Sample build

This example is from the Sexton Omnimetre project. The Omnimetre is actually a cardboard slide rule to begin with. My home printer will not print cardboard; the approach is to first print on paper and glue the paper scale onto a suitable backing.

Many printers will print on reasonably heavy bond paper and at least the lighter card stocks. These would be fine for this application.

Also, the scales are built for letter size paper, and may not fit A4. If that proves to be a problem, feel free to contact me and I'll scale them to fit

Materials

• Transparent plastic for cursor. I source clear acrylic sheets from a local art stores
• Paper, a heavy bond is best.
• glue
• spray varnish/fixative recommended
• Cardboard for backing
• Tools, sharp knife (exacto), heavy shears for cardboard, files or sandpaper

For the build, I'm assuming that paper scales are affixed to cardboard. If you can print to card stock, you may choose to not use a cardboard backing.

Preparing scales and cardboard

For the Omnimeter PDF file, the scales are printed on pages 2,3 and 5.

Page 1 is a composite view of the assembled SR.

Page 2 and 5 are the outer scale, front and back side.

Page 3 is the inner front scale.

The cursor template is on page 4.

You need 2 pieces of cardboard, one for the inner scale and one for the outer scale. You need not cut them into rounds because the scales when glued on the cardboard will provide a cutting template. I recommend you cut the cardboard sheets and the paper into square shapes first.

It's a good idea to protect everything with a couple of coats of spray varnish or fixative (both the paper scales and cardboard faces). This prevents the scales from smearing or rubbing off, and provides some resistance to moisture.

Glue inner and outer front paper scales on cardboard

I use a spray adhesive to attach the inner and outer scales (pages 2,3) to the cardboard. This works better if you apply adhesive to both the paper and the cardboard. I have also used rubber cement for this step, but have had poor results at times. If you are using rubber cement, you will need to keep the coats very thin, and probably coat both the paper and the cardboard. I've read elsewhere that letting rubber cement partially dry improves the strength and longevity of the bond.

Glue back face to back side of outer scale

Before you put the back face (page 5) on the back of the outer scale, you should insert a pin (I use a thumb tack) through the centre of the front scale, and do the same on the printed sheet for the back face. You may want to also scribe a registration mark on the back of the cardboard to align the top end of the front page over to the rear.

Apply the spray adhesive to the back side of the cardboard, and to the back of the rear face. Next push the pin or tack back through the front face so that it sticks out of the back of the cardboard. Use the pin to match to the hole on the back page, and smooth it down.

I find it works best to align the sheet over the cardboard, curl it up at the middle of the page, attach it at the centre, then roll it outward smoothing it down.

You now should have one inner scale and a two-sided outer scale on two pieces of cardboard.

Trim Cut the slide rule disks

Cut the scales along the outer circumference. You must use sturdy scissors if the cardboard is reasonably thick.

Do not drill holes until after the cursor is made.

Make the cursor

Print the cursor template and cut the cursor out, leaving a small area around it (I'll say 6.4 mm / 1/4 inch). Glue the piece onto one edge of a piece of acrylic with rubber cement. Protect the other side with masking tape.

Cut the cursor out of the acrylic, as close to the outlines as possible without cutting into it. For this, I use tin-snips (super heavy scissors). I have used various saws as well, but find the tin-snips much easier to control.

Sand or grind clean the edges.

Using a sharp knife with a ruler, carefully scribe the hairline using the template as a guide. You MUST do this in one stroke; you can’t easily deepen the line with a second pass – that inevitably gives 2 hairlines when I try it. I don't have a more robust technique at this time.

For the hole on the cursor, you must take care to drill through the centre of the inscribed hairline as that is the radial axis line; missing it will that lead to reading errors.

Once the line is scribed, you may find there is an asymmetry in the cursor shape; if so the edges can be ground.

SR Pre-assembly

I test the alignment of the pinhole by looking at the rule with the thumb-tack acting as a temporary support. Here you see I've gone ahead and drilled the cursor hole. If you have a snug fit and have got the hole centred correctly, the scales will line up, I do the usual tests, say, set it for 1 x 2. and check points around the circumference for fit. If you don't have one, you'll need to adjust for it when you enlarge the holes for the central pin that is to be used.

Cut the holes to fit the ‘pins’ : e.g., for my build I used a rivet requiring a 6.6 mm/ .125 inch hole for the cursor, and a 3.18 mm/.26 inch hole for the scales.

In the slide rule faces, I don't generally have the needed dimensions in the PostScript for the hole on the inner and outer scales (because I don't know what diameter pin will be used, to hold it all together). For my build I marked the right sized hole on the face with a pencil and a conveniently sized washer. The hole was then cut with my Exacto knife.