[AMPS] Meter Scale

[Ian White, G3SEK]

Mike Sims wrote:
>
>Just finished a homebrew wattmeter and would like to make a non-linear meter
>scale.  Also, have an amplifier and would like to redo the meter scales to
>directly display the measured values.
>
>Not interested in doing it free hand as when I do it they look like they
>were made with crayons using large lined paper.
>
>Can anyone describe how to make meter scales (faces) using programs that are
>relatively available on the home computer?  The term "relative available"
>means something like the MS products, Draw, Word Perfect, etc.  I've heard
>of making meter scales with various CAD packages.  However, I don't have
>access to any of them.

Products that are intended for document illustration don't hack it in
this application. Sure, you can use them to draw kind of scale, but it
will look very home-made.

What you need is a mechanical drawing program that will obey precise
instructions such as "draw me an arc of radius 1.785 inches", or "draw
me a line passing through this point, at an angle of 23.2 degrees to
that other line".

I've had good results with AutoSketch, which is available at very
reasonable cost and is suposed to be easier to learn than AutoCAD. Corel
Draw is also excellent, although I'm not certain if it can accept
precise instructions fromt the keyboard, which is a very handy feature
for this particular job. There are also many other low-cost and
shareware products.

What you need to do is to measure up the existing scale plate of the
meter to find: 
1. radius of the scale arc
2. radius from the meter pivot (not the same as 1 in most modern meters)
3. total width of the scale arc
4. location of scale arc on the scale plate. 

Measure very carefully, preferably using a vernier gauge. Then you can
re-create the original scale arc on your CAD package. The CAD package
will automatically draw a thickened line for the main arc, which you'll
probably want. Add some small marks to locate the arc relative to the
corners of the scale plate. 

Draw temporary circles from the center of the scale arc to act as length
guides for the short and long tick-marks on the scale.

To draw the tick-marks, you should draw a full radial line from the
center of the meter pivot (not the center of the scale arc) and then cut
each line to length manually using your guide circles.

To mark out a linear scale, AutoSketch will automatically draw a bunch
of equally-spaced radial lines at a specified angle interval. For a non-
linear scale you need to calculate the angles and have the program draw
them accurately with respect to the zero line.

Finally add the numbers and lettering, again using a temporary guide
circle. AutoSketch allows arbitrary rotation of text to sit on the guide
line.

It's tedious, but the results can look really good if you print the
scale on thin white gloss paper and then spray-fix it to the polished
aluminum rear of the scale plate (so that the old scale arc doesn't show
through). Behind the meter glass, it looks totally professional from
more than a few inches away.

It will take time, but by the time you've learned how to drive the CAD
program you'll also have learned a useful new skill. 

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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