Laser Cut Acrylic Nipkow Disc

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Laser Cut Acrylic Nipkow Disc

Postby Robonz » Sat Aug 19, 2017 3:42 pm

I have had a busy day on the laser cutting machine today experimenting . I cut a small pile of test apertures in 6mm thick black acrylic. It was really hard to find a reliable process to cut such fine tolerances with out parallax. I managed to get 25 micron repeatability in the aperture size. I have not tried it all over the laser bed so that could be my next snag but hopefully not Here's my process.

This is how my Coreldraw cad looks. Cut in order of description

Raster engrave layer Green. engraved pocket to a depth of ~4.5mm any deeper and light can shine through the pocket
Cut Brown layer - this is there so I do not end up with a sticky little plug in the aperture
Cut Blue layer - this is the final aperture. It is cut in two passes, 1 cut pass and 1 clean pass. It also reduces heat distortion

Image

Here is the back of the aperture showing the pocket. The pocket is purely there to reduce parallax or shadowing.

Image

Here is the front shown under my measurement microscope. The apertures measure very close to the target of 0.66mm wide.
The aperture measures 28 grads * 25 mircons = 0.7mm, close enough?

Image

I have got my hands on a lathe so I will turn up a 20mm thick acetyl hub. I plan to mount the hup directly on the motor shaft. Any hints on reducing wobble are extremely welcome!

Cheers
Keith
Last edited by Robonz on Sun Aug 20, 2017 5:14 pm, edited 2 times in total.
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Re: Laser cut Nipkow Disc

Postby Klaas Robers » Sat Aug 19, 2017 7:21 pm

If you can laser cut thin metal, do so. A thin metal disc stretches more or less automatically when spinning.

But of more importance: black acryl might be not black enough. You should realise that the apertures (one at the time) have a transparency of only 0.1%. The remaining 99.9% is the black acryl. If that has itself a transparency of 99.9%, which is VERY dark for acryl, then the "brightness" of the whole viewing window is equal to the white of the aperture picture. So the contrast is no better than 1:2.

Metal gives a much better attenuation of light. And a thin disc has a much lower inertia, which is nice for synchronisation. Don't make spokes in the disc, they introduce wobble. In the past these were made in an effort to reduce inertia, but some clever guys, glued drawing paper in the spaces in between the spokes, which helped. A metal, e.g. aluminium disc of 0.5 mm to 1 mm will do.
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Re: Laser cut Nipkow Disc

Postby Robonz » Sat Aug 19, 2017 9:00 pm

Hi Klaas

Very nice to talk with you. You raise some interesting points and have got my brain thinking. Firstly like a lot of people here they use the tools and materials they have on hand, so that's why I have chosen this path. Also I have not seen anyone make a rigid acrylic disc yet, so its great to try a new solution. I never thought about thin metal self flattening as the result of centripetal force. Unfortunately metal cutting lasers that can accurately cut apertures are very expensive. But that said PCB stencil companies can cut thin stainless steel perfectly at cheap prices.

Yes, I did the calculations last night and did come to the conclusion I would only get about 40mW of light coming out of the screen if stick a huge 30 watt led light source behind the disc. I didn't stop to think about the resulting contrast ratio. I have now tested some 6mm acrylic with a 1 watt led behind it in a pitch black room. I am pleased to say there is no light to be seen so I might be okay there, phew.

Some other thoughts I have had are. I can speckle engrave the front to give a nice matte finish as the acrylic is very shiny which I am sure is bad. I also thought if the disc is not flat enough I can stick it in the oven on a sheet of glass and anneal it.

Yes, I did note that spokes would be bad for flatness and also be audibly noisy so was not going to use them. As far a inertia goes I thought a high inertia would be good as the speed would be more steady when maintaining the speed. My experience with un-perturbed mass has shown inertia is good. e.g. high quality turntables typically have very heavy platters to maintain speed more accurately. It is interesting as I have just completed a direct drive motor control for a commercial turntable customer and we used a 5Kg disc. We did however use a very expensive slotless brushless motor and controller that had a lead lag filter. I can see I may need to implement a lead lag filter in the Arduino if it does lock well.

Cheers Keith
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Re: Laser cut Nipkow Disc

Postby Andrew Davie » Sun Aug 20, 2017 1:28 am

Robonz wrote:Here is the front shown under my measurement microscope. The apertures measure very close to the target of 0.66mm wide.
The aperture measures 28 grads * 25 mircons = 0.7mm, close enough?


