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Author Topic: Anybody heard of skeleton molds?  (Read 6700 times)

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Offline aa

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Re: Anybody heard of skeleton molds?
« Reply #80 on: September 06, 2008, 07:56:02 PM »
After reading the patent, it seems to me the flywheel and cord were meant to rotate the mold smoothly and rewind it. 

Agreed

Pulling a cord once is a quicker, easier, one-step way to rotate the mold than turning it by hand - a single person could set it in motion then put the bubble into the mold. 

That may have been the intention. However, I don't agree that pulling the cord would necessarily give a smoother rotation. That is just an opinion based on conjecture. It may be right but I think that it is important, when trying to analyse things "backwards' to consider all the options. That is, presumably, the essence of research.

There are not that many parallels between glass and ceramics but here http://www.youtube.com/watch?v=CwFtg8mBW3s you can see one of the most accomplished potters ever, Shoji Hamada, working on a wheel that is hand rotated on a weighted flywheel - you will see the motion of the rotation which was done by using a stick that was placed in a groove on the wheel.

This illustrates that you can achieve a very satisfactory constant level of rotation by hand-turning a flywheel.

I am not sure whether the cord-pulling method would be practical because you I think you would have to let go of the cord as soon as you had pulled it, and then there could be a risk that it would not rewind evenly. If you held on to it, you would risk losing your fingers. There would obviously been a boy (assistant) to pull it, but they could just as easily spin the flywheel. It might be easier to get sufficient torque by doing this then by pulling a cord. We used to have a electric motor on a flat bed grinder that required so much torque to start that it is best to hand spin it a couple of times and then switch off the motor, rather than risk it cutting out at the breaker.

It would be very interesting to learn if anyone has actually seen one of these in action. For all we know it was actually an idea that never made it off the drawing board. I am not actually sure what benefits would be achieved by using it as I think there are probably simpler ways of achieving the same result.


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Offline Frank

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Re: Anybody heard of skeleton molds?
« Reply #81 on: September 06, 2008, 08:11:40 PM »
Pull the cord to get it turning, hold on to it until it is rewound then let go, solves simply that problem - visions of a boy wrapped around a spininng shaft  :o

In time the patents in the study can be annotated as to their use, I have some data to start that process but at the moment concentrating on the digitising. It is also clear that patents were being created just to try and profit from a process, as is common today too and it is up to the examiners to prevent too much of that getting through. Patents can also be challenged later. Originally patents were issued by the crown or similar in a fairly arbitrary way based on personal relationships... a study of 17th Century glass patents is fascinating as they were granted and cancelled at a whim - often in return for some form of substantial incentive.

No doubt more will appear in the skeleton mould story and there are still unanswered questions on the subject.
Frank A.
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Offline aa

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Re: Anybody heard of skeleton molds?
« Reply #82 on: September 06, 2008, 08:20:02 PM »
Try this. Assuming you have a bicycle, turn it upside down on the floor. Attach a piece of cord to the pedals. Pull it. See what happens. Now try spinning the wheel by hand. See what happens. Let us know which works better.  ;D
Hello & Welcome to the Board! Sometimes my replies are short & succinct, other times lengthy. Apologies in advance if they are not to your satisfaction; my main concern is to be accurate for posterity & to share my limited knowledge
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Offline krsilber

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Re: Anybody heard of skeleton molds?
« Reply #83 on: September 06, 2008, 09:06:30 PM »
Quote
However, I don't agree that pulling the cord would necessarily give a smoother rotation. That is just an opinion based on conjecture. It may be right but I think that it is important, when trying to analyse things "backwards' to consider all the options.

Certainly, one must consider the options.  Whether a cord or some other manipulation produces a more constant motion isn't entirely conjecture, though, but also a matter of experience.  Personally, I would find using a cord easier.

Once the cord was pulled and released, the only thing preventing a constant rotation would be friction, which you'd expect to be constant (unlike that when throwing a pot).  Using a cord would mean a greater speed could be achieved, just as you pull a cord on a gyroscope to start it turning, or on a lawnmower.  It seems to me it would be easier to keep the tension on the cord pretty steady when you're setting a flywheel in motion that way than to turn one by hand, when it would be natural to use more force at some point in its rotation (as one is pushing down, for instance).  

