (let's say 3mm thick, 3 layers tall carbon 90x90mm carriage vs 3mm thick, 7075 aluminum 90x90 carriage)?

Also I've read about the "2:1 rule" when designing carriages. Could someone please elaborate? I can't find any info.

It is about as important to have light carriages as it is to have light diagonal rods. Actually you can argue it is more important to have light carriages (especially when the print head is far away from the corresponding tower). On the other side, carriage weight is not important when the print head is near to the corresponding tower. It is harder to accelerate linearly than to accelerate mostly rotationally and only partially linearly. That supports importance for lighter carriages compared to arms. On the other side, you cannot make carriages as light as arms since carriages will require steel bearings while arms can use light plastic ball joints. Do as good as you can. At the end it only depends on how high accelerations you want to use and how much ringing (print error) you can tolerate in your prints.

As for as the material thickness: Use such thickness which cannot be noticeably bent by the for forces from your steppers. E.g. you use 42 Ncm steppers and pulleys with radius of 6.4 mm; then you need some carriage material which will not noticeably bent under force 42/6.4e-1 = 66 N.

I do not know your 2;1 rule.

thank you, that was very helpful

Could someone please point me in the direction of some nice literature or arms spacing? I'm building a delta with outside radius of about 500mm and height of about 1500mm and I wonder if 100mm.
Thanks

Put arms in one pair as far away as possible (i.e. as your platform size allows or as much as you are willing to enlarge it). The father away the arms are the less your printer will be sensitive to errors in the arm length and arm mounting point positions on carriages/platform. The bigger distance also provides bigger lever for forces (which means lower stress on the arms and the mounting point locations).
Of course, increasing platform will result in decrease of the printing area. E.g. my printer has build area about 20x20 cm and arm distance 7.5 cm. So you do not want to increase your platform size way too much.

How about the "Given that diagonal rod length, choose the rod separation. I suggest about 1/6 of the rod length." rule that originated on David Crocker's Solutions blog?

Rod length does not have any direct influence on the optimal rod separation. 1/6 proposal is just pulled out of thin air. The important decision is how big platform are you willing to use. The distance of platform middle to an line connecting two corresponding rod ends is equal to the half of horizontal space your printer will be wasting because of the platform size.
The rules are:

• printer size influences rod length
• printer size influences maximum reasonable platform size (to limit wasting of space)
• platform size puts the top limit on the rod separation
• rods should be as much separated as the platform allows

Is there a downside to big (30-35 degrees) minimum angle other than lowered print height?