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The last several days have been spending time in the garage on the lathe making these fittings for the scope.
Both these fittings are shown upside down to make the threads visible. On the left is an adapter sleeve which is placed over the base illuminator opening. A telescope eyepiece filter is screwed into the top. Before this was made, the eyepiece filter was resting on top of the upper lens in the illuminator, not the best arrangement.

On the right is a threaded plug that goes into the vertical tube port on the trinocular head. Before this was made, was leaving the tube screwed in with a plastic cap on top to keep dust out of the prisms in the head. Now the tube only needs to be mounted when doing photography.

The filter sleeve on top of the illuminator:
The vertical tube port plug installed on the trinocular head:
Noticed something interesting about the threads on the telescope filters. They are made in Korea for 1-1/4 inch telescope eyepieces made in China. I figured the threads would probably be metric. After not being able to get a close fit with metric thread pitch gauges, went to the U.S. thread pitch gauges. Got a very close but not exact fit with 40 threads per inch. Did some checking and discovered 40 TPI is very close to a .65mm metric thread, but that gauge leaf was not in the metric thread pitch set. I guess that thread pitch is not commonly used. Anyway, it was a fortunate coincidence, because my lathe cannot cut metric threads without buying additional gears to mount on the quadrant in the head.

And this time was careful to fasten the compound rest at the correct 29 degree angle, so the threads came out a lot better looking, but not perfect. Still a bit ragged in places. Anyway, the parts fit smoothly without binding or excessive looseness.

For what it's worth, Rick ... The vast majority of filter threads are 0.75mm pitch.

This [of course] may, or may not, be true of Korean ones for Chinese telescopes.

MichaelG.
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Edit: ... Mmm ... I've just found this:
https://www.cloudynights.com/topic/3401 ... eyepieces/

MichaelG. wrote:For what it's worth, Rick ... The vast majority of filter threads are 0.75mm pitch.

This [of course] may, or may not, be true of Korean ones for Chinese telescopes.

MichaelG.

Yup, I tried a .75mm gauge when checking the threads, and the fit was very poor. The 40 TPI American thread gauge fit was almost exact. Since the fit wasn't perfect, I'm guessing these filters are made with a .65mm thread pitch, which I don't have a gauge for to check exact fit.

I wonder. At one time I had a circular ND glass filter, without a metal sleeve, just the bare glass, installed in a general-purpose Newport or Oriel round filter holder. When placed on top of the illumination port, there was a vertical distance of about 10mm (roughly) between the filter and the port top glass. It proved the wrong way to go: All dust particles on the filter were visible as dark objects within the FOV. When I removed the filter holder and again placed the bare glass filter flat on the illumination port top glass, all was well again, the dust became invisible.

desertrat wrote:I'm guessing these filters are made with a .65mm thread pitch, which I don't have a gauge for to check exact fit.

Do you have any 4BA screws ?
The pitch is right, although the thread angle is only 47.5°

https://www.fastenerdata.co.uk/ba-threads

MichaelG.

I severely doubt that the ubiquitous filters for cameras and such as are used worldwide by the millions would use an oddball thread with a 47.5 degree thread angle... The world standard thread angle is 60 degrees...

BillT

billbillt wrote:I severely doubt that the ubiquitous filters for cameras and such as are used worldwide by the millions would use an oddball thread with a 47.5 degree thread angle... The world standard thread angle is 60 degrees...

BillT

Bill ... That is precisely why I mentioned the angle !!
I was suggesting using 4BA to check the pitch only.

MichaelG.

Digital readout, dial or vernier calipers can quickly and accurately measure the pitch on ANY thread..

BillT

Hobbyst46 wrote:I wonder. At one time I had a circular ND glass filter, without a metal sleeve, just the bare glass, installed in a general-purpose Newport or Oriel round filter holder. When placed on top of the illumination port, there was a vertical distance of about 10mm (roughly) between the filter and the port top glass. It proved the wrong way to go: All dust particles on the filter were visible as dark objects within the FOV. When I removed the filter holder and again placed the bare glass filter flat on the illumination port top glass, all was well again, the dust became invisible.

That's an important point, and I'm lucky that didn't happen with this adapter. I would be really bummed if I had to throw it out after the time and effort it took to make it!

billbillt wrote:Digital readout, dial or vernier calipers can quickly and accurately measure the pitch on ANY thread..

