Advice required - how to dimension a snap in cylindrical part?

[DJBLINX]

I have a cylindrical injection moulded part that has a snap fit feature on the end that 'snaps' inside another cylindrical part and we are struggling how to dimension it. The problem being is that there are 4 gates equi-spaced around the perimeter and the part is not round, it is a sqircle (technical term for a square circle ). This isn't a problem as the parts still go together because there are two diameters at 90° that are the correct size (within tolerance) and the rest is small. So my question is, how can this be shown on the drawing? I realise there may be other factors not considered, but out of curiosity, is it possible to dimension a circle to show that at least two diameters are within a given tolerance?

Hopefully that is explained well enough and some one has a good idea of how to tackle the problem.

Thanks in advance

David.

 

[MikeHalloran]

In, (e.g.) AutoCAD, you might draw the perimeter as a series of lines and arcs, or as a polyline.

The set of elements that you had to draw to make an image of the part are also the elements that you have to dimension and tolerance, perhaps indirectly.

I don't see your difficulty.


Mike Halloran
Pembroke Pines, FL, USA

 

[pmarc]

Question #1. Is there a chance to see at least simplified sketch of what you are dealing with?
Question #2. Which GD&T standard are you working to - ASME or ISO?

 

[DJBLINX]

Q#1 Hopefully the attached will help.

Q#2 BS8888 (ISO)

 

[DJBLINX]

Any one out there have any advice?

Thanks

David.

 

[CheckerHater]

The way I understand it, your part is theoretically correct before it’s removed from the mold (or is it mould?).
Then, under normal working condition, it is forced into snap-fit with mating part.

Between these two conditions your part is in Free State Condition, and I believe ISO has a symbol for that.

Now you have option to specify:
Out-of roundness in free state.
MIN, MAX, and/or Average diameter in free state.
Emphasize 4-lobed shape of the part by specifying angle at which measurements are taken.
Resort to textual note explaining what you have explained to us.
Combination of all of the above in some form.

 

[DJBLINX]

Thanks for the reply, I had not thought of the free state condition. I'll look into it.