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Polygon mirror made on the laser cutter

PostPosted: Mon Sep 10, 2018 10:47 pm
by Robonz
I had a go at making a polygon mirror today for my next mechanical TV. It did not come out true enough. It does look good though haha.

Enjoy the video of my fail and please give some contructive ideas on how it might be made much more accurately especially if it uses a laser cutter


youtu.be/oAM9mOTiqgY

Cheers
Keith

Re: Polygon mirror made on the laser cutter

PostPosted: Tue Sep 11, 2018 12:26 am
by Harry Dalek
Robonz wrote:I had a go at making a polygon mirror today for my next mechanical TV. It did not come out true enough. It does look good though haha.

Enjoy the video of my fail and please give some contructive ideas on how it might be made much more accurately especially if it uses a laser cutter
Cheers
Keith


Excellent mirror design Keith very jealous you can do this stuff !
Having tried the idea with glass precut mirrors making my own mirror drums there's lots of ways it can go wrong ...your test is right to test that each mirror is over lapping the laser line's .
i am seeing a wobble from the mirror drum housing i found sticking any thing like this on a motor shaft you tend to get the same results ...may be if the length thickness of the mirror drum housing were thicker it would perhaps rotate truer .
So i am not so much thinking its your mirror drum as more its attachment system to the motor shaft .
I would use either a vcr head or use it on it a laser printer polygon motor both are very accurate then you would know if it were your mirrors being off or not .
Some thing you are not doing and is a no no is trying to glue a mirror to a flat surface no matter how little glue you use you will always get the same result each mirror will be off a touch so i really like your design...don't give up i think you are on a winner /

Re: Polygon mirror made on the laser cutter

PostPosted: Tue Sep 11, 2018 12:57 am
by Andrew Davie
Only just starting to think of ways to do this. But, how about a monolithic slab of material (say, aluminium) first drill a center hole and then cut or preferably lathe to a circle of the given diameter. Now we need a jig - to be able to abrade the 18 sides. I don't know about the abrasion bit, but the angles could be very precisely handled using just a set of dividers. I saw this method on the "clickspring" antikithera build I posted a few days back. Scribe a large circle (say, 1m) on a board, and roughly calculate circumference/18. Set the dividers to this width, and mark a starting point on the circle. "Walk" the dividers around the circle, counting 18 steps. If you end up short, move the dividers a bit wider. If you're over, narrow them. Repeat until your last step ends exactly on your starting point. now you have an exact 1/18th arc width, and you can permanently mark around the circumference those 18 positions. Drill a hole precisely at each point in the board. Build an "arm" with pegs on the end which insert into the center hole of the board on one end, and to a circumference hole on the other. Now drill a hole somewhere on the aluminium near the border. Match this position with another peg on the "arm" we just built. Now we can fairly EXACTLY step the aluminium blank in 1/18th rotations, just moving the outer peg to each of the holes on the circumference in turn - and the "blank" rotates the right amount. This caters for our exact angles. Now we need something setup in the rig to grind down the mirrored surface on each of those 18 facets required in the aluminium blank. Anything positioned with a vertical grinding surface would do. it could even be done with a manual file, provided there was a guide there. Doesn't matter about its angle, as long as it is rigid - for any angle at all is still going to be tangent to the blank. Just grind/file down a bit, step the jig to the next position, repeat. Keep going until the edges of your facets meet. And bob's your uncle. I'm sure there are better/easier ways, but this is how the ancient greeks would have made their 18-sided mirror drum ;)

Re: Polygon mirror made on the laser cutter

PostPosted: Tue Sep 11, 2018 8:10 pm
by Harry Dalek
A picture explains all ...
Removing the mirror and replacing it with the mirror drum it not only has the motor shaft to hold it in place but a flat surface which also rotates to stop any wobble
Getting the mirror drum right is only half the problem you see .
download (11).jpg

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 14, 2018 12:07 pm
by Robonz
Thanks for the comments. I am still not sure how to proceed on this but here's today's experiment. Harry you were quite right that that I need to solve the run out. Even if I went with a large jig like Andrew was talking about, I still need to mount this thing with pretty much no wobble. I am pretty keen to use this motor but I do have some polygon and vcr motors on standby.

Anyway I have solved the wobble by putting a 4 point adjustment on to the hub. It seems to work really well. It has rubber wahsers that I laser cut that make a compression spring arrangement. Here is a video and some photos.


youtu.be/Ma9gDBLY7kQ

DSC00085.JPG


DSC00084.JPG

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 14, 2018 12:14 pm
by Andrew Davie
Robonz wrote:Anyway I have solved the wobble by putting a 4 point adjustment on to the hub. It seems to work really well. It has rubber wahsers that I laser cut that make a compression spring arrangement. Here is a video and some photos.


Clever, but the mathematician in me cringes when I see "4 point" levelling for a plane. They do it all the time in 3D printing, with the rare exception. Any three points define a plane, and if you add a fourth, then you most likely no longer have all points accurately on that plane. In other words, you are far, far, better having just 3 points for adjusting, not 4. Just saying. :)

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 14, 2018 12:21 pm
by Robonz
Yes, you are correct Andrew. I did consider 3 points when I made it. The thing about 4 points is that it is very easy to adjust so that's why I went that way. Another note, If 4 points were bad then all the "precision engineers" would not be using 4 jaw chucks. Another issue with three points e.g. with 3d printers is you lose support on the bed corners which are good for rigidity. Its a tricky one with pros and cons everywhere.

