Optimum Arm Spacing

Hi All, been a while.

I have read enough to calculate optimum arm length for delta radius on a large(ish) printer (1000mm height, 300mm build plate) but I can't seem to find anything on the ideal space between arms on the carriage and effector.

What is the suggested distance?

I am using Openbuilds mini v-wheel carriages so I can either work in or out from 50mm.

I am planning on using an E3D Chimera hotend so not a lot of space on the effector would be required.

P.S. does anyone have any experience with CNC Milling aluminium frame parts?

The arm spacing is a compromise. Larger spacing makes the structure more rigid. But larger spacing requires a bigger effector, so the nozzle can't get so close to the towers and the print radius is reduced. My printer is about the same size as yours and the arm spacing is about 45mm, but a larger spacing would be better for this size printer. I suggest about 60mm.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Yep, make them as far apart as possible for you platform and carriages. I use 77 mm. I should have done it 90 mm, it would still fit, just barely.

Would that be 60mm to center of arms, given my rid ends are 12mm then if they were on the outside the plate would be 48mm?

Yes, my suggestion of 60mm was between arm centres. But as hercek says, larger is better if you don't mind the loss of print radius, or if there are other reasons why you need a larger effector.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Effector size also depends on whether you want the heat sink above or below the effector. If you have it above, then you need enough space to add a print cooling fan without interfering with the arms.

My delta has slightly over 300mm build circle, 350mm carbon fiber arms, and heat sink above the effector. The arms are 60mm apart and the 14mm balls on the effector have a minimum distance between them of 1.25mm.

Everything except the extrusions was done on a CNC router.

I am a little confused. Are your arm spacings on the effector closer than on the carriage? Should they not be parallel in this plane?

mine is appx 90mm center to center for arm spacing

This should make it clear: [img.photobucket.com]

With you now. What thickness is your effector, I am trying to design to be as light as possible without compromising rigidity? Also, I would be interested in seeing what you did for frame corners as I am trying to come up with a simple design to CNC mill now?

This will probably give you a better idea of how the effector was made:


It's 6.35mm thick, but 4mm would probably be rigid enough. The aluminum portion weighs 23 grams and the ball bearings and magnets add another 160 grams.

I used 30 degree extrusions from Misumi to build the corner brackets. Misumi doesn't have any pre-made connectors to use these, so I through bolted them to 2550 extrusions. I had to make several jigs to hold the 30 degree extrusions in place while bolting them. While the frame ended up being perfectly aligned (at least as well as I can measure it ), I hesitate to recommend doing the corners this way. The motor mounts were made from 1.5"x3" rectangular tubing and bolted to the lower top and bottom plates and to the vertical extrusion, so add a lot of rigidity to the frame.

The carriages were also CNC'd:

Quote
Dave_robinson_022
With you now. What thickness is your effector, I am trying to design to be as light as possible without compromising rigidity? Also, I would be interested in seeing what you did for frame corners as I am trying to come up with a simple design to CNC mill now?

For metal corners you should look into these: [github.com]

Edited 1 time(s). Last edit at 09/29/2015 03:21AM by Koenig.

When I see, how much weight the ball bearings/magnets add, I would go for hollow balls and spring loaded rods instead.

Has anyone tried to drill a hole in the balls? It´s mild steel ...Are they available in aluminum?
Then place the ball with the hole aiming away from the rods contact area.
-Olaf

Just go for proper angle ball joints. Not TRAXXAS. Find a local store with parts for RC model cars/planes/helicopters. There will be a lot of high quality assembled ball joints there. They are extremely light, strong, and tight. No springs are needed. No heavy magnets which are going to rattle or even disconnect from time to time at higher accelerations.

I referred to etfrench´s effector. You can hardly use any RC ball joints with his geometry. The balls are too close.
-Olaf

There was a typo in my last post, the effector with 14mm balls and .5in ring magnets weighs 110 grams, not 180 grams

I looked at using Traxxas balls, but the ones I purchased weren't very spherical. They probably would work fine as the contact area is pretty small. I also thought about using a single ball at each corner, but it would weigh quite a bit more. This would be the perfect candidate for hollowing.

The 25000 corners should work well, but they're not really very good for CNC'ing unless you also have a brake.

My effector and carriage design have a provision for adding a tensioning spring if the magnets don't provide enough holding force.

Would there be enough attraction between a hollowed ball and a magnet? My reply was meant for spring loaded rods only. ( eg. aluminum ball(s) )
-Olaf