Reduce thread on telescope eyepiece ? How best to ...

My perfectly serviceable but ~12 year old birding telescope (good quality with ED glass) has a female Imperial threaded socket for the eyepiece,
~28.42mm OD measured on a mating eyepiece male thread. I've just bought a newer, more powerful eyepiece with a slightly larger metric thread, 28.95mm male OD (+0.5mm), and having emailed several likely places without even a reply, I am wondering how best to get my new eyepiece to fit my older telescope. The pitch of both of the threads is very close to 32tpi. (Sorry about the mixed dimensions!) I think the material of the metric eyepiece thread is aluminium, as also I suspect is the Imperial threaded socket on the telescope. (Doesn't react to a magnet.)
(The importers of the telescope and eyepieces have offered to change the telescope thread to metric, but then my older Imperial eyepieces will need changing.)
I don't have a lathe, and haven't been able to find suitable tpi dies large enough; but do have time, a machine vice, small files etc. and am looking for practical advice on how best to reduce the larger thread. (As far as I'm aware I can't remove the optical cell, so any cutting would best be done without oily lubricant.)
Thanks for any ideas
--
M Stewart
Milton Keynes, UK
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Malcolm Stewart wrote:

That should be a 0.75mm pitch. But optical threads are often funny.

You won't find a die that size. And if, it would be a pain to get it straight onto without a lathe. Don't even think about trying that!
If you try to recut the M29 x 0.75, you won't have luck. The thread is 0.75mm deep (per side), not enough meat. And don't try to make a dual-purpose thread. :-)
Nick
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snip
Thanks Nick Not having any thread gauges, or similar, I tried to measure the threads by placing both eyepieces and a tape measure on the glass of my scanner. I then took the image into Photoshop, and using the <Shift> + pencil/draw tool, drew accurately parallel lines from the threads to the markings on the tape measure scale. At least there shouldn't be any optical distortion creeping in which would have been likely, if I'd simply photographed the items.
--
M Stewart
Milton Keynes, UK
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Hi Malcolm - more used to seeing you elsewhere!
How thick is the wall of the tube of the new eyepiece? If there is enough meat on it, it may be possible to turn off the thread and re-cut on the smaller diameter. This would be easy if the thread pitch were the same, but they won't be, which will be a problem as the depth of the old threads will be slightly greater than the difference in OD.
Also, is there a decent un-threaded cylindrical surface which can be used for chucking the eyepiece? This would make life a lot easier. Make sure you use a collet or some shim to protect the tube surface though.
Find someone with a shadowgraph or toolmakers microscope, or at the very least some proper thread gauges, to check the thread pitches and depths before doing anything drastic. If you come near this part of the world (Bromley, SE London) you are welcome to use mine (or have me use it for you).
Aluminium is usually cut with paraffin lubricant, but AFAIK this is mainly to reduce the build-up of welded Al on the tool tip, and for screwcutting of this type you would be strongly advised to turn the lathe by hand anyway. Brass is usually fine when cut dry anyway.
I did dry cut a 0.75mm (IIRC) pitch thread on a couple of microscope eyepieces once; the brass one cut like a dream, the aluminium one had torn crests (but worked fine).
Attacking it with a file is the last thing you should do!
--
David Littlewood

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Malcolm Stewart wrote:

Unless the pitch is near-identical you will find it impossible to recut the thread with any accuracy, the difference in size is too small.
Hold two males together, and check the pitches. Is there more than a quarter thread mismatch over their working length? If so, you will not be able to recut the thread. Less than an eighth would be better.
Metric dies that size are in fact available, but they probably cost quite a bit more than a new lens - best part of 100 new and maybe more depending on quality, and unusual sizes tend to be only available in the more expensive ranges. But there was a 29 x 0.75 mm die on eBay about 4 months ago, in a lot with some smaller dies - fetched 150 Euros iirc.
Imperial dies, on the other hand ..
One other possibility is what I believe is called a Coventry die head, but I may be wrong about that. It has sets of blades which cut teeth of a certain pitch, but the diameter is adjustable. Using one by hand you may be able to thin the thread down, and if you use it carefully it will align with the old threads. But again, this depends on the pitch matching very closely.
But I don't think you will have much luck recutting the new lens' thread. Unless of course it is 29mm x 32 tpi - stranger things have hapened in the telescope world!

This might be the best solution - in future when you buy new stuff it is likely to be metric, and will then fit.
If you can get to keep the original imperial telescope female fitting as well, you might be able to change the fitting rather then the lens?
Another possibility is that the Imperial male lenses will fit loosely but satisfactorally into the metric female fitting - again, only if the pitches are close enough. Could be a suitable case for a bit of Rizla though ..
--
Peter Fairbrother


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On Fri, 4 May 2007 10:33:20 +0100, "Malcolm Stewart"
Worth speaking to these guys:
http://www.srbfilm.co.uk /
They specialize in photographic filters and adapters, & are geared up to make specials for just the kind of problem you are describing. Worth giving them a call to see what they would suggest & how much they would charge you. For example, a potential solution would be to get them to machine up an adapter with a male thread to fit the scope body and a female to fit the eyepiece. This will result in the eyepiece standing off rather further than it would normally, by a coupe of mm more than its male thread length; however, if the adjustment in the focusing mechanism on the scope has enough travel, it should still work (you could probably check this first by offering the new eyepiece up to the scope & seeing if it still focuses out to infinity. If it does, this may be a goer.
Regards, Tony
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On Fri, 4 May 2007 10:33:20 +0100, "Malcolm Stewart"

