Using a single pattern of features as datum BOTH as a group, and idividually, for different FCFs

[cbrf23]

The quick:

I've come up with a handful of possible options for how to structure the DRFs and FCFs for this part. I've narrowed it down to the two shown below, but I don't know which is better, or if there is maybe another way of doing this I'm overlooking. I believe both are compliant to Y14.5-2009, and I'm leaning towards option 1, but I wanted to get another set of eyes here to make sure the requirements are clearly communicated and as easy to understand as possible.

The dirty:

This is a cast part. It is cast to near net shape and then machined to final specifications. I need to establish both the relationship of the machined features to each other and some of the machined features to the rough casting.

For relating the machined features to each other, I've simply declared the first pattern as a datum feature, and created a compound true position FCF for the other pattern - using the first pattern as the secondary datum.

Now, for checking the features from the first pattern to the casting, I need to check each machined feature individually to it's cast member. I've read through section 7.4.8 of ASME Y14.5-2009 (which defines the use of the "INDIVIDUALLY" moniker - shown in figures 7-26 and 7-37) and I think both options I've constructed above would satisfy the standard, so I'm wondering if one is more clear than the other, if one would be preferred over the other for some reason, or if there's some other way of doing this that I'm overlooking.

 

[powerhound]

I want to look at this more but one thing that jumps out at me is that you didn't need to customize your datum reference frame. Just switch the secondary and tertiary datum references and it does the same thing.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[2JL]

Woah! There is a ton of things to look at. I will also need more time. So for now, one question: are datums B and C 2 of the 4 holes Ø1.035+/-.002? If yes, they are at the same time datum F.

2JL

 

[3DDave]

They are individually datum F to the counterbores.

Mostly it is too clever and allows for convoluted tolerance stacks that aren't likely to be understood or analyzed.
For example B and C are used to define E, but E is the basis for B and C.

There's also no good method for determining the effect of variation in the surface used for D.
For certain there is no mating part that is constrained to this one according to the given FCFs.
RFS is great for CMM checking of requirements, but usually fails to capture the function or sensitivity of the function of the part to variation.

 

[cbrf23]

Having trouble replying from my phone - I'll add a better response tomorrow from the office.

 

[cbrf23]

Wow, apparently having trouble replying from a computer as well.
Just typed out a nice detailed response, and when I hit post it just gave me a blank page that said "CCC" and when I went back, all content was lost.

@powerhound: That makes sense and is much simpler. One of the things I never liked about this print was the use of a custom DRF - I feel that since this is not commonly seen, it could easily be misconstrued.

@3DDave: You've made some good observations I think warrant an explanation.
(None of these things really have anything to do with the question I originally asked - regarding the best way to communicate the positional requirement relating the machined bores - I.D.s - to the casting counterparts - O.D.s.)

RFS for the 2.004 bores was chosen because of the mating part. These bores are hydraulic chambers, and the mating part is a piston. This piston has t-seals that seal against the internal surface of the cylinder and ride against it as the piston travels in it's stroke. The nature of these t-seals is such that they compress evenly all around the cylinder to take up any "slop" between the piston and the cylinder. So due to the elastic nature of the t-seals, there is no MMC bonus to be had here; the mating part's axis will always be concentric (as much as theoretically possible) to the axis of the bores, regardless of feature size.

-D- does not constrain anything in this assembly, it was chosen as the X,Y origin point for basic dimension for a handful of reasons (mostly related to the machining setup for this part and end use of the part) but also do characteristics of the casting design itself. As for irregularities in the surface, this is actually covered in Y14.5 section 4.11.2, where it says

"If irregularities on a datum feature are such that the part is unstable (that is, it rocks) when brought into contact with the corresponding datum feature simulator, the default stabilization procedure is per the candidate datum set as outlined in ASME Y14.5.1M. If a different procedure is desired (Chebychev, least squares, translational least squares, etc.), it must be specified."

This is handled by the CMM software. In other cases where cast datum features have presented a particular problem (e.g. due to an area that commonly sinks or twists during cooling, etc) I have used datum targets to define the area in which CMM points are to be taken from, however with this being a newly developed casting, we are still working through this process of identifying problem areas on the part - which couldn't really be done until physical parts were in hand

-B- and -C- were chosen after much back and forth with the supplier of the castings. It's a long story, but originally I had just -B-, which was defined as the inside width between the "legs" of the horse shoe (essentially the vertical centerline of the part). The DRF was then |A|B|D| essentially. We had some pre-production prodotypes cast from SLA models, and irregularities in the physical cast part made using these legs very unreliable; we would get drastically different results between facilities. I tried assigning datum targets on the legs to try to get measurements from the same general areas and while this helped, it wasn't really cutting it. I discussed with the supplier, and we decided to scrap this DRF as this would be a problem in production too. After discussion I changed the DRF to use the two top-most cylinders that are part of the current -G- datum. These became -B- and -C-; however this was short lived as when we went to production, our supplier discovered they needed to add a cross bar to the wax across these points to prevent twisting during cooling. This gets cut off and ground by hand down to the original surface, so with this hand grinding we decided these were not good datum features either. We cannot use the lower two cylinders as there are inlet ports on these (not seen in the screenshots above) which are measured in the CMM setup as well. We did discuss using v-blocks on the OD of what is the current -B- and -C- (the cast surfaces which are the FULL RADIUS of the legs) - however this would have required really tall v-blocks, and we decided together that it was easier to just use the machined holes, since the true position to -G- requirement is what actually controls rotation of the pattern in relation to the casting.

 

[fsincox]

To your your original question. I have always thought this type of thing might apply for two dowel pin holes with individual concentric features (like, pre-drilled steps) where the dowels are then used as a secondary in a framework for all other features. I see nothing conceptually wrong with the idea.
Frank

 

[3DDave]

G isn't a well defined feature. It isn't a feature of size and has undefined extent.

It is still unclear what locates this part to the mating part. Is it the hydraulic bores or the smaller holes?

Which ever one it is I'd make that group the secondary datum and add profile tolerances to the cast portions to control them, with refinements as required for the bosses, just as if the entire part was a hogout. You are describing the finished part, not a manufacturing step.

 

[fsincox]

3DDave,
I assume you meant the "G" in option 1? Option 2 seems clear to me.
Frank

 

[2JL]

Cbrf23

Option 2 is similar to what we are doing (or trying to do) currently to control during inspection the location of the "critical" as-cast surfaces relative to machined datums. The remaining as-cast faces are tied to the machined datums through a loose general profile tolerance.
I don't know how well it will do but we had to drop the use of datum targets because none of our suppliers was using them as intended to locate the cast part before machining (even though the target areas where suggested by them!). I'm not 100% confident about this method since there is case where we are calling out a true position on a reference diameter taken from a conical hole or boss (as probably yours).

2JL

 

[cbrf23]

@2JL - we've had the same type of issues with our suppliers -> they suggest something, then we come to find out that's not how they are checking it at all.
That's why I took the approach I did here; I've been working with our supplier's from the get go - having them in our facility, and us in theirs - so we could agree on common inspection methods, fixturing, etc.
Now, I'm basically trying to work backwards from the agreed upon inspection to create my geometric tolerancing.

@3DDave - there are mating parts to both features. Pistons go in the larger bores, these power a hydraulically actuated part. Guide rails go into the smaller holes, which the aforementioned part travels along. That's the simplest explanation I can think of.

@All - thank you all for the input. I really appreciate all your time and advice,
I've decided to go with Option 2, with the modification to DRF structure suggested by Powerhound.

Cheers!