Hi, Hopefully someone can help 1

[Rwelch9]

Hi,

Hopefully someone can help me out i have two very small cylindrical pockets only 2mm deep.

They are coaxial cylinders on the drawing it reads 2x Ø20H9 with a Datum C call out next to it.

So i am assuming this means the constructed axis between these two features is the Datum C.

Does anyone have the correct way to dimension two cylinders that are a common axis ?

Any suggestions how to create these on my CMM

I am thinking of taking circles projected into the bottom planes of the pockets then constructing a line between the center points of the circles ?

Thanks

 

[Rwelch9]

Seasonlee.

Regarding the illustration about Datum feature simulators.

That makes total sense.

However taking a plane on the actual surface of the component that has a high quality machined finish. I always check the surface for surface finish and check the flatness using a surface table etc.

Unfortunately my software only uses the least squared method for calculating planes. I believe due to the high quality of machining i very rarely see anymore than 10microns on the CMM with regards to flatness this would be taking a decent amount of points.

The other issue i see potentially with using physical Datum simulators is it will not just be the component you have to keep clean. it will be all the simulators that come in contact with the part etc.

As i says i think i may be stuck in my ways with regards to just using the CMM most of the time. So seeing how you guys think is definitely making me question my methods.

 

[Rwelch9]

Dean,

What CMM software do you use ?

 

[Dean Watts]

Rwelch9,

Evan has much more hands-on CMM software use than I do. The only hands-on experience I have is with OGP's MeasureMind 3D software, used on their multi-sensor machines with vision and touch-probe and laser. I also worked with a good metrology engineer closely for about 20 years, so I learned a lot from him. He used several types of CMM software over the years, ending up with PC-DMIS. Since 1999 I've been a GD&T trainer and consultant/coach. I've also worked with some other very helpful metrology focused people while working as part of the ASME Y14.45 committee. That standard is titled Measurement Data Reporting. It should be released for public review within the next few weeks, and I hope it will be published before the end of the year.

Good luck with your part. It sounds like you're doing fine with it, and on a path with your software plans to do even better.

Dean

http://www.validate-3d.com

 

[SeasonLee]

Glad to know Y14.45 Measurement Data Reporting will be released by end of this year, I heard it about four years ago and the title was "Measurement Data Reporting Practices".

Season

 

[Rwelch9]

Dean,

I saw you mentioned on my previous post. Regarding the job we have been discussing . Obviously i have two pockets which the customer is wanting two Datum features as one common Datum.

Can you call this out as for example Datum C-D .

Can you then have a true position tolerance from Datum Feature D back to C ?

Thanks

 

[Dean Watts]

Rwelch9,

Since those cylindrical pockets are so shallow, I still think the really tight coaxiality control your customer wants may not be necessary. I'll assume they know better than I what the functional requirements are though.

In your sketch datum feature A is a planar surface. Datum A will constrain two rotations and one translation as the datum reference frame is established. I'll call the translation that A constrains the Z direction, so the Z axis would be normal to datum A.

Datum feature B is a pair of small planar surfaces and the size (width) tolerance between them. It will do a fine job of constraining one translational degree of freedom, but not a very good job of constraining rotation about Z because both of those planar surfaces are small. Your datum feature C, or C-D if you like, being from the two cylindrical pockets, is much more capable of constraining rotation about Z. A thought that came to mind, but what I didn't mention in yesterday's comments, is that the two cylindrical pockets would be a better choice for datum feature B. That would just mean switching B and C.

Tolerancing one pocket relative to the other would be a nice thing to do, but the pockets don't have enough depth for either one to constrain rotational degrees of freedom (to orient a datum reference frame) by itself. The coaxiality of the two cylindrical pockets can be toleranced, but it would need to be relative to the datum axis established from both cylindrical pockets. If the pockets are specified as datum feature B, they will need |position|diameter x.xx|A| placed under the 2X diameter tolerance, so they're controlled relative to datum A. You could then add another feature control frame to refine the coaxiality tolerance. Stacked below the first feature control frame that is below the 2X diameter tolerance would be |position|diameter x.xx|B|. The x.xx would be a very tight tolerance value, since the pockets are so shallow. You could also leave off the reference to datum feature B if you like. If that coaxiality control seems odd then please take a look at Figures 4-24 and 4-25 in Y14.5-2009.

Datum feature C would then be the 50 mm width. A |perpendicularity|x.xx|A|B| would be applied under the size (width) tolerance, or you could apply a perpendicularity to each of the two planar surfaces by having the perpendicularity feature control frames not under the width dimension, with a leader that points to each of the two planar surfaces.

