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
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[KENAT]

I think this '2 hole datum' was discussed in depth on another thread, perhaps thread1103-236974

I had trouble getting my nogging around it so my solution was to quit following the thread!

KENAT,

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

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

 

[fcsuper]

Paul,

OK, I understand the usage of the pattern as the datum. I'm still not clear on how rotation of the pattern to the datum is controlled. Does using a "mid-point" assume the existance of unstated perpendicular planes (where B and C would be), and assume a reference to those?

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]

On last question (and hopefully my previous one gets answered by someone ), would this be an OK composite post tol for the holes in the 64x pattern (given my last example):

Composite TB|dia.002M|A|D
Composite TB|dia.001M|A|D

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,

It would be a mistake for you to propose some new way of dimensioning if you couldn’t fully defend your reason for doing so! I looked at your new sketch and read your typo capitulation to referencing the 64X holes from A|D rather than C|D but… I am confused as to why there are slots??? in your sketch if the plate registers on two round pins in the two round holes D whether rotated right-side-up or up-side-down 180 degrees while still on surface A? What do the slots do???

As for your questions about using composite FCF’s… Why??? The lower segment of a composite… controls pattern integrity to itself and just orientation of the pattern to the datum features… what is its benefit with this design where the 64X pattern needs to be repeatable and 180 degree reversible at its approximate 40 degree angle to the 2X secondary pattern???

Sorry for the delayed response. I had to go play racquetball with the younger-faster-more agile engineers in our old league… Old age and treachery struggling to overcome youth and determination kinda stuff! I lament… I took off the whole winter block to ski and teach little rascals to ski… golf is beckoning… oh my knees, oh my back… it’s hard engaging again!

paul

 

[KENAT]

Paul, from how I understood it in either orientation only one set of hole and slot are used to engage the pins. I dont' think fcsuper ever said 2 holes were engaging the pins.

KENAT,

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

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

 

[PaulJackson]

Oh also features toleranced from A|D do not originate from the center of the part... but rather... the center of the 2X pattern "D" perpendicular to the primary "A".

Paul

 

[PaulJackson]

Wow Kenat,

It appears that I misunderstood a great deal of this exchange if your insights are correct. I think that I clarified my assumptions 14 posts ago!

Paul

 

[KENAT]

It's OK, I only realized there were 2 slots after fcsupers last image. I thought there were 2 holes and one slot before that, with only one of the holes being used in either direction.

I thought you were proposing replacing the slot with a hole, or something like that. Or else relating the pattern to the 2 holes, even though only one hole is used at a time functionally, as an approximation.

KENAT,

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

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

 

[fcsuper]

Sorry for not being clear enough (though I do think I did state what I meant, save for my typos and such).

OK Paul, so does this change your impression of how to properly control the pattern? ..cuz I think both KENAT and I have questions about your proposal (which I did try to implement).

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, kenat,

My first mark-up of your picture had only 2 round holes "upper-left and lower right" as datum feature pattern "B" omitting the slot that I assumed was "lower right." It wasn't until I saw your re-sketch tonight that I questioned the difference in our assumptions! If you re-read my comments in that light you will understand my advice to make the plate 180 degree reversable and repeatable with the least marginable shift or interference.

paul

 

[PaulJackson]

I'll cave on the advice of "56x [.500] (= 7x 3.5)" if it is clear to both of you but I am an old inspector and the basic spec that was detailed in fcsuper's first sketch was clear to me!!! I modified it in my sketch as I interpreted it without "words". I have to admit that I would have to do some mental gymnastics to interpret "56x [.500] (= 7x 3.5)".

This stuff isn't all science... there is a lot of art and communication in it.

paul

 

[fcsuper]

I believe I have done this already, and I guess I'm looking for a sanity check on what I have done. It looks like KENAT and I both have the same open questions about how to control the orientation of the pattern with just a center axis.

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]

So, I looked through the thread again and looked at the sketches you (Paul) attatched.