I wondered about what happens to the cord once it was released, too.  Presumably the turning of the shaft, if fast enough, would hold it in place.

I didn't watch the whole Hamada video, but I didn't see any fly wheel there, only the one the pottery is made on.  Hamada's wheel slows quickly as soon as he stops turning it, suggesting there isn't one, and a lot of time is spent just getting the thing going.  These days (in America, anyway) person-powered potters' wheels are connected to a flywheel that's kicked.  In either case, one benefit is that the speed can be controlled easily, but you have to keep working at it to make it go the speed you want.  In the case of the patent, you put the bubble in the mold, pull the cord, and you're done.  Theoretically, anyway!

(written while I was writing this post:)
Try this. Assuming you have a bicycle, turn it upside down on the floor. Attach a piece of cord to the pedals. Pull it. See what happens. Now try spinning the wheel by hand. See what happens. Let us know which works better.  ;D
Well that wouldn't work!  Attach it to the pedals?  You'd have to attach it to the sprocket, and wind it up, pull...then see which works better!
Kristi


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Offline Frank

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Re: Anybody heard of skeleton molds?
« Reply #84 on: September 06, 2008, 09:24:51 PM »
One benefit of the cord is that it would give a more consistent speed than pushing the flywheel. Theother use of the flywheel would be to help overcome the friction drag.

The cup was made of graphite or other materials - what would be the impact of blowing the bubble to hard... little bits of hot glass sliced off and flung away  :huh:
Frank A.
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Offline aa

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Re: Anybody heard of skeleton molds?
« Reply #85 on: September 06, 2008, 09:54:51 PM »
Gear or direct drive?

On a lawnmower or an outboard motor, a cord works because there is a flywheel to which the cord is attached.  The purpose of the flywheel in those examples is to make it easier to pull. The flywheel in those examples works as a gear, which means that not too much effort is needed to start the motor.  Which is why you perceive it to be easier to pull a cord.

The whole point is that the diagram shows a cord attached to the drive shaft and as Patrick pointed out originally, relatively the flywheel looks rather heavy. It doesn't make sense to use a cord to start it. It will require much more strength than a lawnmower.  Do you have a car, or a bike? Do they have gears or are they direct drive?

In the Hamada example, there is a weighted flywheel underneath the working surface. His wheel was a pre-cursor to the kick start wheels that you refer to. He managed to work out that by pushing the edge of the wheel, it required less torque or strength to turn the wheel than to put crank it from a central drive. This is also a form of gearing. Imagine if he had a hole near the centre of the wheel to put his stick into. Which would have been easier?

You are quite right, attaching a string to the pedal would not work. Well spotted! As you say, you would need to attach the cord to the sprocket, which, again is a gear!! Maybe, what you should do is to attach a piece of steel to the axle of the bicycle and then wrap the cord around that. See if you get a better result than spinning the wheel from the edge.

I am not really an expert in this, but so far as I know, direct drive requires more effort than gears. But I am not an engineer, so I am not going to add much more. Instead, I am hoping that there is an engineer out there who will be able to confirm that it would require less effort and torque to spin the fly-wheel from the edge, rather than pull the cord.

However, I would be equally happy if an engineer can show that that pulling a direct drive cord requires less force.  Smiley

I would also be very interested for that engineer to provide an explanation that shows that a using a cord would give a more constant speed than spinning the flywheel, because my very basic memory of studying this stuff about 35 years ago is that the fluctuation of the speed will be primarily related to the weight of the flywheel and any variation to the speed will be related to that as soon as the force is removed. In other words, when you stop pulling the cord, or spinning the flywheel, the deceleration will follow a constant curve.

