Starting at the beginning with Mirror Screws

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Postby gary » Tue Sep 13, 2011 10:22 am

Viewmaster wrote:To reduce noise further a simple ball bearing, housed in a centre drilled cone in the bottom of the shaft as a thrush bearing would be quieter (and cheaper!) than a ball race thrust bearing.


Panrock wrote:Interesting idea. The positioning and the profile of the internal cone(s) would clearly need to be spot-on for this. Would there be cones both above and below the support ball - for location?
Steve O


I think turning the cone on a lathe would be accurate enough, but Albert I can't get my head around how that's put together - any chance you could post a rough sketch?

Cheers,

Gary

(very interesting thread BTW guys)
gary
 

Postby Viewmaster » Tue Sep 13, 2011 6:39 pm

gary wrote:
Viewmaster wrote:To reduce noise further a simple ball bearing, housed in a centre drilled cone in the bottom of the shaft as a thrush bearing would be quieter (and cheaper!) than a ball race thrust bearing.


Panrock wrote:Interesting idea. The positioning and the profile of the internal cone(s) would clearly need to be spot-on for this. Would there be cones both above and below the support ball - for location?
Steve O


I think turning the cone on a lathe would be accurate enough, but Albert I can't get my head around how that's put together - any chance you could post a rough sketch?


...........see sketch

By 'centre drilled' I meant using a 60 degree centre drill used in turning
between centres etc. These drills are available in various sizes.
Images here......

http://www.drill-service.co.uk/Tools.as ... 0000000000

Steve wrote.......
How did you achieve dimensional accuracy on your screw?

Not sure what you mean (hole position on slats or slat angular alignment?)
If the latter see this thread.......
http://www.taswegian.com/NBTV/forum/vie ... rror+screw

BTW, if you search on this forum for 'mirror screws' you will find many posts on this topic by many better qualified than I am.

Another thought...Also if colour will there be 3 rows of LEDs ? if so, will that give some kinda angular problem to the eye position I wonder ?

As Corp Jones would have said in Dad's Army,
"Sorry to worry you again." :-)


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Postby Panrock » Tue Sep 13, 2011 7:26 pm

Viewmaster wrote:For anyone contemplating building a colour mirror screw
(who might that be I wonder? )
See this wonderful site.......

http://www.earlytelevision.org/color_mirror_screw.html

Yes this was one of the first places I went to! You'll see there's mention there of a discussion between Steve McVoy and me about how to create a suitable high-intensity colour line of light.

Viewmaster wrote:

...........see sketch

I still think another V in the bottom plate would be highly advisable, to locate the bottom of the shaft properly via the ball. I'm minded that the slightest imbalance is a matter of concern, and the whole thing would need to be firmly held, top and bottom, to 'soak up' any transverse forces resulting from imbalance... But knowing me, have I missed something here?

Viewmaster wrote:
Steve wrote.......How did you achieve dimensional accuracy on your screw?

Not sure what you mean (hole position on slats or slat angular alignment?)
If the latter see this thread.......
http://www.taswegian.com/NBTV/forum/vie ... rror+screw


AFAICS, you seem to have skilfully made these parts up yourself in the workshop and set their positions manually. I wouldn't be that brave. As regards your clamps, I would have been slightly concerned about 'streamlining' issues. I'm expecting 'wind resistance' (and resulting noise) on my mirror screw to be a bit grim as it is. Another thing, being a screw, will it try to 'take off' - or 'burrow into the earth' - like some sort of Victorian machine? :shock:

Viewmaster wrote:Another thought...Also if colour will there be 3 rows of LEDs ? if so, will that give some kinda angular problem to the eye position I wonder ?

See Steve McVoy's and my discussion on the page you quoted for more on this.

Steve O
Last edited by Panrock on Tue Sep 13, 2011 9:51 pm, edited 1 time in total.
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Postby Panrock » Tue Sep 13, 2011 8:17 pm

I thought it a good idea to post my envisaged spec here, so the experts can give it a grilling and point out any 'howlers' - and there may well be some!

Horizontal scanning will be used.

Basically, I propose using 1.5mm stainless steel sheet for the 'slats'. These could be mounted singly for 120 lines, or mounted in pairs for 60-lines, should 120 lines prove impracticable for any reason.

The slats will no longer have curved rears, but will be strictly symmetrical in shape, for critical balance.

120-line mode

Slat every 3 deg
Picture subtends 6 deg horizontally
Aspect ratio 4:3
With 1.5mm sheet, picture height = 180mm
Picture width = 240mm

Sin 6 degrees = 0.1045
240/.1045 gives viewing distance of 229.6 cm


Working out peg and hole positions, and allowable 'slop' in their engagement for 1 pixel horizontal error...

