Help understanding metric / imperial dimensions

[FLY-FAST]

Hello All-

Many thanks to the community for helping me understand precise measurements.  

I have attached two pictures, one measured in metric and the other imperial.

I'm trying to compare apples to apples here.  As an example, in the first picture you will see that it calls for a dimension 24".  I got a hold of a set of metric plans, image 2.  

 

Looking at the same cut, the metric plans call for a dimension of 609.6.  So, 24" = 609.6?  How can this be?  What am I missing here?  What do I need to measure this?  

Sorry if this comes across as a stupid question - I know I've got a lot to learn... Any help understanding the conversion would be much appreciated!

[lewisc]

A lot of the plans I follow are in imperial measurements which makes it tricky because metric is what I know.  When I get round to starting the Guild Rocker, I'll need to work out if converting the measurements or staying in imperial will be the best way to proceed. 

I times the imperial measurements by 25.4 (1" = 25.4mm). Not many of the conversions are a whole number so I generally round everything up or down making sure the project is kept in proportion. The side rails on a table I made recently were meant to be 40 1/2" which converted to 1028.7mm. It was simpler to just measure and cut the pieces at 1030mm 

Of course, if you need something to be exactly what the plan says, this method won't help much.

[Brendon_t]

Yes,  24 inches equals 609.something millimeters.

I can easily build in either either but most often use imperial  because my favorite tape happens to be imperial.  given that the metric plans aren't rounded to the nearest millimeter, I would guess the plans were drawn in imperial then converted exactly for the smart people who don't have to have their own fancy system. 

I'd say choose one and stick with it. 

[Cochese]

I'm genuinely not understanding the question. The 609.6mm measurement does equal 24".

[wdwerker]

609.6 MM is easy, just eyeball halfway below 610. A estimated difference might be 1/256th off so no big deal.

[Brendon_t]

2 minutes ago, Cochese said:

I'm genuinely not understanding the question. The 609.6mm measurement does equal 24".

I think he was asking what unit of measurement the 609.6 represented.  which is millimeter 

[Cochese]

Just now, Brendon_t said:

I think he was asking what unit of measurement the 609.9 represented.  which is millimeter 

Could be. The use of a comma in the metric numbers made me think the OP was familiar with it. 

[krtwood]

People were suggesting that since the plans he was referencing had some oddball measurements in them that they had been converted from metric.  Since it's a cnc there might be some components it uses that have metric dimensions and could still correlate in specific parts to a metric dimension.  Overall though it seems to have been designed in imperial so you are better off sticking with those plans.

[Tpt life]

6 hours ago, Cochese said:

Could be. The use of a comma in the metric numbers made me think the OP was familiar with it. 

Wha?

[wtnhighlander]

Wha?

Commas, rather than periods, are typically used as decimal indicators in non-North American countries where metric is the standard. Then they use periods as thousands seperators. Go figure.

[davestanton]

If the plans are in Metric, then that is the measurement. If the plans are imperial, then that is the measurement. The advantage in imperial is that fractions come into play. The advantage of metric is that it is base 10. Both have their place in precision engineering but for me working with timber that is very forgiving, 24 inches is "close enough" to 610mm.

[gee-dub]

1 hour ago, wtnhighlander said:

Commas, rather than periods, are typically used as decimal indicators in non-North American countries where metric is the standard. Then they use periods as thousands seperators. Go figure.

And we drive on the right side of the road too.  Americans; natural born rebels .

 

I survived two attempts by America to convert to metric and my school years were spent learning and un-learning the "new" math methods.  On the negative side I had to take basic math as an elective in high school because I thought I might actually need to know how to do long division later in life. On the positive side I can use metric or imperial pretty fluidly.

As davestanton says, the measurement method is what it is.  Pick one and move along.  There appears to be some reason for your plans for either method to have very small increments to arrive at the final result.

[Cochese]

If you are working with converted plans or working with mixed units in general, a combination tape measure might be a good choice. I picked up a FastCap one the other day. I'm nowhere near working in all metric (I'd actually like that, because base 10 doesn't make my brain hurt), but working with the 32mm cabinet system I've had to make some adjustments.

