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SG-focus rack and pinion

 
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d.s.



Joined: 19 Aug 2011
Posts: 9
Location: outer banks, nc

PostPosted: Sat Aug 20, 2011 7:39 pm    Post subject: SG-focus rack and pinion Reply with quote

On a speed graphic, should there be grease on the focus rack and pinion where the gears mesh or in the groves that rack slides in? I'm cleaning with a q-tip and there is black stuff coming off. I can't tell if it's grease, graphite, or just crud. I'm going to remove the focus rack and give it all a thorough cleaning. It could need it after 54 years. Just wondering what it needs before reassembly.

d.s.
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45PSS



Joined: 28 Sep 2001
Posts: 3255
Location: Mid Peninsula, Ca.

PostPosted: Sat Aug 20, 2011 8:17 pm    Post subject: Reply with quote

White lithium grease applied sparingly will be fine. Spec says use grease AN-G-6. Running the rails out fully, cleaning the gear teeth and relubing should be sufficient. The bed guides and bed blocks must be removed to remove the yoke which must be removed to remove the focus pinion. Once the bed guide and bed blocks have been removed the bed brace springs will be exposed. The bed brace springs are under high tension and can pop out and injure you.
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d.s.



Joined: 19 Aug 2011
Posts: 9
Location: outer banks, nc

PostPosted: Sat Aug 20, 2011 9:01 pm    Post subject: Reply with quote

Cool, white lith it is.
I've already removed the bed guides and bed blocks, cleaned everything and reassembled them. I left one bed guide off for reassembly of the yoke.

Putting the bed blocks back in is a little like putting the bolt back in an M-14, once you get the angle right it goes right in. The key is looking at the bottom and seeing that certain screw holes are flared for a reason.

d.s.
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45PSS



Joined: 28 Sep 2001
Posts: 3255
Location: Mid Peninsula, Ca.

PostPosted: Sat Aug 20, 2011 11:09 pm    Post subject: Reply with quote

getting the yoke square and even tensioned the full travel is a lot of fun.
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d.s.



Joined: 19 Aug 2011
Posts: 9
Location: outer banks, nc

PostPosted: Sun Aug 21, 2011 3:59 am    Post subject: Reply with quote

The yoke is in. Travel fore and aft is smooth without hard spots. I checked with camera vertical and the yoke does not fall toward or away from the case. No lens was mounted so I might have to make an adjustment for it later. I rubbed some bees wax on the yoke where the front standard moves, and it slides smooth without chatter.

The coupling lever for the range finder was dragging across the bed brace plate when focusing the yoke forward slowing it down. The yoke could move ahead of the lever and the lever would slowly catch up, so I finessed a little bit of a curve in the lever away from the plate so it does not drag against it. Now the lever follows the eccentric screw until it is blocked by the bed brace from following the eccentric screw any farther. How far should the lever travel, and is it suppose to go past the bed brace?

Thanks, you guys are great! This forum is lucky to have such knowledgeable folks with the patience to help people like me.

d.s.
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45PSS



Joined: 28 Sep 2001
Posts: 3255
Location: Mid Peninsula, Ca.

PostPosted: Sun Aug 21, 2011 5:03 am    Post subject: Reply with quote

The rangefinder arm should move freely the full travel of the rangefinder shaft which is around 40 to 50 degrees of arc.
The rangefinder has enough movement for lens to focus from infinity to the lens close focusing distance which is around 6 feet for 135mm. There is a formula somewhere to calculate the close focus distance of a lens.
The yoke should have around 1 to 1.25 inches of travel with the rangefinder arm in contact with the rangefinder bracket.
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Last edited by 45PSS on Tue Aug 23, 2011 8:36 am; edited 1 time in total
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bartbob



Joined: 30 Oct 2010
Posts: 102

PostPosted: Mon Aug 22, 2011 3:11 pm    Post subject: Reply with quote

45PSS wrote:
There is a formula somewhere to calculate the close focus distance of a lens.
I've used the following formula to calculate the lens-to-film distance for a given lens' focal length and lens-to-subject distance:

1/((1/lens-to-subject distance)+(1/focal length))

I subtract the lens focal length from film-to-subject distance to improve the accuracy of the formula. For example, a 3.5 inch (90mm) lens' focal lenght subtracted from 1200 inches (100 feet) gives 1196.5 inches to use in the formula.

Also, use the exact known focal length of the lens. If your lens has a rangefinder cam, use the cam number to determine the lens' focal length. This make a big difference as subject distance gets shorter and/or lens focal length gets larger. A 1mm error in not using the exact focal length of a lens marked "135mm" results in about 2/10ths of an inch error in how far the lens has to move forward from its infinity position to focus at 5 feet.

Answers will be close enough for most purposes. If you make a spreadsheet with the formula then add another colum to enter an approximate lens-to-film distance, then subtract that from the film-to-subject distance. when the calculated answer for lens-to-film distnace equals what your approximation is, the results are the most accurate.
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