Spinning mirror to make cone. Help

[polishedball]

Can someone explain the basics of using a spinning mirror to produce a cone of light. Recommend RPM, and what makes the cone wider or more narrow. Looking to a a couple of these as targeted beam effects, but have no idea where to start on building them. Figure asking will save a little experimenting time.


The commercial product appears to be this http://www.ebay.com/itm/Laser-stage-...item232e46654a

But as mentioned I'd like to also know how the cone size can be determined or built in.

Thanks

[laserist]

Can someone explain the basics of using a spinning mirror to produce a cone of light. Recommend RPM, and what makes the cone wider or more narrow. Looking to a a couple of these as targeted beam effects, but have no idea where to start on building them. Figure asking will save a little experimenting time.


The commercial product appears to be this http://www.ebay.com/itm/Laser-stage-...item232e46654a

But as mentioned I'd like to also know how the cone size can be determined or built in.

Thanks

If the mirror surface is perfectly perpendicular to the rotation axis the reflection would a beam (maybe a bit shakey depending on the bearings involved.) as the angle between of the mirror surface and the axis of rotation moves away from perpendicular the cone forms. See the drawing below. The angle between the axis of rotation and the mirror in the drawing case is 5 degrees so the angular change of the axis perpendicular to the mirror as it rotates is 10 degrees, and the angular diameter of the cone is twice that value or 20 degrees in this case.




Oh and the speed depends on what you're trying to achieve. Really fast if you don't want flicker. Don't assume that a motor that does what you want will have bearings that will stand up to an unbalanced condition for long…

[polishedball]

Thanks for all the info, pretty much cleared up all I was looking for. I have some old AC rack fan motors I plan to use, also have a large supply of rebuild bearings to go with them

If the mirror surface is perfectly perpendicular to the rotation axis the reflection would a beam (maybe a bit shakey depending on the bearings involved.) as the angle between of the mirror surface and the axis of rotation moves away from perpendicular the cone forms. See the drawing below. The angle between the axis of rotation and the mirror in the drawing case is 5 degrees so the angular change of the axis perpendicular to the mirror as it rotates is 10 degrees, and the angular diameter of the cone is twice that value or 20 degrees in this case.




Oh and the speed depends on what you're trying to achieve. Really fast if you don't want flicker. Don't assume that a motor that does what you want will have bearings that will stand up to an unbalanced condition for long…

[The_Doctor]

Don't assume that a motor that does what you want will have bearings that will stand up to an unbalanced condition for long…

I'll back that up, it becomes maybe the most important point, because one of the simplest ways to set cone angle is to have the mass slightly eccentric, spring balanced against centripetal force, and you can change cone angle purely with change in motor speed, but what you gain in ease and control, you lose in paying for a motor that can stand this off-axis force. Also, it may take some careful work to make it operate smoothly at all speeds without becoming chaotic and drawing ugly blurred cone walls. You'll also want to mount the mirror such that worst case mirror movement is strictly limited because otherwise at some point it's almost guaranteed to spin of with startling results. It's worth spending plenty of time with something like SketchUp planning a small mount for this. If you just want to stick it on the end of the motor shaft and get it done, make it very tiny..

I had a bit more thought on this..
First, polishedball, you mention fan motors.. I'm not sure what those will be like but I guess you might want more speed and power than a small fan motor. Maybe a cheapo PCB drill off eBay, those might have the right combination of sturdy shaft and high speed and low vibration needed. A quick way to make a safe fast scanner could be this:
Get a bit of thin 5mm square front surface mirror... and acetal rod offcut (cheap and easily had on eBay), diameter anywhere from 1/2 to 3/4 an inch wide, and cut a stub about half an inch long. Drill axially through it to tightly fit the motor shaft, and widen that hole half-way way through from one end with a 10mm drill bit. Once the acetal is superglued onto the drill shaft put a small blob of silicone bath sealant into the big hole and press the small mirror onto that, so it's deep enough in that hole to be restrained by the wall at extreme speeds. To set it up. low power laser, slow motor.. Stop the motor and poke the mirror edges with a pin till you get the narrowest cone you want. (Bit narrower, maybe). Then let it set for at least 24 hours before testing at high speeds. This method is fast cheap and easy but does not tightly control the relation between cone angle and motor speed, but once you've built a couple you'll be able to gauge that better.

