Disassemble the front group of a Dallmeyer Pentac, 8" F/2.9

[ralf_j.]

Hello -

 

I received a Dallmeyer Pentac and it desparately needs cleaning. I was able to remove the rear group and the front

group from the barrel and cleaned their surfaces, but do not see a way to disassemble the front group which has 3

elemtents I believe. Usually a few specs of dust do not bother me, but this group is really dirty and it is begging for a

wipe down?

 

Can anyone help me on how to disassemble the front group, please? Thank you very much in advance.

 

RJ

[colin carron]

Can you post a couple of close-upshots of the front group? Pentacs were made by a number of manufacturers and in different materials. The aluminium mounts were the more common air recon type but they were also made in brass mounts.

 

The front group should consist of a doublet then an air space (where you are trying to get) then a single element. The design effectively works as a 5 element triplet like some Heliars.

[ralf_j.]

Hi Colin - I will post a picture of the beast tonight, it is the aluminum barrel. Please let me know what you think and would like to thank you for your time.

[ralf_j.]

Colin - here is the picture:<div></div>

[lauren_macintosh]

Ralf: Remove the front elements from the barrel and study the rear of the lens , you are going to need a pair of rubber gloves like ones used for garden work [ its a cotton glove but has latex rubber on palm]

great for holding lens with and to break apart the lens : Look for some sign that there is a joint there then you will know how to take it apart: Other wise the front ring on the lens will have to be remove to seperate the elements of the group! Good luck

[cdewen]

Here are some photos of a disassembled 12" Pentac in My Portfolio: http://www.photo.net/photo/7599494

[ralf_j.]

Thank you Charles and Lauren. I tried with bare hands, but the front group has been put together very tightly. It will need some sort of a vice or machinery to exert the pressure required. I am beyond frustrated at this point. I was going to try heat, however I still need something to grip it after it gets hot.

[colin carron]

Charles' photo shows a nice Pentac in a brass mount. These are quite easy to disassemble. However the aluminium mount lenses were mass produced wartime production for air recon use so are designed not to come apart under severe vibration.

 

I have not had time to have a good look a my Pentac and other ali-mount WWII lenses but I suspect there are two factors against disassembly. 1) they are designed not to come apart easily and were probably regarded as disposable items at the time. 2) the aluminium mounts have probably corroded together to some extent making them even more difficult to undo.

 

Ralf, I will have a go at seeing whether I have any luck with my Pentac but at the moment have not had a chance to get to them. I also have a 36 inch WWII recon lens which has the same problem and will have a look at that too.

 

cheers - Colin

[jim_galli4]

Aerial lenses often had mechanical means to hold elements tightly like tiny set screws etc. Sometimes beneath the paint even. If no locking setscrews are found you may just have a problem with dissimilar metal corrosion. Cut some patches from an inner tube to use as grips. Get some "rubber buffer" from an Auto Parts store. It's used for patching tires but it's a wonderful cleaner that leaves no residue. Sometimes you can get a little of it in the threaded space to help lube. It's also fantastic for freeing up sticky nasty old diaphragms. Give it a bit of a soak then grab the 2 pieces with the rubber 'grips' and give it a helluva twist. Once the group is out, it should split in 2 threaded pieces.

[ralf_j.]

Thanks Jim. I will give it a go.

[matt_smith34]

<p>Hi all,<br>

This is my first post here, I was unsure if it was better to revive this old thread or start a new one - so please let me know if I should start a fresh topic?</p>

<p>I have the same issue as above - Haze between the elements in the front group of a dallmeyer/pentac 8" aerial lens - did anyone ever manage to separate the elements as above or do I have to live with the haze? any other suggestions?<br>

thanks and apologies again if I have posted incorrectly.<br>

best wishes,<br>

Matt</p>

[raul_casal2]

<p>hello Matt , i have this lens its easy to desassembly all the glass elements but , the dicoult is (in my case) to remove the douplets out of the metal ring , </p>

<p>and another matter i can see is , that the haze that you can see could be a polish rouge on the external surface please if you abserve this with a 6x magnifier you can see </p>

<p> in other hand , i would like to know how remove the glass from the metal ring </p>

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[michael_parker7]

<p>I can add something to the WWII Pentac database. The design was pre war but came in very handy for high speed aerial surveillance. The 5x5 cameras were often fitted in pairs in place of guns in the wing bays of Spitfire fighter planes; sometimes required to do high speed daylight runs in unarmed planes attempting to get in and out before they could be intercepted. Steroscopic photo analysis was critical in locating V2 missile sights and undoubtedly changed the course of the war. <br>

Based on a small sampling of lenses I have come across there was lots of variation in construction of the WWII 8 inch Pentac. Most were aluminum although Ross built some out of brass; beautiful but very heavy. Some had integral mounting flanges that can't be removed. And of course most of them have been harshly used or allowed to corrode making restoration difficult. Most of the war time lenses had no makers name on them; perhaps to avoid giving the enemy any extra clues as to where they were being made. <br>

The Dallmeyer factory Pentacs were usually better finished and most importantly, could be disassembled for cleaning. (once you find all the locking screws) The critical point is whether the second element in the front group can be unscrewed or is "swedged" into place with a thin metal strip that is rolled up over the edge of the lens. Regardless of how nice a 80 year old lens looks, unless it is taken apart and cleaned it won't be much good for photography. It can be difficult at first to tell the difference because even the swedged versions have a knurled edge that looks like you could grab hold and unscrew. If you have one that isn't threaded together you must remove the thin metal strip holding the lens element in place. Here is how I do that.<br>

First you need a laythe; OK I know most people don't have access to one but most communities have machine shops and it is really easy. First you gently chuck the front assembly (I generally wrap tape around it to avoid damage) and adjust it until it turns true with no wobble. Then, with the lens turning slowly, take a fine sharp file and place the edge at the base of the swedged metal and carefully file away the metal till it starts to break apart. Once you have that off the lens can be loosened by turning against a sheet of rubber and should pop out. <br>

You now have access to all air/glass surfaces and can clean to your hearts content. To reassemble all I do is place the lens back in it's socket and hold it with two or three dabs of gel "crazy glue". Should you ever need to open it up again a little acetone should do the trick; after all it is not going to be bolted back into a Spitfire with a 1600hp V12 Rolls Royce engine that could shake the fillings out of your teeth. </p>

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