Blueprint Notation Question - Print dimension like 4.312 g +.002 -.001

[Eric C.]

I could use some help with and issue on reading blueprints. I should probably know this, but haven't been formally trained and have never seen this before.

When you have a print dimension like 4.312 g +.002 -.001

I'm guessing that the "g" means ground
an "m" means milled
a "t" means turned

What would an "h" mean?

Any help is greatly appreciated.

[True Position]

While I've never seen that notation, h could be 'honed'. Odds would be against honing unless you're dealing with a surface where the finish was very important but it's the only process that comes to mind that starts with an h.

Normally I see just a dimension and a surface finish tolerance. From what I see when someone wants a specific process/machining direction they specify it fairly clearly on the drawing. 'Grind this surface counterclockwise' and the finish symbol 0.3. etc.

[Stijloor]

In Reply to Parent Post by Tiger Woods

I could use some help with and issue on reading blueprints. I should probably know this, but haven't been formally trained and have never seen this before.

When you have a print dimension like 4.312 g +.002 -.001

I'm guessing that the "g" means ground
an "m" means milled
a "t" means turned

What would an "h" mean?

Any help is greatly appreciated.

Welcome to The Cove Forums!

Well.....let's not guess....

Can you attach the actual document or part of it so that we can take a look at the actual call-out?

Stijloor.

[Eric C.]

Thanks for the quick responses and the welcome.

Let me see what I can do about getting a part of the actual call-out on here.

[Eric C.]

Here is a portion of the blueprint that I can show.

I have circled the two dimensions that I'm not sure about.

One have an "h" and the other has an "N".

[Stijloor]

In Reply to Parent Post by Tiger Woods

Here is a portion of the blueprint that I can show.

I have circled the two dimensions that I'm not sure about.

One have an "h" and the other has an "N".

OK. That clarifies it.

The letters do not indicate particular machining operations but are part of a call-out related to what "The Machinery's Handbook" calls "Allowances and Tolerances."
There is also an ANSI Standard for this: ANSI/ASME B4.2-1978(R2004)

In your example from the attached drawing....

100 N7 means the following:

• "100" is the nominal dimension.
• "N" Indicates the location of the tolerance zone relative to the nominal. You can see that the tolerance zone is located below the nominal. In addition, a capital letter also means that this applies to an internal diameter.
• The number "7" indicates a tolerance grade.

Next:

112 h8 means the following:

• "112" is the nominal dimension
• "h" indicates the location of the tolerance zone relative to the nominal.
• "8" indicates the tolerance grade. The larger the number, the more tolerance.

There's a lot more to it then I can explain here, but if you have access to "The Machinery's Handbook", it explains it in great detail.

[Miner]

This sounds a lot like the various classes of threads with letter designations for the fit and upper/lower case for internal/external threads.

[Stijloor]

In Reply to Parent Post by Miner

This sounds a lot like the various classes of threads with letter designations for the fit and upper/lower case for internal/external threads.

That is correct. The 'Thread Standards" people borrowed some parts of the system for thread tolerance designations.

Stijloor.