I threw the image into a paint package and counted pixels. Looks to me like it's 0.725 mm - that left edge, though, is difficult to locate exactly. I then shifted the ruler so that the 0 was exactly on the left edge as close as I could eyeball it. Then the right edge is at 29 grads - and 29/40 = 0.725 mm too. So I reckon it's 1/40th mm larger than you think :)
Nice work!!
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Re: Laser cut Nipkow Disc

Postby Robonz » Sun Aug 20, 2017 4:57 pm

Yes Andrew you are quite right about reading the scale, but in the optics world corners are never sharp and apertures have thickness and surfaces are not smooth so that's why I dropped that 25 microns. Klaas the black acrylic is showing a perfect contrast which is great.

I made a 110mm x 16 aperture test disc with the laser cutter today. I also did a bodge setup to test it. I found it very hard to take photos and videos of it in action. Most of the lines look pretty good but the spacing on the outer lines may be overlapping a bit. I guess I am hitting the accuracy of he laser cutter. I hope this is good enough to make a decent screen

Here is the set up
Image

Here are some results with an unmodulated light errr 50hz mains modulated light I should say. This picture show the cross over between the diffuser (dim light) and the direct led globe bulb. Note the dim parts are the 50hz mains showing up.
Image

Here is the most crappy video I could do. I will work on that

http://edns.co.nz/televisor/100_7968.MOV

I hope the real motor shows up soon so I can start building!

Cheers
Keith
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Re: Laser cut Nipkow Disc

Postby gary » Sun Aug 20, 2017 11:13 pm

Robonz wrote:Also I have not seen anyone make a rigid acrylic disc yet, so its great to try a new solution. Cheers Keith


I used a rigid acrylic disc for my bead disk:
viewtopic.php?f=1&t=713&p=6306&hilit=+bead+disk+#p6306
download/file.php?id=2756


youtu.be/CJrbRdFWgtc
Perfecting an NBTV system is like trying to slam a revolving door...
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Re: Laser Cut Acrylic Nipkow Disc

Postby Harry Dalek » Sun Aug 20, 2017 11:38 pm

Good work Keith looks good have you a picture of your laser cnc thing ,what laser power are you using ...if you know of Doms nbtv software you can adjust to different aperture shapes for the holes and gives the effect of a video using that hole shape might be some thing you could try since you have a laser cutting machine all the rest of us can only do is make a round hole you could do that for real
http://www.authorityfile.co.uk/nbsc/
Last edited by Harry Dalek on Mon Aug 21, 2017 4:39 pm, edited 1 time in total.
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Re: Laser Cut Acrylic Nipkow Disc

Postby Robonz » Mon Aug 21, 2017 12:03 pm

Hi Gary, your TV looks like it has a really good picture. Getting all those beads to line up so well must have been fun. Using beads would make the apertures a lot bigger. The challenge I am tackling is cutting accurate optical apertures which seems to be working well using the engraved pockets. I am sure your bead system is much brighter which must be great. Is there a thread that shows your build etc? I also have to say thank you for all your amazing software. I have downloaded and played with all of it.

Hi Harry, I have gone for rectangular apertures as shown in the beginning of the thread. My laser cutter is a Universal VLS60 = 60 watts. It is not a cheap Chinese machine so it does perform quite well. Now I have worked it out, it's actually quite easy to cad a Nipkow disc from scratch. I use Gary's Nipkow DXF software to make sure my numbers look right when doing the cad. I have to keep focused but the 2 drum compact looks like a good one to laser cut. I have the perfect material for doing slits.

Harry, here is a picture in may garage from a few years ago
Image

Thanks for the feedback I am really enjoying this Baird experience.
Cheers Keith
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Re: Laser Cut Acrylic Nipkow Disc

Postby gary » Mon Aug 21, 2017 1:54 pm

Robonz wrote:Getting all those beads to line up so well must have been fun. Cheers Keith


Well the only real difficulty is getting the to all lie in the same plane, and even then there is a good degree of forgiveness providing one views it straight on.

Robonz wrote:Using beads would make the apertures a lot bigger Cheers Keith


It does, but no less precise, the centre of the bead needs to be exactly on the centre of the aperture.

In fact, I found that *drilling* the holes was not accurate enough because of run out in the drill bit itself, and the fact that the length of the drill bit amplified the run out in the spindle - they had to be cut as a pocket using an end mill.

