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[ortwin]

...
What is the diameter of the knurled knob?

The knurled knob has a diameter of 10.5 mm - dictated by availability.  I wanted a larger diameter, 12 to 13 mm.
That number is based on the diameter of the stem of my "Quark Top".  With that I can reach starting speeds of above 1500 RPM.
Therefore I think the smaller torque I can apply with the thinner stem/knob is limiting my starting speed with JBB to the observed values of below 800 RPM, mostly.

[ortwin]

JBB 28:07

The new hardware I mentioned in a post above is doing well, but that was not the cause for this improvement in spin time.
With the "JBB hardware update" my best time improved only by three seconds to 26:25. It was clear that further better balancing would be needed to get to the improvement of about  2 minutes I was expecting from that hardware update.
But then on Monday, the  two sapphire watch glasses arrived that I ordered for cheap from China a few weeks ago.

 

They are flat and too thick to be made concave with the vacuum trick. That means the base has to be leveled carefully for the top not to wander off the sapphire. The tungsten carbide ball tip with the 0.5 mm small diameter helps in that respect because the top is not moving around too much. Nevertheless, I had to adjust the leveling of the base during that 28:07 run once, at around two minutes to keep JBB from going for a hike. (I guess that disqualifies that spin from being a "legit" one, in my book at least.)
But the combination of a tungsten carbide ball tip and sapphire really seems to work well! More tests are needed regarding the lubing and so on....
First I will try to get JBB really good balanced, that is why I put those nuts back on the spokes. If I manage to get back to that nearly perfect balance, I am confident that JBB can spin past the half hour barrier on sapphire.



 The starting speed for the 28:07 run was ~800 RPM, toppling at ~88 RPM . I did not record  a full spin decay curve for this, I just noted some values on paper. On a glass mirror surface I have a SDC of 26:25 should any one be interested. Since there is still some clear progress on the spin times without close inspection of the spin decay curves, that is not my major focus for now.
 

[ta0]

They are flat and too thick to be made concave with the vacuum trick. That means the base has to be leveled carefully for the top not to wander off the sapphire. The tungsten carbide ball tip with the 0.5 mm small diameter helps in that respect because the top is not moving around too much. Nevertheless, I had to adjust the leveling of the base during that 28:07 run once, at around two minutes to keep JBB from going for a hike. (I guess that disqualifies that spin from being a "legit" one, in my book at least.)

Good progress!
In my book slowly adjusting the base so it doesn't spin out is legit.

[Jeremy McCreary]

In my book slowly adjusting the base so it doesn't spin out is legit.

I agree. You're just suppressing travel dynamically. Any effect on spin decay rate would be upward. No regeneration involved.

[ortwin]

In my book slowly adjusting the base so it doesn't spin out is legit.

I agree. You're just suppressing travel dynamically. Any effect on spin decay rate would be upward. No regeneration involved.

Not everything that is not regeneration is fine. For example: I could start the top and at the same time the stop watch. But then I realize that the top is precessing too much for a good end result. I touch the top somehow to kill most of the precession. This certainly only slows the top down. Do I let the stop watch running? Do I start it at zero again after the last touch to the top? Should I just stop the run and try a better one?

Yes, it is fine for now. I will be happy to reach the 30 minutes goal with some leveling in between. But after that the next goals will not be a longer spin time, it will be to go past the half hour in a clean way: no leveling or otherwise messing around with the base (lubing, pumping vacuum ) during the spin. No touching the top after the initial twirl. Recording a video  of the whole spin. Recording a spin decay curve of the whole spin. No pedestal, make sure that the contact point is always the lowest point of the top. Except for the spin decay curve that video of the Kemner "top" that spins for almost 50 minutes provides all that. 
 
 In the meantime JBB has taken a spin for 28:36 , with quite some leveling of the base and some vacuum pumping to keep the sapphire fixed in place. during that time.

[ortwin]

JBB 28:59
Today a double domed sapphire watch glass of 30 mm in diameter arrived.It works great as base for JBB! The friction seems to be as small as with the flat sapphire. But with this I don't have to work on the leveling screws during the spin any longer. And  in addition to that, it is more forgiving with twirls that give the top some lateral momentum at the start: instead of sliding straight off the base, the top just comes back to the center. That leads to a higher average starting velocity. For the 28:59 run the start was at about 820 RPM, topple speed around 79 RPM.
The minute that is missing to the half hour, I hope to get from still better balancing (~1/2 minute) and from a higher starting speed (~1/2 minute). I might need a different stem of larger diameter though, to reach a higher starting speed.
 Since the sapphire does not look that much different in a photo  from the flat one in reply #212, and on JBB basically only the positions of the balancing nuts changed a bit, I am not posting any pictures here.

[Jeremy McCreary]

Congrats on the continued progress!

Your current 10.5 mm knurled stem is already quite thick. By all means, keep some kind of knurling. But given JBB's substantial but not extreme AMI, you might also experiment with thinner stems -- especially if you're allowing multiple twirls.

Or better yet, try some tapered or stepped stems. All of my high-AMI tops have stems stepping down from about 7 to 5 mm, with my fingers starting on the thicker part and moving up to finish on the thinner. Without the thinner step, their release speeds are considerably lower.

