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Author Topic: Educational size rotational molding machine  (Read 49784 times)

ta0

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Re: Educational size rotational molding machine
« Reply #150 on: July 26, 2019, 07:36:54 PM »

I have wondered if I could print a top out of a soft material. They have ninja flex. I wonder if it would deform while spinning🤔
I have a couple of different flex materials but have not been able to optimize the 3D printing yet. I upgraded the extruder to the latest version to do this, but I still have to work on it. Right now I can print very nice flexible mats  ::)
Diabolos have soft shells but, of course, spin much slower.
My guess is that the top would work.  Centripetal force would tend to keep it round. The concern would be that it develops some vibration/flex modes.
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jmadrigal

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Re: Educational size rotational molding machine
« Reply #151 on: July 27, 2019, 07:30:38 AM »

Possibly some internal cross bracing would be needed to prevent deformation during the spin?
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Jeremy McCreary

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Re: Educational size rotational molding machine
« Reply #152 on: July 29, 2019, 02:50:43 AM »

I have wondered if I could print a top out of a soft material. They have ninja flex. I wonder if it would deform while spinning🤔

My guess is that the top would work.  Centripetal force would tend to keep it round. The concern would be that it develops some vibration/flex modes.

Some of my LEGO tops have more flexible structures than others. When these deform under the abrupt acceleration of spin-up, they can "ring" for some time after release. Especially susceptible to these vibrations are tops carrying much of their mass in spokes with outer ends free to flap and twist.

By structure and material stiffness, metal, hard plastic, and wooden throwing tops are generally much more rigid than my tops. But I can see a hard landing or a jerk of the string exciting lasting vibrations in a throwing top of softer materials.

LEGO precision molding guarantees static balance when all parts are fully seated in a symmetrical arrangement. In addition, tops with parts designed to move under centrifugal force usually self-balance quite nicely. (Might also be true of a flexible throwing top.)

So most of my wobbles end up coming from excited vibrations. Gentle spin-ups can help, but the only real cure is to change the mass distribution or structural stiffness or damping to shift mode resonances away from operating speeds. Problem is, that may not be possible when you're going for a certain look or behavior.
« Last Edit: July 29, 2019, 03:24:50 AM by Jeremy McCreary »
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Jeremy McCreary

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Re: Educational size rotational molding machine
« Reply #153 on: July 29, 2019, 03:13:49 AM »

I have wondered if a rubber roto-molded top would be possible . . .

Some of my LEGO top rotors are based on wheels. And I sometimes fit them with rubber tires to increase AMI. (The rubber is a good bit denser than the ABS plastic.)

Problem is, tops like this don't recover from scrapes. The rotor grabs the ground or the hand instead of bouncing/slipping, and the spin comes to an abrupt halt.

May be more of an issue in finger tops, but it might come up when correcting a throwing top. String coming in contact with the body might also grab on instead of sliding smoothly toward the notch in the tip.

Still, if anyone could overcome these challenges, it would be you.
« Last Edit: July 29, 2019, 03:19:17 AM by Jeremy McCreary »
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the Earl of Whirl

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Re: Educational size rotational molding machine
« Reply #154 on: November 24, 2020, 10:02:29 PM »

Here is a blast from the past.  I did not realize it has been six years since we started working on this and helping Don get started on downsizing.  It is very interesting to reread how it all went down.  I forgot a lot of it.

I bring this up because I now have a few more of these large plastic tops that Don originally made with the rotomolder.  It makes me laugh to see them all in a row!!!
« Last Edit: November 24, 2020, 10:05:19 PM by the Earl of Whirl »
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