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!

 

[TugboatEng]

Machine shops often want nominal dimensions for parts which complicates things when you have to make metric to standard translations. In these cases I give the acceptable range instead of nominal ±. This really annoys the machinists until I explain that I don't care what the diameter is as long as it's in the range. The machinists want a target so they can try to score a bullseye.

 

[rb1957]

@mfgenggear, NP, apologies, didn't mean to be insulting. Sure, in your business that degree of precision is typical. But like someone above said, the importance of rounding is to ensure the design still works, so a GD&T exercise. In my business I'd go to the nearest practical drill size, to save making a special drill (if not needed).

Of course one answer to the OP would be "if you bought a metric drawing, use metric tools". And as you allude, do you have the design authority to make the part different ? unless you check each dim'n produced imperially to verify that it is within the original metric tolerance ?

 

[mfgenggear]

@mfgenggear, NP, apologies, didn't mean to be insulting. Sure, in your business that degree of precision is typical. But like someone above said, the importance of rounding is to ensure the design still works, so a GD&T exercise. In my business I'd go to the nearest practical drill size, to save making a special drill (if not needed).

Of course one answer to the OP would be "if you bought a metric drawing, use metric tools". And as you allude, do you have the design authority to make the part different ? unless you check each dim'n produced imperially to verify that it is within the original metric tolerance ?

During the beginning
of my career. It was contract work so no design authority.
I would as project engineer I would quote to
In the project obtain the correct tools.
But it was not always in the budget.
The work I did was just nasty, very tight tolerance, nasty heat treats would distort
Terrible. And was always a challenge.
I delt with exotic, carburized and nitride
Case harden surfaces.
Customers would try to produce, but failed
So they would farm it out us.
And was not unusual to tighten tolerance
For stack up, distortion from heat treat.