Tolerance symmetric Dimension to centerline

[321GO]

Hi Guys,

I need to tolerance two opposing symmetric features with regards to their common centerline. The tolerance must be equal for both features.

Suppose i placed the tolerance only on one feature. Would it be obvious for the machinist that the opposing feature has the same tolerance (with regard to the centerline)?

Or better tolerance both features independent with regards to the centerline?


Thank you all!

 

[Enginerd9]

I think I'd have to see an example to be sure, but from what it sounds like you may need to tolerance both sides.

Or, you could try making the center line a datum and dimensioning the total span with symmetry within the confines of your total allowable tolerance? The ASME Y14.5-2009 book shows figure C-2 in the appendix, and there is a symbol for symmetry in that table. Additionally, Section 7.7.2 discusses the use of that symbol and shows some examples (Fig. 7-66).

If GD&T isn't an option, I would say you'd need to place a tolerance on both sides.

-Brad

 

[Enginerd9]

On second thought, using a center line as a datum for symmetry is a silly idea. I don't know what I was thinking.

However, you can still use the GD&T "symmetry" method as long as you have another feature to use as the datum.

Also, after third thoughts, a center line should imply symmetry about itself. You may not even have to worry about it. What I assume is that when you place a dimension on opposing features that share a center line, the dimension and accompanying tolerance are evenly split by convention. I can't back that assumption up with a citation at the moment... but I'm pretty confident it's correct.

 

[pmarc]

321GO,

Assuming you are following Y14.5 standard, it offers some options to do this properly, but I agree it would be really good to see at least a simple sketch of considered situation.

For sure avoid placing datum feature symbol on a centerline - it is not allowed in Y14.5 std.

Also, even that you are trying to control symmetry of features, I would recommend position tolerance, but to give further details a sketch would be helpful.

 

[KENAT]

As others say, post a sketch. Just because things are shown inline or symmetrical etc. doesn't usually place a control on how inline or symmetrical they are. You normally need to place an explicit requirement.

Posting guidelines faq731-376

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

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

 

[drawoh]

321GO,

When I have features symmetric about a centreline, I dimension across the centre from feature to feature. I use whatever feature defines the centreline, as a datum. I Apply positional or profile tolerances to the features.

I try to avoid dimensioning from centrelines, because these do not exist and you cannot measure from them. I consider a ± tolerance from a centreline to be meaningless.

JHG

 

[fcsuper]

The key to drawoh's approach is the use of position and profile gd&t. Using a centerline to establish symmetry is not supported by Y14.5. I think others are right when they state that the centerline doesn't exist and is unreliable as a specification. In the case of symmetry, a centerline is the result of a feature of size or related features. As such, the centerline cannot really drive the locations of those features. It's a bit like the tail wagging the dog.

However, I will dimension off a centerline sometimes to establish a relationship between features. When I do this, I still provide a dimension to that centerline, and I understand that my tolerances are effectively doubled (not halved, as is commonly misbelieved) when doing this.

It seems to me that many informally trained drafters (i.e., engineers) confuse the function of nearly defunct Symmetrical Outline with that of GD&T's almost as nearly defunct symmetry. Instead of using Symmetrical Outline, I would use Profile. Instead of using symmetry, I would use Positional Tolerance.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[Enginerd9]

What about drafters who decided to go one step further and become engineers? Specifically, design engineers?

 

[321GO]

Pls see attachment, i think this would be ok.

p.s. the plate has a tight fit with a flattened axle.

 

[Enginerd9]

It'd be difficult to measure the 11.6 dim from an imaginary circle. I might suggest placing the dim between the two flats at +0.0 / -0.2, maybe?

As dimensioned, the flats can be 23.2 to 23.4 apart. If you follow my suggestion, you'll get 23.2 to 23.4 also, and add a location tolerance wrt the outer diameter to achieve your specific location need, etc.

It's the end of the day and my brain's fried. I might be totally wrong.

Thanks!

 

[KENAT]

OK, give us some functional information 'cause that dimensioning scheme looks a bit odd.

Posting guidelines faq731-376

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

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

 

[ewh]

I agree that it looks odd... did you allow for the tolerance on the diameter (really two radii)? It may possibly end up being much tighter than you intended.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Enginerd9]

I'm still trying to wrap my head around measuring to an imaginary diameter... seems like we're back to square one by specifying the 11.6 distance from removed diameter to flat. It really should be dimensioned to features that can be measured, and the easiest to measure (and thus hold) is flat-to-flat, with a position tolerance w.r.t. the outer diameter.

Am I wrong? Maybe at least partially correct?

 

[321GO]

Pls see attachment. (Don't mind the tol. value's)

To my knowledge this is viable, but i'm not 100% sure.

What do you guys think?

p.s. i'm using the German standard

 

[321GO]

oeps...

 

[Enginerd9]

It seems better, imho. However, you're still using an imaginary diameter for a datum reference. I'd suggest placing a dimension on the outer diameter of the whole plate, and using that to specify a position tolerance using the cross-hairs symbol, or something like it.

Then again, I'm no expert. I'm just not convinced it's easy to measure to that imaginary diameter created when the flats are added.

 

[Enginerd9]

Oh, I just also noticed...

You're calling for symmetry about "A" datum which in this case it physically impossible. "A" datum is one of your flats, and it can't be symmetric about itself. I could be wrong, but it doesn't seem to work out in my head.

I'll work on a quick sketch of my idea and post it up in a few...

 

[ewh]

Is there any reason you can't use the opening between flats as the datum and use a true position of the diameter to that?
Regardless, I think true position would be a much better choice than symmetry.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Enginerd9]

Actually, just reference ASME Y14.5-2009, Section 7.6.1. It deals with coaxial positioning and such. It may help to clarify what exactly you want to accomplish... my assumption is that you want the center of your cut to be coaxial with the axis of your outer diameter within a specified tolerance zone.

Thanks!

 

[KENAT]

Not sure they use ASME Y14.5-2009 in Germany, DIN/ISO I believe.

I seem to recall symmetry is one of the places ISO is different from ASME and that it may be used more there.

However, I can't recall details.

Certainly I believe you still need a real feature as your datum feature, not an imaginary one.

The outside diameter looks tempting but I don't know function and I'm not up on ISO either.

To 14.5 I'd probably look to use position as ewh says.

Posting guidelines faq731-376

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

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