Delta geometry, effector offset, rod lenght

Hi,
I just got second hand Kossel xl with replacement rods and effector
So I have to replace rods and effector. Print Hight is 280mm and radius around
250mm. Towers are 223mm from center and Rods (Traxxas joints) are 320mm long
Original effector has 40mm arm spacing and 33mm offset, replacement
has the same 40mm arms pace and shorter 22mm offset
Here are my questions:
Should I replace Effector with shorter offset and why not?
Do I have to change Rods Length to compensate for shorter
Effector offset ?
Any help is much appreciated

Regards
Mario

Quote
novio8
Towers are 223mm from center

Is that the distance from the mid point of the joints on the carriages to the centre (which is the dimension we need in order to advise you), or something else?

A rule of thumb is that when the nozzle it at the edge of the bed opposite a tower, the rods to that tower should be at an angle of at least 20 degrees to the horizontal. If this angle would be smaller, use longer rods.

Edited 1 time(s). Last edit at 05/12/2019 03:25PM by dc42.

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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].

dc42, thank you for replay
distance from the mid point of the joints on the carriages to the centre is 193mm
Mario

Quote
novio8
dc42, thank you for replay
distance from the mid point of the joints on the carriages to the centre is 193mm
Mario

I assume that the 20mm offset you refer to is the distance between the centre of the effector and the mid point of a pair of bearings on the effector. That makes your delta radius 193-22 = 171mm. The rod length between bearing centres is typically chosen to be about twice the delta radius, which suggests 342mm rods

As the tower bearing centres are only 193mm from the centre, I don't see how your print radius can be as high as 250mm. Unless the belts are well behind the carriage bearings, it will be a little less than 171mm before the effector bumps into the belts. Perhaps you meant that the bed diameter (not radius) is 250mm?

The maximum print radius R without violating the 20 degree rule will be given by:

cos(20 degrees) = (171 + R)/320

This gives R = 320 * cos(20 degrees) - 171 = 130mm.

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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].

dc42,
yes bed diameter is 250mm, sorry.

As I understang one has to change rod lenghts, to compensate for shorter Effector offset.
I have 6mm OD Carbon rods and Traxxas Rod ends.
Does new Rod lenght change stabillity or performance ?
Is smaller effector less prone to tilt ?

My calculations indicate that with a bed of up to 130mm radius bed you wouldn't violate the 20 degree rule using the existing rods. So you don't need to get new rods unless there is something wrong with the old ones.

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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].

dc42 thank you,
I searched for math formula to calculate maximum angel that Rod Ends have
at very end of Bed to verify if Traxxas can provide that kid of Swivel. I did not find any,
Do you now how ?
Rods are OK,and Traxxas have no play, despite being 4 years old, that is quite supriseing
as Delta is enclosed and heated to about 50C to print ABS

Edited 1 time(s). Last edit at 05/14/2019 04:07AM by novio8.