Hole pattern dimensions question 1

[boottmills]

I have a flat sheet metal part with 2 different size hole patterns. One is a .75 hole in a 9x6 array with a large tolerance, the other is a .177 fastener hole in a 3x5 array with a tight tolerance. Some of these holes are in the same ordinate plain and I am having difficulty figuring out how I should dimension this to show the tolerances. We currently are looking at ordinate dimensioning of this part, but I'm having difficulty showing the tolerance. Would you recommend adding a view to dimension the fastener holes, or is there another way you would go about this?

Regards,
Boottmills

~Boottmills

 

[KENAT]

You 'should' use positional tolerancing - at least per ASME Y14.5M-1994.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

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

 

[JNieman]

Agreed with Kenat.

Ordinate dimension, or provide a coordinate origin, and supply a hole table with X/Y values. Depends on what suits drawing clarity.

A Feature Control Frame should be applied to both of your diameter callouts to indicate positional tolerance. That would be my approach. The ordinate dimensions are [Basic] dimensions, with the positional tolerance controlling the location.

That is most commonly the desired condition of the part, to suit it's fit, form and function.

 

[ModulusCT]

I like the idea of an x-y table with a note describing general hole position tolerance.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[MintJulep]

I know that this is "wrong", but...

Flat sheet metal part.

It's going to be all done on the same machine in the same set-up.

So you are going to get the same process capability for both patterns, and that capability will have to be able to achieve your "tight" tolerance pattern.

So just apply the "tight" tolerances to everything.

 

[CheckerHater]

I actually agree with what everyone is saying.

Let me just arrange the suggestion in the order of preference:
1. Use GD&T. Establish datums and attach FCFs (feature control frames) to the holes. This is the least ambiguous, standard-compliant way to control the position of the hole.
2. If your company just outright rejects use of GD&T and you are forced to use directly toleranced ordinate dimensions, use tabulated dimensions. This way you can clearly distinguish that hole A is located at 20+/- 0.25 mm and hole B is located at 20+/-0.10 mm.
3. If your company is so deeply entrenched in “we always did it this way” attitude that even tabulated dimensions are out of question, just use the same tolerance for all locations. You can even do it in form of general note like “unless otherwise specified, all hole locations are +/-0.10”.
4. Another possibility, not mentioned yet is to use ANSI B4.2 / ISO 286 designation like “location tolerances +/- IT12/2” where IT is “ISO tolerance”. But I can imagine your company is not very fond of ISO 286 either.

 

[drawoh]

boottmills,

I agree too.

A problem with dimensioning holes is that different holes can wind up on the same dimension line, as you note. This is why the positioning tolerance should be attached to the hole specification.

The GD&T positional tolerance accomplishes this. The positional dimension is basic. The feature control frame is attached to the hole. The linear position dimension can represent several tolerances.

--
JHG

 

[boottmills]

Thanks for your help folks! I agree with using a positional tolerance and will eventually move that way, once I convince my boss that GD&T is not as scary as it looks.

I wish I could show you how horribly these are dimensioned currently...Ordinate dims mixed in with pitch dimensions on the same views is just blowing my mind. Makes me wonder how a place has managed to survive this long with so few problems.

~Boottmills

 

[Tunalover]

If two or more positional tolerance feature control frames are used with cylindrical tolerance zones and the same datums in the same order of precedence, a simultaneous requirement is established. If the two hole patterns are not functionally related then you can make it less restrictive by saying SEPARATE REQUIREMENT or SEP REQT. No sense making things more restrictive than they have to be!

Tunalover