metric conversion of a plan

[al ladd]

This question is mostly for woodworkers in metric lands, especially Australia and the UK. How available are 1/2", 3/8", and 1/4" router bits for you?

In converting imperial measurements to metric for woodworking plan do I typically round to nearest mm? Can you buy supplies from Mcmaster Carr?...or if not, is there a comprehensive industrial supplier you use?
Thanks!

[Rod Sheridan]

This question is mostly for woodworkers in metric lands, especially Australia and the UK. How available are 1/2", 3/8", and 1/4" router bits for you?

In converting imperial measurements to metric for woodworking plan do I typically round to nearest mm? Can you buy supplies from Mcmaster Carr?...or if not, is there a comprehensive industrial supplier you use?
Thanks!

Hello Al, router bits are available in all standard Imperial and Metric sizes.

When I convert plans from Imperial to Metric, unless there's a burning reason to stick to the Imperial sizes, I do the following.

If it's a 3/4" thick shelf, make it 20mm, if it's a 2" square table leg make it 50mm.

You get the idea, make it easy and simple.

Fortunately sheet goods in North America have been metric in thickness since the 1970's so metric bits are available, that said plywood varies from sheet to sheet due to sanding tolerances so it's always a measure to fit approach.

Lee Valley sell metric and imperial router bits, and they ship by mail which is pretty convenient.

I went metric decades ago, it's so much easier than dealing with fractions, and nobody can tell what measurement system you use.

Some stuff is easier in metric, such as the 32mm system for cabinets and hardware.

Regards, Rod.

[Jamie Buxton]

If you don't want to change the plans at all, just convert the inch measurements to metric. 1"=25.4mm For instance, if a dimension on the plans is 28", the metric version is 711.2mm. You can probably ignore the .2mm, and call it 711mm. 711mm might look wonky, but it is a perfectly good number. Your metric measuring tape can show it to you as easy as some less-wonky-looking number.

[Bill Dufour]

Make sure you know what normal size lumber is in that country. Designing around 4x8 plywood sheets will not work if theirs is just a few cm smaller. I very much doubt they have 2x4 lumber for cheap.
Bill D.
Edit Australia plywood is 7.87 x 3.9 feet.

[Greg Quenneville]

Router bit shaft sizes are the same except some European brands. I don’t think I have ever seen a metric collet except as an option on Festool routers.

McMaster-Carr won’t sell overseas unless you have an old account. Which is vexing since I have industrial hobbies, and I would prefer to have a single vendor.

I can get most things locally or online. Many of my router bits came from the U.S., but my shaper tooling and saw blades are all European.

When converting plans I would round to the nearest millimeter.

2x4 stud here is 45 x 90. = 1 3/4 x 3 1/2”. (35 x 90 also available and is a more commonly used size). An 8’ 2x4 here costs US$ 11.38. 25% cheaper than last year.

[Richard Coers]

If your plans call for plywood, I have no idea how you can spec the grooves when sheet good thicknesses are all over the place. I thought 8mm shanks were pretty common.

[Joe Cowan]

The math is much easier with metric. I am trying to get my mind around it as I just started using the Panto Router machine and having to determine the center on all pieces, dividing by 2 is much easier with metric.

[roger wiegand]

Years of working in the lab taught me to avoid conversion at all costs. It is an incredibly error prone process (just ask the space telescope guys). I'd redraw the plans using sensible mm based measurements (eg even millimeters) so that it looks right while being easy to build, and not mess with the weird decimal values that come from conversion.

[Jim Becker]

Years of working in the lab taught me to avoid conversion at all costs. It is an incredibly error prone process (just ask the space telescope guys). I'd redraw the plans using sensible mm based measurements (eg even millimeters) so that it looks right while being easy to build, and not mess with the weird decimal values that come from conversion.
I kinda agree with this and it's exactly how I migrated to metric a few years ago. Even if it was something I've previously built in inches, I'll re-design for metric. At that point, the only time any kind of "fudging" is required is because of variable sheet goods thickness when I'm doing take-off for panels that fit between panels within the overall "sane" outside dimensions. And I don't get more granular than a half-mm by eye unless this is something CNC related.

[Lee Schierer]

Why not just convert to a metric tape measure and scales for the project. Then numbers are numbers and there is less chance of error. If the drawing is in CAD or other digital drawing media, it is relatively easy to convert all dimensions to metric or US inches.

[Chris Parks]

I made a chair from imperial plans and using a tape with both metric and imperial on it just went for the nearest sensible metric number. I do have a good sense of the main equivalents though, 12mm is near enough to 1/2'', 3/4" is close to 19mm etc so that does help.

[Curt Harms]

If your plans call for plywood, I have no idea how you can spec the grooves when sheet good thicknesses are all over the place. I thought 8mm shanks were pretty common.

That's why I prefer a jig where you use a piece of plywood scrap to set the width of the groove. Correct width every time. Jig doesn't work everywhere - dados lengthwise on narrow pieces is a no-go for one.