Millimeters vs inches

I was very loosely making this same point. I was rebutting the idea that the machine cut in millimetres by saying it cut using yet another measurement. I left the connection that there would therefore need to be a unit conversion, no matter what your controller logically measured in, to the reader.

Plus, it was bedtime:)

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As a machinist, I really don’t care much about HOW a machine works so much that it works when I take care of it. It is, however, useful to know how to fix it when things go wrong, and similarly how to optimize my process to best take advantage of its features.

This is why I am totally confused by debates like this. I feel like doing anything in inches in CNC is like wearing lead underwear. Who does it benefit? What possible advantage could it be?

I think everybody agrees that some people prefer it for whatever reason, but other than that it exists, why would you CHOOSE it?

Familiarity and experience. Same reason you’d probably prefer to read in your native language than another (though there are interesting exceptions to that).

In this group, most are not simply operating a CNC. They are building things. So if they’ve 30+ years of experience thinking and measuring objects in inches, inches are going to be easier to think about and work when they come to designing and building.

Also, the CNC might produce a part that fits into another thing, and that other thing might be 12" long because of history or region.

I work with metric exclusively, and find anything smaller than an inch confusing. But if others find it easier to work in inches because of their history, machines are happy to help them. And that’s what machines are for - to make things easier for people.

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So true, except when they don’t! :rofl: :rofl:

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All of these units are based on the same thing, so fundamentally, I don’t think this argument has much of a point:

  • An inch is defined as 25.4mm
  • A millimetre is defined as 1/1000th of a metre
  • A metre is defined as the distance traveled at the speed of light in a vaccuum in 1/299792458th of a second

An inch is just some number of millimetres. If you want to think in units of 25.4mm, feel free. If you want to think in units of 10mm, feel free. Hide it how you like, you’re using metric whether you like it or not.

These abstractions are just ways to help our puny human minds communicate. They allow us to say “this part should be 3 inches wide” instead of “the width of this part should be the distance travelled by light in a vacuum in 1/3934284225.72 seconds”.

The purpose is to help our puny human minds so use whichever abstraction best allows your puny human mind to get the job done.

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I find this topic interesting.
All of this technical debate over a simple question.
I personally use both Metric and Imperial to help keep my brain active. I struggle sometimes remembering the smallest thing, or what I did with the allen wrench I was just using.
Sometimes over complicating things do not help answer a simple question that are abundant in this forum.
Not everyone even understands totally what you are saying.

My brain hurts… :tired_face: :grin:

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When I first got my Shapeoko I made a conscious decision to work all projects in millimeters. Converting back and forth to Imperial units is a extra step I don’t want. Just have to remember a 3/4" sheet of plywood is 19.05 mm.

I’m glad people are so gung-ho about using metric. In school for physics and engineering we used exclusively metric, then later at work it was all metric (except for pilot display panels which were in knots and feet, but that’s another discussion).

When I built up my amateur wood shop I went with metric. I got a metric tape measure and did the CAD in mm/kg/s. But then I bought 2x4s and plywood sheets and they were in inches. I got a table saw, and the ruler was in inches. Drill bits, hole saws, blades, kerfs, etc, all inches. It was super annoying, especially dealing with fractions. Almost the only thing in metric was my CAD, and I got tired of making conversions all the time so I unhappily switched to imperial and it has been easier.

Just wondering if people had similar experiences as me, and how they dealt with it and think about the whole situation. Maybe I should just go back to metric, what do you think? Do you guys all get metric endmills or what?

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Sure; my endmills are 6.35mm!

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It’s an easier decision with ER collet chucks, but either way I think of a 1/4” end mill as a 6.35mm end mill, just like I think of 3/4” plywood as 19.05mm plywood.

It’s not a crime to call it a 1/4” end mill, though. It is what it is.

To break down and start using fractions in your designs is not the same value call, though. It can get frustrating when it seems like the deck is stacked against you, but it’s not all that hard to find metric shop supplies if you exclude big box hardware stores.

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It also depends on your type of project. If you are making furniture with stock lumber, converting to metric is a pain. I tend to make smaller items on my CNC, like routing templates or fancy boxes, so I can stick with millimeters without needing to convert.

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Ok, since nobody but @WillAdams can cite a reference to their own personal choices, I’ll post this link from one of Will’s recommendations on reading. (Does anyone remember reading actual books for an education? :smiley: ) I bought the book and found that it enlightened me on the beginnings of precision in the machine world.

