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Milling tolerances for Jaegers objective cell

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#1 terraclarke

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Posted 06 February 2013 - 09:40 PM

I am beginning to machine aluminum objective cells for two Jaegers objectives, an 83mm x 1219mm achromat and a 103mm x 1575mm objective. My question is what should the tolerance be between the cell's i.d. and the objective's o.d. Several of you in this group have just fabricated telescopes with these same objectives. Is 1 mm to much? 0.5 mm? Also, I was considering seating the objective in the cell on a thin cork ring. Is it better to seat the objective directly on the aluminum surface?

Thank you much for your advice and willingness to share your experience.

#2 starman876

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Posted 06 February 2013 - 09:46 PM

Hmmmm, I would need to get a caliper and measure the Jeagers I have in a cell. Would that help? First I need to get a caliper :foreheadslap:

#3 terraclarke

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Posted 06 February 2013 - 09:49 PM

We'll have a caliper at the machine shop Friday as well as the lens and the tube. But shouldn't there be a tiny bit of play between the edge of the lens and the wall of the cell?

Terra

#4 starman876

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Posted 06 February 2013 - 10:05 PM

But.... I will not be at the machine shop with the cell. There has to be a bit of play, Otherwise you would never get the objective out of the cell without chipping it. Also, the cell would pinch the optics if it used outside in the winter because of contraction because of the the cold temperature. Glass and metal does not contract at the same rate.

#5 dan77

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Posted 06 February 2013 - 10:06 PM

Hi Terra, First of all you'll need to establish the roundness as well as the outer diameter of the lens. Measure the diameter at several points and add .01-.015 to the largest measurement and bore your cell to that. Good luck!
Dan

#6 terraclarke

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Posted 07 February 2013 - 07:51 AM

Thank you very much Dan. Thats exactly what i needed to know. Me and my machinist friend are going to work on it tomorrow. I'll post pics when it's done.

Terra

#7 bremms

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Posted 07 February 2013 - 12:43 PM

0.010 is more than you need for a 3" lens. The difference in expansion gives a delta of 0.0028" for a 50 C temperature change. 0.005 is more than adequate. 23-7 ppm per deg c is 16ppm x3.5" x 50 deg is 2800x10-6 or 0.0028". 0.005 is plenty. I was being conservative with all values.
Rule of thumb is minimum of 0.001" per inch

#8 bremms

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Posted 07 February 2013 - 12:52 PM

Add a little for paint if need be. If its being anodized it would be the post anodizing dimension. If it is a little too big, black masking tape spacers around the edge are easy peasy

#9 DAVIDG

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Posted 07 February 2013 - 02:10 PM

We make them 0.010" over sized for two reason. One reason is that the glass itself may not be perfectly round and the second is that when your inserting or removing the glass, it allows for a bit of room for any tipping of the elements without them jamming in the cell and/or chipping an edge.
If your lenses are air spaced with individual spacers at 120 degrees, you want three raised areas on inner rim of the cell also at 120 degrees. The reason is that one wants even pressure on the spacers to maintain an even air gap. I have come across a number of vintage objectives were they have been dissassembled and then reassembled with the spacers not position over the raised areas on the rear of the cell. This results is a triangular shaped diffraction pattern at high magnifaction from the elements being pinched.
I wouldn't use cork but instead three small pieces of Kapton tape at 120 degree centers. Kapton is very uniform in thickness, won't change dimension with humidity and will last much longer then a natural material.
Your objective should have very slight rattle when shook side to side and then turn 90 degrees and shook again. I restored a 6" Schupmann refractor for a friend were the objective rattled nicely in one direction but when the cell was turned 90 degrees the glass was being pinched. When the air temperature dropped astigmatism developed. I chucked up cell on the lathe and turn it out a few more thousands and astigmatism was gone. So check your objective in both directions when it mounted in it's cell.

- Dave

#10 bremms

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Posted 07 February 2013 - 02:35 PM

0.010 makes sense, I machined mine 0.005 over the very largest dimension. the lens rests on a PVC ring and the retainer is PVC and spring loaded with VERY slight pressure. This ensures even retention and no stress in the objective. Kapton would great be for a spacer. Have a few thicknesses here at work. The raised area under the spacers is something I've seen for large objectives, with three and six spacers. It would help with even distortion of the elements. I'll post a pic of my spring loaded retainers.