(Word about acetal: I like it because it's easily machined, and very tough. There may be better choices but whatever you use it shouldn't fracture under impact or stress. Rules out polycarbonate! That stand impact but is very poor with long term stress.)

[mixedgas]

See me at SELEM and I'll draw this out for you on a piece of paper. The secret is not to use a small mirror, the glass is too thin and subject to breakage from vibration. Small mirrors then become a pain in the neck to hit from any distance. You actually need something of moderate diameter with substantial inertia. My last down-cone was for a Pink Floyd tribute band in St. Louis. It had to work flawlessly for three nights up in the rigging, with no adjustments. It did, and was not expensive.


The OSLS unit in the auction is actually a good design, because the mirror is large enough you can hit it. Its tight fitting case around the mirror acts as a nice beam block if your pointing accuracy is marginal.

Steve

[The_Doctor]

My method wasn't assuming you wanted to aim at it from across a room. It's intended to be mounted with a small laser in one box. In this case it should be small to limit inertia and stress. No harm in using a thicker bit of mirror, but too much mass in this method is bad because most of the speeds for a decent choice in cone angle will be high. Get it right, and never mind three nights, it could easily last three years.

[Displaser]

I used hot glue (don't laugh at me too much, ok?) to secure small mirrors (<25mm diameter) on Maxon DC tape transport motors for years. It allowed for adjustment while the glue was hardening and I never threw a mirror over the course of several years use. I purchased first surface mirrors from Edmund/Anchor Optical.

Anchor Optical

Greg

My apologies is this not relevant to any of our readers...

[Photonbeam]

Three nights? Three years? How about three decades? The history of laser cones - Here is the English band Genesis, 1978 with upward cone from SP171 krypton laser. 1983 - The Canadian band Loverboy saw me do a single laser cone for someone and insisted on five cones. That was stretching the 20W SP171 argon and you can see how important the smoke was. I still have one of these units and it still works. Basically a DC/tach motor with 5" spinning mirror on adjustable hub. It needs to spin at least 1200 RPM to look solid and not flicker. Later, chopping and color mod produced interesting effects. Remember the cone will be an ellipse if you are hitting the mirror off-angle.

Here is a simple one I made with ceiling mounting bracket and 2.5" FS mirror on 12VDC motor for bedroom use-

[The_Doctor]

I used hot glue (don't laugh at me too much, ok?)

Would I ever do that? Seriously, nothing wrong with it. Good mix of elasticity and ductility for many things. I suggested silicone for variable cone angle though because elasticity is very high and ductility very low, it has an extreme ability to recover precise original shape. Makes a great spring as well as good bond.. With that much elasticity and speed, there is a chance of losing a mirror fast, hence that bit about the acetal holder. Maybe overkill if the whole thing is secured in a box though.

Maxon DC tape transport motors...

Like the little ones in most small tape decks? Those are great for regular speed and extremely low vibration and long life. I had a box of old motors like that, but sold them on eBay when in the kind of mood that makes small excited children look for more things to put on a fire. I'm not sure what the cheapest repeatable source of motors with low vibration and high (and very variable) speed is. Some kind of models-and-toys supplier, maybe. Something with an eight-inch shaft.. Like laser scanners, stiffness helps if you want real speed. The little tape motors I had were all about 1.5mm or so.

[mixedgas]

I bow to Photonbeam on this one.
Wow, Genesis... SP171 driven downcone, drool.

Anyways. 1250 or 1650 RPM AC 1/4 HP motors are a good start.

I work on things that positively, absolutely, have to be there all night, which leads itself to sturdier construction then the angled mirrors of a spinning mirror spirograph.

Steve