The CNC has a nominal resolution of 1/5000 of an inch (it is difficult to precisely measure the runout error of the spindle. I can only say it was well under a thou).

No, I don't have a build thread I am afraid, other than the beads the build was pretty much standard NBTV issue using a bistable based speed control to lock to sync.

It never really got beyond a lash up prototype (as do most of my builds) but worked quite well, and yes, just one high brightness LED gave me the brightest picture I have ever obtained.

Cheers, I envy you having a laser cutter.
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Re: Laser Cut Acrylic Nipkow Disc

Postby Harry Dalek » Mon Aug 21, 2017 4:55 pm

There are a few different ways i see now on the forum of prototyping something this case the Nipkow ,i think Gary mentioned limits are on the cnc are drill bit size and i should think there would be a limit on the 3D printing size of something small ...
The laser i am wondering what materials it can cut ? made for this hobby for sure :wink:
Yes Square holes a seems good idea.
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Re: Laser Cut Acrylic Nipkow Disc

Postby Robonz » Mon Aug 21, 2017 5:08 pm

Hi Harry, my machine can cut almost anything except metal. You can get metal cut cheap through Chinese PCB suppliers for around $100 USD. If you or Gary want something cut and can send me good CAD artwork I will cut if for free. You just pay for the material and shipping. It only took 15 minutes to cut the small test disc. Probably an hour for a full size disc. Even better if my disc works well I can send a copy of that. Laser cutters are definitely the best machine IMO as they are super fast. Not too much noise and no dust etc. When I cut the full size disc we will find out if it is actually good enough.

Cheers
Keith
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Re: Laser Cut Acrylic Nipkow Disc

Postby Harry Dalek » Mon Aug 21, 2017 11:22 pm

Robonz wrote:Hi Harry, my machine can cut almost anything except metal. You can get metal cut cheap through Chinese PCB suppliers for around $100 USD. If you or Gary want something cut and can send me good CAD artwork I will cut if for free. You just pay for the material and shipping. It only took 15 minutes to cut the small test disc. Probably an hour for a full size disc. Even better if my disc works well I can send a copy of that. Laser cutters are definitely the best machine IMO as they are super fast. Not too much noise and no dust etc. When I cut the full size disc we will find out if it is actually good enough.

Cheers
Keith


Seems the way to go Keith if anything for the Accuracy making the Nipkow holes .
I am fine to watch at the moment ,i have always just hand made the disks but its interesting to me to see it done so well here .
Watch your build i will.
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Re: Laser Cut Acrylic Nipkow Disc

Postby Robonz » Fri Aug 25, 2017 8:57 pm

I got my 30 watt led unit today. Measured the current, it was a lousy 160mA so it can go a lot brighter, I just need to hack the electronics out as it is not DC and not driving very hard. I will just take the whole PCB out and chuck a resistor in series. I can adjust the brightness with PWM and voltage supply. I will use the supply so I can have a small resistor as I dont want a lot of unnecessary heat.

Here is a picuture of it running at just 160mA. Nice and bright. The lines are caused by the switchmode PSU in side the light source. Its quite good as I can see the disc working well. Note the double lines are caused by the magnifying glass. It is in focus quite well about 1/3 down the image.
Image

I cut the full size disc today. 360mm diameter. 32 holes 0.7mm x 0.5mm. I forgot the sync slot and re-cut it after.

Image


On the front I put a 50hz 750 RPM strobe pattern. You cant see the 32 holes on this side but they are there!

Image

More progress. Turning the hub is next, I have cut the blanks ready for the lathe from 17mm thick white acetyl. Its a shame my mate with lathe doesn't drink. I could have used a couple of dozen beer to expedite it haha. MY goal is to talk him into it this weekend.


Cheers
Keith
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Re: Laser Cut Acrylic Nipkow Disc

Postby Harry Dalek » Fri Aug 25, 2017 10:39 pm

The hardest part of mechanical tv for me is always the mounting of the disk ,
i always had trouble till i cheated and used either Cd or dvd motors and there disks as they just clip on or vcr heads hard drives no wobble with most of those cd dvd ok for monitor but as i found out last year not great for a camera for the sync part of it the tiny wobble cause a wavy raster line if you use the mechanical sync :roll:
Good work i see the disk is ready to be mounted on the coming motor .
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