Position tolerance relation to datum inside face of hole

[umatrix]

I have a part with three holes, with a diametric 1 mm positional tolerance defined at datum A, B, C for the two top holes.
I am having difficulty seeing how the tolerance is applied to datum C , being the inside face of that hole?
Datum A and B are clear to me, I.e., can vary diametrically 1mm from datum A and B.

 

[3DDave]

Something else is wrong as the datum features A and B are given the (M) modifier so the Feature Control Frames make no sense.

This looks like an example question from a textbook asking to specify what is wrong with it.

 

[mfgenggear]

datums a & b establish centerline of of two holes, the two holes must be perpendicular to datum c
a cmm would take care of all the constraints

 

[mfgenggear]

and it depends how it is dimensioned

 

[umatrix]

Hi mfgenggear,

Thank you. You mention that it depends of how its dimensioned, can you elaborate ?

Note, that this image is just a quick model of the print I made for the post, because I can not share the actual print.

 

[drawoh]

umatrix,

Your question belongs in forum1103. Also, it is hard to read your figure. The fonts are too small.

Your question is practical rather than theoretical. Your datum features completely constrain your part. Your datum feature[ ]C is a feature of size, and may have to be called up at MMC. Fixturing to your hole will be difficult, although not impossible. Can you pick up the front or back face of your part instead?

--
JHG

 

[mfgenggear]

it is actually easier to machine than to inspect.
I would use a 4 or 5 axis mill .
square up the block machine datums A, B and C simultaneously.
that way nothing has to be moved. then lay on a A & B pick up C or use a Diamond pin to locate.
datum c as drawn is not restrained but need to be held to Datum A & B and on center line of part
in order for two holes to fall in Datums A, B & C have to be held true position, that allows
two holes to fall in.
this is very doable.

 

[3DDave]

I just want to know what workbook this is from.

 

[umatrix]

3DDave,

This is not from a workbook. I created it in solidworks, it is just a striped down version of the positional tolerance I was trying to understand. The aim was to understand the positional tolerance.

I was having trouble understanding how/why the callouts were referencing datum C. mfgenggear cleared it up.

On another note , I am happy you think my 3D drafting skills are publication quality. But then again you did say it was drawing of "what not to do", so I should not rejoice that much.

 

[3DDave]

Uhh, anyone can self publish. I was mislead by what looks like the shaded edge from a poorly done copier scan. I wanted to know what author to avoid.

So if you created it why are there (M) symbols applied to features that it cannot apply to? Why do diametral feature not have diametral tolerance zones?

Glad to see you take ownership, but baffled why you printed it and then scanned it.

I take it back - a scan would not skew the image, so this is from some other source. Since you did it you can provide a clean un-skew PNG with no trouble.

 

[umatrix]

I suspected that may be why you thought it was from a book.

I printed a paper copy, and didn't save the model. So I scanned the paper with my phone scanner (Tiny Scanner it is called) to pdf in order to upload.

About the (M) symbols , that is the way the actual part is specified by the previous engineer.

Unfortunately, I am weak in terms of GD&T, so I am trying to understand the basics of positional tolerance in reference to datums.

Part of the difficulty is not being able to share the actual print.

 

[3DDave]

That's a terrible phone app to do a scan that badly. But to a PDF and then from that to JPG?

Anyway - advice to those weak in Y14.5 Feature Control Frames - buy the ASME Y14.5 standard and read it. Come back with questions you have about that book before dealing with anything else. Otherwise it's just like

https://thedailywtf.com/

where people try to guess what prior workers might have meant with senseless FCFs (Feature Control Frames)

 

[umatrix]

Thank you 3DDave