Unbranded Petzval lens, rack&pinion screw missing.

Topsy · Apr 2, 2017

Hello!

I'm new here and I hope I've found the right corner of this forum to ask.

I recently acquired an unbranded magic lantern/carte de visite (?) petzval design lens to play around with/do some experiments and potentially build a camera around it for fun. It's got no aperture or waterhouse slot.

Overall the lens is not in a bad shape for it's age and it came with the lensboard which was a plus for me.

It is however missing one of the small screws that attaches the pinion to the rack.

It's thread is 3mm long (~1/8"), 2.35mm (~3/32") in diameter and has a pitch of, possibly ~48-50tpi?. I can't figure it out accurately.

Here's an image of the lens:

And here one of the screw that fits. I'll add the dimensions under this image again so that you don't have to scroll up again.

"It's thread is 3mm long (~1/8"), 2.35mm (~3/32") in diameter."

Does anyone know what kind of standard (if any!) this screw might be? I got the lens from England, so I thought it might be a Whitworth or a #5 BA, but I can't currently find anything in the appropriate sizes.
Knowing the rough age of the lens (or in general what it would have been used for) might help too, and would be interesting to me.

I could of course always re-tap the hole but where' the fun in that? Besides I don't really want to modify it at all.

If anybody has any ideas, please let me know!
Many thanks,
Topsy

Leigh B · Apr 2, 2017

Hi Topsy, and welcome aboard.

The threadform in your photo is unlike any modern 60° US or metric thread.
Modern threadforms are intersecting triangles, pointed at top and bottom with straight sides.

It may be a 55° Whitworth, which was common in England.
That form has radiused peak and valley, although not as significant as yours IIRC.

A 3/32" 48TPI Whitworth is standard. You could buy such a screw and try it.
Wikipedia page: https://en.wikipedia.org/wiki/British_Standard_Whitworth

- Leigh

Ian Grant · Apr 2, 2017

I have a similar Petzval it's a magic lantern projection lens, there would have been a brass piece that pivoted to use like a lens cap with a screw at the edge, you have the holes on the top ring and the second hole had a screw so that the brass circle would sit against it closed.

Not sure if I have any screws or not, it would be a day or so before I can look. My lens isn't very practical it covers just over 6x6cm but not at infinity and with no slot for a Waterhouse stop.

I have a much larger Petzval which covers 5x4 and I use it with a Speed Graphic, it came mounted as an enlarger lens with a big brass mount but would have been converted from old unsold stock after Rapid Rectilinears became dominant, as it does have the slots for stops.

Ian

shutterfinger · Apr 2, 2017

Diving the number of peaks or valleys by the length gives the Threads Per Inch. 6 peaks or 6 valleys divided by 1/8 = 48. Now measure the diameter of the screw at the peaks and look up screw thread limits to find what size screw it is.

Leigh B · Apr 2, 2017

shutterfinger said:
Diving the number of peaks or valleys by the length gives the Threads Per Inch.

Your algorithm is correct if you start counting the peaks or valleys at 0.
If you start counting at 1, you must subtract 1 from the total before doing the division.

For example, you can count the same threads as
0-1-2-3-4-5-6 where 6/(1/8)=48
or
1-2-3-4-5-6-7 where (7-1)/(1/8)= 48

What you actually want is the nmber of intervals _between_ the peaks or valleys.

- Leigh

Topsy · Apr 2, 2017

Leigh B said:
Hi Topsy, and welcome aboard.

The threadform in your photo is unlike any modern 60° US or metric thread.
Modern threadforms are intersecting triangles, pointed at top and bottom with straight sides.

It may be a 55° Whitworth, which was common in England.
That form has radiused peak and valley, although not as significant as yours IIRC.

A 3/32" 48TPI Whitworth is standard. You could buy such a screw and try it.
Wikipedia page: https://en.wikipedia.org/wiki/British_Standard_Whitworth

- Leigh

Yes, I do believe this might be the fitting screw, however finding a 3/32 BSW seems to be easier said than done!
Up to now I've found lots of 3/32taps and dies but no 3/32 screw yet.. I might have to make one myself if I can't find one somewhere.. I'd be quite happy to find a used and slightly rusty one though, to match the other one I have!

Ian Grant said:
I have a similar Petzval it's a magic lantern projection lens, there would have been a brass piece that pivoted to use like a lens cap with a screw at the edge, you have the holes on the top ring and the second hole had a screw so that the brass circle would sit against it closed.

Not sure if I have any screws or not, it would be a day or so before I can look. My lens isn't very practical it covers just over 6x6cm but not at infinity and with no slot for a Waterhouse stop.

I have a much larger Petzval which covers 5x4 and I use it with a Speed Graphic, it came mounted as an enlarger lens with a big brass mount but would have been converted from old unsold stock after Rapid Rectilinears became dominant, as it does have the slots for stops.

Ian

Sounds very much like the one I've got here, this one roughly covers a bit less than 6x6 at infinity too..

I'm currently working on a new pivoting brass lens cap for it, though it's pretty hard to find the right materials here in Switzerland for some reason.. Or it's me that constantly keeps choosing the 'wrong' projects..

If you'd have a fitting screw lying around unused it'd be brilliant!

Thanks for all your replies, it's much appreciated!

John Koehrer · Apr 2, 2017

You could verify the size if you just try a tap you have access to.
Obviously you don't want to go out & buy a tap for testing. Oh Well.

shutterfinger · Apr 3, 2017

Over here a 3/32 diameter screw is called a #3, perhaps searching for a #3-48 will turn something up.
ebay search #3-48 bsw screw turns up lots of hits on ebay U.S. and ebay UK.

