How to accurately and quickly convert inches to millimeters in technical drawings? 2

[vutuankhanh]

Hello, fellow mechanical engineers!

I need your help with a problem that has been driving me nuts for the past month. If any of you have faced the same issue, please share your wisdom with me!

I’m a mechanical engineer from Southeast Asia—specifically Vietnam—working at a company that deals with American clients. And here’s my struggle: in the U.S., they love their inches, while at my company, we live and breathe millimeters. So, every time I receive a technical drawing from a client, I have to go through the oh-so-fun process of unit conversion.

My go-to method? Importing the PDF into AutoCAD to convert the units. But let me tell you, it’s mind-numbingly tedious and eats up way too much time. Sometimes, I skip the conversion altogether and just redraw the 3D model in NX before sending it for manufacturing.

Now, here’s where things get scary:
I’ve noticed that American drawings love rounding up dimensions—sometimes aggressively! A dimension like 10.4 inches can magically become 11 inches. And since 1 inch = 25.4 mm, even small rounding errors can snowball into huge discrepancies between my drawings and the client’s. That’s a serious risk when sending designs for fabrication!

So, my question is: How can I convert units as accurately as possible without manually dimensioning every single measurement?

Any tips, tricks, or magic spells would be greatly appreciated!

 

[rb1957]

do you import your American Autocad model and change the units ?

I've never experienced "American drawings love rounding up dimensions" ... do you mean they overwrite the model dim'n ?

personally I've seen way too often people retaining decimal places in their dimensions when they don't want/need to, just because the modelling s/ware will print 3 decimal places.

 

[mfgenggear]

I did the opposite. My guys are use to inches not MM. We would contract with company with Metric system. I would grab my red pen. With a macro start converting numbers from mertic to inches. Red line the cross over. Then digitize it.
Then later on red line the pdf file.

 

[CWB1]

I'd be surprised if there wasnt an app that provided a real-time conversion using your phone's camera, or a plug-in for Adobe. I use the built-in translate function in my phone's camera app regularly.

Regardless, you should always request a 3d model if not provided with the 2d. That prevents modeling mistakes and will provide most** of the nominal dimensions. Tolerances should be very repetitive for many dims, so converting shouldnt take much effort.
**Features with +/+ and -/- fit tolerances are often modeled below or above the nominal, which I suspect is the cause of your misconception about "rounding." Frankly, I've never heard of rounding on a print nor see how it would make sense.

 

[mfgenggear]

I never had an issue. and really it depends on how tight the tolerance is. , and I pinched the manufacturing size to make sure it fell in.

 

[SWComposites]

https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Convert-imperial-unit-drawing-to-metric-units.html

 

[TugboatEng]

Inches and mm should be directly interchangeable.... That is unless you're using nominal dimensions.

 

[3DDave]

I had the chance to work on a full automation line drawing package to convert from mm to inch because the USPS wants it that way.

The greater pain was the use of Italian/European material standards that have no perfect equivalent in the US.

I'll take a pocket calculator for unit conversions over that materials conversion any day.

" A dimension like 10.4 inches can magically become 11 inches" doesn't make sense to me. That sounds like a problem from a specific client rather than a US practice.

 

[dgeesaman]

I create and receive drawings across the globe. My domestic customers (and our production team) need inches. My international customers want mm. Dual dimensions are a very reasonable solution.

Converting from 10.4 to 11 is dropping at least one significant figure and very incorrect. I suggest your specs require at least an equal number of significant figures in the converted measurement and make it clear which unit system governs the contract.

 

[ctopher]

Whenever I do drawings for mfg in Asia, I use dual units. "in, [mm]"
I have never seen a dim round from 10.4 to 11. That would be from someone that doesn't know how.

 

[mfgenggear]

Take an example 12 +/_ .2mm
12÷25.4=.4724409449
.2÷25.4=.0078740157

I would round off to .4724 ÷/_ .007

 

[rb1957]

that's a crap load of precision ! maybe 0.47 +/-0.01 ? ... or 15/32" +/- 1/64"

 

[ctopher]

I would also make it an easier dim to follow.
Also, tolerance and dim should be the same dec places, otherwise you could have rounding issues again. I have seen it.

 

[JStephen]

If you can get the CAD files, you can fix some of this in dimension styles, etc., assuming drawings are to scale.
One big decision: Are you trying to reproduce the same exact thing either way, or an equivalent item?
Example: screw threads. You can get similar sizes in US and metric, but those sizes can't be interchanged, either, so you don't just change the dimensions on a bolt to make the conversion.
Example: Do you convert 150mm to 6" (similar, and rounded dimension) or 5-7/8" (more exact but less handy) or 5.9055" or what?
In my work, where things are measured with a tape measure, you never see any fractional dimensions beyond 1/16"- so never any 1/32", 1/64", etc. But that's going to vary by industry and user.

 

[mfgenggear]

Listen fellas , here's the deal.
Here's what matters
Do you have design authority or not
The intent and the application

I am in aerospace where it has to be to the drawing. I work with aerospace projects.
People fly in my projects, so there is safety concerns .
To do the job right it takes metric calibrated
Machinery and inspection equipment.
Next best would be to maintain the print with pinched tolerances

If you can live with more liberal dimensions.
Than do it.

Normally I just block people with insults but
Figure some one is having a bad day.

 

[ctopher]

I didn't see any insults.

 

[mfgenggear]

I didn't see any insults.

RB post, but its all good

 

[mfgenggear]

that's a crap load of precision ! maybe 0.47 +/-0.01 ? ... or 15/32" +/- 1/64"

RB let me explain I work with precision ground parts, tolerances are typically +/- .010, +/- .005 , .0005 Max or less
even had .0003 tolerance

 

[JStephen]

My brother once worked at a pacemaker company. Very highly regulated, medical field.
The pacemakers came with a wire lead that the surgeon would trim when he installed the pacemaker.
So it didn't much matter how long it was (as long as it was long enough!), but still had to be shown precisely on the engineering end of things.
So the motto for that part was "Measure it with a micrometer, mark it with a piece of chalk, cut it with a hatchet."

 

[ctopher]

My brother once worked at a pacemaker company. Very highly regulated, medical field.
The pacemakers came with a wire lead that the surgeon would trim when he installed the pacemaker.
So it didn't much matter how long it was (as long as it was long enough!), but still had to be shown precisely on the engineering end of things.
So the motto for that part was "Measure it with a micrometer, mark it with a piece of chalk, cut it with a hatchet."

Haha. Or "cut to fit, paint to match".