BillT

That's a very good point, and one I didn't think about. But because of the ledge right next to the threads on the filter and the shape of my dial calipers' jaws, I probably wouldn't be able to get a good enough measurement to show the difference between 40 TPI and 0.65mm threads.

And after looking on the 'net a bit, I found a company in New York that sells these 4BA fasteners (I don't have any), and the prices aren't bad (not counting shipping or minimum order).

But since the filters fit nicely in the adapter, I'm not feeling very inclined to try to solve the mystery..

"And this time was careful to fasten the compound rest at the correct 29 degree angle, so the threads came out a lot better looking, but not perfect. Still a bit ragged in places. Anyway, the parts fit smoothly without binding or excessive looseness."

Ragged threads are usually caused by not enough side clearance on the threading tool.. I speak from years of experience in solving this and a myriad of other machining problems that I was in charge of..

Regards,
BillT

desertrat wrote: But since the filters fit nicely in the adapter, I'm not feeling very inclined to try to solve the mystery..

... and that's what matters : Good result.

MichaelG.

Metric - imperial conversion:
My metric lathe has a 63 tooth gear. 63 is close to the half of 127 which is half of 254. A 63 or 127 tooth gear might help in your case too, Rick.
There are computer programs to calculate gear settings based on what gears you have available. The sometimes come to many additional combinations over what the manual of the lathe suggests and state the error based on the nominal pitch you want to cut.
For my old lathe I made some additional threading gears from the gearbox of a hand drill with odd tooth counts.

With short, fine threads I find it difficult to determine the right pitch with caliper or gage. It is handy to have access to a measuring microscope here.
I had a couple of cases where I got my pitch wrong.

Bob

Hi Bob,

Some American lathes are designed to be fitted with a 127 tooth conversion gear that allows for exact conversions to metric threads. My old Atlas lathe wasn't designed to use that. For it, some extra gears used in the quick change box can be bought and are fitted to the movable quadrant outside the gearbox. These aren't exact conversions, the average pitch error is about 1 in 3000, which is OK most of the time unless real long thread engagement is needed. I have the user manual which has a complete chapter on these conversions for all the commonly used metric threads.

In the near future, it looks like the only metric threads I'll be cutting are for filters, which have an engagement of only a few threads. For these, and since no mechanical load bearing is involved, the nearest U.S. thread approximations can be made to work, especially since the filter is available to try fitting during the last several thread cuts.

The reason the 127 tooth gear is an exact conversion is because early in the 20th century, the USA redefined the American inch to be exactly 25.4 millimeters. I don't know what the conversion was before that happened, but the change was apparently so small that dimensions and tolerances for machinery were not affected.

desertrat wrote:The reason the 127 tooth gear is an exact conversion is because early in the 20th century, the USA redefined the American inch to be exactly 25.4 millimeters. I don't know what the conversion was before that happened, but the change was apparently so small that dimensions and tolerances for machinery were not affected.

Wikipedia ... the fount of all knowledge:
https://en.m.wikipedia.org/wiki/Inch
... well, it's convenient anyway.

MichaelG.

"But because of the ledge right next to the threads on the filter and the shape of my dial calipers' jaws, I probably wouldn't be able to get a good enough measurement to show the difference between 40 TPI and 0.65mm threads.

You can carefully take a "cast" of the thread with a small amount of modeling clay and measure the pitch from that..

To measure inside threads it is also a way to use a piece of wood or cardboard and transfer some paint or dirt marks of the thread onto it. Paper is not ideal as it deforms when pressed onto the thread. Bills cast method is more precise but usually I'm too much in a hurry!

Bob

I'm certainly no engineer but this is fascinating, the OPs industriousness and lathe skills are to be applauded, the trivia on measurements fascinating and I love the practical measuring techniques discussed latterly. Bravo one and all.

I always was a bit strange though, don't worry for me.

Just came upon this thread and have to applaud desertrat for doing a great job of fabricating a part that is not likely to be found easily or could be found at all. I've always had great respect for people who are able to do this kind of stuff. Great job and thanks for sharing, now I have a source for manufacturing hard to find parts . Cheers. -Norm