mm vs inches


  • I'm 75.  When in 3rd grade they told us that they were going to teach us metric because the country was converting to metric.  They never did.  It never did.
  • So I slipped through life using inches...ignorant of metric.
  • In 2016 I built a street organ.  I ordered the plans from London, England.  The plans came hand-drawn and with a combination of inches and metric.  This was my metric baptism.
  • It's now 2020.  I have a 3D printer on order and am trying to slip into that (metric) world.
I'm posting this thread to perhaps start a discussion about metric vs inches.  I know that those of you not in the US most likely use metric (and are therefore fortunate).  I'm finding myself in the position of needing to convert my thinking into metric.  At first I thought I was too old to change horses...but the metric world is becoming more dominant and I'm needing to accept reality.  I've got some tools already that include both inches and metric.  A digital micrometer, for example.  And many of my rulers have both graduations.  And I DO like metric.  Once you get used to it, it's easier.
So I've decided to create all of my 3D print files using the metric system.  And I do have some supplies that are inches (ball bearing assemblies and stainless steel shafting, for example) that I will need to consume and then replace with metric.  I've got many pounds of fasteners #2, #4, #6, and #8.  I'm probably going to continue using the inch fasteners.
Anyway, curious to know if there are others out there that are considering making this change to metric?
-Jm Coffee-


Hey Jim! 

I've been thinking about this for a while. I grew up using metric (in South America) but I moved to the U.S almost 20 years ago
and here I've worked a lot in construction and fabrication related fields, and oh boy, was it a rough transition at the beginning. 
I remember buying measuring tapes that included metric and imperial and I even carried a calculator that could do fractions and 
conversions easily into job sites. But the inevitable happened and I just got used to thinking in feet and inches and fractions of
inches, and you know, I actually find it quite intuitive by now if the size of your project easily relates to a human body. 

I think the number 12 (inches in a foot) is so nice to work with. It divides easily by 2, 3, 4, and 6. And dividing an inch into fractions
for objects nearly the size of a hand now makes sense to me. I just wish there were more things with base 12. 

At this point it's easier for me to have a mental image of what's a foot, or 10 feet, or a mile  than it is to have a mental image
of a centimeter, a meter of a kilometer. I think that kind of matters since real world references inform my understanding of things.

That said, there are some instances where metric does make more sense. 3D printing is certainly one of those, most of the software
I've been using has less issues when using metric as a rule. For example, I was designing things in fusion360 using inches, then
I would export a file into the slicer and the dimensions where all wrong, so it was simpler just to work in mm across the board 
rather than mess around with conversions. That's why I think your decision to create the 3D printing files using metric is very wise. 

That said, at this point I kind of consider myself "bilingual" when it comes to measuring systems. I certainly have wished that everyone
adopted the metric system and standards, but I also think it's fine to speak multiple languages (disclaimer, my native language is 
Spanish so there is some background there). And there are some things about using inches and feet that I actually enjoy. 

Then there is the issue of fasteners, I also have a collection of #2, #4, #6, #8, #10 and 1/4" and sometimes I've thought about 
switching over to metric but the prices in the US are often almost double for metric, so I tend to stick with what I have. 

To end with some practical suggestions I'd say:
  • definitely have a metric ruler for at hand when you are designing, for reference.
  • SometimesI also sketch on 5mm grid paper (the common grid paper in the us is 1/4"), I feel that "forces" some of the design decisions into a metric reference.
  • Digital calipers are wonderful for going back and forth between metric and imperial, or if you are staying in metric they still make life easier. 
  • Make reference your own materials like the hole plank below.

I like to make a hole reference print that I keep on my desk. It's just a plank with labeled holes in a certain range.
The one in the picture goes between 2.0mm and 4.9mm in increments of 0.1 mm. This is useful because sometimes I just want to know what
a whole of certain size looks like, or what hole has the right dimensions for a certain fastener or axle. It's also useful because the 3D printer is a bit 
more fuzzy when it comes to getting the dimensions you want, things will change depending on calibration, material choice, location of the feature, etc.
But this kind of reference made on your own printer can give you a pretty decent idea of what you need. 

Hope some of this is helpful or interesting, Cheers! :)