Shaft GD&T questions 2

[BodyBagger]

Hello all,
I would appreciate some input on the following situation. I have a shaft with 2 bearing seats about 2.5 inches apart. The back bearing is pressed on to the shaft up to a shoulder and then the shaft and bearing are pressed into a housing up to a shoulder in the housing. Then the front bearing is pressed in against a shoulder in the housing. I am a bit confused about calling the GD&T for the bearing seats. What I have done is call out perpendicularity to the shoulder for the first and bearing seats and total runout to the centerline of the shaft. Does this sound proper?

SEE ATTACHED PIC FOR CLARITY
Recommendations welcome.

Thanks,BB

 

[KENAT]

Copy of relevant posts from thread286-269341 where this started.

ewh (Aerospace) 9 Apr 10 9:34

KENAT is pointing you in the right direction.
One thing that jumps out at me is that you need to identify which feature is datum A; an imaginary plane would be frowned upon, and a line may be the axis of datum A, but can't be A in and of itself.

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

KENAT (Mechanical) 9 Apr 10 10:13

Well, you haven't reposted so I'll try and give you some pointers.

1. What drawing standards are you working to? Most regular GD&T posters here, including me, work to ASME, mostly Y14.5-1994 though 2009 is increasing in popularity. So most responses will be based on those stds.

2. Your thread is not correctly specified. Should I believe be 1.2500-12 UNF-2A (or maybe -3A or something). You should also reference the relevant thread spec.

3. As ewh points out, you can't make 'center-lines' as such datum features. You need to pick a real feature from which the axis to be used as a datum is then derived. Also, depending on function using just one dia as -A- may not work well, you may want to pick up diameters at either end and have runout to the 2 datum features like in figure 6-49 section 6.7.1.3.2

4. Your perpendicularity call out makes no sense when referenced to A. Your cylinder is 'coaxial' to A, not perpendicular to it. If the perpendicularity is meant to be to the '0' face then that needs to be a datum feature and the FCF changed accordingly. You might actually want to put a runout control on the diameter relative to the '0' face, this is shown in figure 6-50 section 6.7.1.3.3.

5. Is the orientation of the flat at one end to the .281 cross holes significant? If so one of them needs to be a clocking datum feature and the other features related back to it. Also it looks like it should be 2X Ø.281.

6. You may not need total run out, but I'm no expert.

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[dingy2]

I was reviewing the drawing and just couldn't figure out datum A. Could you please let me know the feature that created datum A (cannot be a center line)?

Dave D.

http://www.qmsi.ca

 

[BodyBagger]

Hi dingy2,
I was using the C/L as Datum -A-. See attached picture for application info. Hope this will help.

Thanks,
BB

 

[ewh]

That begs the question "The centerline of what"? A feature is required to use its axis as a datum.

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

 

[BodyBagger]

Hi ewh,
The intended feature is the shaft centerline. My mistake if this is not a viable option to use as a datum.

Thanks,
BB

 

[KENAT]

The point being, you have to pick a specific diameter, or as I mentioned before possibly 2 diameters, to derive the actual datum axis from.

This is what all 3 of us have tried to point out now. Do you have a copy of a drawing standard to look at?

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[BodyBagger]

Kenat,

We do not have a copy of the standard to reference.

Regards,
BB

 

[drawoh]

BodyBagger,

You need to read the ASME standard. Datums must be applied to real features so that fabricators and inspectors can reference everything off them. Your datums will be used to fixture your part. They must render it immobile.

In your case, the face at 0 (zero) would be a good datum_A, and the Ø2.5000_0/-.0005 next to it would be a good datum_B. If you have any rotation issues, the 2.500 flat face would be a good datum_C. Your second accurate Ø2.5000 would be located by a positional tolerance to |A|B|. In this datum configuration, concentricity is not meaningful.

Alternately, I would be tempted to make the first accurate Ø2.500 datum_A, the second accurate Ø2.500 datum_B, and the zero face datum_C. Your flat face would be datum_D, if required. The two diameters work together to define a centre. Your flat face must be perpendicular to |A-B|. Note how there is no need for a concentricity or true position tolerance.

I do not think your .0005 run-out specifications apply any control to +0/-.0005 diameters. Also, your 63[μ]in finish is pretty rough. If the fabricator can achieve your diameter tolerances, the finish will be better than that.

JHG

 

[KENAT]

You apparently have little understanding of the topic, and apparently no references to refer to, there's only so much help we can give here. I suggest you get the relevant standard, maybe some kind of training material and/or training course.

You might try web-searching for some freely available material, perhaps you'll have more time to spend on it than I & might find something.

http://www.tec-ease.com/?gclid=CIHQi4SY-qACFRQWawodt3Q6uQ

has some pretty good free resources.

http://www.draftingzone.com/

has a little to though most you need a membership for.

Some bearing suppliers actually given dimensioning information in their catalogs so you might get something there just be careful of any ISO V ASME differences.

Your second post actually starts to help with answering functional information, if you could maybe give a bit more description that might help too.

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[dgallup]

Trying to apply geometric tolerancing without a standard is kind of like teaching a pig to sing. It's a waste of time and annoys the pig.

 

[BodyBagger]

Thank you very much for your input. That is the kind of constuctive help I was looking for. I am glad to have picked up a few things I did not know today.

Regards,
BB

 

[SeasonLee]

BB

Parts do not have conterlines, pls read the link below :

http://www.tec-ease.com/gdt-tips-view.php?q=130

SeasonLee

 

[BodyBagger]

Hello SeasonLee,
Examples are great, thanks for the confirmation.

BB

 

[fsincox]

If working to the 2009 standard you may possibly now even use a cylindricity callout to the (2) bearing lands and add the "continuous feature" symbol. The ISO based manufactures, ex: SKF and FAG, catalogs have used this type of designation for 20 or 30 some years, much to my dismay as I was not being allowed to do that. Our traditional way for standard grade (3 & 5) bearings in machine tool spindles was total runout to A-B kind of like you showed. I will leave the grade 9 precision spindles to a concentricity discussion on some other day.
Frank