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Smooth rods and diagonal rods length dimension -Rostock

Posted by pdesigns 
Smooth rods and diagonal rods length dimension -Rostock
February 21, 2013 06:11AM
Hi

I´m trying to build a Rostock, and i have doubts on the dimensions of the rods and the resulting printing area.
I have 500mm smooth rods, and i wanted to ask how can i determine the diagonal rods length and the resulting printing area?

If possible i would like to get something arround 200x200x200.How can i see if its possible to get this printing envelop?
VDX
Re: Smooth rods and diagonal rods length dimension -Rostock
February 21, 2013 06:37AM
... you'll get a good estimate with drawing a sketch with one arm positioned horizontal - then the center of the toolhead should be able to draw an arc from one of the opposing struts to the other.

You won't get this far, but longer arms make the bot more wobbly and much shorter will loose the (maybe comming, optional) ability to print 'between the struts' with a rectangular build base bigger than the common circular footprints ...


Viktor
Re: Smooth rods and diagonal rods length dimension -Rostock
February 21, 2013 12:02PM
Thanks VDX

I shall look into it later on tonight!
Re: Smooth rods and diagonal rods length dimension -Rostock
February 22, 2013 03:26AM
I did a rough sketch of how to look at the build area in this post.

I suggest you aim for a rounded triangle, with each vertex close to the base of the closest pillar. That way, there are no points in the travel of any arm that are excluded by mechanical constraints. To do this, you need to set up the ratios so that one arm is completely vertical when the other two are completely horizontal. I can do up the math for you if you want, but I don't have time to look at it just now.
Re: Smooth rods and diagonal rods length dimension -Rostock
February 22, 2013 11:24AM
Ok. I have 500mm smoth rods, i did the diagonal rods with 210mm (from center to center of the ball ends) and i can get one arm completely vertical when the other 2 are completely horizontal.

Now i have 410mm where the vertical carriages can travel, so if i´m not wrong i should be able to get something around 150mm of printing height.
I will also do the math to see what i get.
Re: Smooth rods and diagonal rods length dimension -Rostock
February 22, 2013 01:11PM
I did the work to the scad model to make rod lengths and frame size parametric and set the full model up so that it could be animated with the head moving in a circle. I think johann pulled the changes. I found that playing with the numbers using that gave a good feel for how to balance platform size, rod length and build height
Re: Smooth rods and diagonal rods length dimension -Rostock
February 22, 2013 01:57PM
I will have to look at scad first (never used it) and see the rostock scad files. I´ve been wanting to try and use it, but i´ve been missing a little motivation..I´m more a man of nurbs and sufaces modelling.. Nothing like necessity to give us a little push smiling smiley

But this give me the idea of doing one article/tutorial about the rostock and/or kossel. Aimed at the noob level..a mix between something like the "prusa mendel visual instructions" and an easy explanation on how to dimension the bot if we go bigger or smaller..

If theres any interest in joining some efforts it would be great stuff for the second issue of the reprapmagazine!
Re: Smooth rods and diagonal rods length dimension -Rostock
February 22, 2013 10:01PM
Anything that helps me understand the math behind this amazing design, I'd love to read.


I can't be of much help to you as I took the lazy way and am building a kit. ;-)
Re: Smooth rods and diagonal rods length dimension -Rostock
February 22, 2013 11:37PM
Here is an animation I made from the model. The distance of the hinge from the smooth rods makes a difference to the achievable build area for a given strut length. The further from the rods the better, because you can use shorter struts which means you get greater build height, but this will put torsional loads on the bearings which may not be good. In the example, I had made that distance parametric, but had not updated the model for the carriages to deal that, so there is a gap between the hinge and the bearing mounts.



Edited 2 time(s). Last edit at 02/22/2013 11:48PM by Greg Frost.
Re: Smooth rods and diagonal rods length dimension -Rostock
February 23, 2013 06:45AM
I think i´m beginning to understand the principals.

Having that gap between the hinge and the bearings optimizes the build space, but has the down side of the torsional loads you mentioned.To reduce this torsional effects i would have to bring the smooth rods closer to the center of the printer.
If i put the hinge right next to the bearings (without the gap) the build area will be smaller (on the horizontal plane). If i then increase the diagonal rods to increase the printing area i will reduce the maximum print height.
Re: Smooth rods and diagonal rods length dimension -Rostock
February 24, 2013 10:27AM
The radius of the flying carriage also factors into the horizontal reach. This is easiest to visualize when one set of rods is at the low extreme and are near horizontal. The flying carriage effectively increases the rod length by its radius.
Re: Smooth rods and diagonal rods length dimension -Rostock
February 25, 2013 01:34AM
@xclusive585, the math behind rostock is in the post I referenced above. If you have any questions about it, I'd be happy to explain.
Re: Smooth rods and diagonal rods length dimension -Rostock
February 27, 2013 04:23PM
So i ended up with this settings:

// Center-to-center distance of the holes in the diagonal push rods.
#define DELTA_DIAGONAL_ROD 222.0 // mm

// Horizontal offset from middle of printer to smooth rod center.
#define DELTA_SMOOTH_ROD_OFFSET 175.0 // mm

// Horizontal offset of the universal joints on the end effector.
#define DELTA_EFFECTOR_OFFSET 33.0 // mm

// Horizontal offset of the universal joints on the carriages.
#define DELTA_CARRIAGE_OFFSET 22.0 // mm

with
#define MANUAL_Z_HOME_POS 175

i already did some printing tests on a 20x20mm and its base come out 20.20mm..some calibration needed...and the distance between the nozzle and the bed is fine at the center, but not on the edges. Still some work to do.
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