Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Jeremy McCreary]

Interesting variability in your speeds at first scrape.

[ortwin]

Interesting variability in your speeds at first scrape.

Since balancing is not perfect and not exactly the same each time, plus the scraping angle varies and also the distance from contact point to CM, it is not THAT surprising.
Well, I guess that is what you get for free if that many adjustments and changes are possible in a modular system.

[ortwin]

JBB 22:57
 

 

 

You can see that I took the balancing nuts off. That is certainly not why the time improved, but I got it balanced pretty good only by adjusting the spokes. In fact I don't think balancing could be much better now. Since there was some luck involved getting to that point, I think I won't touch those screws for a few days now. Critical speed is now consistently 71 +/- 1 RPM.
Visually it is still not running perfectly smooth. The reason for that is, that the stem is not perfectly at a right angle to the plane of the ring. But that does not seem to hurt much, the principal axis of inertia is running through the contact point, and that is the main thing!
Now that JBB is running stable, I could test different surfaces. I have different concave mirrors, that touch screen from a tablet computer, the bottom of jars, jar lids, bottle caps. As lube I used that "forehead lube" recommended here in the forum. The tip is a 4 mm (white) ceramic ball from a ball bearing.
And I was surprised to find these big differences: On most "good surfaces" it took about eight minutes to spin down from 170 RPM to 72 RPM when it toppled. But one little (only very slightly ) concave mirror stood out, here it took a whole 2 minutes longer!
With that one I got that 22:57. It is visible in the photo sitting on a larger mirror that has  a blue frame. The problem is to get the top to high starting speeds on this small area. It is not so much the scraping that is the problem, the problem is that if it has too much tilt or lateral speed upon release, it will wander off the mirror: the brass ring will touch the rim of the mirror and the run is over. So with the setting as it is, I get get to about 600 RPM spinning stable on the little mirror, with lets say trying ten times, but on different surfaces I had it already running for two minutes when speed was down to 600 RPM.
That means for me that I look around for larger surfaces that behave as well as that specific mirror. If I get those two extra minutes at the high speed end, a goal is reached.

[Jeremy McCreary]

Wow, only 2:03 away from your goal!

I've also found that not all glass is created equally when it comes to tip resistance. No clue why or how to tell the good stuff short of testing.

[ortwin]

JBB 24:05

No substantial changes done to the equipment this time. The improvement in spin time came mostly from the the higher starting speed. It must have been about 650 RPM. I could not get a good measurement of speed in the first few seconds. When speed was down to 600 RPM, the clock (and the top) was running for 48 seconds already.

I will try bit further, before I change much with the top itself. Maybe I get a lucky start with more than 750 RPM !

[ta0]

You almost reached your goal of 25 minutes! You can already put the champagne in the ice bucket.

[ortwin]

It will not be champagne! I promised myself as reward, that I will order that "Recording laser Tachometer" that Jeremy reviewed in the other topic.
And of course I start thinking of how I tackle the next obvious goal: nylon spokes again? 0.7 mm tungsten carbide ball as tip? Recessed tip?

[Iacopo]

JBB 24:05

This is a very good spin, better than 23:56, which is my longest spin on a glass surface, (brass, single twirl and external tip).
But I use glass rarely, I almost always spin on tungsten carbide spinning surfaces.
 

[ortwin]

IACOPO: Maybe one of these days I will ask you for advice on how to get that tungsten carbide surface, or maybe we can make some deal and you send me one.
But before that I try with different glass, hardened glass as in display protectors or even sapphire if I can get hold of something reasonable.

[Iacopo]

IACOPO: Maybe one of these days I will ask you for advice on how to get that tungsten carbide surface, or maybe we can make some deal and you send me one.

I buy tungsten carbide rods, diameter 5 mm, which I cut in pieces and attach to my inox steel horns. 
I use a diamond ball bit diameter 8 mm for to shape the concavity in it, then I polish the surface with diamond pastes.

[ortwin]

With what I have now, I will not get past the 25 minutes.  The glass surface of that small mirror that gave me the best times, is deteriorating faster than I can improve the starting velocity! Small scratches accumulate and that slows  down my top faster. 
 
