Starting Datums

[Troy442]

Seems like everytime we have a review of drawings there are many opinions of what the 3 "primary" datums should be. I am looking for some ideas from this group. I have attached a pdf of a single injection molded part. It will be inspected with a CMM touch probe. I have simplified the part for this discussion. There would be draft and ribs and small rounds etc. for molding.

This part holds a PC board with display and connector. There is another molded part that completes the enclosure and mounts to a wall.

So what are the thougts for A-B-C? Would you have a D or E?

Thanks, Troy GDTP-T-0386

 

[ctopher]

Your attached dwg does not show datums.

Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

 

[drawoh]

Troy442,

The best datums, most of the time, are your intended mounting points. Since your fabricator and inspectors are required to fixture to your datums, they should be good fixturing points.

It sounds like you are looking for some simple rules for everyone to follow. A simple, documented process is how you learn to make french fries at McDonald's. If your people are too dumb to place datums, they are too dumb to do drafting and design.

JHG

 

[SeasonLee]

The front surface is good for primary datum, but there are five mounting bosses for PCB and four mounting bosses for the bracket, it will cause some unavoidable shrinkage and/or warpage post molding on the front surface, therefore datum target (point or area) should be applied on the suitable area.

I will recommend the LCD opening as secondary datum based on the practical application, because of the physical geometry of the square hole opening, it will constrain translations x and y and orientation w as well, all DOF would be constrained, however it is necessary to use the other feature catching the clocking on this case, I will recommend the connector hole as tertiary datum.

SeasonLee

 

[Belanger]

Injection-molded parts are a classic example of something where you will never get everyone to agree on what the datums should be. The manufacturing folks will usually want the parting line to be a datum -- that makes some sense, because the dimensions on the tool are important from that parting line.

But guess what? Once the part is popped out of the mold, the parting line is meaningless. So the product engineers usually want the main datum to be some other feature that has functional importance. Both parties are correct in a sense, but it is true that the functional interface points make the best datums.

You aren't going to be able to get agreement; sometimes the squeakiest wheel gets their way. Sometimes on these plastic parts you could create "interim datums" -- that means you'll have x-y-z datums to please the manufacturing folks, and then a-b-c datums for the final functional checks. The new 2009 standard sort of alludes to this in paragraph 4.8.1.



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

 

[Troy442]

Thanks for the Friday afternoon responses. Our people here are fair with GDT, it really is the starting datums that are argued the most.

***Redrawing the part I see I simplified too much. The front surface (with LCD window) is a complex curve. It is not a nice flat.***

The meeting at work with 6 mechanical engineers (no machinists, molders, or inspectors) had the following mis-matched suggestions:
Datum A
1) In the side view the four bosses becomes a flat datum "A"
2) In the side view a single boss becomes a flat datum "A" and the other 3 bosses are ferenced to it (profile of surface)
3) In the side view the large perimeter (gasket area) becomes the datum "A"

Datum B:
4) Two of the yellow bosses become "B"
5) One yellow boss becomes "B"
6) Two datum targets are used on the long main outside edge to become "B"
7) The entire LCD window becomes "B"
8) A long horizontal edge of the LCD window becomes "B"

Datum C
9) If #8 above then a short vertical edge of the LCD window beomes "C"
10) One yellow boss becomes "C"
11) One datum target on the vertical outside perimeter becomes "C"

Datum D
Some green fetures that are tied to the PC Board become "D" and other green feures are dimensioned from "D"

Some people saw no need for "D".

Thanks for those comments, I would like to hear more. For me I am a fan of a B and C being able to be referenced as the part is set on the CMM from either the front or back. So a LCD window, or the port circle, or both.

I liked the comment that the 4 tall bosses will warp and might not be a good choice. I guess a built fixture for inspections in that case.

Troy

 

[PeterStock]

One more opinion, I would use the surfaces mating to the PCB as A and the holes of those bosses as a pattern datum B, everything would be related to A|B(M). If you needed to hold the gasket surface to the bracket mounting, you could use the bracket mounting surfaces as C and the the bracket mounting holes as a pattern datum D. C and D would be related back to A|B(M). The Gasket seal groove and the surfaces around it would be related to C|D(M).

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[i2xtreme]

Troy,

We have the same issues. Picking functional datums on components which have very few straight edges can be a challenge. Often times we find using the functional surfaces will not provide a repeatable results. In your case I would use the following if you want to check it unrestrained.

The 3 boss surfaces as datum A with the 4th controlled with profile. Then the long LCD edge as -B- and short as -C-. This will give you possibly the most repeatable measurement method off a cmm, plus it is far easier to set up.

The other option is a fixture, but sounds like that may want to be avoided.

Good luck.

 

[PaulJackson]

I tell the designers that ask my advice:
1. Identify the functional primary(that which constrains the most degrees of freedom functionally) then the secondary likewise(if one is necessary), then the tertiary likewise…
2. Imagine gage contacting the part on those features and in that order and ask yourself :
a. Is it stable and repeatable enough for inspection GR&R?
b. Are there desired alternatives that enhance measurement or process step measurements that people are willing to accept stacked functional tolerance allocations instead?
c. Does it make sense?
3. Stack up the critical functional requirements from the identified functional datum feature(s), establish the tolerance requirements, and then allocate those tolerance values between the alternate Datum references the functional datum references and the critical feature (location, orientation, and/or clearance) if necessary.