½ length from centre -20mm slat = 100mm
required tolerance better than 3/120 deg = 1½ minutes
Sin 1½ minutes= .0005
1½ minutes at 100mm = max permissible positional error of .05mm


Working out practicability of cutting all 120 slats from a 2M x 1M x 1.5mm stainless steel sheet...

No of slats along sheet: 8
Rows: 15
Max slat width: 60mm
Recommended slat width: 50mm


Line of light width (1½ minutes at 460cm) = 2.3mm

The above figure assumes the line of light is as far away from the mirror screw as the observer. But I still don't really understand line of light positioning effects.


60-line mode (fallback)

Slat every 6 deg
Picture subtends 12 deg horizontally
Aspect ratio 4:3
With doubled 1.5mm sheet, picture height = 180mm
Picture width = 240mm
Sin 12 degrees = 0.2079
240/.2079 gives viewing distance of 115.4 cm

½ length from centre -20mm slat = 100mm
required tolerance better than 6/60 deg = 6 minutes
Sin 6 minutes= .0017
6 minutes at 100mm = max permissible error of .17mm


Line of light width (6 minutes at 230.8cm)= 3.9mm


Material: stainless steel 304.
Hopefully the laser cut edges can be polished - but I'll check this. Softer grade stainless steel should polish easier than harder.

Weight of mirror screw, less shaft: 38 lbs (17¼ Kg).

A thumping great DC motor will be needed to spin this heavy great thing up to 1500 rpm and regulate its speed.

Steve O
Last edited by Panrock on Fri Sep 16, 2011 6:21 pm, edited 3 times in total.
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Postby gary » Tue Sep 13, 2011 8:22 pm

Viewmaster wrote:...........see sketch


Thanks Albert, for some reason my mind wasn't picturing a SINGLE ball.
gary
 

Postby Viewmaster » Tue Sep 13, 2011 9:32 pm

If memory serves me I believe that the Sutton Coldfield TV
transmitting mast sits on a single steel ball too about 2 inches (?) diameter. Of course it's not rotating at 1500 RPM ! :-)
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Postby Viewmaster » Tue Sep 13, 2011 9:52 pm

Panrock wrote:
Viewmaster wrote:For anyone contemplating building a colour mirror screw
(who might that be I wonder? )
See this wonderful site.......

http://www.earlytelevision.org/color_mirror_screw.html

Yes this was one of the first places I went to! You'll see there's mention there of a discussion between me at Steve McVoy regarding how to create a suitable high-intensity colour line of light.

Viewmaster wrote:

...........see sketch

I still think another V in the bottom plate would be highly advisable, to locate the bottom of the shaft properly via the ball. I'm minded that the slightest imbalance is a matter of concern, and the whole thing would need to be firmly held, top and bottom, to 'soak up' any transverse forces resulting from imbalance... But knowing me, have I missed something here?

Viewmaster wrote:
Steve wrote.......How did you achieve dimensional accuracy on your screw?

Not sure what you mean (hole position on slats or slat angular alignment?)
If the latter see this thread.......
http://www.taswegian.com/NBTV/forum/vie ... rror+screw


AFAICS, you seem to have skilfully made these parts up yourself in the workshop and set their positions manually. I wouldn't be that brave. As regards your clamps, I would have been slightly concerned about 'streamlining' issues. I'm expecting 'wind resistance' (and resulting noise) on my mirror screw to be a bit grim as it is. Another thing, being a screw, will it try to 'take off' - or 'burrow into the earth' - like some sort of Victorian machine? :shock:

Viewmaster wrote:Another thought...Also if colour will there be 3 rows of LEDs ? if so, will that give some kinda angular problem to the eye position I wonder ?

See Steve McVoy's and my discussion on the page you quoted for more on this.

Steve O


Another V at bottom in the base plate is not required. The radial location of the shaft is done by the main bearings. That ball only takes care of vertical thrust, although a small retainer piece could be added at the TOP of the shaft to prevent it rising up and becoming the first mirror screw on the International Space Station ! :-)
My idea was to have a free float shaft and gravity will rest it on the ball, using oilite pillar blocks which allow the shaft to float down onto the ball.
IMPORTANT...If you use ballraces as bearings then the shaft is retained and surely NO ball is needed.
Yer pays yer money and takes yer choice.

Re the clamps...they were ONLY used to set up the angular position of the slats.
A retaining clamping nut at the end of the shaft was finally tightened and all the clamps were then removed.

BTW, I balanced each slat by putting then on a slightly loose horizontal shaft and see which way they 'see sawed' ! Filed off a bit off the heavy end and re tested. Not very accurate as I still had rumble at 750 revs.
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Postby Viewmaster » Tue Sep 13, 2011 10:05 pm

Panrock wrote:A thumping great DC motor will be needed to spin this heavy great thing up to 1500 rpm and regulate its speed.

