Arc as a datum?

• Jun 18, 2020

• #2

greenimi

Mechanical

Nov 30, 2011

2,296

Maybe you can use B as a datum target point or datum target line. Just saying.

 

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• Jun 18, 2020

• #3

Burunduk

Mechanical

May 2, 2019

2,404

Agree with greenimi that datum feature B can better be defined by a datum target.
Without knowing the function, I dare to guess that probably it is better to change the datum precedence order to make the width that is currently datum feature C the secondary after A. The arc-shaped feature should probably only constrain one translational DOF and be tertiary.

 

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• Jun 18, 2020

• #4

CheckerHater

Mechanical

Sep 22, 2009

2,882

Here is one way to use arc as a datum:

Here is another:

Simply pointing to the arc and saying "this is a datum" doesn't work.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

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• Jun 18, 2020

• Thread starter
• #5

dthom0425

Mechanical

Dec 6, 2018

47

Thanks for the quick responses...agree with all of you.

Just going to pull on this thread a little bit more...so if datum feature B were to stay as it is shown in the picture and the FCF changes to A|C|B.

Datum feature A - locks 3, Datum feature C locks 2, Datum feature B locks 1 and would theoretically just be line contact on whatever hard gage is used. As a secondary datum feature though, datum B, as shown, would generally be confusing and not truly lock the appropriate DOF. Agree?

I do like the profile surface method shown above - thanks for that.

 

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• Jun 18, 2020

• #6

drawoh

Mechanical

Oct 1, 2002

8,927

dthom0425,

Can you swap datum features[ ]B and[ ]C? The arc makes a perfectly good tertiary datum.

--
JHG

 

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• Jun 18, 2020

• #7

Burunduk

Mechanical

May 2, 2019

2,404

dthom0425 said:

Datum feature A - locks 3, Datum feature C locks 2, Datum feature B locks 1 and would theoretically just be line contact on whatever hard gage is used. As a secondary datum feature though, datum B, as shown, would generally be confusing and not truly lock the appropriate DOF. Agree?

Click to expand...

Exactly.

dthom0425 said:

I do like the profile surface method shown above - thanks for that.

Click to expand...

That method will require an arc-shaped datum feature simulator. Think about how you want that thing fixtured/inspected. If it is sufficient to make line contact with a planar fixture component, specify a datum target line.

 

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• Jun 18, 2020

• #8

chez311

Automotive

Mar 24, 2015

1,426

While I agree datum targets may be a good option, I'm not sure that specifying a datum target line with a cylindrical feature by default implies the datum target simulator is planar even if thats what you desire. A note would make that explicit. Alternately, you could utilize datum target line(s) that are perpendicular to the axis of the arc like CH's second figure.

 

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• Jun 18, 2020

• #9

Burunduk

Mechanical

May 2, 2019

2,404

The usual application of datum target line is making line contact with a pin, which may be a good option too. What I have in mind is a pin normal to datum feature simulator A (the primary) and centered on what is now C (and should be the secondary). The datum target line will be specified on the drawing on the extremity of the arc and represented like a point on the left-hand view where the B extension line is currently tangent to. In this specific case, it will probably not matter if a planar or cylindrical datum feature simulator is used to simulate the datum target line, because the type of contact will be line contact anyway.

 

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• Jun 18, 2020

• #10

chez311

Automotive

Mar 24, 2015

1,426

It will of course be line contact whether a planar or cylindrical datum target simulator is used. A pin parallel to the arc axis will not capture the most extreme point on a real part - but it may be close enough for OP's purposes.

 

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• Jun 18, 2020

• #11

Burunduk

Mechanical

May 2, 2019

2,404

As long as it only has to constrain translation parallel to datum plane C, it will suffice.

Of course, we can't be 100% certain that this is the best scheme without knowing how the part functions, but it surely makes more sense than what is currently shown.

 

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• Jun 18, 2020

• #12

Tarator

Automotive

Sep 20, 2013

176

dthom0425,

The short answer is "any (mathematically defined) feature can be a datum feature". But it is also important how to select the datum features, and the order thereof. Something tells me that your DRF may not be right (not saying it is not). Of course, it is hard to know your design intent without seeing the mating part(s). Would you be able to sketch the mating part(s)?

 

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