#11 dan77

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Posted 07 February 2013 - 04:30 PM

0.010 is more than you need for a 3" lens. The difference in expansion gives a delta of 0.0028" for a 50 C temperature change. 0.005 is more than adequate. 23-7 ppm per deg c is 16ppm x3.5" x 50 deg is 2800x10-6 or 0.0028". 0.005 is plenty. I was being conservative with all values.
Rule of thumb is minimum of 0.001" per inch



I made my first lens cell (a 5"dia) .005 over and when checking the fit with the lens got it into a bind and chipped a huge scallop in the crown destroying the lens. Every since then I go at least .010 over and haven't had any problems.In theory .003-.005 is enough but practically is not.
Dan

#12 terraclarke

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Posted 07 February 2013 - 05:25 PM

Thanks all. My biggest fear is that it would be so close, that a slight tilt would cause it to jam and flake the edge of the lens. I am going with .5 mm over the lens diameter. That would be about .01 inch all around. If its slightly to much jiggle it can be accommodated with tape. I have a metal spacer ring that came with the lens. Will seat it on Kapton.

#13 actionhac

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Posted 07 February 2013 - 06:08 PM

Terra how about this:
http://www.cloudynig...p?product=73843

Robert

#14 terraclarke

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Posted 08 February 2013 - 07:36 AM

That's a nice one Bob. That's actually a friend of mine selling it. He's quite a telescope builder. The cell I am making is unique in that it is being custom machined for an empty 3 inch Unitron ota that I have. So it has to be made to fit not only my Jaegers objective, but the Unitron tube and its pre-drilled holes on one side, and the threaded Unitron dew shield on the other. Thus it will be a Jaegertron. It will be fun to set it up along with complete 3 inch Unitron and compare the views.

Terra

#15 terraclarke

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Posted 09 February 2013 - 10:37 AM

We spent several hours yesterday afternoon in the shop. Checked all measurements on the plans I drew up against the actual parts with calipers and made a few minor adjustments. Then we selected a piece of aircraft grade aluminum stock and cut it.

Terra

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#16 terraclarke

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Posted 09 February 2013 - 10:41 AM

The original stock was four inch diameter. The next step was to place it in the three clamp jaw of the lathe's headstock and face it off, then roughly take it down to the largest outer diameter. That done, be begin to hollow it out. Here's the first drill cut.

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#17 terraclarke

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Posted 09 February 2013 - 10:43 AM

Now that we have the center hole, we begin the small bore. The first bore was about 3/4 inch. Here's the second bore. We are using a 1 and 1/2 inch bit in the tailstock's chuck here.

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#18 terraclarke

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Posted 09 February 2013 - 10:51 AM

After the second bore, we back off the tailstock and place a cutting tool in the tool post. Then we move the cross slide in and center the tool to begin to widen the cut.

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#19 terraclarke

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Posted 09 February 2013 - 10:54 AM

The last step for yesterday was to widen our center cut to roughly approximate the largest inner diameter of the cell. Now the main body of cell is roughed out. We'll finish it next week.

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#20 bremms

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Posted 09 February 2013 - 05:19 PM

Terra, I take it to be the smallest inside diameter? Your pics are rotated.. I was using my phone to look at them and I rotated and was like.. Oh yea that's right, then the image on the phone flipped.. It was kind of funny, I kept spinning the phone and had it cocked a certain way. Was in public too.Just the local farmer's market, but still.

#21 terraclarke

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Posted 09 February 2013 - 05:23 PM

Yes, that's a mistype. It is currently taken down to its smallest i.d. and its largest o.d.

#22 bremms

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Posted 09 February 2013 - 05:25 PM

Since the spacer is a ring use a ring for the bottom as well.
I've made quite a few cells, never had a real problem with a stuck lens. It could happen. You develop a feel for taking the lens in and out. If you have a decent gap on the sides you need to take it up, otherwise you can get decentering of the lens.

#23 terraclarke

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Posted 09 February 2013 - 05:41 PM

We are going for 0.01 inch tolerance. I think that will be perfect. If its too much a small bit of foil tape or drafting tape can take up the slack. I was thinking of using a gasket ring for the base. Have to see what I come up with. We will hopefully finish the cell one day next week. We still have quite a bit to do on the lathe, then cutting the threads, drilling the holes to mount it in (on really) the Unitron tube. (It will fit to the tube in the same way as will the original Unitron dew shield which will thread to the outside of the cell.

#24 bremms

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Posted 09 February 2013 - 08:18 PM

Terra, here are a couple of shots of my 83mm cell. This one is very simple did this during 3 lunch times at work.

This one is a PVC retaining ring using 4 2-56 screws with or without springs. You can use no springs and make the screws a finger tight with some locktite

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#25 bremms

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Posted 09 February 2013 - 08:19 PM

Cell with lens. You will see what the big eyepiece is for.

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