O-Ring Dove Tail Groove Definition

[jimbod20]

Hello,

I need to modify or clarify the dove tail groove geometry on one of the parts currently being processed in the shop.

The modified dove tail groove geometry, using the Parker O-Ring Handbook as a guide, is provided in the attached sketch. The Parker handbook does not define the bottom (.015) or top (.005) radius size tolerance so the tolerance I define is twice the cutting tool radius size tolerance. Have a look at the attached sketch.

Are there any recommendations or clarifications needed to define the groove geometry beyond what I have defined in the attached? Is there experienced based dove tail groove geometry you can share that should be incorporated into my sketch?

 

[chez311]

jimbod20,

As far as the drawing specifications, if possible I think you'll find that a profile tolerance will much more reliably control the dimensions of a tapered groove like this. +/- dimensions look simple, but especially when applied to a tapered feature like a dovetail they could potentially allow a surprising amount of variation you're not expecting and is hard to quantify.

As far as the design itself, as long as you follow the recommended gland fill percentage requirements you should be fine (ie: smallest possible groove does not exceed maximum fill percentage). With the dovetail design you've got two additional factors you have to contend with/balance and thats how effectively the O-Ring is retained (ie: largest groove doesn't allow it to fall out under whatever assembly/shipping conditions) and how much force it takes to install it.

Why did you have to modify it? Did you have trouble with any of the above requirements in the field or did it need to be modified for manufacturing reasons?

 

[3DDave]

It is a mix. The groove is likely to be cut with a form tool, which means a precision machining of the tool. The real problem is that the section area of the groove is what is most important to be controlled, but the form tools are good enough that Parker doesn't seem to see the need to specify it. So, while a profile tolerance could be applied, it's like applying a profile tolerance to a screw thread. Almost no one does it as the form tools are good enough.

I always have mixed feelings - a profile tolerance is more precise and offers an easier analysis of the situation, but theses grooves are expensive to check and almost entirely determined by the form tool - and I know of no industry leader who has found a pressing need to use a profile tolerance.

tl;dr - best of luck to you.

 

[chez311]

3DDave,

You make a fair point. As long as the total depth is within tolerance as well as the width at the neck, you're probably good to go. Though strictly speaking the omission of a profile tolerance doesn't really technically free them from having to verify all those other dimensions and angles, it just means they probably won't as the requirement will likely be much less stringent without it.

 

[jimbod20]

Many Thanks for the input.

Chez311,
We have manufactured the part on three previous runs. Inspection found the groove width to sharp corners smaller than print allowable on the current manufacturing run resulting in an investigation into what we produced/shipped previously and why small on this current production run. Reviewing groove design guidance and the engineering drawing led me to the sketch I provided and the message thread here requesting input/guidance.

3DDave,
Your explanation makes sense to me. "The form tools are good enough that Parker doesn't seem to see the need to specify it.". Based upon this note, I'm going to define the engineering drawing radius tolerance to be twice tool specified size control.