The OpenROV body: adapting to EU sizes

As everything that gets built and designed in the US, especially DIY things, there is always the problem of the conversion between the imperial system (inches and feets) to the SI (International System of Units, with mm, cm, meters and so on).

The usual Imperial vs SI problem

Lucky the designs of the OpenROV are in in mm, not in inches and fractions of inches, but there is still a little problem: not all parts of the body are cut to size. All the tubes are bought in standard sizes, and being the designers in the US, of course, they only had standard sizes in inches. Unfortunately in Europe the standard sizes are in mm and obviously the 2 standards don’t match.

Electronics tube

The electronics tube is 4″ OD/ 3″-3/4 ID which is 101,6 mm OD – 95,2 mm ID converted to SI. Obviously nobody is that crazy to make a standard size that is such a weird number, so the EU size is 100 mm OD - 94 mm ID. It wouldn’t be such a problem if nothing had to go inside and fit to the mm. Unfortunately the tube has to be sealed with the caps, that have to fit perfectly in the tube. And these caps are exactly 94mm of diameter, and I think this might be a bit tight. But I’ll cut to the right size and then see if this is a problem: otherwise I’ll slightly reduce the size of the caps.

Props shrouds

The same problem is for the shrouds of the propellers: original specs are  3″-1/4 – 3″ (82,5 mm – 76,2 mm) while the standard size is 80 mm – 74 mm. This has 2 problems:

• the propeller is exactly 3″ (76,2), so it won’t fit inside the 74 mm guard.
• The shroud has to be glued to a arc guide whose radius is 41,25mm, so the 40mm radius of the tube won’t fit perfectly.

To overcome the first problem I’ll buy a smaller prop. And for the matching with the brace, I’ll see when I assemble everything: maybe a bit of epoxy might do the trick. Or if not I’ll modify the laser cutting pattern.

Ordering acrylics

The standard of acrylics sheets sold in US are as well in inches, and come in fixed sizes. Here in Germany I found a producer that sells Plexiglas sheets cut to exact size. So I don’t need to buy standard sizes and pay twice to plastic I don’t need.

So, here are the types and sizes of acrylic I’m going to buy:

• Blue, Length: 610 mm Width: 460 mm, 20,86€
• Clear, Length: 480 mm Width: 200 mm, 11,38€
• White, Length: 200 mm Width: 240 mm, 9,89€
• Electronics Tube 100mm OD x 94mm ID, 180mm Length, 14,23€
• Shrouds Tube 80mm OD x 74mm ID, 150mm Length, 7,93€

A European Bill of Materials

To help other makers that would like to source all parts themselves from Europe, I forked the original BOM and made one with EU suppliers (or suppliers that ship to EU at least): OpenROV 2.3 – EU suppliers.

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