Are you now saying:

1. Use a 'conventional' hole & slot secondary & tertiary datum using one set of hole & slot. Make the tolerances on the other hole & slot close enough so they still provide adequate location when the part is reversed and they become the functional datums.

Or:

2. Make the 64X pattern the second & tertiary datums per 4.5.8 & figure 4-22 of 14.5M-1994 then relate the 2 hole/slot pairs back to this datum. While this is 'backward' from a function point of view to me it's equivalent and allows the different hole/slot pairs to act as the true functional datums in the 2 different orientations.

I think both options have been suggested by several people now but we got lost due to some misunderstandings. Option 2 seems more technically correct to me, however depending what tolerance is really required 1 may be adequate and perhaps simpler to understand.

I think anyway;-)

KENAT,

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

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

 

[KENAT]

Sorry posting at the same time, fcsuper take a look at 4.5.8 & figure 4-22, I think I'm starting to get it.

I was advocating leaving off the (=7X 3.5) bit or variations there, on as it's confusing to me too, just have the [56X .500].

([] indicates basic for anyone feeling picky;-))

KENAT,

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

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

 

[fcsuper]

I've been looking at 4.22 all day. lol

I'll upload a new example tomorrow. Once we get this sorted out this may be a good topic to make a FAQ.

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]

Kenat,

No!!! my advice was to loose the slot!!! that I thought was "lower right" and replace it with a round hole... In doing so... the pins "roll pins or solid... each with chamfered ends" will interfere slightly or substantially restrict rotation and shift of the plate to the fixture or mating part that it assembles with.

Paul

 

[KENAT]

Ah, once again at cross purposes.

fcsuper, when you talk about rotating 180° and engaging in pins, is this tiff something like what you're talking about? I added a chamfer on one corner to try and understand the orientation better.

If this is correct then Paul, [pretend the slots are holes, and the pins have some give if need be] and answer 1 or 2 per my above options or something else, if you have the time to try and educate a dullard like me.

KENAT,

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

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

 

[PaulJackson]

fcsuper,

I just looked at 4-22 "hole pattern as a datum feature" and I understand your apprehension in employing it (as others in this forum have lamented in previous posts)... Where in fact is the origin of an imperfect geometric pattern???

That is not "so much" a question with a tightly toleranced 2X patterns "round dowels fitting with round holes with slip-fit or interference fit clearances" where 90% of the variation is dissipated in a straight line between the dowels. These assembly locators win wars, help us travel through space, taxi us through the air at 400+ miles/hour, and enable us to get groceries in the blink of an eye, yada, yada, yada.

No matter whether the CAD designer, draftsman, or detailer depicts the “two round holes and two round pins” as “secondary and tertiary” or “pattern secondary” they operate the same way! When one spread is long and the other short they tend toward interference… no matter what the size differences are!!! A slot naturally eliminates the potential interference but… we consistently design precision assemblies with “two round holes and two round pins” to achieve the least possible variation in location and orientation of one feature to another, hence my advice for your plate.

paul

 

[fcsuper]

KENAT,

Yes, that's what've I've been saying.

Paul,

So, my last example is OK now if I continue to use the pattern secondary?

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]

Paul, I think what I'm asking is how do you suggest dealing with the fact that fcsupers plate (as I understand it) has 2 different sets of alignment holes (or less preferably but shown in his & my sketch hole-slots) for use in 2 different orientation.

Do you

Option A: Arbitrarily pick a pair for your secondary/tertiary datum and rely on the tolerances on the other pair giving adequate relation to the main hole pattern when in service they are providing alignment.

Option B: Make the main hole pattern the datum (all 64 holes) and relate both sets of alignments holes (or hole-slots) to that pattern.

Option C: something cleverer/simpler I haven't listed.

I never meant to argue that hole-slot is better than hole-hole if you can use it - you convinced me of that on an earlier thread some time back.

KENAT,

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

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