[Immortan D]

2 hours ago, wtnhighlander said:

Commas, rather than periods, are typically used as decimal indicators in non-North American countries where metric is the standard. Then they use periods as thousands seperators. Go figure.

I use tha floating-point.

[TerryMcK]

There are 25.4 millimetres in 1 inch. So 24" multiplied by 25.4 equals 609.6 mm What's difficult about that?

Some European countries use commas instead of periods in metric numerical notation measurements but I don't think the OP mentioned that (even though it was on the drawing).

11 hours ago, FLY-FAST said:

 So, 24" = 609.6?  How can this be?  What am I missing here?  What do I need to measure this?  

 

What do you need to measure this? You need a measuring device that has a millimetre scale.

http://www.homedepot.com/p/Stanley-26-ft-Tape-Measure-33-428/100152858

http://www.homedepot.com/p/Kapro-36-in-Aluminum-Ruler-with-Conversion-Tables-with-English-Metric-Graduations-1-16-and-mm-306-36/203325515

Alternatively just do it in inches

[RichardA]

As nearly everyone here agrees.."pick one" and move on.  Wood construction, in 99% of the work is not precise, so pick the one you are most comfortable with and get the project done... and remember.. on this forum unlike most others  " if there is no picture, it didn't happen" !

[Immortan D]

I don't think you need exactly 609.6 mm, 610 mm will do and that's easy measured with a mm or cm ruler. But if you want it to be more accurate, measure 609 mm and scribe a line with a 0.5mm mechanical pencil, then cut to the waste side of that line and you'll end up with about 609.5 mm, in theory.

[Tpt life]

4 hours ago, wtnhighlander said:

Commas, rather than periods, are typically used as decimal indicators in non-North American countries where metric is the standard. Then they use periods as thousands seperators. Go figure.

Well aware of this. The OP did not use a comma. 

[Cochese]

1 minute ago, C Shaffer said:

Well aware of this. The OP did not use a comma. 

I was looking at the screenshots.

[Tpt life]

Just now, Cochese said:

I was looking at the screenshots.

Figuring that is the way the plans came and without labels. Not sure why the assumption the OP knows metric. Doesn't matter. Not trying to start a fight. Just hoping not to jump on a wrong foot with the OP. 

[Cochese]

1 minute ago, C Shaffer said:

Figuring that is the way the plans came and without labels. Not sure why the assumption the OP knows metric. Doesn't matter. Not trying to start a fight. Just hoping not to jump on a wrong foot with the OP. 

I think my assumption was born from the fact that imperial plans were available, but then they were looking to the metric ones as well. I have no idea.

[Hurricane Dry]

The imperial drawing is using all common imperial whole number measurements and common fractional measurements. 

The metric drawing is using the exact conversion of the imperial dimensions into oddball metric dimensions with decimals.indicating the designer probably designed in imperial and then produced a metric drawing using the CAD programs unit conversion for the dimensions.

Easiest thing to do is to build to the imperial drawing using imperial measurement tools.

You didn't say what system of measurement you are used to using.

[AJ_Engineer]

2 hours ago, Hurricane Dry said:

The imperial drawing is using all common imperial whole number measurements and common fractional measurements. 

The metric drawing is using the exact conversion of the imperial dimensions into oddball metric dimensions with decimals.indicating the designer probably designed in imperial and then produced a metric drawing using the CAD programs unit conversion for the dimensions.

Easiest thing to do is to build to the imperial drawing using imperial measurement tools.

You didn't say what system of measurement you are used to using.

This is what happened. I design for items that go north of the border which is nominally a metric country. So our default drawing units are then metric. We run into an issue though in that all of the steel and most of the lumber is based around USA sourced/dimensioned items. So when I call out a 5/16" weld I'm not calling out a 7.9375mm weld, I call out 8mm. And while I specify 13mm plate, what actually arrives is 1/2" since it is rolled in the USA. In the woodworking realm we see the opposite happen with foreign manufactured plywood (actual dimension thickness set in millimeters, nominally sold as fractional inches).

At the end of the day if you are using metric you have to look at drawings like this and do a bit of further rounding/conversion to round numbers that make sense. Understand for your application which dimensions are critical (hole distances likely) and realize that all others may not be as important so it would be acceptable to change them to suit.