 :hb1: ;D

Hello & Welcome to the Board! Sometimes my replies are short & succinct, other times lengthy. Apologies in advance if they are not to your satisfaction; my main concern is to be accurate for posterity & to share my limited knowledge
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Offline aa

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Re: Anybody heard of skeleton molds?
« Reply #86 on: September 06, 2008, 10:05:45 PM »
 ;)
This is probably getting too technical for some people, so here is a simpler explanation - you can dilute almost everything down to a culinary example:

Take two egg whites and put each in a separate mixing bowl, each bowl being the same size.

Using a hand whisk or fork, beat the eggs. In the first bowl rotate your whisk in the centre of the bowl on one spot, all the time. In the second bowl, rotate your whisk around the outside of the bowl all the time. Time how long it takes to get the egg whites to meringue consistency. Which is faster? Which is more tiring and uses more effort?

You don't actually have to try it, if you already know the answer!! ;D
Hello & Welcome to the Board! Sometimes my replies are short & succinct, other times lengthy. Apologies in advance if they are not to your satisfaction; my main concern is to be accurate for posterity & to share my limited knowledge
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Offline Patrick

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Re: Anybody heard of skeleton molds?
« Reply #87 on: September 06, 2008, 11:38:07 PM »
Yes, part of the description
Quote
The shaft D is then caused to rotate by sharply pulling a cord or strap K coiled upon it. A flywheel H mounted on the shaft serves, by its impetus, to re-wind the cord K

Hi,
 As I said before I think the answer lies in " sharply pulling " as in a toy Gyroscope ( Kristi ) . The ENERGY used in the pulling is STORED in the flywheel .
 For the bars of graphite to work they must be rotated at a good speed, a slow speed would be no good.
 The neatly wound cord in the diagram is just to show the principle, in reality the cord when let go would go more into a ball but still work when it was re-pulled. As I said if I had my Meccano set that had a cast flywheel it would be so easy to demonstrate.

Best wishes,  Patrick.


Offline KevinH

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Re: Anybody heard of skeleton molds?
« Reply #88 on: September 07, 2008, 12:09:53 AM »
I have only just seen all this discussion on flywheels and cords, and thought I'd suggest that there is an important point to consider (which has, in fact, already been hinted at but seems to have been forgotten in the discussion of the engineering principles).

I see nothing in the patent diagram or its descriptions to say that the cord is permanently attached to the shaft. I feel sure that the idea is for the cord to be merely wound around it and that it comes free at the end of its pull. The part of the description that states, "A flywheel H mounted on the shaft serves, by its impetus, to re-wind the cord K" simply means to me that when each operation of shaping is done, the flywheel / shaft is braked by some means and the cord is then reapplied to the shaft and the flywheel slowly turned by hand to rewind the cord.

If there is no clutch mechanism (which appears to be case from the diagram) then having the cord attached to the shaft would be highly dangerous, possibly resulting, as Frank suggested, in a boy assistant being wrapped around the shaft (or at least having his fingers broken or ripped off).

Personally I don't have a bicycle and neither do I have a whisk so I can't try the suggested experiments.

However, I do know that a rotating shaft in machinery can have enough energy to quickly wrap long hair around it and rip a whole patch out the human head leaving a 2 inch diameter area of damge - as proven by my brother in the days when his hair was as long as Frank's was in 2005 :o :o :o
KevinH


Sklounion

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Re: Anybody heard of skeleton molds?
« Reply #89 on: September 07, 2008, 12:56:45 AM »
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it is smooth on the inside, with no depressions behind the bosses.  Perhaps that means it wasn't made using the same type of mold,
So all round the Wrekin, to arrive at the point of departure. This topic has shown an impotent lack of progress, the height of archeological necromancy, that is akin to fortune -telling, and now we know less about so-called skeleton moulds, due to a lack of consideration for the opinions of glass-makers.
Yes I know what the patent says, but all too often, there are flaws which cannot be resolved. After all, it took others to find the real benefits of a patent taken out by the Reverend Stirling, regarding endothermic engines, a mere two hundred years, before a tangible use for his patent was found.
("What I know about glass I can write on the back of a postage stamp ( and imho, that is the way it should stay")) ;D ;D
Bah... humbug....
Regards,
Marcus

 

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