working with multiple profiles of surfaces

[Pupsocket]

... or is "surface profiles" the better plural?

Hi group,
I'm Karl, new here, long time tech/mechanical designer, very limited GD&T experience. I have a concept that must be fabricated, and while I can describe it to a machine shop with plenty of words and hand waving (the way it was described to me), I'd rather specify it in such a way that they don't tack on a $1000 design line item on their quote just for an hour's work interpreting my hand waving.

My issue involves proprietary items and designs, but I've simplified the CAD model. Here goes:

The concept consists of three tapering parts of complex cross section that taper consistently smaller as they go up. These three parts are held in alignment by being pressed lightly (transitional fit) into a base containing complementary holes. Parts will be either tool steel or some type of stainless, most likely made with wire EDM. The example shown has a 3/4" base, 3" of exposed tapers, and the tip profiles are offset in .1" from their form at the top of the base.

Criticals:
Form of each taper's sides (functional requirement, ±.0005)
Spacing of tapers and holes relative to themselves (functional requirement, ±.001)
Form of each hole (assembly requirement ±.0025 ±.00025 relative to tapers)

Non-critical:
Overall size of base

Question:
I've used relatively uncomplicated GD&T callouts on relatively simple part drawings. This one is beyond my experience, and searching online I couldn't find helpful examples. I am thinking profile of surface for each taper, but I'm at a loss for the base holes. Specifying two side datums and controlling the holes seems unnecessary if there's a way of specifying a datum based collectively on multiple surface profile features.

What would be a good strategy to capturing the above intents? Let me know if anything important is ambiguous; I'm trying to balance details while not exposing IP.

Thanks in advance!

Karl

 

[Burunduk]

Are the holes shaped like the tapered features? It could be easier to design round holes and matching cylindrical bosses at the bases of the tapered towers.
How critical is the mutual rotational orientation about an axis normal to the base?

 

[3DDave]

There will be a simultaneous requirement among all the profile tolerance Feature Control Frames (not GD&T). You can use all of the projections collectively as a datum feature. Best of luck doing that and making your inspectors happy.

 

[Pupsocket]

Hi Burunduk,
NOTE: typo in OP: base hole tolerance is ±.00025, not .0025.

This is a static assembly, and matching tapered holes, or cavities, in the base allow for an unchanging alignment, once all tapers are pressed in from below.

The actual tapered parts can have very small forms, making pinned or keyed and bolted attachments more complex to fab and less robust.

The primary purpose of the assembly is a go-no gauge to check vinyl extrusions for inner wall form by inserting it into the end of an extrusion and noting how far it goes. Optical scans of cross sections are also done at a lesser frequency.

Karl

 

[3DDave]

Unless this gauge is only 1 inch wide then 0.00025 on vinyl seems ambitious. Even a few degrees temperature change will make this useless.

 

[Burunduk]

I am assuming ASME Y14.5 as the drawing standard.
To capture the transition fit, you can design the tapered parts and the mating holes with identical basic dimensions and use unequally disposed profile tolerances (circled U) of the appropriate values to have the type of fit you want. Use the all around symbol for the profile feature control frames.
For the base, you can use one large face as a datum feature and reference it for the profile of each hole. This will take care of the orientation of the holes and the tall parts assembled into them relative to the base, and of the mutual location and orientation between the holes due to a simultaneous requirement. The outside edges of the base can be profiled with a loose tolerance relative to the same datum face as the holes and this will take care of the mutual location between the holes and the base edges, also due to a simultaneous requirement.

 

[Pupsocket]

Hi 3DDave,

The.00025 is the tolerance of each tapered hole in the steel base to its corresponding steel tapered shaft, in order to achieve a light press fit.

In my representative model, the .1 offset used to create the taper is driven by the vinyl profile tolerance. In this example the vinyl's tolerance zone is slightly less than .1, so that the gauge's range slightly exceeds that tolerance. The 30 to 1 taper angle magnifies the tolerance, like a wedge gap gauge.

Karl

 

[3DDave]

Karl,

I get the picture and am far ahead of you. Those Starrett gauges are for relatively harder metals than vinyl that have similar thermal expansion coefficients. Vinyl will easily deform and small temperature changes will produce large comparable variations. It's tough enough on a single feature, much more difficult on a group.

Just put 3X [profile | .0001] all over on the exposed surfaces. (The "3X" is the secret handshake that all the surfaces are simultaneously meeting profile and location at the same time.)

 

[Pupsocket]

Thank you Burunduk and 3DDave, I should be able to make intent understood to a fabricator thanks to your input.

While this concept wasn't my own design, I was assigned to procure it in part to experience a wire EDM design flow, something about old dogs and new tricks. Like learning gd&t when it's not even used internally.

Karl