But let me quote Jeremy : "Spin time is not everything." and: "The good news is:" That type of mirror is still available and it is not expensive. So if I get another one and prepare it with the knowledge I gained with the current mirror, this top configuration might still prove suited to spin longer than 25 minutes.

...

I've also found that not all glass is created equally when it comes to tip resistance. No clue why or how to tell the good stuff short of testing.

Maybe that is the case, but with my equipment it could also just be that the different glass surfaces experienced different wear: That little mirror, my best performer, I have hardly ever used until a week ago. The other surfaces I compared it to, had to endure a lot of spins already by than. So that could be an explanation of the differences and also why the performance differences become smaller now.
I am looking forward to get another new mirror of the same type as well as to some display covers that I ordered: Those claim to be scratch resistant, "hardness 9", but no sapphire yet.
We will see, I mean I will see and then report for you to read.

[Jeremy McCreary]

Hardness 9 sounds like a good idea, because digging or drilling into the support during the spin could be a more important contibutor to total tip resistance than simple sliding friction or rolling resistance.

Concave lenses are also good sources of glass concave surfaces. They're very precise and, if coated, could be quite scratch-resistant. The longer the focal length, the smaller the curvature.

Best of all, if the diameter of the lens is significantly smaller than the rotor's, the lens will just be a pedestal, and your scrape angle and spin times will increase accordingly.

[ortwin]

...
Concave lenses are also good sources of glass concave surfaces. They're very precise and, if coated, could be quite scratch-resistant....

Yes I saw you use those concave lenses sometimes. I could not get a hold of one up to now. Maybe if I manage to take that old large camera lens apart, I can find a concave lens in there. But up to now it does not want to be opened with my unsuited tools. And I do not really want to use saw and hammer!
The coating is usually for the minimization of reflection losses, not for hardness or scratch resistivity, so it could be counter productive.  That is also why usually sapphire glasses of watches are only anti-reflection coated on the  inner side.

 

...
Best of all, if the diameter of the lens is significantly smaller than the rotor's, the lens will just be a pedestal, and your scrape angle and spin times will increase accordingly.

That is how that mirror you see in replies #155 and #153 acts already. And it is important to get those good times. Ultimately I want to reach my goals without that helpful "pedestal" or "dedicated base" effect, but for now it would be fine with me.  By the way, the contact point in those runs is about 2 mm downwards from the lowest point of the ring.

[ortwin]

No further improvement.
JBB lost its almost perfect balance, and I was not so lucky this time in re-establishing it.
I got a few more (new) mirrors and a protective smartphone cover as base surfaces, I have different lubes that I can try, I improved the stem so I could even start it with more than 800 RPM - but all that is nothing  if you want more than 25 minutes and the balance is poor.
 The problem might be one that can not be solved with this version of the top, maybe only  another lucky punch in setting it up is possible.
The skew stem, with respect to the ring, is where I think the main problem is. Plus the direction of where the stem/tip is pointing to, changes every time I twist one of the centering crews/spokes with the screwdriver. That means I still do not have enough independent control of the settings in the top: it seems I need to be able to control both, the position of the axle as well as the direction it is pointing to.
I will try that in a next version with the aluminum flywheel of "Easy Listening" using six adjustable nylon spokes. But I doubt the spokes will be stable enough to allow for a good fast start by a stem.

[ortwin]

JBB 24:08

 

Only a very marginal improvement by three seconds over the previous best with this top in reply #155.
Start with 668 RPM, first scrape at 86 RPM.
As in reply #145, I used the short knurled stem directly fitted with the ball point pen tip. Although the starting speed and total time is very similar to my old best run, the spin down curve is quite different. In this run I lose about two minutes at the low RPM end, but that time  was made good over the total rest of the RPM span.
One of those plastic curtain rings that I so often used before as flywheel, made an appearance here as part of the pedestal:

 

The balancing nuts I use here are different from the ones I used before. I can adjust them now easily with my fingers, no need for tools. But they really seem to cost some time: when I had them close to the ring noticeable more even. That is why I moved them towards the stem. That observation leaves hope for further improvement when moving to thin lines as spokes... we will see.