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[ortwin]

...
My optimal stem is 4 mm, not 5. 
...

Ah, I took 5 mm because in reply #222 you gave the highest speed to the stem with 5 mm.
3 mm to 4 mm is the range where I want to start experiments with the multiple twirl technique with two palms. 
But right now I want to do  a run in vacuum.

[ortwin]

Alright, JBB took a spin in the vacuum. It lasted well over one hour. Maybe one hour and 11 minutes. I did not watch it all the time, but that is what I get from the video footage I took and the spin decay curve ( available upon request). Starting speed something above 600 RPM. Toppling seems to be above 100 RPM, no proper balancing, contact point to CM distance quite big in comparison to the best spins of JBB in air.

The video is showing only about the first two minutes and the last two minutes. Next time I need to place a clock visibly in the video.



Another Q&D production ... I hope you don't mind...

[Jeremy McCreary]

That darn air -- can't live with it, can't live without it!

Well done!

[ortwin]

3 mm to 4 mm is the range where I want to start experiments with the multiple twirl technique with two palms. 
...

It turned out to start at more like 6 mm. I cut the head of a M5 screw and put some black heat shrink tube over most of it.
JBB could also stand for "Japanese Brass Band"
To me it is the most elegant looking version of JBB so far. As others said before: brass and black go together well.


Did I get it right, that palm shear starting technique is particularly common in Japan?


I got JBB up to more then 1700 RPM  this way, in the video it is 1250 RPM. So at that end I win at least three minutes but I lose a lot more at the low RPM end so far:
-can't do it on the sapphire yet
- contact point to CM distance is a lot higher
- not properly balanced yet
...
So it is still below 20 minutes in both timed spins it performed so far.

It is a totally different category anyways than the single twirl with one hand. I think if I come back to this in more seriousness, the goal should be 45 minutes.

[ta0]

Very nice!

Do you have any idea what is the vacuum that you can get with the hand pump? I wonder if the decay is now completely determined by the tip friction or if there is still some residual air drag.

Did I get it right, that palm shear starting technique is particularly common in Japan?

It's common with Kyukogoma tops.

[ortwin]

...
Do you have any idea what is the vacuum that you can get with the hand pump? I wonder if the decay is now completely determined by the tip friction or if there is still some residual air drag.
...

I did not measure the pressure directly but my guess is something like 3 -7 mbar. It is from an "Ice-Quick" set
  .
Since it  works in freezing the water, the pump must be able to  make a vacuum better than 6 mbar which is the saturation vapor pressure of water at the freezing point.There is certainly still plenty of air drag! 6 mbar is very good for a hand pump, but it actually more at the upper end of "medium vacuum".There is a measurement method for vacuum that makes use of exactly that air drag  on a levitating ball. Martin (Levitron) Simon pointed that out to me when I was fooling around with that levitating magnetic sphere:
"
Nice try.  With that pump I think you are still in the heavy
air friction dominated damping region.  Maglev rotatingspheres are used as pressure gauges.  Measure the dampingrate and you know your pressure, from 1 - 10^-6 mbar(Torr).
https://www.mksinst.com/product/product.aspx?ProductID=283 "

You see, that method is applied at some orders of magnitude lower pressure than we deal with here.

[Jeremy McCreary]

The aerodynamic braking torque in the von Karman case is proportional to air density and the square root of the air's kinematic viscosity.

The good news: The density is proportional to pressure at constant temperature.

The bad news: To get a significant reduction in the kinematic viscosity, you'd need to approach the kind of vacuums used in particle accelerators.

ortwin, how far are you from CERN?

[ortwin]

...
ortwin, how far are you from CERN?

If I travel the distance I am away from CERN not to Switzerland but straight upwards, I should also reach some decent vacuum.But the vacuum is only another silly diversion, I mean we all know that things can spin quite long when there is nothing braking them. Take our mother planet: 4.7 billion years or something?

[ortwin]

JBB 29:19
Not much changed from reply #216 or #212, tiny bit faster start (comes from all that practicing I guess) ~825 RPM. Topple at ~79 RPM since I took more time and care balancing the top while making sure those balancing nuts where as much as possible inward on the spokes. That reduction in air drag resulting from the position change of those tiny nuts really seems to make a difference of a few seconds (that is why I plan a next version with even thinner spokes and better balancing and centering options).
So if those larger diameter knurls, that I am expecting by the end of the week, really help me to get starting speeds of up to 900 RPM as I hope for, I should be able to report some time above the half hour next week.

[Jeremy McCreary]

Your coming upgrades sound promising.

[Iacopo]

JBB 29:19

Congratulations ! 
Now you are very near to my best spin of a brass top/external tip/single twirl, (with carbide contact points):

29m 38s

Better than this with a single twirl/external tip I have only the spin obtained with my tungsten top, (36m 11s).

You are doing very well !

[ortwin]

...

Congratulations ! 
Now you are very near to my best spin of a brass top/external tip/single twirl, (with carbide contact points):

...

Thank you Iacopo!
Please send me your postal address via private message. I want to mail one of those flat sapphire watch glasses to you. I am curios how your tops will perform on that surface.

[Jeremy McCreary]

I got JBB up to more then 1700 RPM  this way [palm twirl], in the video it is 1250 RPM. So at that end I win at least three minutes but I lose a lot more at the low RPM end so far:
-can't do it on the sapphire yet
- contact point to CM distance is a lot higher
- not properly balanced yet
...
So it is still below 20 minutes in both timed spins it performed so far.

Tough trade-off. Unfortunately, that long metal stem increased both central TMI/AMI ratio and CM height to the detriment of critical speed, as you observed. Also put more weight on the tip.

Following the teachings of Iacopo-san, you might try a low-density wooden stem instead. For single finger twirls, the wooden stem could be as short as 24-32 mm, preferably tapering upward.

The tapering end of a cheap disposable chopstick comes to mind. Not so elegant, but the unfinished texture should be good for grip.

[ortwin]

... For single finger twirls, ...

I would like to see that performance! Can you make Q&D video? Which finger are you using? 

[Jeremy McCreary]

... For single finger twirls, ...

I would like to see that performance! Can you make Q&D video? Which finger are you using? 

Pinkie. No video -- top-secret technique.