How to specify on drawing a dead center hole on a rectangle 1

[TOMLIN66888]

Hi Everyone,
I am new to posting but have been learning quite a lot by reading the Forum. I have two question regarding to GD&T.
In the drawing , I think method 1 would work but
Q1: I am wondering in practice how would QC people setup the datum simulator to measure it. Would it require a coordinate measuring machine (CMM) or is there a way to setup jig for measuring.

Q2: is there any other to specify a dead center hole? I have tried to use the hole as secondary datum feature but I do not know how to make the width and length center to the secondary datum feature.

Thank you for you time to respond

Tom

 

[drawoh]

TOMLIN66888,

As per ASME Y14.5, your drawing is completely meaningful. The hole obviously is to be centred.

Inspection could be interesting. I like FOS datum features to be very much more accurate than the tolerances specified using them. In your case, you are trying to locate something inside a 0.1mm diameter from features that are [±]0.1mm, and that are allowed to be at an angle with respect to each other.

A better dimensioning scheme might be for the hole to be datum[ ]B, and one edge to be datum[ ]C. Apply a profile tolerance all around the outside.

--
JHG

 

[KENAT]

You have specified the hole to be centered on the rectangular part in an acceptable way to Y14.5 as Drawoh says.

The other way to get similar result as Drawoh alludes is to make the center hole the datum and center the rectangular outside profile on it - doesn't have to be surface profile it can be achieved with position tol possibly using "boundary" call out.

Warning, I'm not an inspection guy but as I understand it...

With your sketch, Per ASME definitions of datum features you can't really inspect using just CMM - you'd arguably need to sandwich it between 2 flat datum simulators/fixtures then use the CMM to derive the centerplane from them. ASME doesn't really support just taking a bunch of CMM points on a surface and either averaging them or picking the 'worst' of them or similar to derive the datums.

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mkcski]

Your Method 1 accomplishes your requirement. This is "feature located from the edges". The alternative profile option is OK too. This is "edges located from the features" In either method I would recommend adding flatness to the "bottom" face and consider MMC for the hole. For Method 1, I would consider using MMB concepts for the width features of size (FOS).

Certified Sr. GD&T Professional

 

[greenimi]

1.) Why method 2 is locating the hole in the middle?
2.) Hole as a datum feature or the width as a datum feature or anything else..... In my opinion just speculations ....since the OP did not tell us how this part works

 

[dgallup]

I would inspect method 1 with an optical comparator or measuring microscope. Either one can pick up the high points to establish the datums. It's reasonably simple to find the middle of the flats and the center of the hole and calculate the true position error.

Only thing I would change is extend the center lines beyond the boundary of the rectangle.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[TOMLIN66888]

Thank you everyone for the reply

I got this question from a friend challenging me on how to make a hole dead center regardless of the width and length variance. Therefore I do not have the application for this part in mind.

to drawoh
In your suggestion using the profile control, let's say left side end up on the innerest side of envelope and right side on the outest envelope, then the hole is not center anymore, correct?

to KENAT
I am not familiar to CMM neither. As I understand that in usual case that uses a side as datum feature, you slide the part until it hits that side with the datum(a machined piece as flat as possible) and you measure from that machined datum to your feature of interest. In the case of center plane of a width, how do you do that? That's why I thought you might need to do the measure in virtual space and simulate the imaginary center plane.

to greenimi
No, the second method is not finished. I was trying to come up with an alternative using the hole as datum but I don't know how the specify the width centered to hole datum feature.

to dgallup
Thanks for the suggestion.

 

[greenimi]

TOMLIN66888,

Okay.
Let me ask you what means "dead center"?

 

[JNieman]

A rudimentary method I've seen before is to label the overall width as "Dimension A" and the length as "Dimension B" with the location dimensioned to a corner, reading "A/2 +/-.001" and B/2 likewise.

Crude. But it works in some situations.

Just throwing out "another way" - not necessarily a better way. Know your audience.

 

[drawoh]

to drawoh
In your suggestion using the profile control, let's say left side end up on the innerest side of envelope and right side on the outest envelope, then the hole is not center anymore, correct?

Correct.

--
JHG

 

[TOMLIN66888]

to greenimi
My goal is that
1. in the case of width being 9.9mm (10mm-0.1mm), the center of hole lies on 4.95mm(basic dimension) from one side
2. in the case of width being 10.1mm (10mm+0.1mm), the center of hole lies on 5.05mm(basic dimension) from one side

with the positional tolerance, the hole center can wonder from the basic center

 

[greenimi]

Tomlin66888,
Then make the hole a datum feature and control both sides with profile with the same datum reference frame.