Steve O


I too used a DC motor with pot control. No real sync as the rig was just to play around with a mirror screw myself.

What thought have you given to achieving reliable locked sync at 1500 RPM with such a relatively large mass of the mirror screw and the motor rotor combined?
I wonder how others who have built a mirror screw here have finally faired in their own builds at 750RPM?
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Postby Panrock » Tue Sep 13, 2011 10:47 pm

Viewmaster wrote: What thought have you given to achieving reliable locked sync at 1500 RPM with such a relatively large mass of the mirror screw and the motor rotor combined?
I sort-er imagined using the standard 'club' circuit with a beefed up output stage... a bit like I already have on my 'fork lift truck' Grosvenor monitor and which works very well at about 4 amps. To be beefed up yet more on that I suspect...

The mirror screw will have substantial inertia, which hopefully should reduce hunting.

Steve O
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Postby Panrock » Fri Sep 16, 2011 5:17 am

An update: today I've created the CAD .dwg file for the slats, and shaved 20% off the projected weight too! I've also done some sourcing on silver steel rod for the 25mm OD shaft and bearings. Tomorrow, I'll get a provisional price from the laser cutters - then I'll know if this is to go ahead...

Steve O
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Postby kareno » Fri Sep 16, 2011 5:07 pm

Sorry I'm late with this but I do like your 'peg and locating hole' method of ensuring correct angular displacement of the slats, Steve!
kareno
 

Postby Viewmaster » Fri Sep 16, 2011 6:24 pm

Panrock wrote:An update: I've also done some sourcing on silver steel rod for the 25mm OD shaft and bearings. Steve O


Silver steel is a very strong tool steel. Please realize it's difficult to machine and turn. You will be in near heavy engineering to machine it if required ! :-)
If I were making this machine my gut feeling is that 3/4 inch dia
silver steel is more than OK for this.

After all, where is the stress on the shaft to be? ............
Not on any weight carrying as you said only 40lbs and now have cut it down further.
If well balanced then no lateral stresses.
Certainly no great torque requirement to get the machine up to 1500RPM.

Check out the properties of SS though before making the final decision.
As I said 3/4 dia (18mm) is about £9 from, say, Kirjeng supplies plus postage.
25mm will be much dearer and in my humble opinion way over the top.

Check out what other large mirror screws use as shaft material and diameters on the web, before a final commitment.
Good luck.
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Postby Panrock » Fri Sep 16, 2011 6:44 pm

Thanks Karen and Albert!

Karen, I now think all this stuff in my original diagram about holes and pegs facing in various opposing directions was a load of nonsense. Why not do the whole thing with holes and just stick the pegs through paired up holes when needed? The CAD file shows the 'new' narrower slats with holes on either side at 3 and 6 degrees. These will both allow the slats to be locked together flush (in pairs if 60 lines is used or 'all together' when initially polishing all the faces) and also locked in stagger fashion for standard operation - sorry haven't explained that very clearly!

I have some (accurately measured) 4mm brass tubing from which I can make the 'pegs'. This largish size should allow accurately round holes to be produced by the laser cutting and yet they still fit nicely within the 3 degree increment space.

Albert, I tend to prefer to over-engineer things when I don't really know what I'm doing! I've sourced the various bits now and the silver steel is 330mm long so it won't need cutting. I still have to find suitable pulleys and belt though - and the motor.

Steve O
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Postby Viewmaster » Fri Sep 16, 2011 7:33 pm

Panrock wrote: I still have to find suitable pulleys and belt though - and the motor.

Steve O

I bought high quality toothed belts and pulleys from HPC Gears Lts for my Edikow machine.
Very pricey, very high postage, but very good quality.
My piggy bank was emptied . :-)
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Postby gary » Fri Sep 16, 2011 7:33 pm

Albert, silver steel (or drill rod in the US) is usually supplied in the annealed state where it's workable at something like 25 to 30 Rockwell (I could be more specific but all my engineering books are packed away pending my forthcoming move). I use it all the time on my little Taig (Peatol in UK) lathe for making spindles, tommy bars, fly cutters, gear cutters, etc. When hardened it has a Rockwell of between 60 and 70. That hardening is the hardest thing for me because I am red-green colour blind, so I had to build an electric foundry so I could get the temperatures right (I mean what colour is "straw"? for goodness sake, and how many shades of red do "cherries" have?).

In Steve's case I would have thought that the hardening process would also be the hardest part (no pun intended).

Steve, this is probably teaching grandmother to suck eggs, but when specifying the holes make sure either your specified size or the laser cutter operator (via a specification) takes the laser kerf into account otherwise the pins will drop straight through or will wobble in the hole.
gary
 

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