On diagonal rod length

So, I'm building a scaled-up Kossel Mini, with ~310mm horizontal extrusions. How long should the carbon rods be?

I've heard two 'rules of thumb': 0.8x the aluminum extrusions, and twice the delta_radius. However, both of these seem quite imprecise. What are the pros and cons of shorter and longer rods?

I will be using a reuleaux-triangle build plate that just barely fits inside the belts (with the corners of the triangle at the corners of the printer). I need to be able to reach a bit farther to retract the bed probe (right?), but that could just be one spot. So, how long should the carbon rods be?

You can select your diagonal rods whatever length you like, but:

• If you want to reach with your platform to the tower base (so that the corresponding diagonal rod is vertical) then you need your rods to be longer than sqrt(3) * deltaRadius. If you select the diagonal rod length just at this low boundary then you will be barely able to print behind the triangle defined by the towers.
• If you want your built area to cover a full circle with radius deltaRadius then you need your diagonal rod long at least 2 * deltaRadius.

The longer rods you select the more far away from the center you will be able to print (at the locations where towers are not in the way).
The longer rods you select the lower your print area height will be.

i did 300mm horizontal and 250 mm rods give me a build are of 110mm radius to be safe. current build volume is 220mm D x 580mm

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hercek
You can select your diagonal rods whatever length you like, but:

• If you want to reach with your platform to the tower base (so that the corresponding diagonal rod is vertical) then you need your rods to be longer than sqrt(3) * deltaRadius. If you select the diagonal rod length just at this low boundary then you will be barely able to print behind the triangle defined by the towers.
• If you want your built area to cover a full circle with radius deltaRadius then you need your diagonal rod long at least 2 * deltaRadius.

The longer rods you select the more far away from the center you will be able to print (at the locations where towers are not in the way).
The longer rods you select the lower your print area height will be.

DeltaRadius is the horizontal distance that the carbon rods span when the effector is centered, correct?

If so, it seems to me that having rods twice as long as deltaRadius would trace a reuleaux triangle with a minimum radius of deltaRadius, and having rods sqrt(3) times deltaRadius would give you a reuleaux triangle with the corners just touching the towers. Are these two assumptions correct?

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epicepee
DeltaRadius is the horizontal distance that the carbon rods span when the effector is centered, correct?

Correct. It is also computed as towerOffset - effectorOffset - carriageOffset.

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epicepee
If so, it seems to me that having rods twice as long as deltaRadius would trace a reuleaux triangle with a minimum radius of deltaRadius,

I do not know what radius of reuleaux triangle means. Also it would not be reuleaux triangle any more since:

• the curved sides of the shape are somewhat more "fat" than they should be (the radius of the arch would be smaller than it should be)
• the center of a arch of a curved side of the shape is inside it (not at its perimeter)

But the shape would be quite similar to a reuleaux triangle and a circle with radius of deltaRadius would just fit in it.
The actuall build area would be the shape with some area around towers cut off since the platform and rods cannot move through the towers. What the cut-off area look like depends on the geometry of your towers/carriages/platform.

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epicepee
... and having rods sqrt(3) times deltaRadius would give you a reuleaux triangle with the corners just touching the towers.

Correct.

Okay, that makes sense. How much of a precision hit do I take by making the rods longer?

Also, am I correct in saying that I need to be able to move outside the build volume in order to retract the bed probe?

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epicepee
Okay, that makes sense. How much of a precision hit do I take by making the rods longer?

I did not investigate how precision changes around built area. But around all the available built area it will by always about the same as your stepper motor single step length. The only thing which will change is how one carriage position error will distribute into hotend tip cartesian X/Y/Z errors.

On deltabot google groups, some german blogger claimed that errors around towers are bigger and that how big they are depends on the diagonal rod length. I believe he was wrong. I even responded to him to explain me why he thinks he is right but google have eaten my response instead of showing it up. I did not care enough to try to write it again.

I think the only thing which changes with different position around the build area is how one carriage position erorr translates to the cartesian X/Y/Z errors. This does not have much to do with diagonal rod length. If your sigle step corresponds to the change of 0.01mm on a tower. Then (for any of the cartesian coordinates separately) this will not lead to bigger error than 0.01mm. Regardless of the location on the build area.

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epicepee
Also, am I correct in saying that I need to be able to move outside the build volume in order to retract the bed probe?

I do not know. Depends on the exact design of the probe. Check some videos to see how much you need to move when deploying/retracting probe.
As I indicated in a different thread I do not use permanently mounted probe. If you calibrate your printer correctly there is no need for it after callibration. Maybe only if you are changing bed all the time or when the bed is bent or some other strange reasons.