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Graphic Rangefinder Cam

 
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45PSS



Joined: 28 Sep 2001
Posts: 4011
Location: Mid Peninsula, Ca.

PostPosted: Thu Jun 11, 2020 7:52 am    Post subject: Graphic Rangefinder Cam Reply with quote

1. Calibrate your rangefinder according to the instructions in the factory service manual. http://www.southbristolviews.com/pics/Graphic/manual-pdf/TRFService.pdf
The manual says to use a target at least 500 feet or more away. The further the better to include using a celestial object. When adjusting the movable mirror (7) shim the rangefinder arm (11) so that it will not move up or down when the screw (12) is loosened. Coincidence on the target should be maintained when the screw (12) is tightened. 95% coincidence on a celestial target is acceptable. Fig 1 pdf page 8 used for reference.

2. Test camera a 1970 Crown Graphic equipped with a 135mm Raptar, P31 cam, 30881-10 focus scale. Infinity on the ground glass set with the yoke .040 inch forward of fully retraced. Aperture blade front to the ground glass measured to be 130mm; front of the shutter blades to be 131mm. A p31 cam is for a 132mm optically measured lens.

3. The lens wide open is in sharp focus on the ground glass with an 8x loupe when the calibrated rangefinder is used to focus at the measured distances listed below. Therefore it is assumed to be a lens to cam focal length match.

4. This cam measures 1.03 inch at the base, 4 inward tilt at the rear and a 8 inward tilt at the front. The peak of the cam is .447 inch. The cam moves .966 inch from fully retracted to end of travel of the actuator plunger (28 ). [(34) rangefinder actuating bracket just touching the plunger.]
The bed moves .681 inch from fully retracted to the rangefinder actuating bracket just touching the actuating plunger.

5. Using the measurements in #4 we get a .704968 movement factor for the cam to bed movement for focusing. This cam measures from the rangefinder contact to the cam tract base at:
Infinity .440
25 feet .422
15 feet .3915
10 feet .372
6 feet .363
end of cam travel/4 feet focused distance .343
All measurements in inch. Multiply the inch by 25.4 to get the millimeter equivalent.

These distances should help those that are trying to make their own cams for the Pacemaker Top Rangefinder. All measurements stated in this post should be within .001 inch of perfect, use them as a guide.

The focus scale is for a 135mm with a -1.4 variance. The focus scale is aligned at infinity and the indicator line width behind the 6 foot mark when focused at 6 feet.

The rangefinder actuating bracket is set to position the rangefinder arm on the edge of the cam without coming off when the yoke is fully retracted.
Measure the forward movement of the bed at the listed distance, multiply by the movement factor, subtract that result from the cam travel to get the distance at the base inward from the left (front) edge of the cam for the focus height measurement. Example the bed moved .380 from fully retracted to in focus at 25 feet. .704968*.380=.2678878 .966-.268=.698 in from the left edge at the base with a .422 height. Rounding earlier may give position errors.
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alt.kafka



Joined: 16 Nov 2019
Posts: 6

PostPosted: Thu Mar 18, 2021 2:18 pm    Post subject: Re: Graphic Rangefinder Cam Reply with quote

45PSS wrote:
1. Calibrate your rangefinder according to the instructions in the factory service manual. http://www.southbristolviews.com/pics/Graphic/manual-pdf/TRFService.pdf
The manual says to use a target at least 500 feet or more away. The further the better to include using a celestial object. When adjusting the movable mirror (7) shim the rangefinder arm (11) so that it will not move up or down when the screw (12) is loosened. Coincidence on the target should be maintained when the screw (12) is tightened. 95% coincidence on a celestial target is acceptable. Fig 1 pdf page 8 used for reference.


I used the target method** for calibrating infinity on my Crown Graphic, and I think it should be at least as accurate, and more convenient than using distant objects. I created a target that looked like this:

->| |<-

where the two vertical lines are the same distance apart as the centers of the rangefinder lenses. In the case of my Crown Graphic, 3.130". Distance to the target doesn't matter, but closer should be better. In my case, less than a foot. I then turned my mirror until the arrows were coincident.

When turning the mirror, it's important to have enough friction from the set screw that fine adjustments are possible. Also, tightening the set screw will throw the mirror off, so be careful to maintain alignment when doing this.

I just set my existing cam to infinity to do this adjustment, but the process was easy enough to repeat that I'll just do it again when I get around to making a master cam.

Also, vertical alignment of the mirrors is accomplished via a screw on the fixed mirror. This is described on page 3, section 5 of the manual, but for some reason they don't use the words "vertical alignment", so it's a little unclear what they're talking about.


**Adapted from the following resource https://learncamerarepair.com/downloads/pdf/Graflex-SuperGraphic-InfinityTarget.pdf
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