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Symmetrical Part problem!

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var10

Mechanical
Apr 4, 2013
188
My part is symmetrical throughout except for two cutouts. I want to use symmetrical dimensions to minimise my time spent and make it look good. So now I just have a note in my drawing which reads "the part is symmetrical about the centre line except for connector cutout" and on the drawing view I have an arrow saying these are the two cutout locations. Is this acceptable? or are there any other options which will be time saving and easy to read?
 
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What drawing standard do you work to?

Do you make use of geometric controls?

What you've done doesn't sound like it is explicitly supported by drawings standards I'm familiar with so it may be misunderstood/misinterpreted.

If nothing else think about this - how symmetric is symmetric? I.E. how far off from symmetry can the features be and relative to what datum?

ASME Y14.5M-1994 does have some suggestions for general notes such as "PERFECT LOCATION OF SYMMETRICAL FEATURES AT MMC REQUIRED FOR RELATED FEATURES." take a look at section 2.7.3 if you work to this standard.

What can seem like a nice drawing short cut doesn't always seem so smart when parts come in wrong;-).

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484
 
var10,

If you show features that appears to be symmetrical across a centre line, I would assume that they are nominally symmetrical. This is especially true if you dimension one side only from the centre line, or show dimensions across the centre line. I strongly prefer the latter, since the dimensions are measurable.

Use profile tolerances to control the symmetrical faces. I claim that the symmetry is of extremely limited use.

Asymmetrical features are no problem. Just show them.

--
JHG
 
Is it acceptable to dimension features from a centerline as long as the dimensions include the outside (envelope) geometry dimension?

lightweight, cheap, strong... pick 2
 
In regarding the original post, would it be better if in his notes he replaced the word symmetrical with positioned? (from an inspection point of view....)

lightweight, cheap, strong... pick 2
 
Koda94 said:
Is it acceptable to dimension features from a centerline as long as the dimensions include the outside (envelope) geometry dimension?

This is a separate question. How about starting a new thread?

--
JHG
 
If you really mean: "is it acceptable to dimension the ouside features from a centerline defined by some inside feature datum reference franework?" I would say, yes.
 
I got tired of answering questions about symmetrical parts decades ago, because every damn department or company that gets the print "just picks up the phone" to check that it really is symmetrical, etc.

Nowadays, I would dimension the part (not from centerlines) AS IF it were not symmetrical in any way.


Oh. Also, if the few asymmetrical features on your part are not OBVIOUSLY asymmetrical, I would make them so, or add otherwise vestigial features that are fluorescently asymmetrical.





Mike Halloran
Pembroke Pines, FL, USA
 
the part is symmetrical about the centre line except for connector cutout

Your note tells the reader what the part is NOT. Drawings must define what the part IS.

Consider a dimension that states "This dimension is not 123 mm". Not very useful is it?

Drawings need to be complete, clear and unambiguous.

"Easy to draw" is way down the list of important requirements.
 
Koda, if it's your question then new thread would probably be better. If you're posting it as a solution to the OP's dilemma then what you're proposing may have a negative impact on tolerances depending how they're applied.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484
 
Koda94,

Your drawing looks fine to me. It conforms to my first suggestion, above. To apply form controls, you must select features that define your centre lines.

--
JHG
 
My apologies for the confusion, my questions were with regards to finding a solution for the OP, not my own. The thing about the drawing I presented is I am not certain if the envelope dimensions should be basic or if that matters in this case.

The OP is asking if his note is acceptable or if there is a better way. My direct answers are; the note is only acceptable if it’s clear (requires no phone call) and the drawing example I presented is IMO a better way because it’s clear and does not require a note at all.

The problem here is the abuse of overusing the word symmetrical to describe a part who’s features only need to be positioned about a centerplane. The OP does not need to include a note at all.


lightweight, cheap, strong... pick 2
 
Thanks for all the suggestions. The reason why I have it this way is

1. According to CASA (atleast that's what I was told) every dimension on the drawing needs to go through FAI (first batch) & Goods receiving inspection everytime we receive (check 10% of parts again all dims). So it's easy to put dim's on the drawings but in reality I can't get my instruments to measure those dims. (we have calipers, micrometer, hole & bore guages for now)

2. Koda94's drawing would be another way but when you need the part toleranced from the centre ordinates are not recommended (plus I do not want overalls as they are don't affect anything so I want give the supplier some freedom). Plus too many features for my part and it will look very messy (both hard for machinist and the inspector).

3. We are currently putting in place various standards and I am setting up design drafting standards. To answer Kenat's question , No we are not following any standards as of now.

4. I only have one boss with fillets not symmetrical in the entire part - currently showing them with an arrow to refer to the notes. (Still going against the standards which I do not wish to do in future when we have established standards to work towards)

Mintjulep gave me a good point to think about.

Cheers,
 
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