Datum Features - edges of a hexagon to determine center plane 1

[jleeper]

I have a hexagon shaped part where holes are drilled through one of the surfaces (not the ends). A co-worker believes he can use the edges of the hexagon to determine the center plane of the feature and locate the holes. My understanding is that two planer surfaces can be used to create a center plane but not edges. I could not find an example in ASME Y14.5 showing the use of edges to determine a center plane.
Not sure what the right answer is...

 

[drawoh]

jleeper,

Is this a hexagonal part, or is there a shaft or thread?

I am very careful about using a Feature Of Size (FOS) as a datum. A crude rule of thumb is that your fixture should be ten times as accurate as the feature to be fabricated and/or inspected. You really do not want to use a sloppy feature as an FOS datum.

If your two faces are parallel, they make an FOS[ ]datum[ ]B, with the limitations noted above. If the top face of your hex is your primary datum[ ]A feature, your specification would look like this...

[box][⌖][/box][box][⌀]0.2[Ⓜ][/box][box]A[/box][box]B[Ⓜ][/box]​

This scheme with your secondary datum and your holes called up at MMC is fixturable and inspectable. Alternately, you can make the significant hex width dimensionally very accurate. If none of this is functional, you need a new datum scheme.

Hole patterns often are the most accurate feature of a part, thus, they make good datum features.

--
JHG

 

[jleeper]

drawoh,
The part is a stock size hexagonal shape, no shaft or thread. I understand two parallel faces making a datum, however my coworker is saying we can use two edges to create a datum.

 

[drawoh]

jleeper,

How are you going to fixture to two edges? If this part is machined, somebody will take a file to those edges. Is this a precise enough process for you to be able to use it as a fixture?

How are you mounting this thing? Using your mount faces as datum features is good design practise.

You can show an end-on view of your hex faces and apply datum targets to the corners that represent your edges. All of my objections above, continue to apply.

Why does your co-worker think two edges makes a good datum feature?

--
JHG

 

[3DDave]

I suggest just letting them apply datum symbols to the corners and sending it to the factory. Let the makers and inspectors explain to your coworker why not to do this. It's tiring to dissuade those you cannot fire from doing senseless things.

 

[drawoh]

3DDave,

I hope that is how it turns out. The fabricators can conclude the the OP's co-worker is an idiot, or they can conclude that we all are idiots.

--
JHG

 

[mfgenggear]

jleeper

what is the purpose of the cross holes. and how accurate to the hexagon. is there an assembly were it has to line up with pins or fasteners.
that the holes line up with a mating part. it would be easier to mill the hex then drill & ream. the holes would be zero from the end of the face. using 4 th or 5 axis
picking up a hex is not easy. if using a hex stock now relying on a stock surface for precision, using a hex collet. the hex it self could be indicated in to pick up the center line.
indicating across two opposite flats. it would be easier and faster to mill and drill all at once.

 

[jleeper]

All,
Thanks for the input. I finally convinced the department manager that my co-worker is wrong; hard to do as he is highly regarded in the department (and in his own mind). So many hours wasted on this drawing...