Let’s not try to revise history just because there are folks that don’t know the history.

The following quote by Will is part of the linked thread and is a reference to the book that one should read before starting a discussion like this current thread:

My current reading is also topical: The Perfectionists: How Precision Engineers Created the Modern World by Simon Winchester — highly recommended.

Just buy the book and put yer money where yer mouth is! :smiley:

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Thanks.

I will note that our software will accept either Imperial or metric, and aside from everything getting mapped to a grid defined by the pitch of the pulleys and belts, and the preference for smaller units being used for calculating arcs, which unit is used shouldn’t really matter.

For folks who want to improve this, the two notable options which don’t really have much in the way of drawbacks are replacing the motors w/ 0.9 degree ones, and higher tooth count pulleys (though that will slow the machine a bit) — but it’s not likely you’ll realize noticeably better results since it still comes down to interpolation.

Hmm, I don’t get this one. If you increase the tooth count of the pulley, you increase its circumference. If you increase the circumference, you increase the distance traveled in a revolution. If the distance traveled in a revolution increases, so does the distance traveled in a step, so the resolution goes down.

Don’t you want the opposite?

Or is the goal here just to make the pulley circumference a cleaner multiple of the step count?

You’re right, it should increase the speed — yes, ideal would be a cleaner multiple, either 16 or 32 I think would work out for typical dimensions (see attempt at math above), but as folks have noted, that goes out the window if one calibrates for belt stretch.

Other option would be a reduction pulley system, but that would increase backlash.

I’ve not been able to measure the backlash in my reduction pulley system, I’ve tried but all I can measure is the motor deflection :wink:

The backlash in the Gates GT2 belt profile is rather small, see pg 11 of the Gates Light Power and Precision Drive Manual, in the region of 0.0025mm (0.0001in) per pulley in your system. You can have quite a few of these before that is noticeable over the belt tension deflection and stepper torque deflection.

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I love books like this. I’m going to read it!

I read “The Invention of Air” by Steven Johnson right when I got out of the Army, when I was trying to figure out what my life was going to look like after two wars and some dreams. I also read “Brilliant” by Jane Brox at about the same time. I would bet that these authors are all talking about the same thing, which is the story of how we were delivered into the modern era.

It took some pretty incredible weirdos to get us to today.

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I have been using millimeters instead of inches, but it’s inconvenient when you use inches for everything else. My question is, why do they prefer metric, and does imperial give worse results? Does anyone know where this comes from?

Not to beat a dead horse, but at work I use both (imperial fractional inches & mm). Sure it would be nice if everything was one or the other. There are many advantages to SI over Imperial, but your comfort level with CAD-CAM software available to you is probably what will drive which way you go. In my case I’m using a machine that was built in Italy and am comfortable with using mm and inches interchangeably. However everything else at work is done in fractional inches because that is what MOST workers in our industry (construction) are familiar with. Yes the machine controller can be set to accept imperial unit G-code, but we actually have another machine built in France where choosing imperial (in) caused problems and it was just safer to leave it in SI (mm). Then you get to the whole “precision” side of things and since most SI (metric) tape measures are in mm and most imperial are in 1/16" (in) for the smallest division. Some imperial only measuring devices (tapes, combo squares, etc) will have a portion down to 1/32". At the standard tape measure out past a foot you are almost always going to be comparing mm to 1/16". 1/16" is about 1.59mm. Metal machinists will work in thousandths of an inch (or hundredth of a mm). 0.001" is between 0.02mm and 0.03mm, but most woodworking does not require that level of precision.

My comfort with SI is from college where I got a Engineering degree. We had to do problems in both Imperial and SI units. SI became a defacto preference for most if not all of my classmates. When I was in grade school there had been a push towards the US adopting SI units and the road signs (at least that I remember) had both mi & km (for distance to the next few towns/cities) ie

Denver 100mi/160km
Colorado Springs 160mi/256km
etc

Do I wish the US would go SI? Absolutely. Do I expect it to happen in my lifetime? Unfortunately not.

In conclusion SI gets the nod over Imperial just about any way you slice it except individual comfort level. I would suspect a lot of people raised with Imperial units will prefer them over SI units simply due to comfort.

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I think this is pretty much spot on.

At this point in time, it’s pretty accurate to say that nobody outside the US is being raised or schooled on imperial measurements, or has been for the last 20 to 30 years. So billions of people on SI versus a few hundred million on Imperial.

It’s not a big deal to be metric. The only rubbish thing is useful factors of 10 are fewer.

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