Topsy · Apr 3, 2017

shutterfinger said:
Over here a 3/32 diameter screw is called a #3, perhaps searching for a #3-48 will turn something up.
ebay search #3-48 bsw screw turns up lots of hits on ebay U.S. and ebay UK.

Thank you very much, it's not easy to get to know the right technical terms for a thread standard I've never knowingly been confronted with before.
I do see a few screws turn up on ebay but they're all located in the US (also those turning up on ebay UK) so I might after all end up getting a die and making my own. And hope that it is the correct screw!

by the way, what era would this lens have been made in? I've been trying to judge it by the design and lensboard and have spent a considerable amount of time looking at magic lanterns, but it's not easy for me. I am looking forward to expanding my knowledge on these topics though.

shutterfinger · Apr 3, 2017

Topsy said:
by the way, what era would this lens have been made in?

My guess is 1860-1900.
Many of the ebay listings for the screws are shipped from China. My only experence with British screws was 20 years ago on a floor standing wood shaper. What a PITA.

Ian Grant · Apr 4, 2017

I'd put the possibledate as as a bit later mid to late 1880's as that coincides with commercial dry plates and lantern slides here in the UK. they were still using Projection Petzvals on Lantern slide projectors in 1910 (I have adverts) I'd guess they switched to better lenses after WWI.

These days getting British Imperial screws etc new here in the UK is impossible, we've been Metric too long. I've some old 16mm ex RAF cine projectors which I'll strip for the Imperial screws etc.

Ian

Topsy · Apr 6, 2017

Thanks a lot for the info guys!

I've ordered a bsw 3/32 - 48 die and am hoping it's the right one.

In the meantime I've made an adapter for the lensboard and will try it on my spotmatic*

*when that is fixed..

shutterfinger · Apr 6, 2017

It the die is the correct one the existing screw should thread into it but it may clean some of the buildup off the screw, not the metal.

Topsy · Apr 19, 2017

Just reporting back.. Received my BSW 3/32 -48 die and made a screw out of brass.

(I cleaned up the start of the thread, don't worry)

It's close. It sort of works, but it does not seem to be the right thread and only threads in 3/4 turns, just enough though. It allows the pinion to be turned slowly and carefully. It's prone to skipping when turned faster currently. Maybe a shim under the screw can help with that.

Still very interested in what kind of screw the "original" (If it is!) is and if it's some strange standard from the past or if it's a thread type only one factory used! It would seem to have 4-6 tpi less.

Edit: Would the barrel usually be lubricated with something? I know the lens smells slightly of old grease but I'm not sure what would be best to use?

shutterfinger · Apr 19, 2017

Topsy said:
Would the barrel usually be lubricated with something? I know the lens smells slightly of old grease but I'm not sure what would be best to use?

Atrace on the inside of the fixed barrel, slightly more on the pinion threads and gear teeth.
A light weight grease such as White Lithium or similar. I started using Finish Line Fett grease with teflon https://www.amazon.com/Finish-Line-...1&keywords=finish+line+fett+grease&th=1&psc=1

shutterfinger · Apr 19, 2017

Topsy said:
It's close. It sort of works, but it does not seem to be the right thread and only threads in 3/4 turns

Does the origonal/existing screw thread into both holes easily?

A screw can be turned into a cleaning tap by cutting three groves equidistant apart and 90° to the screw shaft in the end of the screw with a taper file cutting deeper at the screw lead edge and shallower several threads back.
3 threads back on your example should be sufficent. Once the groves are cut run the taper file with the threads over the grove to remove any burs.

Early screws were hand made and there is always a slight varrance between screws of a given size.

Topsy · Apr 19, 2017

shutterfinger said:
Atrace on the inside of the fixed barrel, slightly more on the pinion threads and gear teeth.
A light weight grease such as White Lithium or similar. I started using Finish Line Fett grease with teflon https://www.amazon.com/Finish-Line-Premium-Grease-Fluoropolymer/dp/B002L5YYYA/ref=sr_1_fkmr1_1?ie=UTF8&qid=1492636048&sr=8-1-fkmr1&keywords=finish+line+fett+grease&th=1&psc=1

In that case I'll visit my bicycle mechanic friend. I think he actually has that exact tub knocking about somewhere!

shutterfinger said:
Does the origonal/existing screw thread into both holes easily?

A screw can be turned into a cleaning tap by cutting three groves equidistant apart and 90° to the screw shaft in the end of the screw with a taper file cutting deeper at the screw lead edge and shallower several threads back.
3 threads back on your example should be sufficent. Once the groves are cut run the taper file with the threads over the grove to remove any burs.

Early screws were hand made and there is always a slight varrance between screws of a given size.

Yes, the existing screw threads in fine and the internal threads seem to be clean.

Yeah, it might have been. Coming to think of it I suppose it might have been made with a hand chaser and thus wouldn't be bound to any diametre restrictions really.

Topsy · Jun 7, 2017

Just a small heads up for anyone intersted.

The BSW 3/32 -48 screw worked after all, I'm not sure if it simply adjusted itself to the threads or if it cleaned them or something.. Doesn't matter now, I'm happy!
The rack has received its dose of grease and the pinion got two small shims as the 'plate' that pushes it towards the rack didn't 'push' enough anymore. A .1mm shimstock piece on both sides was enough.

I applied the woodfinish to my adapter two weeks ago and it's currently attached to a Pentax ME-Super with FP4 Plus 125 loaded. I did try it on D too. Couldn't wait! It's IQ is surprisingly brilliant and unique near zero purple fringing, nice swirly and absolutely brilliant rendering. I also made an aperture to attach to the front of the lens which makes it razor sharp.

I don't think I've ever had this much fun with cameras and lenses. Slippery slope though, I want to build more things like this now!

Unbranded Petzval lens, rack&pinion screw missing.

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