color mirror screw

[roger63]

Hello everybody,
I am glad to share with you some data related to my experiments with mirror screw and color. Here after, you will find two schematics.
One shows the physical structure of my dual monochrome/color light source module for a 60 lines mirror. Notice that a sandy glass is necessary to avoid dark areas to spread horizontally along the picture in monochrome mode and also to smoothly spread the colors in color mode. The cost for using it, is a considerable loss of light. Hopefully, the styren rods partly correct that by concentrating the light. When using the RGB piranias leds, an additional dedicated styren rod is necessary and viewing in darkness is mandatory.
The other schematics, shows my mirror screw color amplifier using the WC-01 Aurora converter as input. Output are for RGB cms piranias leds but any other rgb leds are ok, given they gives very hight brightness .
Note: the external styren rod in front of the sandy glass is 20mm diameter and not 10mm.
Rogerl

[Viewmaster]

Many thanks Roger for sharing this with us.
I am very much more a mechanical man than electronics so am rather interested in your photo, "Miroirs en cours d'ajustement angulaire."

When I made, but a humble 32 line mirror screw, I used a 96 tooth gear to index for each mirror piece (using every 3rd tooth), and a fixed datum point to set to with clamps on each of the 32 slats. (See previous thread)

In your photo there appears to be a kind of datum lever but I was wondering how you got the correct 6 degree indexing and how you clamped the slats already set?

I used BMS for the mirror pieces and they still shine after about 2 years, so has your aluminium tarnished since making it ?

[roger63]

Hello,
I used a large 30 centimeters diameter protractor to adjust the plates to 6° each. Once the plate is correctly positionned in relation to the previous one, I glue it , just a drop at the 20mm borders. Then I wait for the glue to dry and 1 hour after, I do the same thing for the next plate. It is a boring work and you are never sure to get the absolute angular accuracy. In fact, with this system, I am never been able to get a perfect angular accuracy on my miror screw.
The archives tells that the angular accuracy should be better than 1/50°. I am far from that.
Up to now the aluminum polishing of my mirror screw stay ok after a year. When I it began to tarnish I will use a polishing cream for aluminum and use a coton tip to pas onto the plates edges.
If I could replace the protractor with a 60 theeth wheel on the toll's shatf, I will surely get better results.
Roger

[Viewmaster]

I suppose Roger, that if you used a tiny dab of Superglue you would cut the waiting time down from 1 hour.

Yes, a 60 tooth good quality large diameter gear would, I am sure, give you very good results.
I use gears for indexing all manner of constructions where accurate angular dimensions are required.
All very much cheaper than a dividing head .
There are 60 tooth gears for sale in Ebay most of the time, or 120 teeth (using every other tooth).

Thanks again for your drawings and input on all this.

[roger63]

Hi back,
Thanks for your message. No, I don't want to use superglue for the mirror plates as it is very difficult to make further angular positionning correction without damaging the plates. I use a kind of resine-based glue instead, that gives a chance to eventually unglue the plates for re-adjustment.
I will look Ebay for the 60 theeth gear you mention. Do you have an idea of accuracy of such gear?
I think a 30 centimeter diameter whould be a minimum.
I would be so happy if I could get my mirror perfectly adjusted.
Have a nice week end!
Riger

[Viewmaster]

Hi back,

I will look Ebay for the 60 theeth gear you mention. Do you have an idea of accuracy of such gear?
I think a 30 centimeter diameter whould be a minimum.
Riger

The best gears are machine cut, whilst the cheaper plastic ones are moulded. In either case it is likely that the gear tooth spacing would be better than that achieved by using a protractor. The teeth do not suffer from cumulative errors either.

30 cm dia is very large for a 60 toth gear and you will have difficulty in finding such I would have thought.
I had more in mind about 10cm as a practical gear. All the teeth must be cleaned of any particals which may alter the central position of each tooth.

As your mirror is greater than 10cm then errors will be magnified of course, by the ratio of gear dia to mirror dia.

But with a turned peg to fit the tooth inner profile on a screwed rod (without ANY side play !) you should have a good results.
I'm sure, Roger, you will know that backlash, side wobble, loose bearings etc. will all contribute to the overall errors, so each must be eliminated.

Out of interest, the professionals in the old days used a dividing head to index the mirror screw with an illuminated hair line observed through a microscope to zero in on (see image) to set their mirror screws. (Dividing head at the bottom)

machine cut gears also use a dividing head of sorts to get the correct tooth spacing, so one is using the image, as it were, of the dividing head accuracy, if using a good quality gear.

added......gears are also cut with a hob, but do noit know the accuracy

[roger63]

Thank you very much for all those data.
Effectively a 30 cm gear would be far too large. I think also that around 10 cm would be ok, as the mirror screw wide is 12 cm.
I have contacted a french manufacturer to see if He can make the gear at a reasonable price (!) for me.

I have made an adjusting tool that is not that bad but of course, the protractor is not allowing enough precision.
I believe that with a 60 tooth gear mounted on the shaft and a metal piece fitting exactly in the gear hole for each tooth, should give much better results.
I give you the plan of my adjusment tool (the one you see on my website) on a next message.
Roger

[roger63]

Hello,
This would be my mirror screw adjustment tool improvment I am thinking about. Mainly, replcing the protractor by the toothed gear.
The A, B and C shafts are precisely aligned and so, the D piece shows no play in sliding along the A shaft.
Roger

[Viewmaster]

Yes, that all looks very good.
In my case I cut out all the shafting A and B and used a simple stop resting on the base plate of the whole assembly.

The stop was moved along under each slat in turn to set it up.
The extra problem there was that the mirror screw shaft (your C) had to be exactly parallel to the base in order to get constancy in the stop position.
But in this manner there were less bearings to introduce errors.

In place of your sprung loaded pawl I used a tapered end screwed rod screwed into a gear tooth each time. The taper fitted nicely, the profile of the inner tooth shape.

I guess one pays one's money and takes one's choice.

Good luck with your gear, although you may find a purpose made gear that you are getting, rather pricey.

It occurs to me that if you did get a special made it need not be a gear as such. Just a 60 division blank with the 60 places accurately cut for you to index into. Much cheaper to make.

[roger63]

Thanks. I would be interested in seeing a plan of your adjusment tool.
I would think of a 12cm wheel with 60 "V" shaped cuts to fit with a "v" shaped blocking piece.

On my tool, the front and back vertical supports cut at the same time, have their shaft holes being drilled together, so no misalignment may occur .
During alignment process, the mirror shaft rests horizontaly on V shaped cut in the front and back support.

One remark about the mirror screws I saw here. They almost all have upper and bottom round plates to maintain the mirror assembly. I first used this, but quicly noticed that it produced a shadow on the upper and lower part of the mirror screw. So, I replaced it by a thick bar at each end and no more shadow appear.

[Viewmaster]

Thanks. I would be interested in seeing a plan of your adjusment tool.
.

I have no drawings as I make things on the hoof (probably why some never work! )
but it is shown here on this thread......... Picture 210.jpg

http://www.taswegian.com/NBTV/forum/vie ... c&start=60

[roger63]

Thanks my friend.
Personnally, I will discard nylon-type gear and use a copper wheel instead.
I will inform about modifs of my aligment tool if significant improvment occur. But before that, I must unglue all the mirror plates and this is a boring and delicate operation I am not ready to do in short yet.

[Steve_McVoy]

You may be aware of the color mirror screw project we worked on a couple of years ago:

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