- Published: Sunday, 19 February 2012
- Written by Jon Chandler
- Hits: 4833
I recently found a great old Westinghouse AC Ammeter at Goodwill for a few bucks. I love the look of these old meters but this one had a cracked lens and a 50 year accumulation of cigarette smoke and grim on the case. I didn't take ant "before" pictures but here's how it looks cleaned up, minus a lens.
The meter face was almost brown from the accumulation of grime and cigarette smoke. I took the front cover off the meter and scrubbed it. It looks pretty good after cleaning as the above picture shows.
This article will show a great method of producing a life-size drawing of oddly shaped parts and document the less-than-successful process of replacing the lens.
The next order of business was the meter lens. The thin glass was cracked in several places. It's an irregular shape and my initial efforts to sketch an outline didn't work very well. Then I had a moment of inspiration. I used a sticky label to hold all the pieces together and laid it on the glass of my scanner to scan it. The result was a life-size scan of the broken lens.
This picture shows the label side of the lens. You can see how dirty the meter was from the edges of the lens.
Next, I imported the scan into Viso. It remained life-size so the scale in Visio was exactly right. I traced around the lens using the curve tool and took effort to adjust the curve as close as possible to the scan. There is some room for error inside the meter, so slight variations are ok.
The Visio drawing was saved as an SVG (scalable vector graphic) that the laser cutter at Metrix can understand. A lens was cut from 1/16" thick clear acrylic. I was ignoring the vices in my head that said this might not work. More on that in a minute.
The protective plastic is still on the lens, giving it the blue color. Next, I installed it in the meter cover using four clips.
Nice fit if I do say so myself!
The picture below shows the meter without the cover. The dark strip under the meter scale is a mirror. By looking at the reflection of the meter needle in the mirror, you can make sure you're viewing it straight on with no parallax error. At the bottom center of the picture is the meter zero linkage. An offset pin from the front panel screw fits in the slot to allow exact adjustment of the needle to zero. It's essential that the pin fit in the slot when the meter is reassembled.
I carefully re-assemled the meter. Those voices in my head that I had been ignoring got louder. When I assembled the meter, being careful to align the zero pin, what I saw is shown below.
The meter pointer was far from zero. Furthermore, touching the acrylic lens caused the pointer to move across the scale! What's happening?
This is an extremely sensitive meter and it takes almost no force to move the pointer. The electrostatic charge on the lens is enough to move the pointer across the scale. Well crap. The voices in my head had been right all along!
This wasn't a totally wasted effort. I have a pattern to cut a glass lens, maybe from the thin glass in a picture frame. And the trick to scan an object to make a life-size drawing is worth remembering. Experience!
Here's one last picture showing parallax error. If you look in the mirror just to the left of the needle, you can se its reflection. This means you're not looking at the scale straight-on, and the reading will be in error.