Motor required for spin casting, 10-100rpm

[jim hardy]

Red and black, aye !

 

[jim hardy]

great photos by the way.

How do you post them?

 

[tiviny]

Torque is good. It really only needs to overcome air resistance in the end. How does my motor being bigger affect torque?

There is enough space to make the capacitor fit easily. I put 4 of these capacitors on the board, and the last one is supposed to go across the motor terminals.

I just use the attach file option to post photos.

While I am here i might as well ask, could you think of anything that would attatch to the shaft thread? Its only meant for a worm gear, I not sure how to secure anything to it.

cheers jim

 

[jim hardy]

I think you'll want to attach to the shaft coming out of nylon gear. In the automobile the windshield wiper is driven by an arm held on by the nut.
The worm drive screw on main motor shaft is pretty high speed, that's why i envisioned instead using the same output shaft as automaker did, after worm reduction..
Worm gear drive is pretty smooth and gives high torque at about the speed you need.
I see from your photo that shaft has flats on it where wiper drive arm was attached..
One of those shaft extenders for electric motor should grab it (unless it's metric), or epoxy a friction wheel to it.


"..motor being bigger affect torque? " i don't follow.. torque will be in proportion to motor current.

The two empirical equations for any DC motor are:
Voltage across motor = (K * flux) * rpm
where flux is field, in your case from permanent magnet so relatively constant
and K is proportionality constant for your specific armature.

so if you measure voltage at any speed you know product (K * flux)
and
Torque = 7.04 * same(K * flux) * Ia , where Ia is motor current and torque is ft-lbs.
Are you in a metric country?

Keep us posted.. This is interesting...!

 

[sophiecentaur]

I'm not keen on the idea of a base, I think hanging will give the best stability all round.
Even once I get the spinning plate level it is near impossible to correctly place the container right in the middle. Even the slightest bit out is not good enough. Hanging the container (accurately!) eliminates both this problem, and the issue of having the container pointing at zenith. Even with no sections of material in the container to aid the plaster in spinning up, waves do not occur, so I assume the same will be so hanging.

Do you mean dirt-mud with cement? I'm pretty sure I just realized, however, that the easiest thing to do would be to just do a final layer of watery resin on top of the plaster, as this needs to be SMOOTH, and the grains in the plaster are too rough for a good reflecting surface.

If you hang the container, loosely, and you don't get the centre of mass right on axis there is nothing to stop the whole thing precessing and going wild. The thing needs to be constrained so it won't do that. If you use a firm, massive base then the accuracy you need for mass distribution is much less. In fact, the only 'accurate' centring you need is for the geometry of the paraboloid. That would be very easy to arrange as long as the container is a good circle. Let's face it, a lathe will give you a good circular shape, whatever the shape or position of the workpiece. You don't need the workpiece to be 'balanced' if the chuck holds it firmly enough. I can't think anyone would try to 'turn' a chair leg by hanging it from a rotating bungee support (for these reasons).
I understand that you may have had problems mounting your spinning plate satisfactorily but how would this be easier with it hanging? You still need to be able to adjust the position of the centre plus two tilt angles. A baseplate with diagonal slots can be used for side to side adjustment and three screws can give tilt adjustment. It may sound over the top; it's not a trivial thing you're proposing. But all this could be a lot easier than other methods of producing a paraboloid. What is the required accuracy of the shape? Is it for an optical, microwave or sound application? Will you be measuring the paraboloid as you spin it up or will you rely on the speed to define the focal length?

 

[tiviny]

"..motor being bigger affect torque? " i don't follow.. torque will be in proportion to motor current.

I asked because you said this

How's the torque feel to you? Your machine is bigger that i thought, still those wiper motors are stout.

I wasnt sure what you were getting at here.

As regards attatching, the nylon gear spins too slow and doesn't have enough RPM range. The motor shaft spins at at a good speed with good range, but slowed down to where the good range is it vibrates more than I would like. It looks like I will end up attatching a pulley system using the nylon gear. That would give a good rpm range and isolate the vibrations somewhat.

sophiecentaur, I've tried a few sorts of levelling bases and basically my conclusion is I want to at least solidly try the hanging method. I assumed that getting something to spin would be a very simple thing, not so. The 3 issues are getting the spinning base to spin true relative to its shaft, getting the shaft true to vertical, and getting the container to spin accurately around its axis. Each one sounds relatively simple to fix, but each is really a whole problem of its own, and combined they make for a poor parabola.

Hanging will ideally (naively?) bypass the first 2 problems, however your concept of a very tough, nicely spinning base attatched to the axis of a container is sound, provided I had funds for such a machine. I've seen videos of purpose built ones and they indeed do look nice, but Ill post a photo tommorow of the best I've managed to come up with. 'Baseplate with diagonal slots' is a good idea.

I haven't properly considered how to attatch the container to the support to allow for adjustability, but I have ideas, mainly based on using screws.

The required accuracy is "very" accurate, haha. As best as possible. Its going to be used for visible and maybe IR wavelengths. Focal length/size isn't important, I am just going to use a container that's about the size I want, and spin it up until the liquid is just below the rim. I can get a potter to make a nice container. Ballpark is good enough, because as long as its a parabolic shape it has a focal point somewhere, and that is all i need.

 

[sophiecentaur]

Practical choices are personal in the end and the method anyone really fancies will be good in many ways.

I gather you are more interested in 'focussing energy' than forming a good image. The accuracy needed will depend largely on target size, I guess.

 

[jim hardy]

I asked because you said this

I see - the old disconnect - I'm mildly Aspergers and miss a lot .

I meant to convey that the rotating assembly you are building is larger than i thought it would be.

Actually your windshield wiper motor is somewhat shorter than the one in my Ford Ranger..
Still i think it'll make plenty of torque.

Sorry for the disconnect at my end , \sheepish grin icon\

old jim

 

[jim hardy]

"""The 3 issues are getting the spinning base to spin true relative to its shaft, getting the shaft true to vertical, and getting the container to spin accurately around its axis. Each one sounds relatively simple to fix, but each is really a whole problem of its own,"""

I think Sophie is saying at some deviation from vertical a hanging, rotating load could emulate a slingshot...

so experiment as you go.

keep us posted - this is interesting.