Cheers
Keith

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 14, 2018 1:53 pm
by Harry Dalek
Keith i would mount the drum on the vcr head and use the motor either direct drive or belt ,i tend to use a pulley system.
That's if you want to use that motor this way you will have a precision mount for your drum this way no wobble ...i have a old u matic vcr drum head and that's how they drove it .
With a pulley system you can increase the drums speed much more than a direct drive system ..with as many mirror faces as you have you will have no problems doing 60 64 line with freeNBTV software you could make a video file to display your perhaps coming monitor ....to your monitors limits ....very very cool me thinks ! :wink:
Or if you can use the VCR motor on the drum its self i would say you would get some nice speeds out of it ! i have only played with these motors a bit tended to have to kick start them but off they go then !
I know George who was on the forum posting he's mirror drum design used his type of motor or a HD drive motor circuit driver shown.
i would have a look at that as the design it is some thing you can make with your cnc .
viewtopic.php?f=1&t=2090&hilit=mirror+drum

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 14, 2018 2:30 pm
by Robonz
Hi Harry

I just tested the surface with a laser at 45 degrees and I only get 1mm of wobble 30 cm away so that meets my spec. At 10 to15 cm that's only 0.3 to 0.5mm of wobble. And I have not tried hard to adjust it well so I can probably do better.

The question is, can I laser cut a mirror drum to mount on this with good specs. I will do that experiment next. If that fails I intend to surface grind some 5mm stainless steel. I hope I do not need to.

The motor is intended to run at 5000 rpm for 60 lines at 25fps.

Cheers
Keith

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 14, 2018 7:05 pm
by Harry Dalek
Robonz wrote:Hi Harry

I just tested the surface with a laser at 45 degrees and I only get 1mm of wobble 30 cm away so that meets my spec. At 10 to15 cm that's only 0.3 to 0.5mm of wobble. And I have not tried hard to adjust it well so I can probably do better.

The question is, can I laser cut a mirror drum to mount on this with good specs. I will do that experiment next. If that fails I intend to surface grind some 5mm stainless steel. I hope I do not need to.

The motor is intended to run at 5000 rpm for 60 lines at 25fps.

Cheers
Keith


It does not stop it working another contact i know Georg Zimmermann did it on 625 line a slight wobble or shake to the picture ,but what the hell it worked not sure this is georges same idea
.https://hackaday.com/2011/12/31/full-color-laser-tv/
But this is on link's on this page
viewtopic.php?f=1&t=2486\
https://www.youtube.com/watch?v=Owc9EFnEA7o&t=7s
Next you will need a frame drum on my octagon the laser i used go's to the line mirror then to the frame drum i used a stepper motor for that one .
viewtopic.php?f=1&t=1557&hilit=octagon&start=15

Polygon mirror Version 2

PostPosted: Wed Sep 19, 2018 10:01 pm
by Robonz
Love the links Harry

I made a lot of very small changes over the last few days. This is fresh off the laser cutter without the adjustable hub fitted. I spent about 6 hours tuning the laser cutter so it could cut much more precisely than normal. This will be useful for everything going forward. I finally figured out the laser cutter optics using the ray simulator- >link
https://ricktu288.github.io/ray-optics/simulator/

Anyway, enough blabbering. Here is a couple of videos showing that the new polygon mirror is getting very close. There is barely a laser beam worth of wobble and I know I can get it better with some more perseverance.

New Mirror

youtu.be/frK15O2ABAQ

Laser Line

youtu.be/2ywgVK6igEY

Re: Polygon mirror made on the laser cutter

PostPosted: Thu Sep 20, 2018 9:35 pm
by Harry Dalek
Yes that does look much better Keith nothing worry about now .
What are your plans for the drum ? its a 18 mirror yes ...so 2 turns per frame 32 line 4 turns 64 line .
just have to make that frame drum which would be easy to just do 12 mirrors or half 6 / but some times you want it to run faster as it is harder to run some thing slow at precise speed than touch faster with fewer mirrors .
On the reflecting of the mirrors i made both the line and frame drums adjustable on the mount so just viewed the result and adjusted the result to get that right for the reflection raster .
Just run the laser low as you can to just see it and when you get a correct screen result ramp up the laser .
heres an experiment or 2 i was trying once
viewtopic.php?f=1&t=1768&hilit=Resonance
viewtopic.php?f=23&t=1890

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Sep 21, 2018 10:14 am
by M3DVQ
Andrew Davie wrote:I'm sure there are better/easier ways, but this is how the ancient greeks would have made their 18-sided mirror drum ;)


I think the modern way, avoiding walking dividers, and assuming a proper machine shop would be a dividing head on a surface grinder. I suspect anyone with a machine shop with a suitably surface grinder and dividing head for this sort of precision work would have already thought to use it though :)

EDIT: oh, grinding a polygon is mentioned further down the thread :oops:
I'm slowly catching up on all these cool mechanical projects after somehow forgetting to read the forum for months (years?)!

Re: Polygon mirror made on the laser cutter

PostPosted: Fri Nov 23, 2018 6:46 pm
by Robonz
To answer you questions Harry, here are some calculations/specs I made today.

They are not set in concrete, just an experiment for now but I will keep you updated. Its all about whether I can make some polygon and drum mirrors good enough. It certainly is a fun challenge. One thing that inspires me is I have not seen a good working Scophony replica. Maybe I missed it but that's good motivation for me. !938 style.

Cheers
Keith

Scophony_dims.png


Scophony_dims.pdf
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Re: Polygon mirror made on the laser cutter

PostPosted: Fri Nov 23, 2018 7:49 pm
by gary
Scophony replica? Didn't happen unless it includes a Jeffree Cell ;-)