Iacopo's stems taper down to 3 mm.

[ortwin]

....you might also experiment with thinner stems ...

Thinner ?  Should my reasoning from reply #211 be so terribly wrong?

...
The knurled knob has a diameter of 10.5 mm - dictated by availability.  I wanted a larger diameter, 12 to 13 mm.
That number is based on the diameter of the stem of my "Quark Top".  With that I can reach starting speeds of above 1500 RPM.
Therefore I think the smaller torque I can apply with the thinner stem/knob is limiting my starting speed with JBB to the observed values of below 800 RPM, mostly.

To my understanding the diameter of the stem sets an upper limit to the max. attainable speed. The thinner the stem, the higher the possible speed. Since I reach above 1500 RPM on a top with smaller AMI than JBB, and a 12 mm stem, that diameter can't be too large to reach the goal of 1000 RPM with a top of larger AMI (JBB). Therefore the acceleration  I can provide for the top over the short time of a twirl must be too small if I can only get to about 820 RPM. For larger acceleration a need a stem of larger diameter. Tomorrow I want to look for something with a 15 mm diameter - unless  someone is telling me where I am wrong.
 

... By all means, keep some kind of knurling. ...

If possible yes, but the trick with the rough sandpaper glued to the stem is also working very well.
 

...-- especially if you're allowing multiple twirls....

Multiple finger twirls don't work for me as I said, also that is Iacopo's territory  .But I am considering -as a different endurance category-  multiple twirls with two palms.  It is easy to attach the according stem to JBB. But what is the "according" stem for that type of start? About 10 cm long I would guess. But what thickness? Which material? Maybe some carbon fibre, maybe wood?

[Iacopo]

....you might also experiment with thinner stems ...

Thinner ? 

I agree with Jeremy and I think it is worth to experiment with different diameters, also thinner ones.
In fact, even if a larger knurl diameter provides a larger acceleration, the acceleration phase lasts less time with it, so it is not certain that the starting speed will be higher with a larger knurl. 

[Jeremy McCreary]

I've found the topmaking biz quite humbling. It's always good to plan your next design iteration carefully. But top dynamics are highly non-linear. So are twirling dynamics. Even the most educated guesses may not pan out in practice.

So it's also a good idea to test outside your expectations.

[ortwin]

With JBB I used basically three different stem diameters: 7.5 mm, 10.5 mm and ~12.5 mm.
The 7.5 mm stem brought JBB across the 25 minutes by providing a starting speed of 720 RPM. In reply #207 I then reported with the same stem 730 RPM.

 With the 10.5 mm knurled stem I saw starting speeds up to 820 RPM as in reply #216.
With the stem of ~12.5 mm and the sand paper glued to it I reached as well about up to 820 RPM. But this stem seems too tall for being in good balance with JBB. It always loses at the low RPM end a lot of time.

So for me things are pointing at the moment to a short stem with a diameter of about 15 mm.
I go and look for something like this now:



It might not be the right path, but I need to try.

[Iacopo]

In my personal experience, I have better starting speeds with less large stems, even with single twirl spins.
Maybe it is not the same for everybody.  Maybe there are persons with hands that are more agile and less strong that the mine, and who can find easier to spin a top with a larger stem.  Probably the optimal stem diameter for tops with a larger AMI than the one I tested here, could be a bit larger.

This is a test I made;
I turned an extension for the stem, and I turned its diameter progressively more and more narrow, from 12.5 mm to 3 mm.
I spun the top hard with the extension, many times, these are the highest starting speeds I had with the different diameters of the extension:



The grip of the extension for the fingers was perfect for the larger diameters but a bit slippery for the littler ones.
So, adding a knurl would further favour the littler diameters.

3 mm is very thin and I have two problems with it: little surface in contact with the fingers, (so it slips more easily), and also the fingers coming in contact to each other, so part of the energy is lost for friction between the two fingers.  4 mm works best for me, even for single twirl spins, so I make my tops with about this diameter of the knurl. 

[ortwin]

Not necessarily a contradiction to my findings. I would guess JBB has an AMI about as three times as your test top? So things are a bit different.
I will keep you updated on my results. By the end of the week I should receive the knurls with 16 mm diameter.
 

[ortwin]

@Iacopo: Actually your findings go well together with my plans and my simple model for the optimum stem diameter: The optimal stem diameter scales directly with the AMI of the top. Since JBB has about three times the AMI of your test top, the optimal stem diameter should be three times your optimal stem diameter. Your optimal stem diameter is 5 mm, so it will 15 mm for JBB, as I plan at the moment.

[Iacopo]

Your optimal stem diameter is 5 mm.

My optimal stem is 4 mm, not 5. 
Without knurl, the 4 mm tends to slip between my fingers so 5 mm seems better.
With the knurl, 4 mm is best for my tops.
I am not sure that 3 mm could be even better than 4 mm, because I have particular problems with it. Maybe I should make thimbles of some hard rubber to overcome those problems, and test, so I could know it.

Anyway, it makes sense to me that with a larger AMI, the optimal diameter of the stem increases.