Switching datum features B and C as I am recommending would be even more beneficial if other features on the part will be toleranced relative to |A|B|C|.

I hope this helps.

Dean

http://www.validate-3d.com

 

[Kedu]

I would start with a flatness tolerance on datum feature A. Then a perpendicularity tolerance relative to "A" applied to each of the two planar surfaces from which the datum B center plane are found. That perpendicularity could instead be applied to the center plane by placing it under the width dimension between the two planar surfaces, but there's no significant benefit and I think that adds difficulty

A |perpendicularity|x.xx|A|B| would be applied under the size (width) tolerance, or you could apply a perpendicularity to each of the two planar surfaces by having the perpendicularity feature control frames not under the width dimension, with a leader that points to each of the two planar surfaces.

Dean,

Would you mind to explain a little bit what would be the subtle details of these two dimensioning and tolerncing methods.....

If the datum is the middle plane then you CANNOT apply perpendicularity on the two planar surfaces.

If you apply perpendicularity relative to "A" to each of the two planar surfaces THEN which one is the datum ? How to depict the datum (secondary or tertiary in your scheme) IF you are tolerancing the two planar surfaces?

Something does not make sense.

 

[Rwelch9]

Dean,

Yeh the pockets are basically too small to take as cylinders. I have 1mm Stylus on the CMM but would never consider taking a plane with something so small. My thinking behind getting these good was to ensure the parallelism is good between the bottom faces of the cylinders ( the face that make up Datum B )

Then i just take scan the each diameter this will then project in to the relative plane. Then i will check the True position between one another.

I have took your advice and i am checking the that each of these bottom faces of the pockets are Perpendicular to Datum A.

Could you please Advise any books on ASME Y14.5 that would be worth buying. ?

Just to recap if you can have two cylinders one Datum B and one Datum C but together they will B+C. If you can have this can you still call out Datum C positionally to B ?

 

[Kedu]

Quote (Dean Watts)
I would start with a flatness tolerance on datum feature A. Then a perpendicularity tolerance relative to "A" applied to each of the two planar surfaces from which the datum B center plane are found. That perpendicularity could instead be applied to the center plane by placing it under the width dimension between the two planar surfaces, but there's no significant benefit and I think that adds difficulty

Quote (Dean Watts)
A |perpendicularity|x.xx|A|B| would be applied under the size (width) tolerance, or you could apply a perpendicularity to each of the two planar surfaces by having the perpendicularity feature control frames not under the width dimension, with a leader that points to each of the two planar surfaces.


Dean,

Would you mind to explain a little bit what would be the subtle details of these two dimensioning and tolerncing methods.....

If the datum is the middle plane then you CANNOT apply perpendicularity on the two planar surfaces.

If you apply perpendicularity relative to "A" to each of the two planar surfaces THEN which one is the datum ? How to depict the datum (secondary or tertiary in your scheme) IF you are tolerancing the two planar surfaces?

Something does not make sense.

My point is : if datum feature B symbol is aligned to the width dimension, thus the datum is the center plane, then by qualifying the sides of the width - meaning orienting the sides of the opening- (and NOT the center plane) to the primary, will just look weird, very weird, in my opinion. Never seen this approcah before.
I am sure it is workable, therefore I am inquiring for more details, if possible.

Are there any standard's example?

 

[Dean Watts]

Hi Kedu,

Regarding my suggestion that it is OK, or even a bit easier/better, to specify perpendicularity on each of the two planar faces that are used to establish datum B, instead of placing the perpendicularity under the width dimension to control the center plane:

1) Establishing a datum reference frame is (or at least should be) separate and independent from the process of qualifying the datum features. As long as the datum features (both planar surfaces) have all controllable characteristics relative to datum A controlled, so their form and their orientation WRT datum A, then all is completely fine. An example exactly like this case does not need to be shown in Y14.5. What I am recommending is fully supported by the concepts in the standard.

2) Datum B is the center plane, but it were done correctly, it is a center plane that is perfectly perpendicular to datum A. It's a different center plane, the center plane of the feature, which is the center plane of an unrelated actual mating envelope, that a perpendicularity tolerance placed under the width dimension would apply to. That feature center plane would be bounded by the diameter of the cylindrical pockets so the requirement has a level or two of complexity that a perpendicularity that is applied directly to each surface would not have. Especially if the width feature that is included in PC-DMIS (which I believe will fit an unrelated mating envelope to the data, from which a center plane could be found) isn't available to the metrologist, most measurements for perpendicularity of a center plane will be evaluated as perpendicularity of the derived median plane. We know Rwelch9 isn't using PC-DMIS yet, so why recommend a tolerance that his CMM software won't evaluate correctly?