It looks as if your thread sizes are 1 1/8" 32 TPI and 29 mm 0.75mm pitch
Without a lathe the chances of sucessfully recutting, or filing down, the larger metric thread are near zero!.
With access to a screw cutting lathe it should be possible to sufficiently deepen the Imperial thread with a single point tool. For the short length of engagement the difference between 32 TPI and 0.75mm pitch TPI is too small to matter.
The quick and dirty solution is to get the metric version of the telescope and be satisfied with the sloppy fit of the Imperial eyepiece. The OD difference is 1/2 mm but the double depth thread is nearly 1mm so there should be sufficient remaining engagement if carefully installed.
Jim
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Malcolm Stewart wrote:

For completely different reasons (mounting digital cameras on scopes) this company makes/sells all sorts of thread adapters.
http://www.eagleeyeuk.com/erol2eu/erol.html
BugBear
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Nick Mueller wrote:

Probably 0.8 mm, not 0.75 - 1/32" is 0.79375 mm.
0.8 mm pitch is 31.75 tpi - 0.75 pitch is 33.866.. tpi
I'd think he would have noticed the difference, and I don't think he would have described 0.75 mm as "very close to 32 tpi".
This suggests that if the thread length engaged in the fitting is less than perhaps 1/2 inch he might be able to gently scrape the metric thread down with an imperial 32 tpi chaser.
This must be done in a lathe or similar rotary device. It will be a bit loose, but it should do.
But getting a metric fitting on the telescope is probably the better answer.
--
Peter Fairbrother


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Peter Fairbrother wrote:

That doesn't exists in the series of allowable metric pitches. 0.5, 0.75 or 1.0.
But again, optical threads can be really funny and I would not relay on guesses (like mine) but just on precise measurement.
The suggestion to find a specialist is really the best one can do.
Nick
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I suggest you have a look at an M5 ISO metric tap (0.8mm pitch). The ISO metric thread pitches are ... 0.5, 0.6, 0.7, 0.75, 0.8, 1.0.... Why there is such a gap between 0.8 and 1.0 is odd, I guess that's what happens when you let committees define things.
The main feature of the ISO definition is the thread form, and I guess manufacturers are free to make odd combinations if they want to. The "normal" set of diameter/pitch combinations is clearly inadequate for things like photographic filters and other things to connect to lenses, for example.

Indeed.
David
--
David Littlewood

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wrote:

M5 x 0.8
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On Fri, 04 May 2007 18:57:56 +0200, Nick Mueller wrote:

"allowable"? Who's making these silly rules?
I have a tap&die set here with 0.6, 0.7, 0.8, 0.9 as well as 0.5, 0.75, 1.0, etcetera.
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Nick Mueller wrote:

Allowable? 0.8 mm is unusual, but ..
As an example, I possess 18 x .4, 18 x .45, 18 x .5, 18 x .55, 18 x .75, 18 x .8, 18 x 1.0 and 18 x 1.5 mm taps and dies.
I don't have 18 x 1.25, 18 x 2 or 18 x 2.5 sets, but they are made - an 18 x 1.25 dienut is presently offered on eBay.
I believe that 18 x .35 and 18 x .65 are made too.
I have several smaller 0.8 mm pitch taps and/or dies. I also have a complete 36 mm x 0.8 mm set, with all three taps and a die. Don't ask how much it cost though :-(
I don't have a set for 29mm x 0.8 though, unfortunately, nor the imperial equivalent.
--
Peter Fairbrother


(sometime collector of fine metric taps and dies)
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Peter Fairbrother wrote:

Ah, well. You know I'm a stubborn Teutonic! But ... but I stumble, er .. stand corrected! 0.8mm pitch does exist. M5!

That exotic collection of dies (candidate for the Guinness Book of Records?), what do you need them for?
Nick
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<POLITICALLY INCORRECT> It's the French Metric equivalent of the English Imperial Model Engineer's thread series' that have a constant thread pitch for different diameters. This series has a constant diameter but different pitches... </POLITICALLY INCORRECT>
<G>
Mark Rand RTFM
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Malcolm Stewart wrote:

Please post the raw scan!!!
Or send it to me, and I will post it if you can't.
--
Peter Fairbrother


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Scan emailed to you. Thanks
--
M Stewart
Milton Keynes, UK
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Nick Mueller wrote:

Need?
-- Peter Fairbrother
"And you're not a gun freak?"
"Oh no. I'm a collector. Huge difference."
"The difference being?"
"I've got more guns."
(BOFH)
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Malcolm Stewart wrote:

http://www.zenadsl6186.zen.co.uk/Eyepiece_threads.jpg
Tht thread is very short - you will probably be able to convert it with a threadchaser. You will need some kind of rotating thing to fix the eyepiece in if you have not got a lathe.
I still don't know what the pitch is though!
--
Peter Fairbrother


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