Pos Tol applied to pattern (how to?) 6

[fcsuper]

I have a X,Y grid pattern of holes laid out flat plate. Datum A is on the facing surface. Datum B runs through the part in the X direction. Datum C runs through the center of the part in the Y direction. (See the attached drawing.)

I wish to apply positional tolerance to the holes of the pattern.

What's needed, what's basic, what's reference?

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
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[KENAT]

fcsuper, have you had a look through section 5.4 of ASME Y14.5M-1994 to see if anything helps?

KENAT,

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[fcsuper]

"In that effort, I arbitarily choose the part center of the datum to give these a common frame of reference. Is there a better way?"

Just to correct myself, the center datum was not arbitary, as I do want the pattern roughly centered on the part.

KENAT,

Yes. But now that you mentioned it, I guess its time for me to look it over again.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[PaulJackson]

fcsuper,

Let the actual functional features that are used to locate and orient the plate or the holes in the plate as they interact in their operation with other parts determine what should be declared as datum features! If that is not possible detail the most stable substitute DRF and divvy-up the allowable variation in the stack between the substitute DRF an the functional features.

Since we don’t understand the function of the plate, the holes, and the interactions with other parts… any advice we offer is like target practice in total darkness.

paul

 

[axym]

I was preparing a lengthy post on degree of freedom constraint and datum feature selection, and then read Paul's latest post. He said it all in two sentences.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

PaulJackson,

I like your "no locating pins" concept. My primary concern about fcsuper's original drawing is that two of the datums are features of size, and the positional tolerances are very accurate. If the outline is not critical, it should be sloppy, and some other feature should provide datums. This is particularly important on something like a sheet metal part with all four edges bent up.

JHG

 

[fcsuper]

Paul,

I know the feeling. I knew that would be an issue, but I also know there's a lot of smart people on this board.

I've come to the conclusion (so far) that there are two competing interests in the location of the pin holes and the location of the pattern.

The locating pins are the common reference. The problem is that the part is symmetric about the datum B in my example, but the pins only engage one side of the part or the other, but not both simultaneously. The part is ambidextrous. It's almost as though I need the 64x pattern to be equally positioned in relation to each pin hole separately. This sounds like a lot of pitfalls.

The alternative I'm seeing is to keep the pattern and pin holes driven from a common datum (current the center of the part).

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[PaulJackson]

Can you expound on this statement?

but the pins only engage one side of the part or the other, but not both simultaneously. The part is ambidextrous.

 

[fcsuper]

Paul,

This plate rests on a flat surface.

It is located by two pins which engage the pin hole in the upper left, and a slot in the upper right.

The part can be rotated 180deg and engage the same pins in the same manner, just with the other pin hole and slot.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[JLang17]

This was mentioned once, but isn't this a good example of when to use a composite positional control? Specify a larger tol for overall pattern (wrt A|B|C) and then .001 for hole-to-hole tolerancing (wrt A). Make 1.750 basic and add a basic dimension of .500 from any hole to it's adjacent hole in both directions. Seems too simple I'm probably missing something, what do I know?

 

[fcsuper]

JLang,

Yes, composite geo tols look like what I need. I'm just now conflicted about which features to use as the datums.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

If your function will allow it I'd be tempted arbitrarily pick one of the two 'pin holes' and the slot as your secondary & tertiary datums. Rely on the other 'pin hole' being closely toleranced enough to meet functional requirements when it is the functional datum in operation 1/2 the time.

Like I said though depends if your function/tolerances can accomodate this.

I sometimes think a simpler approximation has benefits over a technically more accurate, but somewhhat more complex, solution.

Still reckon its 56X not 7X too;-).

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[fcsuper]

KENAT,

I was afraid you'd say that (if not you, then someone else). I am out of my depth in trying to apply a composite pos tol to a secondary datum based on a hole (and what would the tertriary datum be)?

BTW, it's not 56X, it's more like 1000X That comment was noted and accepted. The example I've give has be simplified.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
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[PaulJackson]

fcsuper,

I think I understand. Either one of the two locating pin holes could be engaged as the secondary datum feature depending upon 180 degree rotation of the part in the plane of datum feature A?

My recommendation then would be to detail it as I did making the 2X pattern datum feature B... and eliminate the slot in the mating part. You may have to restict the position tolerances of the 2X pattern in your part and the mating part 2X X.XX +/- .XXX |TP|dia. 0(M)|A| in both parts and then control the manufacturing variation and subsequent interference fit with the size tolerances on the pressed in pins and mating part clearance holes... or employ roll pins that are mallable to interference fits to account for that expected interference from manufacturing variation.

paul

 

[fcsuper]

Paul,

Two comments:

One, this is not a press fit scenario. It's a slip fit. These parts are frequently placed on two pins which are on the base plate and then removed.

Two, for me to apply your recommendation about datum B, I would have to understand it. I'm not sure, at this point, how to explain my use of the 2x pattern as reference to others. In using both pin holes, doesn't this create rather complex geometry in trying to calculate the position of each instance in the 64x pattern?

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[PaulJackson]

One, this is not a press fit scenario. It's a slip fit. These parts are frequently placed on two pins which are on the base plate and then removed.

This kind of registry is done daily (repetitively) in automotive valve body calibration test fixtures and in the actual assemblies. The DRF origin is in the center of the 2X pattern with slight interference if the spread between the holes and pins are alternately long... short... if they are the same there may be slight freedom to shift.

In using both pin holes, doesn't this create rather complex geometry in trying to calculate the position of each instance in the 64x pattern?

The 64X holes would be controlled symmetrically from the plane of A and the center of the 2X pattern... what is so complicated about that?

paul

 

[fcsuper]

Nothing, I guess. I was just under the impression that the datum would be the plane created by the two axes. Well, this gives me something to chew on.

Thanks!


Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[fcsuper]

Ok, before I go my merry way, I just want to check to see if there are any more comments about this.

I'm calling out the pattern with "56x [.500] (= 7x 3.5)" I am using a delta note similar to "For Reference: this is a 7 x 7 = 49 pattern with 8 x 8 feature instances" to make make sure the pattern callout is clear.

I'm locating the 2x pin holes with TB|.002m|A|B|C and making the 2x pattern Datum D

For the 64x pattern I'm calling out TB|.001|C|D.

See attachment

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

Not sure on your hole callout, should A not still be the primary datum for perpendicularity? I'm also not sure on the order of CD, but I still haven't completely got my head around the 2 hole pattern datum where the midpoint is the datum. Does it create 2 datums, one through the 2 axis and one at the midpoint? If so you don't need C. Also, if you aren't relating the holes to C & D I dont' know if you get the 'implied centered' anymore. This seems to be Pauls area of expertize, I realize he's explained it before but maybe he can use shorter words & more pictures this time;-).

In the spirit of 1.1.4 I'll assume your 2 slots are defined elsewhere;-). I didnt' realize before you had 2 slots, I thought you were re-using the one.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[KENAT]

Sorry, should have been.

"Also, if you aren't relating the holes to B & C I dont' know if you get the 'implied centered' anymore."

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[fcsuper]

KENAT,

Point 1: Yes, that was a typo. The hole should read tb|.001|A|D

As to the rest of your questions, I am still looking into the Y14.5 standard and cannot find where datum D would be the center of the part and not a plane between the axes, yet. Beyond that, I still have to put this on the real drawing in a way that is readily interpretable, which means I may have to consider another way about this anyway.


Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group