3) When we look at the two planar surfaces from a top view (looking down through the part, normal to datum A) the perpendicularity on each of the two planar surfaces does nothing to control whether they are parallel to each other, but that issue is the same for perpendicularity applied the center plane as well. To control the parallelism of the two planar surfaces as viewed from the top, we're relying on the range of the size (width) tolerance. If the whole part looks like a parallelogram when viewed from above, then this may (emphasis on may) get exposed by a position tolerance on the two cylindrical pockets WRT [A,B]. Now I'll bring up again that I like the two cylindrical pockets better as datum feature B. If that change were made then then each of the planar surfaces could have |perp|x.xx|A|B| applied, and their parallelism to each other would be controlled.



Hi Rwelch9,

If the pockets are so shallow that you can probe at only one depth, so a circle is all you can get instead of a cylinder, then your situation is as if you're dealing with holes punched in sheetmetal. With only a center point from each of the two cylinders (circles) you can't measure coaxiality. You can measure the location of each center point relative to the datum reference frame [A,B], with A and B as shown in your sketch, but that's the best you can do. I would not recommend using any other external features of the part as datum features (as I think I saw recommended in past comments) for this measurement, since they're not functional. Data relative to a non-functional datum reference frame is... Non-functional. The A and B you show in your sketch appear to be the best that you have to evaluate the location of the two pockets. That said, I think it would be better overall to switch B and C, as I mentioned in item #3 above.

Regarding a good GD&T book, others here may have a better answer for you. When I read someone else's GD&T book, I look at if from the point of view of trying to make my book better than theirs . As I sit here this morning, my next task is to finish a revision of the book I use when I provide training. I'm converting from something that was limited to me filling in a bunch of missing information with a tablet PC as we go through the course, to a stand-alone book, with all that info included. I'm quite biased of course, so I'll give a general answer that Amazon has some GD&T books listed (mine not yet included there). I hope someone else will offer some suggestions.

Dean

http://www.validate-3d.com

 

[Rwelch9]

Dean,

Yes measuring coaxiality is impossible for me with these two pockets, so i am just creating circles in the case projected into the relative bottom plane of the pocket.

1 thing i am struggling to understand is why use the centre plane as a Datum . I have checked each individual plane to Datum A which is giving me a good indication if the legs of the component have caved in or distorted. They are caving in ever so slightly however as its equal on both sides the mid plane i construct is extremely perpendicular to Datum A .

What purpose would there be to making a Datum the mid plane ?

 

[Rwelch9]

As previously stated i have just started to become with familiar with the ASME Y 14.5 standard. This is due to our company manufacturing parts for Johnson & Johnson which is American based.

Is there no specific book that is the Bible on this subject. I see a lot of people referring to sections of the standard etc. So i was wondering how to get a full copy of the standard.

Thanks

 

[Dean Watts]

Hi Rwelch9,

ASME sells Y14.5 directly at

https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing

It's kind of expensive, but not as much as the 20 or so ISO standards needed to cover the same ground.

A center plane is best if the mating feature is not biased against one surface or the other (so if it fits tightly between the two planar surfaces, or if it floats between them with no bias force in either direction). The center plane is then the best thing to locate other features relative to. What you're trying to avoid is choosing one of the two planar surfaces as the datum feature, then in the real assembly, the mating part ends up touching the other planar surface instead.

If there are not any important features that will need to be located in the direction the center plane constrains, then there would be no benefit to using the center plane. One of the two planar surface would then be the simpler, so better, choice.

Dean

http://www.validate-3d.com

 

[Rwelch9]

Dean

Perfect , in my case here the mating fits between the middle slot in my drawing above. We have a mating part with two pockets 1 on either side with bearings pressed into the pockets.

Then we have Pivoting link axis with a tight limit diameter at the front. So they get screwed into the two pockets we have been disusing then the tight limit diameter goes thru and locates into the bearings on the mating part.

So this makes sense why they have went for the mid plane as the Datum.

What i was trying to suggest if we take my drawing above . The two Plane features the create Datum B . If these both were out to Datum A , but they were both out the same way . The mid plane on my CMM would tell me the perpendicularity was good from the mid plane to Datum A .

Thanks