1. I may be wrong in my assumptions… but I would say that the functional datum feature between the housing’s two halves (there is another half I assume) is the “gasket trough.”
• Are there any other functional datum features for the cover halves fitting together to locate and orient to one another? I say no, assuming location and orientation is done by both halves compressing on the gasket groove.
• Are there any other features required to locate and orient? I say no (the rectangular ellipse constrains all six degrees-of-freedom) see ASME Y14.5-2009 paragraph and figure 4.3 pages 48 and 50 for an explanation.
• Do the four yellow fastener details do any of the locating and/or orienting of the halves to one another? I say ideally no… they have to be located to “gasket trough” as well as the opposite fastener clearance holes such that the fasteners do not exert any translation influence the gasket… just compression.

Are there any other functional system (sub-assembly) datum reference frames to consider? Yes… the circuit board mounting bosses.
• The functional primary being all five pedestal bases (three degrees-of-freedom)
• The secondary being the pattern of all five cored holes. (all remaining degrees-of freedom)
2. Since this appears to be a part injection molded in one piece in a die I would consider a uniform profile tolerance “all-over” and if necessary alternate “all-around” tolerances for the “trim-thru” windows to account for their alternate improved size and location requirements to the 5X circuit board sub-system datum reference frame.
• Is the primary stable and repeatable enough for inspection? I say yes… but if the rectangular ellipse is uniform then there are two possible orientations for the part… someone will have to select the proper orientation.

The whole part can be inspected on a DCC touch/scan CMM or Optical CMM and compared against the CAD model with wisker-plots of the profile deviations from a defined patter or selected points. Failing that a datum feature simulator mirroring the mating cover and gasket can be fashioned such that the part registers in it and selected points are measured for profile conformance with displacement probes zeroed by a nominal gage artifact.
• Are there desired alternatives…? Can not think of a scenario.
• Does it make sense? You decide!

This is the process I recommend all the time in selecting datum features.

Paul

 

[PaulJackson]

This is what I see as functional

Paul

 

[dingy2]

I guess I look at this a bit differently than Paul. I always look at the assembly of the part and would ask which surface is the mounting surface during assembly. That would become datum A.

Which hole is used for assembly during the first assembly operation. If all 5 holes are used for assembly at the same time, then the pattern would become datum B and it would not be necessary for a datum C. If only one hole is used, then it becomes datum B and all dimensions would come from that hole.

The tertiary, used for anti-rotation around datum B, could be 1 of the 5 holes or another hole but make sure that there is a fair amount of distance between the secondary datum hole and the tertiary.

Hope this helps.



Dave D.

http://www.qmsi.ca

 

[PaulJackson]

I assumed the mating joint was B in the description attached. If it is A or B I'll stick by the functional datum feature selection that I posted. Any other choice would require an alternate path stack that nests the mating part variation.

If the mating Joint is C then the 4X "yellow" CORED HOLES pattern would be the functional secondary datum feature on the part. The primary would be the two surfaces adjacent to the groove and the groove itself would be controlled to [A|B]. The 5X pedistal surfaces would be C, the 5X CORED HOLES D and the Trim-Thrus controlled to [C|D].

Paul

 

[Troy442]

Thanks for all of the great responses. We have been reading and discussing all of the ideas.

For the gasket your "B" shape assumption is correct. Groove in the part I had shown. Gasket material, the other molded plastic part (not shown) protrudes into the gasket to get around 62% squeeze.

I have a question on your "Trim Thru" callouts. Are you guessing those are secondary operations? In this case all the features are molded in. No secondary ops.

Great suggestions!

Thanks Troy

 

[PaulJackson]

I am not guessing whether they are installed/improved in a secondary OP... but chances are the appearance gap that exists between them and the features on the circuit board is a direct variation stack through [B|C] involving the circuit board 5X clearance holes, circuit board component positions and the cover's trim-thru window profiles to [B|C]. If the stacks don't yield an acceptable appearance gap some thing has to change... either a secondary op to improve profiles or better component positions on the circuit board and bigger 5X clearance holes or limited translational float of the components on the circuit board or ...

Paul

 

[PaulJackson]

The point is...

Establish your datum features the exactly the way your part functions first (the way that it interacts, orients, and locates relative to its function or assembly) then if you must... make concessions from there.

Don't simply choose one hole to be secondary and another tertiary if the pattern features have equal weight in stopping translation and rotation.

"Functional gages" are never functional if the Datum feature designations, material condition modifiers, and feature controls do not reflect the actual function... they are just attribute gages and they along with the stacks that legitimize their deployment do little to predict actual functional variation.

Paul

 

[PaulJackson]

With a uniform rectangular ellipse as the "primary-secondary&tertiary datum feature all-in-one... you may be wondering where to dimensionally reference the coordinate system X0Y0Z0.

The answer is: anywhere you desire.
I would choose its center tangent either to the bottom of the groove or the adjacent joint surfaces.

No matter where you put it... Inspection would have to find the center of the imperfect measured groove or the center of the mating atifact gage groove and translate to the X0Y0Z0 basic reference displacements to commence interrogation. Why not just leave it in the center so that no one confuses an alternate X0Y0Z0 basic reference that coincides with some other feature with the primary datum feature specified.

Paul

 

[restedup]

Paul,

I have to ask, what is a rectangular ellipse? I don't believe I have seen nor heard that term before.

 

[PaulJackson]

Hard to find but I imagine you will know when you've seen one, sort of like a triangular circle.

Paul

 

[restedup]

Paul,

A sketch would help. An I the only one in the forum who is unfamiliar with the term?

 

[PaulJackson]

Take a business card and put four push pins into a piece of cardboard at the corners of the business card.

Take a string and tie one end near the point of a pencil, wrap the string around the perimeter of the push pins with some marginal loosness and tie the other end around the pencil point.

While keeping tension on the string outward from the rectangle trace an ellipse about the rectangle.