Datum Selection for Block Supported by 3 Pins 1

[hydronicengr]

I am designing a simple block that is supported by 3 pressed-in spherical/rounded head pins. We are using these 3 spherical pins to accommodate the irregularities on the surface that this block sits on.

If the pins were not there, I would choose the bottom surface of the block as datum A as this would be the face that sits on the mating surface. Instead, since the tips/points of the spherical pins will be sitting on the mating surface, would it be feasible to use datum targets (A1, A2, A3) to create datum A (since 3 points make a plane)?

The parallelism of the top surface of the block must be controlled tight relative to datum A.

Thanks in advance

 

[TheTick]

That's what datum targets are for.

 

[KENAT]

Like TheTick says look at Datum Targets, ASME Y14.5M-1994 section 4.6.

The only thing I'm struggling with a little is how to allow for the variation in location of the pins in reality when defining the datum points but I'm sure there's a way around that someone here will remind us of.

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[drawoh]

KENAT,

Three points define a plane. There may be some concern about where the bottom face is, but it sounds like they don't want to care.

If the pins are round headed, they contact the machining fixture and the granite inspection block, and their precise position in X and Y is not important.

--
JHG

 

[Belanger]

Kenat -- those locations would have to be given on the print (or CAD or whatever). The standard then says that those location dimensions may be toleranced or be basic. If they are basic, then "established tooling or gaging tolerances apply" (para. 4.24.7).
Obviously that means somebody somewhere will have established these in-house tooling or gaging tolerances, but that's another matter.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

I am not sure I am on the same page here, so let me ask OP this question:
The "3 pressed-in spherical/rounded head pins" are part of the simple block and the intent is to define 3 pin tips as datum target points A1-A3. Am I understanding it right?

 

[hydronicengr]

Thanks all for the quick replies!

As mentioned by drawoh, I don't care much about the bottom face as long as the pins stick out from that bottom face as equally as possible.

@ pmarc: Correct. These 3 pins are pressed into the block to create a single part. The tips of the spherical pins will be defined as datum target points A1-A3.

When referencing this new datum A in feature control frames, is the standard convention to simply put "A" or should it be made more explicit by stating "A1-A3"?

 

[KENAT]

hydronicengr,

You need to read up on datum targets preferably starting with relevant standard (e.g. ASME Y14.5) and then if necessary looking at secondary sources.

http://eon.sdsu.edu/~johnston/ME102_Lecture_Notes/GTOL Lecture 3 - Datums/Datum Targets.pdf

looked promising but only flicked through it.

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[pmarc]

hydronicengr,
That for the reply. When referencing datum A (derived from datum targets) in feature control frames you should simply put A, not A1-A3.

Out of curiosity:
You said: "The parallelism of the top surface of the block must be controlled tight relative to datum A."
I am wondering how you are planning to define location of that face relative to datum plane A and how you are controlling length of the portions of the pins that stick out from the bottom face. Could you describe?

 

[drawoh]

pmarc,

He inserts the pins, then he machines the top face, and all the other stuff that is referenced to datum[ ]A.

--
JHG

 

[hydronicengr]

@ KENAT
Will do. I'll also look at ASMEY14.5 as well.

@pmarc
That is a good question. I have not decided yet, but I was thinking of using the surface profile tool with a basic dimension the bottom surface relative to datum A (aka the 3 points that define datum A).

 

[pmarc]

drawoh,
I am not that interested to know how the part is made. I am interested to know how the drawing controls relationship between both surfaces (top and bottom) and pin tips.

hydronicengr,
That sounds like a good idea.

What about the top surface? The parallelism callout will be not enough as it will not control location of that face.

 

[drawoh]

pmarc,

I agree that we do not care how the part is made. The sequence does matter. If the datum[ ]A features are three pins, then the pins must be installed before any other manufacturing takes place. The pins must be used for machine fixturing.

--
JHG

 

[pmarc]

drawoh,
What if the block with press-fit holes is made first. And then, by proper adjustment of pin insertion depths the manufacturer tries to meet the requirements for location and orientation of both flat surfaces in question?

 

[drawoh]

pmarc,

I asked this sort of question when I was trained at ASME Y14.5M-1994. I was told that manufacturing and inspection must be done using the specified datums. Of course, if you mount the thing on the pins, you inspect it and pass it, you don't care how they did it.

It occurs to me that this is an excellent solution for rough surfaces on the fabricated parts, such as on a sand casting. If the part is nicely machined and then sat on a rough surface, the position of the top surface will not be well controlled.

--
JHG

 

[Belanger]

I was told that manufacturing and inspection must be done using the specified datums.

Ugh -- I can only hope that you heard the instructor wrong.
Datums merely tell you what reference frame the specified tolerances are anchored to. It doesn't really matter how a part is made as long as it meets the stated specifications.

That instructor was probably of the mindset that GD&T is for machined parts. But what about plastics? How can you be required to use the given datums when the entire part is molded in one shot?
That's a rhetorical question... I am not picking on you Drawoh, but rather I'm picking on your instructor

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

@drawoh: It is not "must" but it's desirable. The most "professionals" tend to take extreme points of view when the truth is somewhere in the middle.

Like Belanger mentioned, not all the parts are machined.

By the way JP, do you think that inspection should ignore the datums as well?

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

 

[Belanger]

By the way JP, do you think that inspection should ignore the datums as well?

No, because inspection is meant to verify the specifications. And those specifications are mandated to the datums.

Everybody thinks about the simple example of a hole drilled through a square block. But who says it has to be drilled? If I go outside, toss the block up in the air, and shoot a bullet through it, then it's possible I might have made a good part! But to inspect it, you'd have to square it up to the given datums, otherwise you can't document that it meets the drawing requirements; you're just guessing.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[mkcski]

The Y14.5-2009 standard is a for "product definition". It in no way implies any manufacturing or inspection methodology/technique. The part must meet the drawing requirements when manufacturing is completed. The ability to simulate the perfection on the drawing with imperfect "processing equipment" is, and always be, the dilemma.

Certified Sr. GD&T Professional

 

[CheckerHater]

@JP: Bad example.

If I take GD&T into consideration when fixturing the part, and then just eyeball the results without taking any measurements; I will have far greater chances producing good part then by shooting it.

Perhaps that's the reason you don't see manufacturers tossing parts into the air very often.

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